FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/module/zfs/arc.c

     /*
      * CDDL HEADER START
      *
      * The contents of this file are subject to the terms of the
      * Common Development and Distribution License (the "License").
      * You may not use this file except in compliance with the License.
      *
      * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
      * or https://opensource.org/licenses/CDDL-1.0.
     * See the License for the specific language governing permissions
     * and limitations under the License.
     *
     * When distributing Covered Code, include this CDDL HEADER in each
     * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
     * If applicable, add the following below this CDDL HEADER, with the
     * fields enclosed by brackets "[]" replaced with your own identifying
     * information: Portions Copyright [yyyy] [name of copyright owner]
     *
     * CDDL HEADER END
     */
    /*
     * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
     * Copyright (c) 2018, Joyent, Inc.
     * Copyright (c) 2011, 2020, Delphix. All rights reserved.
     * Copyright (c) 2014, Saso Kiselkov. All rights reserved.
     * Copyright (c) 2017, Nexenta Systems, Inc.  All rights reserved.
     * Copyright (c) 2019, loli10K <ezomori.nozomu@gmail.com>. All rights reserved.
     * Copyright (c) 2020, George Amanakis. All rights reserved.
     * Copyright (c) 2019, Klara Inc.
     * Copyright (c) 2019, Allan Jude
     * Copyright (c) 2020, The FreeBSD Foundation [1]
     *
     * [1] Portions of this software were developed by Allan Jude
     *     under sponsorship from the FreeBSD Foundation.
     */
    
    /*
     * DVA-based Adjustable Replacement Cache
     *
     * While much of the theory of operation used here is
     * based on the self-tuning, low overhead replacement cache
     * presented by Megiddo and Modha at FAST 2003, there are some
     * significant differences:
     *
     * 1. The Megiddo and Modha model assumes any page is evictable.
     * Pages in its cache cannot be "locked" into memory.  This makes
     * the eviction algorithm simple: evict the last page in the list.
     * This also make the performance characteristics easy to reason
     * about.  Our cache is not so simple.  At any given moment, some
     * subset of the blocks in the cache are un-evictable because we
     * have handed out a reference to them.  Blocks are only evictable
     * when there are no external references active.  This makes
     * eviction far more problematic:  we choose to evict the evictable
     * blocks that are the "lowest" in the list.
     *
     * There are times when it is not possible to evict the requested
     * space.  In these circumstances we are unable to adjust the cache
     * size.  To prevent the cache growing unbounded at these times we
     * implement a "cache throttle" that slows the flow of new data
     * into the cache until we can make space available.
     *
     * 2. The Megiddo and Modha model assumes a fixed cache size.
     * Pages are evicted when the cache is full and there is a cache
     * miss.  Our model has a variable sized cache.  It grows with
     * high use, but also tries to react to memory pressure from the
     * operating system: decreasing its size when system memory is
     * tight.
     *
     * 3. The Megiddo and Modha model assumes a fixed page size. All
     * elements of the cache are therefore exactly the same size.  So
     * when adjusting the cache size following a cache miss, its simply
     * a matter of choosing a single page to evict.  In our model, we
     * have variable sized cache blocks (ranging from 512 bytes to
     * 128K bytes).  We therefore choose a set of blocks to evict to make
     * space for a cache miss that approximates as closely as possible
     * the space used by the new block.
     *
     * See also:  "ARC: A Self-Tuning, Low Overhead Replacement Cache"
     * by N. Megiddo & D. Modha, FAST 2003
     */
    
    /*
     * The locking model:
     *
     * A new reference to a cache buffer can be obtained in two
     * ways: 1) via a hash table lookup using the DVA as a key,
     * or 2) via one of the ARC lists.  The arc_read() interface
     * uses method 1, while the internal ARC algorithms for
     * adjusting the cache use method 2.  We therefore provide two
     * types of locks: 1) the hash table lock array, and 2) the
     * ARC list locks.
     *
     * Buffers do not have their own mutexes, rather they rely on the
     * hash table mutexes for the bulk of their protection (i.e. most
     * fields in the arc_buf_hdr_t are protected by these mutexes).
     *
     * buf_hash_find() returns the appropriate mutex (held) when it
     * locates the requested buffer in the hash table.  It returns
     * NULL for the mutex if the buffer was not in the table.
    *
    * buf_hash_remove() expects the appropriate hash mutex to be
    * already held before it is invoked.
    *
    * Each ARC state also has a mutex which is used to protect the
    * buffer list associated with the state.  When attempting to
    * obtain a hash table lock while holding an ARC list lock you
    * must use: mutex_tryenter() to avoid deadlock.  Also note that
    * the active state mutex must be held before the ghost state mutex.
    *
    * It as also possible to register a callback which is run when the
    * arc_meta_limit is reached and no buffers can be safely evicted.  In
    * this case the arc user should drop a reference on some arc buffers so
    * they can be reclaimed and the arc_meta_limit honored.  For example,
    * when using the ZPL each dentry holds a references on a znode.  These
    * dentries must be pruned before the arc buffer holding the znode can
    * be safely evicted.
    *
    * Note that the majority of the performance stats are manipulated
    * with atomic operations.
    *
    * The L2ARC uses the l2ad_mtx on each vdev for the following:
    *
    *      - L2ARC buflist creation
    *      - L2ARC buflist eviction
    *      - L2ARC write completion, which walks L2ARC buflists
    *      - ARC header destruction, as it removes from L2ARC buflists
    *      - ARC header release, as it removes from L2ARC buflists
    */
   
   /*
    * ARC operation:
    *
    * Every block that is in the ARC is tracked by an arc_buf_hdr_t structure.
    * This structure can point either to a block that is still in the cache or to
    * one that is only accessible in an L2 ARC device, or it can provide
    * information about a block that was recently evicted. If a block is
    * only accessible in the L2ARC, then the arc_buf_hdr_t only has enough
    * information to retrieve it from the L2ARC device. This information is
    * stored in the l2arc_buf_hdr_t sub-structure of the arc_buf_hdr_t. A block
    * that is in this state cannot access the data directly.
    *
    * Blocks that are actively being referenced or have not been evicted
    * are cached in the L1ARC. The L1ARC (l1arc_buf_hdr_t) is a structure within
    * the arc_buf_hdr_t that will point to the data block in memory. A block can
    * only be read by a consumer if it has an l1arc_buf_hdr_t. The L1ARC
    * caches data in two ways -- in a list of ARC buffers (arc_buf_t) and
    * also in the arc_buf_hdr_t's private physical data block pointer (b_pabd).
    *
    * The L1ARC's data pointer may or may not be uncompressed. The ARC has the
    * ability to store the physical data (b_pabd) associated with the DVA of the
    * arc_buf_hdr_t. Since the b_pabd is a copy of the on-disk physical block,
    * it will match its on-disk compression characteristics. This behavior can be
    * disabled by setting 'zfs_compressed_arc_enabled' to B_FALSE. When the
    * compressed ARC functionality is disabled, the b_pabd will point to an
    * uncompressed version of the on-disk data.
    *
    * Data in the L1ARC is not accessed by consumers of the ARC directly. Each
    * arc_buf_hdr_t can have multiple ARC buffers (arc_buf_t) which reference it.
    * Each ARC buffer (arc_buf_t) is being actively accessed by a specific ARC
    * consumer. The ARC will provide references to this data and will keep it
    * cached until it is no longer in use. The ARC caches only the L1ARC's physical
    * data block and will evict any arc_buf_t that is no longer referenced. The
    * amount of memory consumed by the arc_buf_ts' data buffers can be seen via the
    * "overhead_size" kstat.
    *
    * Depending on the consumer, an arc_buf_t can be requested in uncompressed or
    * compressed form. The typical case is that consumers will want uncompressed
    * data, and when that happens a new data buffer is allocated where the data is
    * decompressed for them to use. Currently the only consumer who wants
    * compressed arc_buf_t's is "zfs send", when it streams data exactly as it
    * exists on disk. When this happens, the arc_buf_t's data buffer is shared
    * with the arc_buf_hdr_t.
    *
    * Here is a diagram showing an arc_buf_hdr_t referenced by two arc_buf_t's. The
    * first one is owned by a compressed send consumer (and therefore references
    * the same compressed data buffer as the arc_buf_hdr_t) and the second could be
    * used by any other consumer (and has its own uncompressed copy of the data
    * buffer).
    *
    *   arc_buf_hdr_t
    *   +-----------+
    *   | fields    |
    *   | common to |
    *   | L1- and   |
    *   | L2ARC     |
    *   +-----------+
    *   | l2arc_buf_hdr_t
    *   |           |
    *   +-----------+
    *   | l1arc_buf_hdr_t
    *   |           |              arc_buf_t
    *   | b_buf     +------------>+-----------+      arc_buf_t
    *   | b_pabd    +-+           |b_next     +---->+-----------+
    *   +-----------+ |           |-----------|     |b_next     +-->NULL
    *                 |           |b_comp = T |     +-----------+
    *                 |           |b_data     +-+   |b_comp = F |
    *                 |           +-----------+ |   |b_data     +-+
    *                 +->+------+               |   +-----------+ |
    *        compressed  |      |               |                 |
    *           data     |      |<--------------+                 | uncompressed
    *                    +------+          compressed,            |     data
    *                                        shared               +-->+------+
    *                                         data                    |      |
    *                                                                 |      |
    *                                                                 +------+
    *
    * When a consumer reads a block, the ARC must first look to see if the
    * arc_buf_hdr_t is cached. If the hdr is cached then the ARC allocates a new
    * arc_buf_t and either copies uncompressed data into a new data buffer from an
    * existing uncompressed arc_buf_t, decompresses the hdr's b_pabd buffer into a
    * new data buffer, or shares the hdr's b_pabd buffer, depending on whether the
    * hdr is compressed and the desired compression characteristics of the
    * arc_buf_t consumer. If the arc_buf_t ends up sharing data with the
    * arc_buf_hdr_t and both of them are uncompressed then the arc_buf_t must be
    * the last buffer in the hdr's b_buf list, however a shared compressed buf can
    * be anywhere in the hdr's list.
    *
    * The diagram below shows an example of an uncompressed ARC hdr that is
    * sharing its data with an arc_buf_t (note that the shared uncompressed buf is
    * the last element in the buf list):
    *
    *                arc_buf_hdr_t
    *                +-----------+
    *                |           |
    *                |           |
    *                |           |
    *                +-----------+
    * l2arc_buf_hdr_t|           |
    *                |           |
    *                +-----------+
    * l1arc_buf_hdr_t|           |
    *                |           |                 arc_buf_t    (shared)
    *                |    b_buf  +------------>+---------+      arc_buf_t
    *                |           |             |b_next   +---->+---------+
    *                |  b_pabd   +-+           |---------|     |b_next   +-->NULL
    *                +-----------+ |           |         |     +---------+
    *                              |           |b_data   +-+   |         |
    *                              |           +---------+ |   |b_data   +-+
    *                              +->+------+             |   +---------+ |
    *                                 |      |             |               |
    *                   uncompressed  |      |             |               |
    *                        data     +------+             |               |
    *                                    ^                 +->+------+     |
    *                                    |       uncompressed |      |     |
    *                                    |           data     |      |     |
    *                                    |                    +------+     |
    *                                    +---------------------------------+
    *
    * Writing to the ARC requires that the ARC first discard the hdr's b_pabd
    * since the physical block is about to be rewritten. The new data contents
    * will be contained in the arc_buf_t. As the I/O pipeline performs the write,
    * it may compress the data before writing it to disk. The ARC will be called
    * with the transformed data and will memcpy the transformed on-disk block into
    * a newly allocated b_pabd. Writes are always done into buffers which have
    * either been loaned (and hence are new and don't have other readers) or
    * buffers which have been released (and hence have their own hdr, if there
    * were originally other readers of the buf's original hdr). This ensures that
    * the ARC only needs to update a single buf and its hdr after a write occurs.
    *
    * When the L2ARC is in use, it will also take advantage of the b_pabd. The
    * L2ARC will always write the contents of b_pabd to the L2ARC. This means
    * that when compressed ARC is enabled that the L2ARC blocks are identical
    * to the on-disk block in the main data pool. This provides a significant
    * advantage since the ARC can leverage the bp's checksum when reading from the
    * L2ARC to determine if the contents are valid. However, if the compressed
    * ARC is disabled, then the L2ARC's block must be transformed to look
    * like the physical block in the main data pool before comparing the
    * checksum and determining its validity.
    *
    * The L1ARC has a slightly different system for storing encrypted data.
    * Raw (encrypted + possibly compressed) data has a few subtle differences from
    * data that is just compressed. The biggest difference is that it is not
    * possible to decrypt encrypted data (or vice-versa) if the keys aren't loaded.
    * The other difference is that encryption cannot be treated as a suggestion.
    * If a caller would prefer compressed data, but they actually wind up with
    * uncompressed data the worst thing that could happen is there might be a
    * performance hit. If the caller requests encrypted data, however, we must be
    * sure they actually get it or else secret information could be leaked. Raw
    * data is stored in hdr->b_crypt_hdr.b_rabd. An encrypted header, therefore,
    * may have both an encrypted version and a decrypted version of its data at
    * once. When a caller needs a raw arc_buf_t, it is allocated and the data is
    * copied out of this header. To avoid complications with b_pabd, raw buffers
    * cannot be shared.
    */
   
   #include <sys/spa.h>
   #include <sys/zio.h>
   #include <sys/spa_impl.h>
   #include <sys/zio_compress.h>
   #include <sys/zio_checksum.h>
   #include <sys/zfs_context.h>
   #include <sys/arc.h>
   #include <sys/zfs_refcount.h>
   #include <sys/vdev.h>
   #include <sys/vdev_impl.h>
   #include <sys/dsl_pool.h>
   #include <sys/multilist.h>
   #include <sys/abd.h>
   #include <sys/zil.h>
   #include <sys/fm/fs/zfs.h>
   #include <sys/callb.h>
   #include <sys/kstat.h>
   #include <sys/zthr.h>
   #include <zfs_fletcher.h>
   #include <sys/arc_impl.h>
   #include <sys/trace_zfs.h>
   #include <sys/aggsum.h>
   #include <sys/wmsum.h>
   #include <cityhash.h>
   #include <sys/vdev_trim.h>
   #include <sys/zfs_racct.h>
   #include <sys/zstd/zstd.h>
   
   #ifndef _KERNEL
   /* set with ZFS_DEBUG=watch, to enable watchpoints on frozen buffers */
   boolean_t arc_watch = B_FALSE;
   #endif
   
   /*
    * This thread's job is to keep enough free memory in the system, by
    * calling arc_kmem_reap_soon() plus arc_reduce_target_size(), which improves
    * arc_available_memory().
    */
   static zthr_t *arc_reap_zthr;
   
   /*
    * This thread's job is to keep arc_size under arc_c, by calling
    * arc_evict(), which improves arc_is_overflowing().
    */
   static zthr_t *arc_evict_zthr;
   static arc_buf_hdr_t **arc_state_evict_markers;
   static int arc_state_evict_marker_count;
   
   static kmutex_t arc_evict_lock;
   static boolean_t arc_evict_needed = B_FALSE;
   static clock_t arc_last_uncached_flush;
   
   /*
    * Count of bytes evicted since boot.
    */
   static uint64_t arc_evict_count;
   
   /*
    * List of arc_evict_waiter_t's, representing threads waiting for the
    * arc_evict_count to reach specific values.
    */
   static list_t arc_evict_waiters;
   
   /*
    * When arc_is_overflowing(), arc_get_data_impl() waits for this percent of
    * the requested amount of data to be evicted.  For example, by default for
    * every 2KB that's evicted, 1KB of it may be "reused" by a new allocation.
    * Since this is above 100%, it ensures that progress is made towards getting
    * arc_size under arc_c.  Since this is finite, it ensures that allocations
    * can still happen, even during the potentially long time that arc_size is
    * more than arc_c.
    */
   static uint_t zfs_arc_eviction_pct = 200;
   
   /*
    * The number of headers to evict in arc_evict_state_impl() before
    * dropping the sublist lock and evicting from another sublist. A lower
    * value means we're more likely to evict the "correct" header (i.e. the
    * oldest header in the arc state), but comes with higher overhead
    * (i.e. more invocations of arc_evict_state_impl()).
    */
   static uint_t zfs_arc_evict_batch_limit = 10;
   
   /* number of seconds before growing cache again */
   uint_t arc_grow_retry = 5;
   
   /*
    * Minimum time between calls to arc_kmem_reap_soon().
    */
   static const int arc_kmem_cache_reap_retry_ms = 1000;
   
   /* shift of arc_c for calculating overflow limit in arc_get_data_impl */
   static int zfs_arc_overflow_shift = 8;
   
   /* shift of arc_c for calculating both min and max arc_p */
   static uint_t arc_p_min_shift = 4;
   
   /* log2(fraction of arc to reclaim) */
   uint_t arc_shrink_shift = 7;
   
   /* percent of pagecache to reclaim arc to */
   #ifdef _KERNEL
   uint_t zfs_arc_pc_percent = 0;
   #endif
   
   /*
    * log2(fraction of ARC which must be free to allow growing).
    * I.e. If there is less than arc_c >> arc_no_grow_shift free memory,
    * when reading a new block into the ARC, we will evict an equal-sized block
    * from the ARC.
    *
    * This must be less than arc_shrink_shift, so that when we shrink the ARC,
    * we will still not allow it to grow.
    */
   uint_t          arc_no_grow_shift = 5;
   
   
   /*
    * minimum lifespan of a prefetch block in clock ticks
    * (initialized in arc_init())
    */
   static uint_t           arc_min_prefetch_ms;
   static uint_t           arc_min_prescient_prefetch_ms;
   
   /*
    * If this percent of memory is free, don't throttle.
    */
   uint_t arc_lotsfree_percent = 10;
   
   /*
    * The arc has filled available memory and has now warmed up.
    */
   boolean_t arc_warm;
   
   /*
    * These tunables are for performance analysis.
    */
   uint64_t zfs_arc_max = 0;
   uint64_t zfs_arc_min = 0;
   uint64_t zfs_arc_meta_limit = 0;
   uint64_t zfs_arc_meta_min = 0;
   static uint64_t zfs_arc_dnode_limit = 0;
   static uint_t zfs_arc_dnode_reduce_percent = 10;
   static uint_t zfs_arc_grow_retry = 0;
   static uint_t zfs_arc_shrink_shift = 0;
   static uint_t zfs_arc_p_min_shift = 0;
   uint_t zfs_arc_average_blocksize = 8 * 1024; /* 8KB */
   
   /*
    * ARC dirty data constraints for arc_tempreserve_space() throttle:
    * * total dirty data limit
    * * anon block dirty limit
    * * each pool's anon allowance
    */
   static const unsigned long zfs_arc_dirty_limit_percent = 50;
   static const unsigned long zfs_arc_anon_limit_percent = 25;
   static const unsigned long zfs_arc_pool_dirty_percent = 20;
   
   /*
    * Enable or disable compressed arc buffers.
    */
   int zfs_compressed_arc_enabled = B_TRUE;
   
   /*
    * ARC will evict meta buffers that exceed arc_meta_limit. This
    * tunable make arc_meta_limit adjustable for different workloads.
    */
   static uint64_t zfs_arc_meta_limit_percent = 75;
   
   /*
    * Percentage that can be consumed by dnodes of ARC meta buffers.
    */
   static uint_t zfs_arc_dnode_limit_percent = 10;
   
   /*
    * These tunables are Linux-specific
    */
   static uint64_t zfs_arc_sys_free = 0;
   static uint_t zfs_arc_min_prefetch_ms = 0;
   static uint_t zfs_arc_min_prescient_prefetch_ms = 0;
   static int zfs_arc_p_dampener_disable = 1;
   static uint_t zfs_arc_meta_prune = 10000;
   static uint_t zfs_arc_meta_strategy = ARC_STRATEGY_META_BALANCED;
   static uint_t zfs_arc_meta_adjust_restarts = 4096;
   static uint_t zfs_arc_lotsfree_percent = 10;
   
   /*
    * Number of arc_prune threads
    */
   static int zfs_arc_prune_task_threads = 1;
   
   /* The 7 states: */
   arc_state_t ARC_anon;
   arc_state_t ARC_mru;
   arc_state_t ARC_mru_ghost;
   arc_state_t ARC_mfu;
   arc_state_t ARC_mfu_ghost;
   arc_state_t ARC_l2c_only;
   arc_state_t ARC_uncached;
   
   arc_stats_t arc_stats = {
           { "hits",                       KSTAT_DATA_UINT64 },
           { "iohits",                     KSTAT_DATA_UINT64 },
           { "misses",                     KSTAT_DATA_UINT64 },
           { "demand_data_hits",           KSTAT_DATA_UINT64 },
           { "demand_data_iohits",         KSTAT_DATA_UINT64 },
           { "demand_data_misses",         KSTAT_DATA_UINT64 },
           { "demand_metadata_hits",       KSTAT_DATA_UINT64 },
           { "demand_metadata_iohits",     KSTAT_DATA_UINT64 },
           { "demand_metadata_misses",     KSTAT_DATA_UINT64 },
           { "prefetch_data_hits",         KSTAT_DATA_UINT64 },
           { "prefetch_data_iohits",       KSTAT_DATA_UINT64 },
           { "prefetch_data_misses",       KSTAT_DATA_UINT64 },
           { "prefetch_metadata_hits",     KSTAT_DATA_UINT64 },
           { "prefetch_metadata_iohits",   KSTAT_DATA_UINT64 },
           { "prefetch_metadata_misses",   KSTAT_DATA_UINT64 },
           { "mru_hits",                   KSTAT_DATA_UINT64 },
           { "mru_ghost_hits",             KSTAT_DATA_UINT64 },
           { "mfu_hits",                   KSTAT_DATA_UINT64 },
           { "mfu_ghost_hits",             KSTAT_DATA_UINT64 },
           { "uncached_hits",              KSTAT_DATA_UINT64 },
           { "deleted",                    KSTAT_DATA_UINT64 },
           { "mutex_miss",                 KSTAT_DATA_UINT64 },
           { "access_skip",                KSTAT_DATA_UINT64 },
           { "evict_skip",                 KSTAT_DATA_UINT64 },
           { "evict_not_enough",           KSTAT_DATA_UINT64 },
           { "evict_l2_cached",            KSTAT_DATA_UINT64 },
           { "evict_l2_eligible",          KSTAT_DATA_UINT64 },
           { "evict_l2_eligible_mfu",      KSTAT_DATA_UINT64 },
           { "evict_l2_eligible_mru",      KSTAT_DATA_UINT64 },
           { "evict_l2_ineligible",        KSTAT_DATA_UINT64 },
           { "evict_l2_skip",              KSTAT_DATA_UINT64 },
           { "hash_elements",              KSTAT_DATA_UINT64 },
           { "hash_elements_max",          KSTAT_DATA_UINT64 },
           { "hash_collisions",            KSTAT_DATA_UINT64 },
           { "hash_chains",                KSTAT_DATA_UINT64 },
           { "hash_chain_max",             KSTAT_DATA_UINT64 },
           { "p",                          KSTAT_DATA_UINT64 },
           { "c",                          KSTAT_DATA_UINT64 },
           { "c_min",                      KSTAT_DATA_UINT64 },
           { "c_max",                      KSTAT_DATA_UINT64 },
           { "size",                       KSTAT_DATA_UINT64 },
           { "compressed_size",            KSTAT_DATA_UINT64 },
           { "uncompressed_size",          KSTAT_DATA_UINT64 },
           { "overhead_size",              KSTAT_DATA_UINT64 },
           { "hdr_size",                   KSTAT_DATA_UINT64 },
           { "data_size",                  KSTAT_DATA_UINT64 },
           { "metadata_size",              KSTAT_DATA_UINT64 },
           { "dbuf_size",                  KSTAT_DATA_UINT64 },
           { "dnode_size",                 KSTAT_DATA_UINT64 },
           { "bonus_size",                 KSTAT_DATA_UINT64 },
   #if defined(COMPAT_FREEBSD11)
           { "other_size",                 KSTAT_DATA_UINT64 },
   #endif
           { "anon_size",                  KSTAT_DATA_UINT64 },
           { "anon_evictable_data",        KSTAT_DATA_UINT64 },
           { "anon_evictable_metadata",    KSTAT_DATA_UINT64 },
           { "mru_size",                   KSTAT_DATA_UINT64 },
           { "mru_evictable_data",         KSTAT_DATA_UINT64 },
           { "mru_evictable_metadata",     KSTAT_DATA_UINT64 },
           { "mru_ghost_size",             KSTAT_DATA_UINT64 },
           { "mru_ghost_evictable_data",   KSTAT_DATA_UINT64 },
           { "mru_ghost_evictable_metadata", KSTAT_DATA_UINT64 },
           { "mfu_size",                   KSTAT_DATA_UINT64 },
           { "mfu_evictable_data",         KSTAT_DATA_UINT64 },
           { "mfu_evictable_metadata",     KSTAT_DATA_UINT64 },
           { "mfu_ghost_size",             KSTAT_DATA_UINT64 },
           { "mfu_ghost_evictable_data",   KSTAT_DATA_UINT64 },
           { "mfu_ghost_evictable_metadata", KSTAT_DATA_UINT64 },
           { "uncached_size",              KSTAT_DATA_UINT64 },
           { "uncached_evictable_data",    KSTAT_DATA_UINT64 },
           { "uncached_evictable_metadata", KSTAT_DATA_UINT64 },
           { "l2_hits",                    KSTAT_DATA_UINT64 },
           { "l2_misses",                  KSTAT_DATA_UINT64 },
           { "l2_prefetch_asize",          KSTAT_DATA_UINT64 },
           { "l2_mru_asize",               KSTAT_DATA_UINT64 },
           { "l2_mfu_asize",               KSTAT_DATA_UINT64 },
           { "l2_bufc_data_asize",         KSTAT_DATA_UINT64 },
           { "l2_bufc_metadata_asize",     KSTAT_DATA_UINT64 },
           { "l2_feeds",                   KSTAT_DATA_UINT64 },
           { "l2_rw_clash",                KSTAT_DATA_UINT64 },
           { "l2_read_bytes",              KSTAT_DATA_UINT64 },
           { "l2_write_bytes",             KSTAT_DATA_UINT64 },
           { "l2_writes_sent",             KSTAT_DATA_UINT64 },
           { "l2_writes_done",             KSTAT_DATA_UINT64 },
           { "l2_writes_error",            KSTAT_DATA_UINT64 },
           { "l2_writes_lock_retry",       KSTAT_DATA_UINT64 },
           { "l2_evict_lock_retry",        KSTAT_DATA_UINT64 },
           { "l2_evict_reading",           KSTAT_DATA_UINT64 },
           { "l2_evict_l1cached",          KSTAT_DATA_UINT64 },
           { "l2_free_on_write",           KSTAT_DATA_UINT64 },
           { "l2_abort_lowmem",            KSTAT_DATA_UINT64 },
           { "l2_cksum_bad",               KSTAT_DATA_UINT64 },
           { "l2_io_error",                KSTAT_DATA_UINT64 },
           { "l2_size",                    KSTAT_DATA_UINT64 },
           { "l2_asize",                   KSTAT_DATA_UINT64 },
           { "l2_hdr_size",                KSTAT_DATA_UINT64 },
           { "l2_log_blk_writes",          KSTAT_DATA_UINT64 },
           { "l2_log_blk_avg_asize",       KSTAT_DATA_UINT64 },
           { "l2_log_blk_asize",           KSTAT_DATA_UINT64 },
           { "l2_log_blk_count",           KSTAT_DATA_UINT64 },
           { "l2_data_to_meta_ratio",      KSTAT_DATA_UINT64 },
           { "l2_rebuild_success",         KSTAT_DATA_UINT64 },
           { "l2_rebuild_unsupported",     KSTAT_DATA_UINT64 },
           { "l2_rebuild_io_errors",       KSTAT_DATA_UINT64 },
           { "l2_rebuild_dh_errors",       KSTAT_DATA_UINT64 },
           { "l2_rebuild_cksum_lb_errors", KSTAT_DATA_UINT64 },
           { "l2_rebuild_lowmem",          KSTAT_DATA_UINT64 },
           { "l2_rebuild_size",            KSTAT_DATA_UINT64 },
           { "l2_rebuild_asize",           KSTAT_DATA_UINT64 },
           { "l2_rebuild_bufs",            KSTAT_DATA_UINT64 },
           { "l2_rebuild_bufs_precached",  KSTAT_DATA_UINT64 },
           { "l2_rebuild_log_blks",        KSTAT_DATA_UINT64 },
           { "memory_throttle_count",      KSTAT_DATA_UINT64 },
           { "memory_direct_count",        KSTAT_DATA_UINT64 },
           { "memory_indirect_count",      KSTAT_DATA_UINT64 },
           { "memory_all_bytes",           KSTAT_DATA_UINT64 },
           { "memory_free_bytes",          KSTAT_DATA_UINT64 },
           { "memory_available_bytes",     KSTAT_DATA_INT64 },
           { "arc_no_grow",                KSTAT_DATA_UINT64 },
           { "arc_tempreserve",            KSTAT_DATA_UINT64 },
           { "arc_loaned_bytes",           KSTAT_DATA_UINT64 },
           { "arc_prune",                  KSTAT_DATA_UINT64 },
           { "arc_meta_used",              KSTAT_DATA_UINT64 },
           { "arc_meta_limit",             KSTAT_DATA_UINT64 },
           { "arc_dnode_limit",            KSTAT_DATA_UINT64 },
           { "arc_meta_max",               KSTAT_DATA_UINT64 },
           { "arc_meta_min",               KSTAT_DATA_UINT64 },
           { "async_upgrade_sync",         KSTAT_DATA_UINT64 },
           { "predictive_prefetch", KSTAT_DATA_UINT64 },
           { "demand_hit_predictive_prefetch", KSTAT_DATA_UINT64 },
           { "demand_iohit_predictive_prefetch", KSTAT_DATA_UINT64 },
           { "prescient_prefetch", KSTAT_DATA_UINT64 },
           { "demand_hit_prescient_prefetch", KSTAT_DATA_UINT64 },
           { "demand_iohit_prescient_prefetch", KSTAT_DATA_UINT64 },
           { "arc_need_free",              KSTAT_DATA_UINT64 },
           { "arc_sys_free",               KSTAT_DATA_UINT64 },
           { "arc_raw_size",               KSTAT_DATA_UINT64 },
           { "cached_only_in_progress",    KSTAT_DATA_UINT64 },
           { "abd_chunk_waste_size",       KSTAT_DATA_UINT64 },
   };
   
   arc_sums_t arc_sums;
   
   #define ARCSTAT_MAX(stat, val) {                                        \
           uint64_t m;                                                     \
           while ((val) > (m = arc_stats.stat.value.ui64) &&               \
               (m != atomic_cas_64(&arc_stats.stat.value.ui64, m, (val)))) \
                   continue;                                               \
   }
   
   /*
    * We define a macro to allow ARC hits/misses to be easily broken down by
    * two separate conditions, giving a total of four different subtypes for
    * each of hits and misses (so eight statistics total).
    */
   #define ARCSTAT_CONDSTAT(cond1, stat1, notstat1, cond2, stat2, notstat2, stat) \
           if (cond1) {                                                    \
                   if (cond2) {                                            \
                           ARCSTAT_BUMP(arcstat_##stat1##_##stat2##_##stat); \
                   } else {                                                \
                           ARCSTAT_BUMP(arcstat_##stat1##_##notstat2##_##stat); \
                   }                                                       \
           } else {                                                        \
                   if (cond2) {                                            \
                           ARCSTAT_BUMP(arcstat_##notstat1##_##stat2##_##stat); \
                   } else {                                                \
                           ARCSTAT_BUMP(arcstat_##notstat1##_##notstat2##_##stat);\
                   }                                                       \
           }
   
   /*
    * This macro allows us to use kstats as floating averages. Each time we
    * update this kstat, we first factor it and the update value by
    * ARCSTAT_AVG_FACTOR to shrink the new value's contribution to the overall
    * average. This macro assumes that integer loads and stores are atomic, but
    * is not safe for multiple writers updating the kstat in parallel (only the
    * last writer's update will remain).
    */
   #define ARCSTAT_F_AVG_FACTOR    3
   #define ARCSTAT_F_AVG(stat, value) \
           do { \
                   uint64_t x = ARCSTAT(stat); \
                   x = x - x / ARCSTAT_F_AVG_FACTOR + \
                       (value) / ARCSTAT_F_AVG_FACTOR; \
                   ARCSTAT(stat) = x; \
           } while (0)
   
   static kstat_t                  *arc_ksp;
   
   /*
    * There are several ARC variables that are critical to export as kstats --
    * but we don't want to have to grovel around in the kstat whenever we wish to
    * manipulate them.  For these variables, we therefore define them to be in
    * terms of the statistic variable.  This assures that we are not introducing
    * the possibility of inconsistency by having shadow copies of the variables,
    * while still allowing the code to be readable.
    */
   #define arc_tempreserve ARCSTAT(arcstat_tempreserve)
   #define arc_loaned_bytes        ARCSTAT(arcstat_loaned_bytes)
   #define arc_meta_limit  ARCSTAT(arcstat_meta_limit) /* max size for metadata */
   /* max size for dnodes */
   #define arc_dnode_size_limit    ARCSTAT(arcstat_dnode_limit)
   #define arc_meta_min    ARCSTAT(arcstat_meta_min) /* min size for metadata */
   #define arc_need_free   ARCSTAT(arcstat_need_free) /* waiting to be evicted */
   
   hrtime_t arc_growtime;
   list_t arc_prune_list;
   kmutex_t arc_prune_mtx;
   taskq_t *arc_prune_taskq;
   
   #define GHOST_STATE(state)      \
           ((state) == arc_mru_ghost || (state) == arc_mfu_ghost ||        \
           (state) == arc_l2c_only)
   
   #define HDR_IN_HASH_TABLE(hdr)  ((hdr)->b_flags & ARC_FLAG_IN_HASH_TABLE)
   #define HDR_IO_IN_PROGRESS(hdr) ((hdr)->b_flags & ARC_FLAG_IO_IN_PROGRESS)
   #define HDR_IO_ERROR(hdr)       ((hdr)->b_flags & ARC_FLAG_IO_ERROR)
   #define HDR_PREFETCH(hdr)       ((hdr)->b_flags & ARC_FLAG_PREFETCH)
   #define HDR_PRESCIENT_PREFETCH(hdr)     \
           ((hdr)->b_flags & ARC_FLAG_PRESCIENT_PREFETCH)
   #define HDR_COMPRESSION_ENABLED(hdr)    \
           ((hdr)->b_flags & ARC_FLAG_COMPRESSED_ARC)
   
   #define HDR_L2CACHE(hdr)        ((hdr)->b_flags & ARC_FLAG_L2CACHE)
   #define HDR_UNCACHED(hdr)       ((hdr)->b_flags & ARC_FLAG_UNCACHED)
   #define HDR_L2_READING(hdr)     \
           (((hdr)->b_flags & ARC_FLAG_IO_IN_PROGRESS) &&  \
           ((hdr)->b_flags & ARC_FLAG_HAS_L2HDR))
   #define HDR_L2_WRITING(hdr)     ((hdr)->b_flags & ARC_FLAG_L2_WRITING)
   #define HDR_L2_EVICTED(hdr)     ((hdr)->b_flags & ARC_FLAG_L2_EVICTED)
   #define HDR_L2_WRITE_HEAD(hdr)  ((hdr)->b_flags & ARC_FLAG_L2_WRITE_HEAD)
   #define HDR_PROTECTED(hdr)      ((hdr)->b_flags & ARC_FLAG_PROTECTED)
   #define HDR_NOAUTH(hdr)         ((hdr)->b_flags & ARC_FLAG_NOAUTH)
   #define HDR_SHARED_DATA(hdr)    ((hdr)->b_flags & ARC_FLAG_SHARED_DATA)
   
   #define HDR_ISTYPE_METADATA(hdr)        \
           ((hdr)->b_flags & ARC_FLAG_BUFC_METADATA)
   #define HDR_ISTYPE_DATA(hdr)    (!HDR_ISTYPE_METADATA(hdr))
   
   #define HDR_HAS_L1HDR(hdr)      ((hdr)->b_flags & ARC_FLAG_HAS_L1HDR)
   #define HDR_HAS_L2HDR(hdr)      ((hdr)->b_flags & ARC_FLAG_HAS_L2HDR)
   #define HDR_HAS_RABD(hdr)       \
           (HDR_HAS_L1HDR(hdr) && HDR_PROTECTED(hdr) &&    \
           (hdr)->b_crypt_hdr.b_rabd != NULL)
   #define HDR_ENCRYPTED(hdr)      \
           (HDR_PROTECTED(hdr) && DMU_OT_IS_ENCRYPTED((hdr)->b_crypt_hdr.b_ot))
   #define HDR_AUTHENTICATED(hdr)  \
           (HDR_PROTECTED(hdr) && !DMU_OT_IS_ENCRYPTED((hdr)->b_crypt_hdr.b_ot))
   
   /* For storing compression mode in b_flags */
   #define HDR_COMPRESS_OFFSET     (highbit64(ARC_FLAG_COMPRESS_0) - 1)
   
   #define HDR_GET_COMPRESS(hdr)   ((enum zio_compress)BF32_GET((hdr)->b_flags, \
           HDR_COMPRESS_OFFSET, SPA_COMPRESSBITS))
   #define HDR_SET_COMPRESS(hdr, cmp) BF32_SET((hdr)->b_flags, \
           HDR_COMPRESS_OFFSET, SPA_COMPRESSBITS, (cmp));
   
   #define ARC_BUF_LAST(buf)       ((buf)->b_next == NULL)
   #define ARC_BUF_SHARED(buf)     ((buf)->b_flags & ARC_BUF_FLAG_SHARED)
   #define ARC_BUF_COMPRESSED(buf) ((buf)->b_flags & ARC_BUF_FLAG_COMPRESSED)
   #define ARC_BUF_ENCRYPTED(buf)  ((buf)->b_flags & ARC_BUF_FLAG_ENCRYPTED)
   
   /*
    * Other sizes
    */
   
   #define HDR_FULL_CRYPT_SIZE ((int64_t)sizeof (arc_buf_hdr_t))
   #define HDR_FULL_SIZE ((int64_t)offsetof(arc_buf_hdr_t, b_crypt_hdr))
   #define HDR_L2ONLY_SIZE ((int64_t)offsetof(arc_buf_hdr_t, b_l1hdr))
   
   /*
    * Hash table routines
    */
   
   #define BUF_LOCKS 2048
   typedef struct buf_hash_table {
           uint64_t ht_mask;
           arc_buf_hdr_t **ht_table;
           kmutex_t ht_locks[BUF_LOCKS] ____cacheline_aligned;
   } buf_hash_table_t;
   
   static buf_hash_table_t buf_hash_table;
   
   #define BUF_HASH_INDEX(spa, dva, birth) \
           (buf_hash(spa, dva, birth) & buf_hash_table.ht_mask)
   #define BUF_HASH_LOCK(idx)      (&buf_hash_table.ht_locks[idx & (BUF_LOCKS-1)])
   #define HDR_LOCK(hdr) \
           (BUF_HASH_LOCK(BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth)))
   
   uint64_t zfs_crc64_table[256];
   
   /*
    * Level 2 ARC
    */
   
   #define L2ARC_WRITE_SIZE        (8 * 1024 * 1024)       /* initial write max */
   #define L2ARC_HEADROOM          2                       /* num of writes */
   
   /*
    * If we discover during ARC scan any buffers to be compressed, we boost
    * our headroom for the next scanning cycle by this percentage multiple.
    */
   #define L2ARC_HEADROOM_BOOST    200
   #define L2ARC_FEED_SECS         1               /* caching interval secs */
   #define L2ARC_FEED_MIN_MS       200             /* min caching interval ms */
   
   /*
    * We can feed L2ARC from two states of ARC buffers, mru and mfu,
    * and each of the state has two types: data and metadata.
    */
   #define L2ARC_FEED_TYPES        4
   
   /* L2ARC Performance Tunables */
   uint64_t l2arc_write_max = L2ARC_WRITE_SIZE;    /* def max write size */
   uint64_t l2arc_write_boost = L2ARC_WRITE_SIZE;  /* extra warmup write */
   uint64_t l2arc_headroom = L2ARC_HEADROOM;       /* # of dev writes */
   uint64_t l2arc_headroom_boost = L2ARC_HEADROOM_BOOST;
   uint64_t l2arc_feed_secs = L2ARC_FEED_SECS;     /* interval seconds */
   uint64_t l2arc_feed_min_ms = L2ARC_FEED_MIN_MS; /* min interval msecs */
   int l2arc_noprefetch = B_TRUE;                  /* don't cache prefetch bufs */
   int l2arc_feed_again = B_TRUE;                  /* turbo warmup */
   int l2arc_norw = B_FALSE;                       /* no reads during writes */
   static uint_t l2arc_meta_percent = 33;  /* limit on headers size */
   
   /*
    * L2ARC Internals
    */
   static list_t L2ARC_dev_list;                   /* device list */
   static list_t *l2arc_dev_list;                  /* device list pointer */
   static kmutex_t l2arc_dev_mtx;                  /* device list mutex */
   static l2arc_dev_t *l2arc_dev_last;             /* last device used */
   static list_t L2ARC_free_on_write;              /* free after write buf list */
   static list_t *l2arc_free_on_write;             /* free after write list ptr */
   static kmutex_t l2arc_free_on_write_mtx;        /* mutex for list */
   static uint64_t l2arc_ndev;                     /* number of devices */
   
   typedef struct l2arc_read_callback {
           arc_buf_hdr_t           *l2rcb_hdr;             /* read header */
           blkptr_t                l2rcb_bp;               /* original blkptr */
           zbookmark_phys_t        l2rcb_zb;               /* original bookmark */
           int                     l2rcb_flags;            /* original flags */
           abd_t                   *l2rcb_abd;             /* temporary buffer */
   } l2arc_read_callback_t;
   
   typedef struct l2arc_data_free {
           /* protected by l2arc_free_on_write_mtx */
           abd_t           *l2df_abd;
           size_t          l2df_size;
           arc_buf_contents_t l2df_type;
           list_node_t     l2df_list_node;
   } l2arc_data_free_t;
   
   typedef enum arc_fill_flags {
           ARC_FILL_LOCKED         = 1 << 0, /* hdr lock is held */
           ARC_FILL_COMPRESSED     = 1 << 1, /* fill with compressed data */
           ARC_FILL_ENCRYPTED      = 1 << 2, /* fill with encrypted data */
           ARC_FILL_NOAUTH         = 1 << 3, /* don't attempt to authenticate */
           ARC_FILL_IN_PLACE       = 1 << 4  /* fill in place (special case) */
   } arc_fill_flags_t;
   
   typedef enum arc_ovf_level {
           ARC_OVF_NONE,                   /* ARC within target size. */
           ARC_OVF_SOME,                   /* ARC is slightly overflowed. */
           ARC_OVF_SEVERE                  /* ARC is severely overflowed. */
   } arc_ovf_level_t;
   
   static kmutex_t l2arc_feed_thr_lock;
   static kcondvar_t l2arc_feed_thr_cv;
   static uint8_t l2arc_thread_exit;
   
   static kmutex_t l2arc_rebuild_thr_lock;
   static kcondvar_t l2arc_rebuild_thr_cv;
   
   enum arc_hdr_alloc_flags {
           ARC_HDR_ALLOC_RDATA = 0x1,
           ARC_HDR_DO_ADAPT = 0x2,
           ARC_HDR_USE_RESERVE = 0x4,
           ARC_HDR_ALLOC_LINEAR = 0x8,
   };
   
   
   static abd_t *arc_get_data_abd(arc_buf_hdr_t *, uint64_t, const void *, int);
   static void *arc_get_data_buf(arc_buf_hdr_t *, uint64_t, const void *);
   static void arc_get_data_impl(arc_buf_hdr_t *, uint64_t, const void *, int);
   static void arc_free_data_abd(arc_buf_hdr_t *, abd_t *, uint64_t, const void *);
   static void arc_free_data_buf(arc_buf_hdr_t *, void *, uint64_t, const void *);
   static void arc_free_data_impl(arc_buf_hdr_t *hdr, uint64_t size,
       const void *tag);
   static void arc_hdr_free_abd(arc_buf_hdr_t *, boolean_t);
   static void arc_hdr_alloc_abd(arc_buf_hdr_t *, int);
   static void arc_hdr_destroy(arc_buf_hdr_t *);
   static void arc_access(arc_buf_hdr_t *, arc_flags_t, boolean_t);
   static void arc_buf_watch(arc_buf_t *);
   static void arc_change_state(arc_state_t *, arc_buf_hdr_t *);
   
   static arc_buf_contents_t arc_buf_type(arc_buf_hdr_t *);
   static uint32_t arc_bufc_to_flags(arc_buf_contents_t);
   static inline void arc_hdr_set_flags(arc_buf_hdr_t *hdr, arc_flags_t flags);
   static inline void arc_hdr_clear_flags(arc_buf_hdr_t *hdr, arc_flags_t flags);
   
   static boolean_t l2arc_write_eligible(uint64_t, arc_buf_hdr_t *);
   static void l2arc_read_done(zio_t *);
   static void l2arc_do_free_on_write(void);
   static void l2arc_hdr_arcstats_update(arc_buf_hdr_t *hdr, boolean_t incr,
       boolean_t state_only);
   
   #define l2arc_hdr_arcstats_increment(hdr) \
           l2arc_hdr_arcstats_update((hdr), B_TRUE, B_FALSE)
   #define l2arc_hdr_arcstats_decrement(hdr) \
           l2arc_hdr_arcstats_update((hdr), B_FALSE, B_FALSE)
   #define l2arc_hdr_arcstats_increment_state(hdr) \
           l2arc_hdr_arcstats_update((hdr), B_TRUE, B_TRUE)
   #define l2arc_hdr_arcstats_decrement_state(hdr) \
           l2arc_hdr_arcstats_update((hdr), B_FALSE, B_TRUE)
   
   /*
    * l2arc_exclude_special : A zfs module parameter that controls whether buffers
    *              present on special vdevs are eligibile for caching in L2ARC. If
    *              set to 1, exclude dbufs on special vdevs from being cached to
    *              L2ARC.
    */
   int l2arc_exclude_special = 0;
   
   /*
    * l2arc_mfuonly : A ZFS module parameter that controls whether only MFU
    *              metadata and data are cached from ARC into L2ARC.
    */
   static int l2arc_mfuonly = 0;
   
   /*
    * L2ARC TRIM
    * l2arc_trim_ahead : A ZFS module parameter that controls how much ahead of
    *              the current write size (l2arc_write_max) we should TRIM if we
    *              have filled the device. It is defined as a percentage of the
    *              write size. If set to 100 we trim twice the space required to
    *              accommodate upcoming writes. A minimum of 64MB will be trimmed.
    *              It also enables TRIM of the whole L2ARC device upon creation or
    *              addition to an existing pool or if the header of the device is
    *              invalid upon importing a pool or onlining a cache device. The
    *              default is 0, which disables TRIM on L2ARC altogether as it can
    *              put significant stress on the underlying storage devices. This
    *              will vary depending of how well the specific device handles
    *              these commands.
    */
   static uint64_t l2arc_trim_ahead = 0;
   
   /*
    * Performance tuning of L2ARC persistence:
    *
    * l2arc_rebuild_enabled : A ZFS module parameter that controls whether adding
    *              an L2ARC device (either at pool import or later) will attempt
    *              to rebuild L2ARC buffer contents.
    * l2arc_rebuild_blocks_min_l2size : A ZFS module parameter that controls
    *              whether log blocks are written to the L2ARC device. If the L2ARC
    *              device is less than 1GB, the amount of data l2arc_evict()
    *              evicts is significant compared to the amount of restored L2ARC
    *              data. In this case do not write log blocks in L2ARC in order
    *              not to waste space.
    */
   static int l2arc_rebuild_enabled = B_TRUE;
   static uint64_t l2arc_rebuild_blocks_min_l2size = 1024 * 1024 * 1024;
   
   /* L2ARC persistence rebuild control routines. */
   void l2arc_rebuild_vdev(vdev_t *vd, boolean_t reopen);
   static __attribute__((noreturn)) void l2arc_dev_rebuild_thread(void *arg);
   static int l2arc_rebuild(l2arc_dev_t *dev);
   
   /* L2ARC persistence read I/O routines. */
   static int l2arc_dev_hdr_read(l2arc_dev_t *dev);
   static int l2arc_log_blk_read(l2arc_dev_t *dev,
       const l2arc_log_blkptr_t *this_lp, const l2arc_log_blkptr_t *next_lp,
       l2arc_log_blk_phys_t *this_lb, l2arc_log_blk_phys_t *next_lb,
       zio_t *this_io, zio_t **next_io);
   static zio_t *l2arc_log_blk_fetch(vdev_t *vd,
       const l2arc_log_blkptr_t *lp, l2arc_log_blk_phys_t *lb);
   static void l2arc_log_blk_fetch_abort(zio_t *zio);
   
   /* L2ARC persistence block restoration routines. */
   static void l2arc_log_blk_restore(l2arc_dev_t *dev,
       const l2arc_log_blk_phys_t *lb, uint64_t lb_asize);
   static void l2arc_hdr_restore(const l2arc_log_ent_phys_t *le,
       l2arc_dev_t *dev);
   
   /* L2ARC persistence write I/O routines. */
   static void l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio,
       l2arc_write_callback_t *cb);
   
   /* L2ARC persistence auxiliary routines. */
   boolean_t l2arc_log_blkptr_valid(l2arc_dev_t *dev,
       const l2arc_log_blkptr_t *lbp);
   static boolean_t l2arc_log_blk_insert(l2arc_dev_t *dev,
       const arc_buf_hdr_t *ab);
   boolean_t l2arc_range_check_overlap(uint64_t bottom,
       uint64_t top, uint64_t check);
   static void l2arc_blk_fetch_done(zio_t *zio);
   static inline uint64_t
       l2arc_log_blk_overhead(uint64_t write_sz, l2arc_dev_t *dev);
   
   /*
    * We use Cityhash for this. It's fast, and has good hash properties without
    * requiring any large static buffers.
    */
   static uint64_t
   buf_hash(uint64_t spa, const dva_t *dva, uint64_t birth)
   {
           return (cityhash4(spa, dva->dva_word[0], dva->dva_word[1], birth));
   }
   
   #define HDR_EMPTY(hdr)                                          \
           ((hdr)->b_dva.dva_word[0] == 0 &&                       \
           (hdr)->b_dva.dva_word[1] == 0)
   
   #define HDR_EMPTY_OR_LOCKED(hdr)                                \
          (HDR_EMPTY(hdr) || MUTEX_HELD(HDR_LOCK(hdr)))
  
  #define HDR_EQUAL(spa, dva, birth, hdr)                         \
          ((hdr)->b_dva.dva_word[0] == (dva)->dva_word[0]) &&     \
          ((hdr)->b_dva.dva_word[1] == (dva)->dva_word[1]) &&     \
          ((hdr)->b_birth == birth) && ((hdr)->b_spa == spa)
  
  static void
  buf_discard_identity(arc_buf_hdr_t *hdr)
  {
          hdr->b_dva.dva_word[0] = 0;
          hdr->b_dva.dva_word[1] = 0;
          hdr->b_birth = 0;
  }
  
  static arc_buf_hdr_t *
  buf_hash_find(uint64_t spa, const blkptr_t *bp, kmutex_t **lockp)
  {
          const dva_t *dva = BP_IDENTITY(bp);
          uint64_t birth = BP_PHYSICAL_BIRTH(bp);
          uint64_t idx = BUF_HASH_INDEX(spa, dva, birth);
          kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
          arc_buf_hdr_t *hdr;
  
          mutex_enter(hash_lock);
          for (hdr = buf_hash_table.ht_table[idx]; hdr != NULL;
              hdr = hdr->b_hash_next) {
                  if (HDR_EQUAL(spa, dva, birth, hdr)) {
                          *lockp = hash_lock;
                          return (hdr);
                  }
          }
          mutex_exit(hash_lock);
          *lockp = NULL;
          return (NULL);
  }
  
  /*
   * Insert an entry into the hash table.  If there is already an element
   * equal to elem in the hash table, then the already existing element
   * will be returned and the new element will not be inserted.
   * Otherwise returns NULL.
   * If lockp == NULL, the caller is assumed to already hold the hash lock.
   */
  static arc_buf_hdr_t *
  buf_hash_insert(arc_buf_hdr_t *hdr, kmutex_t **lockp)
  {
          uint64_t idx = BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth);
          kmutex_t *hash_lock = BUF_HASH_LOCK(idx);
          arc_buf_hdr_t *fhdr;
          uint32_t i;
  
          ASSERT(!DVA_IS_EMPTY(&hdr->b_dva));
          ASSERT(hdr->b_birth != 0);
          ASSERT(!HDR_IN_HASH_TABLE(hdr));
  
          if (lockp != NULL) {
                  *lockp = hash_lock;
                  mutex_enter(hash_lock);
          } else {
                  ASSERT(MUTEX_HELD(hash_lock));
          }
  
          for (fhdr = buf_hash_table.ht_table[idx], i = 0; fhdr != NULL;
              fhdr = fhdr->b_hash_next, i++) {
                  if (HDR_EQUAL(hdr->b_spa, &hdr->b_dva, hdr->b_birth, fhdr))
                          return (fhdr);
          }
  
          hdr->b_hash_next = buf_hash_table.ht_table[idx];
          buf_hash_table.ht_table[idx] = hdr;
          arc_hdr_set_flags(hdr, ARC_FLAG_IN_HASH_TABLE);
  
          /* collect some hash table performance data */
          if (i > 0) {
                  ARCSTAT_BUMP(arcstat_hash_collisions);
                  if (i == 1)
                          ARCSTAT_BUMP(arcstat_hash_chains);
  
                  ARCSTAT_MAX(arcstat_hash_chain_max, i);
          }
          uint64_t he = atomic_inc_64_nv(
              &arc_stats.arcstat_hash_elements.value.ui64);
          ARCSTAT_MAX(arcstat_hash_elements_max, he);
  
          return (NULL);
  }
  
  static void
  buf_hash_remove(arc_buf_hdr_t *hdr)
  {
          arc_buf_hdr_t *fhdr, **hdrp;
          uint64_t idx = BUF_HASH_INDEX(hdr->b_spa, &hdr->b_dva, hdr->b_birth);
  
          ASSERT(MUTEX_HELD(BUF_HASH_LOCK(idx)));
          ASSERT(HDR_IN_HASH_TABLE(hdr));
  
          hdrp = &buf_hash_table.ht_table[idx];
          while ((fhdr = *hdrp) != hdr) {
                  ASSERT3P(fhdr, !=, NULL);
                  hdrp = &fhdr->b_hash_next;
          }
          *hdrp = hdr->b_hash_next;
          hdr->b_hash_next = NULL;
          arc_hdr_clear_flags(hdr, ARC_FLAG_IN_HASH_TABLE);
  
          /* collect some hash table performance data */
          atomic_dec_64(&arc_stats.arcstat_hash_elements.value.ui64);
  
          if (buf_hash_table.ht_table[idx] &&
              buf_hash_table.ht_table[idx]->b_hash_next == NULL)
                  ARCSTAT_BUMPDOWN(arcstat_hash_chains);
  }
  
  /*
   * Global data structures and functions for the buf kmem cache.
   */
  
  static kmem_cache_t *hdr_full_cache;
  static kmem_cache_t *hdr_full_crypt_cache;
  static kmem_cache_t *hdr_l2only_cache;
  static kmem_cache_t *buf_cache;
  
  static void
  buf_fini(void)
  {
  #if defined(_KERNEL)
          /*
           * Large allocations which do not require contiguous pages
           * should be using vmem_free() in the linux kernel\
           */
          vmem_free(buf_hash_table.ht_table,
              (buf_hash_table.ht_mask + 1) * sizeof (void *));
  #else
          kmem_free(buf_hash_table.ht_table,
              (buf_hash_table.ht_mask + 1) * sizeof (void *));
  #endif
          for (int i = 0; i < BUF_LOCKS; i++)
                  mutex_destroy(BUF_HASH_LOCK(i));
          kmem_cache_destroy(hdr_full_cache);
          kmem_cache_destroy(hdr_full_crypt_cache);
          kmem_cache_destroy(hdr_l2only_cache);
          kmem_cache_destroy(buf_cache);
  }
  
  /*
   * Constructor callback - called when the cache is empty
   * and a new buf is requested.
   */
  static int
  hdr_full_cons(void *vbuf, void *unused, int kmflag)
  {
          (void) unused, (void) kmflag;
          arc_buf_hdr_t *hdr = vbuf;
  
          memset(hdr, 0, HDR_FULL_SIZE);
          hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
          cv_init(&hdr->b_l1hdr.b_cv, NULL, CV_DEFAULT, NULL);
          zfs_refcount_create(&hdr->b_l1hdr.b_refcnt);
  #ifdef ZFS_DEBUG
          mutex_init(&hdr->b_l1hdr.b_freeze_lock, NULL, MUTEX_DEFAULT, NULL);
  #endif
          multilist_link_init(&hdr->b_l1hdr.b_arc_node);
          list_link_init(&hdr->b_l2hdr.b_l2node);
          arc_space_consume(HDR_FULL_SIZE, ARC_SPACE_HDRS);
  
          return (0);
  }
  
  static int
  hdr_full_crypt_cons(void *vbuf, void *unused, int kmflag)
  {
          (void) unused;
          arc_buf_hdr_t *hdr = vbuf;
  
          hdr_full_cons(vbuf, unused, kmflag);
          memset(&hdr->b_crypt_hdr, 0, sizeof (hdr->b_crypt_hdr));
          arc_space_consume(sizeof (hdr->b_crypt_hdr), ARC_SPACE_HDRS);
  
          return (0);
  }
  
  static int
  hdr_l2only_cons(void *vbuf, void *unused, int kmflag)
  {
          (void) unused, (void) kmflag;
          arc_buf_hdr_t *hdr = vbuf;
  
          memset(hdr, 0, HDR_L2ONLY_SIZE);
          arc_space_consume(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);
  
          return (0);
  }
  
  static int
  buf_cons(void *vbuf, void *unused, int kmflag)
  {
          (void) unused, (void) kmflag;
          arc_buf_t *buf = vbuf;
  
          memset(buf, 0, sizeof (arc_buf_t));
          arc_space_consume(sizeof (arc_buf_t), ARC_SPACE_HDRS);
  
          return (0);
  }
  
  /*
   * Destructor callback - called when a cached buf is
   * no longer required.
   */
  static void
  hdr_full_dest(void *vbuf, void *unused)
  {
          (void) unused;
          arc_buf_hdr_t *hdr = vbuf;
  
          ASSERT(HDR_EMPTY(hdr));
          cv_destroy(&hdr->b_l1hdr.b_cv);
          zfs_refcount_destroy(&hdr->b_l1hdr.b_refcnt);
  #ifdef ZFS_DEBUG
          mutex_destroy(&hdr->b_l1hdr.b_freeze_lock);
  #endif
          ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
          arc_space_return(HDR_FULL_SIZE, ARC_SPACE_HDRS);
  }
  
  static void
  hdr_full_crypt_dest(void *vbuf, void *unused)
  {
          (void) vbuf, (void) unused;
  
          hdr_full_dest(vbuf, unused);
          arc_space_return(sizeof (((arc_buf_hdr_t *)NULL)->b_crypt_hdr),
              ARC_SPACE_HDRS);
  }
  
  static void
  hdr_l2only_dest(void *vbuf, void *unused)
  {
          (void) unused;
          arc_buf_hdr_t *hdr = vbuf;
  
          ASSERT(HDR_EMPTY(hdr));
          arc_space_return(HDR_L2ONLY_SIZE, ARC_SPACE_L2HDRS);
  }
  
  static void
  buf_dest(void *vbuf, void *unused)
  {
          (void) unused;
          (void) vbuf;
  
          arc_space_return(sizeof (arc_buf_t), ARC_SPACE_HDRS);
  }
  
  static void
  buf_init(void)
  {
          uint64_t *ct = NULL;
          uint64_t hsize = 1ULL << 12;
          int i, j;
  
          /*
           * The hash table is big enough to fill all of physical memory
           * with an average block size of zfs_arc_average_blocksize (default 8K).
           * By default, the table will take up
           * totalmem * sizeof(void*) / 8K (1MB per GB with 8-byte pointers).
           */
          while (hsize * zfs_arc_average_blocksize < arc_all_memory())
                  hsize <<= 1;
  retry:
          buf_hash_table.ht_mask = hsize - 1;
  #if defined(_KERNEL)
          /*
           * Large allocations which do not require contiguous pages
           * should be using vmem_alloc() in the linux kernel
           */
          buf_hash_table.ht_table =
              vmem_zalloc(hsize * sizeof (void*), KM_SLEEP);
  #else
          buf_hash_table.ht_table =
              kmem_zalloc(hsize * sizeof (void*), KM_NOSLEEP);
  #endif
          if (buf_hash_table.ht_table == NULL) {
                  ASSERT(hsize > (1ULL << 8));
                  hsize >>= 1;
                  goto retry;
          }
  
          hdr_full_cache = kmem_cache_create("arc_buf_hdr_t_full", HDR_FULL_SIZE,
              0, hdr_full_cons, hdr_full_dest, NULL, NULL, NULL, 0);
          hdr_full_crypt_cache = kmem_cache_create("arc_buf_hdr_t_full_crypt",
              HDR_FULL_CRYPT_SIZE, 0, hdr_full_crypt_cons, hdr_full_crypt_dest,
              NULL, NULL, NULL, 0);
          hdr_l2only_cache = kmem_cache_create("arc_buf_hdr_t_l2only",
              HDR_L2ONLY_SIZE, 0, hdr_l2only_cons, hdr_l2only_dest, NULL,
              NULL, NULL, 0);
          buf_cache = kmem_cache_create("arc_buf_t", sizeof (arc_buf_t),
              0, buf_cons, buf_dest, NULL, NULL, NULL, 0);
  
          for (i = 0; i < 256; i++)
                  for (ct = zfs_crc64_table + i, *ct = i, j = 8; j > 0; j--)
                          *ct = (*ct >> 1) ^ (-(*ct & 1) & ZFS_CRC64_POLY);
  
          for (i = 0; i < BUF_LOCKS; i++)
                  mutex_init(BUF_HASH_LOCK(i), NULL, MUTEX_DEFAULT, NULL);
  }
  
  #define ARC_MINTIME     (hz>>4) /* 62 ms */
  
  /*
   * This is the size that the buf occupies in memory. If the buf is compressed,
   * it will correspond to the compressed size. You should use this method of
   * getting the buf size unless you explicitly need the logical size.
   */
  uint64_t
  arc_buf_size(arc_buf_t *buf)
  {
          return (ARC_BUF_COMPRESSED(buf) ?
              HDR_GET_PSIZE(buf->b_hdr) : HDR_GET_LSIZE(buf->b_hdr));
  }
  
  uint64_t
  arc_buf_lsize(arc_buf_t *buf)
  {
          return (HDR_GET_LSIZE(buf->b_hdr));
  }
  
  /*
   * This function will return B_TRUE if the buffer is encrypted in memory.
   * This buffer can be decrypted by calling arc_untransform().
   */
  boolean_t
  arc_is_encrypted(arc_buf_t *buf)
  {
          return (ARC_BUF_ENCRYPTED(buf) != 0);
  }
  
  /*
   * Returns B_TRUE if the buffer represents data that has not had its MAC
   * verified yet.
   */
  boolean_t
  arc_is_unauthenticated(arc_buf_t *buf)
  {
          return (HDR_NOAUTH(buf->b_hdr) != 0);
  }
  
  void
  arc_get_raw_params(arc_buf_t *buf, boolean_t *byteorder, uint8_t *salt,
      uint8_t *iv, uint8_t *mac)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          ASSERT(HDR_PROTECTED(hdr));
  
          memcpy(salt, hdr->b_crypt_hdr.b_salt, ZIO_DATA_SALT_LEN);
          memcpy(iv, hdr->b_crypt_hdr.b_iv, ZIO_DATA_IV_LEN);
          memcpy(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN);
          *byteorder = (hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS) ?
              ZFS_HOST_BYTEORDER : !ZFS_HOST_BYTEORDER;
  }
  
  /*
   * Indicates how this buffer is compressed in memory. If it is not compressed
   * the value will be ZIO_COMPRESS_OFF. It can be made normally readable with
   * arc_untransform() as long as it is also unencrypted.
   */
  enum zio_compress
  arc_get_compression(arc_buf_t *buf)
  {
          return (ARC_BUF_COMPRESSED(buf) ?
              HDR_GET_COMPRESS(buf->b_hdr) : ZIO_COMPRESS_OFF);
  }
  
  /*
   * Return the compression algorithm used to store this data in the ARC. If ARC
   * compression is enabled or this is an encrypted block, this will be the same
   * as what's used to store it on-disk. Otherwise, this will be ZIO_COMPRESS_OFF.
   */
  static inline enum zio_compress
  arc_hdr_get_compress(arc_buf_hdr_t *hdr)
  {
          return (HDR_COMPRESSION_ENABLED(hdr) ?
              HDR_GET_COMPRESS(hdr) : ZIO_COMPRESS_OFF);
  }
  
  uint8_t
  arc_get_complevel(arc_buf_t *buf)
  {
          return (buf->b_hdr->b_complevel);
  }
  
  static inline boolean_t
  arc_buf_is_shared(arc_buf_t *buf)
  {
          boolean_t shared = (buf->b_data != NULL &&
              buf->b_hdr->b_l1hdr.b_pabd != NULL &&
              abd_is_linear(buf->b_hdr->b_l1hdr.b_pabd) &&
              buf->b_data == abd_to_buf(buf->b_hdr->b_l1hdr.b_pabd));
          IMPLY(shared, HDR_SHARED_DATA(buf->b_hdr));
          IMPLY(shared, ARC_BUF_SHARED(buf));
          IMPLY(shared, ARC_BUF_COMPRESSED(buf) || ARC_BUF_LAST(buf));
  
          /*
           * It would be nice to assert arc_can_share() too, but the "hdr isn't
           * already being shared" requirement prevents us from doing that.
           */
  
          return (shared);
  }
  
  /*
   * Free the checksum associated with this header. If there is no checksum, this
   * is a no-op.
   */
  static inline void
  arc_cksum_free(arc_buf_hdr_t *hdr)
  {
  #ifdef ZFS_DEBUG
          ASSERT(HDR_HAS_L1HDR(hdr));
  
          mutex_enter(&hdr->b_l1hdr.b_freeze_lock);
          if (hdr->b_l1hdr.b_freeze_cksum != NULL) {
                  kmem_free(hdr->b_l1hdr.b_freeze_cksum, sizeof (zio_cksum_t));
                  hdr->b_l1hdr.b_freeze_cksum = NULL;
          }
          mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
  #endif
  }
  
  /*
   * Return true iff at least one of the bufs on hdr is not compressed.
   * Encrypted buffers count as compressed.
   */
  static boolean_t
  arc_hdr_has_uncompressed_buf(arc_buf_hdr_t *hdr)
  {
          ASSERT(hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY_OR_LOCKED(hdr));
  
          for (arc_buf_t *b = hdr->b_l1hdr.b_buf; b != NULL; b = b->b_next) {
                  if (!ARC_BUF_COMPRESSED(b)) {
                          return (B_TRUE);
                  }
          }
          return (B_FALSE);
  }
  
  
  /*
   * If we've turned on the ZFS_DEBUG_MODIFY flag, verify that the buf's data
   * matches the checksum that is stored in the hdr. If there is no checksum,
   * or if the buf is compressed, this is a no-op.
   */
  static void
  arc_cksum_verify(arc_buf_t *buf)
  {
  #ifdef ZFS_DEBUG
          arc_buf_hdr_t *hdr = buf->b_hdr;
          zio_cksum_t zc;
  
          if (!(zfs_flags & ZFS_DEBUG_MODIFY))
                  return;
  
          if (ARC_BUF_COMPRESSED(buf))
                  return;
  
          ASSERT(HDR_HAS_L1HDR(hdr));
  
          mutex_enter(&hdr->b_l1hdr.b_freeze_lock);
  
          if (hdr->b_l1hdr.b_freeze_cksum == NULL || HDR_IO_ERROR(hdr)) {
                  mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
                  return;
          }
  
          fletcher_2_native(buf->b_data, arc_buf_size(buf), NULL, &zc);
          if (!ZIO_CHECKSUM_EQUAL(*hdr->b_l1hdr.b_freeze_cksum, zc))
                  panic("buffer modified while frozen!");
          mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
  #endif
  }
  
  /*
   * This function makes the assumption that data stored in the L2ARC
   * will be transformed exactly as it is in the main pool. Because of
   * this we can verify the checksum against the reading process's bp.
   */
  static boolean_t
  arc_cksum_is_equal(arc_buf_hdr_t *hdr, zio_t *zio)
  {
          ASSERT(!BP_IS_EMBEDDED(zio->io_bp));
          VERIFY3U(BP_GET_PSIZE(zio->io_bp), ==, HDR_GET_PSIZE(hdr));
  
          /*
           * Block pointers always store the checksum for the logical data.
           * If the block pointer has the gang bit set, then the checksum
           * it represents is for the reconstituted data and not for an
           * individual gang member. The zio pipeline, however, must be able to
           * determine the checksum of each of the gang constituents so it
           * treats the checksum comparison differently than what we need
           * for l2arc blocks. This prevents us from using the
           * zio_checksum_error() interface directly. Instead we must call the
           * zio_checksum_error_impl() so that we can ensure the checksum is
           * generated using the correct checksum algorithm and accounts for the
           * logical I/O size and not just a gang fragment.
           */
          return (zio_checksum_error_impl(zio->io_spa, zio->io_bp,
              BP_GET_CHECKSUM(zio->io_bp), zio->io_abd, zio->io_size,
              zio->io_offset, NULL) == 0);
  }
  
  /*
   * Given a buf full of data, if ZFS_DEBUG_MODIFY is enabled this computes a
   * checksum and attaches it to the buf's hdr so that we can ensure that the buf
   * isn't modified later on. If buf is compressed or there is already a checksum
   * on the hdr, this is a no-op (we only checksum uncompressed bufs).
   */
  static void
  arc_cksum_compute(arc_buf_t *buf)
  {
          if (!(zfs_flags & ZFS_DEBUG_MODIFY))
                  return;
  
  #ifdef ZFS_DEBUG
          arc_buf_hdr_t *hdr = buf->b_hdr;
          ASSERT(HDR_HAS_L1HDR(hdr));
          mutex_enter(&hdr->b_l1hdr.b_freeze_lock);
          if (hdr->b_l1hdr.b_freeze_cksum != NULL || ARC_BUF_COMPRESSED(buf)) {
                  mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
                  return;
          }
  
          ASSERT(!ARC_BUF_ENCRYPTED(buf));
          ASSERT(!ARC_BUF_COMPRESSED(buf));
          hdr->b_l1hdr.b_freeze_cksum = kmem_alloc(sizeof (zio_cksum_t),
              KM_SLEEP);
          fletcher_2_native(buf->b_data, arc_buf_size(buf), NULL,
              hdr->b_l1hdr.b_freeze_cksum);
          mutex_exit(&hdr->b_l1hdr.b_freeze_lock);
  #endif
          arc_buf_watch(buf);
  }
  
  #ifndef _KERNEL
  void
  arc_buf_sigsegv(int sig, siginfo_t *si, void *unused)
  {
          (void) sig, (void) unused;
          panic("Got SIGSEGV at address: 0x%lx\n", (long)si->si_addr);
  }
  #endif
  
  static void
  arc_buf_unwatch(arc_buf_t *buf)
  {
  #ifndef _KERNEL
          if (arc_watch) {
                  ASSERT0(mprotect(buf->b_data, arc_buf_size(buf),
                      PROT_READ | PROT_WRITE));
          }
  #else
          (void) buf;
  #endif
  }
  
  static void
  arc_buf_watch(arc_buf_t *buf)
  {
  #ifndef _KERNEL
          if (arc_watch)
                  ASSERT0(mprotect(buf->b_data, arc_buf_size(buf),
                      PROT_READ));
  #else
          (void) buf;
  #endif
  }
  
  static arc_buf_contents_t
  arc_buf_type(arc_buf_hdr_t *hdr)
  {
          arc_buf_contents_t type;
          if (HDR_ISTYPE_METADATA(hdr)) {
                  type = ARC_BUFC_METADATA;
          } else {
                  type = ARC_BUFC_DATA;
          }
          VERIFY3U(hdr->b_type, ==, type);
          return (type);
  }
  
  boolean_t
  arc_is_metadata(arc_buf_t *buf)
  {
          return (HDR_ISTYPE_METADATA(buf->b_hdr) != 0);
  }
  
  static uint32_t
  arc_bufc_to_flags(arc_buf_contents_t type)
  {
          switch (type) {
          case ARC_BUFC_DATA:
                  /* metadata field is 0 if buffer contains normal data */
                  return (0);
          case ARC_BUFC_METADATA:
                  return (ARC_FLAG_BUFC_METADATA);
          default:
                  break;
          }
          panic("undefined ARC buffer type!");
          return ((uint32_t)-1);
  }
  
  void
  arc_buf_thaw(arc_buf_t *buf)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
          ASSERT(!HDR_IO_IN_PROGRESS(hdr));
  
          arc_cksum_verify(buf);
  
          /*
           * Compressed buffers do not manipulate the b_freeze_cksum.
           */
          if (ARC_BUF_COMPRESSED(buf))
                  return;
  
          ASSERT(HDR_HAS_L1HDR(hdr));
          arc_cksum_free(hdr);
          arc_buf_unwatch(buf);
  }
  
  void
  arc_buf_freeze(arc_buf_t *buf)
  {
          if (!(zfs_flags & ZFS_DEBUG_MODIFY))
                  return;
  
          if (ARC_BUF_COMPRESSED(buf))
                  return;
  
          ASSERT(HDR_HAS_L1HDR(buf->b_hdr));
          arc_cksum_compute(buf);
  }
  
  /*
   * The arc_buf_hdr_t's b_flags should never be modified directly. Instead,
   * the following functions should be used to ensure that the flags are
   * updated in a thread-safe way. When manipulating the flags either
   * the hash_lock must be held or the hdr must be undiscoverable. This
   * ensures that we're not racing with any other threads when updating
   * the flags.
   */
  static inline void
  arc_hdr_set_flags(arc_buf_hdr_t *hdr, arc_flags_t flags)
  {
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
          hdr->b_flags |= flags;
  }
  
  static inline void
  arc_hdr_clear_flags(arc_buf_hdr_t *hdr, arc_flags_t flags)
  {
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
          hdr->b_flags &= ~flags;
  }
  
  /*
   * Setting the compression bits in the arc_buf_hdr_t's b_flags is
   * done in a special way since we have to clear and set bits
   * at the same time. Consumers that wish to set the compression bits
   * must use this function to ensure that the flags are updated in
   * thread-safe manner.
   */
  static void
  arc_hdr_set_compress(arc_buf_hdr_t *hdr, enum zio_compress cmp)
  {
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
  
          /*
           * Holes and embedded blocks will always have a psize = 0 so
           * we ignore the compression of the blkptr and set the
           * want to uncompress them. Mark them as uncompressed.
           */
          if (!zfs_compressed_arc_enabled || HDR_GET_PSIZE(hdr) == 0) {
                  arc_hdr_clear_flags(hdr, ARC_FLAG_COMPRESSED_ARC);
                  ASSERT(!HDR_COMPRESSION_ENABLED(hdr));
          } else {
                  arc_hdr_set_flags(hdr, ARC_FLAG_COMPRESSED_ARC);
                  ASSERT(HDR_COMPRESSION_ENABLED(hdr));
          }
  
          HDR_SET_COMPRESS(hdr, cmp);
          ASSERT3U(HDR_GET_COMPRESS(hdr), ==, cmp);
  }
  
  /*
   * Looks for another buf on the same hdr which has the data decompressed, copies
   * from it, and returns true. If no such buf exists, returns false.
   */
  static boolean_t
  arc_buf_try_copy_decompressed_data(arc_buf_t *buf)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
          boolean_t copied = B_FALSE;
  
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT3P(buf->b_data, !=, NULL);
          ASSERT(!ARC_BUF_COMPRESSED(buf));
  
          for (arc_buf_t *from = hdr->b_l1hdr.b_buf; from != NULL;
              from = from->b_next) {
                  /* can't use our own data buffer */
                  if (from == buf) {
                          continue;
                  }
  
                  if (!ARC_BUF_COMPRESSED(from)) {
                          memcpy(buf->b_data, from->b_data, arc_buf_size(buf));
                          copied = B_TRUE;
                          break;
                  }
          }
  
  #ifdef ZFS_DEBUG
          /*
           * There were no decompressed bufs, so there should not be a
           * checksum on the hdr either.
           */
          if (zfs_flags & ZFS_DEBUG_MODIFY)
                  EQUIV(!copied, hdr->b_l1hdr.b_freeze_cksum == NULL);
  #endif
  
          return (copied);
  }
  
  /*
   * Allocates an ARC buf header that's in an evicted & L2-cached state.
   * This is used during l2arc reconstruction to make empty ARC buffers
   * which circumvent the regular disk->arc->l2arc path and instead come
   * into being in the reverse order, i.e. l2arc->arc.
   */
  static arc_buf_hdr_t *
  arc_buf_alloc_l2only(size_t size, arc_buf_contents_t type, l2arc_dev_t *dev,
      dva_t dva, uint64_t daddr, int32_t psize, uint64_t birth,
      enum zio_compress compress, uint8_t complevel, boolean_t protected,
      boolean_t prefetch, arc_state_type_t arcs_state)
  {
          arc_buf_hdr_t   *hdr;
  
          ASSERT(size != 0);
          hdr = kmem_cache_alloc(hdr_l2only_cache, KM_SLEEP);
          hdr->b_birth = birth;
          hdr->b_type = type;
          hdr->b_flags = 0;
          arc_hdr_set_flags(hdr, arc_bufc_to_flags(type) | ARC_FLAG_HAS_L2HDR);
          HDR_SET_LSIZE(hdr, size);
          HDR_SET_PSIZE(hdr, psize);
          arc_hdr_set_compress(hdr, compress);
          hdr->b_complevel = complevel;
          if (protected)
                  arc_hdr_set_flags(hdr, ARC_FLAG_PROTECTED);
          if (prefetch)
                  arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
          hdr->b_spa = spa_load_guid(dev->l2ad_vdev->vdev_spa);
  
          hdr->b_dva = dva;
  
          hdr->b_l2hdr.b_dev = dev;
          hdr->b_l2hdr.b_daddr = daddr;
          hdr->b_l2hdr.b_arcs_state = arcs_state;
  
          return (hdr);
  }
  
  /*
   * Return the size of the block, b_pabd, that is stored in the arc_buf_hdr_t.
   */
  static uint64_t
  arc_hdr_size(arc_buf_hdr_t *hdr)
  {
          uint64_t size;
  
          if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF &&
              HDR_GET_PSIZE(hdr) > 0) {
                  size = HDR_GET_PSIZE(hdr);
          } else {
                  ASSERT3U(HDR_GET_LSIZE(hdr), !=, 0);
                  size = HDR_GET_LSIZE(hdr);
          }
          return (size);
  }
  
  static int
  arc_hdr_authenticate(arc_buf_hdr_t *hdr, spa_t *spa, uint64_t dsobj)
  {
          int ret;
          uint64_t csize;
          uint64_t lsize = HDR_GET_LSIZE(hdr);
          uint64_t psize = HDR_GET_PSIZE(hdr);
          void *tmpbuf = NULL;
          abd_t *abd = hdr->b_l1hdr.b_pabd;
  
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
          ASSERT(HDR_AUTHENTICATED(hdr));
          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
  
          /*
           * The MAC is calculated on the compressed data that is stored on disk.
           * However, if compressed arc is disabled we will only have the
           * decompressed data available to us now. Compress it into a temporary
           * abd so we can verify the MAC. The performance overhead of this will
           * be relatively low, since most objects in an encrypted objset will
           * be encrypted (instead of authenticated) anyway.
           */
          if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
              !HDR_COMPRESSION_ENABLED(hdr)) {
                  tmpbuf = zio_buf_alloc(lsize);
                  abd = abd_get_from_buf(tmpbuf, lsize);
                  abd_take_ownership_of_buf(abd, B_TRUE);
                  csize = zio_compress_data(HDR_GET_COMPRESS(hdr),
                      hdr->b_l1hdr.b_pabd, tmpbuf, lsize, hdr->b_complevel);
                  ASSERT3U(csize, <=, psize);
                  abd_zero_off(abd, csize, psize - csize);
          }
  
          /*
           * Authentication is best effort. We authenticate whenever the key is
           * available. If we succeed we clear ARC_FLAG_NOAUTH.
           */
          if (hdr->b_crypt_hdr.b_ot == DMU_OT_OBJSET) {
                  ASSERT3U(HDR_GET_COMPRESS(hdr), ==, ZIO_COMPRESS_OFF);
                  ASSERT3U(lsize, ==, psize);
                  ret = spa_do_crypt_objset_mac_abd(B_FALSE, spa, dsobj, abd,
                      psize, hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
          } else {
                  ret = spa_do_crypt_mac_abd(B_FALSE, spa, dsobj, abd, psize,
                      hdr->b_crypt_hdr.b_mac);
          }
  
          if (ret == 0)
                  arc_hdr_clear_flags(hdr, ARC_FLAG_NOAUTH);
          else if (ret != ENOENT)
                  goto error;
  
          if (tmpbuf != NULL)
                  abd_free(abd);
  
          return (0);
  
  error:
          if (tmpbuf != NULL)
                  abd_free(abd);
  
          return (ret);
  }
  
  /*
   * This function will take a header that only has raw encrypted data in
   * b_crypt_hdr.b_rabd and decrypt it into a new buffer which is stored in
   * b_l1hdr.b_pabd. If designated in the header flags, this function will
   * also decompress the data.
   */
  static int
  arc_hdr_decrypt(arc_buf_hdr_t *hdr, spa_t *spa, const zbookmark_phys_t *zb)
  {
          int ret;
          abd_t *cabd = NULL;
          void *tmp = NULL;
          boolean_t no_crypt = B_FALSE;
          boolean_t bswap = (hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
  
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
          ASSERT(HDR_ENCRYPTED(hdr));
  
          arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
  
          ret = spa_do_crypt_abd(B_FALSE, spa, zb, hdr->b_crypt_hdr.b_ot,
              B_FALSE, bswap, hdr->b_crypt_hdr.b_salt, hdr->b_crypt_hdr.b_iv,
              hdr->b_crypt_hdr.b_mac, HDR_GET_PSIZE(hdr), hdr->b_l1hdr.b_pabd,
              hdr->b_crypt_hdr.b_rabd, &no_crypt);
          if (ret != 0)
                  goto error;
  
          if (no_crypt) {
                  abd_copy(hdr->b_l1hdr.b_pabd, hdr->b_crypt_hdr.b_rabd,
                      HDR_GET_PSIZE(hdr));
          }
  
          /*
           * If this header has disabled arc compression but the b_pabd is
           * compressed after decrypting it, we need to decompress the newly
           * decrypted data.
           */
          if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
              !HDR_COMPRESSION_ENABLED(hdr)) {
                  /*
                   * We want to make sure that we are correctly honoring the
                   * zfs_abd_scatter_enabled setting, so we allocate an abd here
                   * and then loan a buffer from it, rather than allocating a
                   * linear buffer and wrapping it in an abd later.
                   */
                  cabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr,
                      ARC_HDR_DO_ADAPT);
                  tmp = abd_borrow_buf(cabd, arc_hdr_size(hdr));
  
                  ret = zio_decompress_data(HDR_GET_COMPRESS(hdr),
                      hdr->b_l1hdr.b_pabd, tmp, HDR_GET_PSIZE(hdr),
                      HDR_GET_LSIZE(hdr), &hdr->b_complevel);
                  if (ret != 0) {
                          abd_return_buf(cabd, tmp, arc_hdr_size(hdr));
                          goto error;
                  }
  
                  abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
                  arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
                      arc_hdr_size(hdr), hdr);
                  hdr->b_l1hdr.b_pabd = cabd;
          }
  
          return (0);
  
  error:
          arc_hdr_free_abd(hdr, B_FALSE);
          if (cabd != NULL)
                  arc_free_data_buf(hdr, cabd, arc_hdr_size(hdr), hdr);
  
          return (ret);
  }
  
  /*
   * This function is called during arc_buf_fill() to prepare the header's
   * abd plaintext pointer for use. This involves authenticated protected
   * data and decrypting encrypted data into the plaintext abd.
   */
  static int
  arc_fill_hdr_crypt(arc_buf_hdr_t *hdr, kmutex_t *hash_lock, spa_t *spa,
      const zbookmark_phys_t *zb, boolean_t noauth)
  {
          int ret;
  
          ASSERT(HDR_PROTECTED(hdr));
  
          if (hash_lock != NULL)
                  mutex_enter(hash_lock);
  
          if (HDR_NOAUTH(hdr) && !noauth) {
                  /*
                   * The caller requested authenticated data but our data has
                   * not been authenticated yet. Verify the MAC now if we can.
                   */
                  ret = arc_hdr_authenticate(hdr, spa, zb->zb_objset);
                  if (ret != 0)
                          goto error;
          } else if (HDR_HAS_RABD(hdr) && hdr->b_l1hdr.b_pabd == NULL) {
                  /*
                   * If we only have the encrypted version of the data, but the
                   * unencrypted version was requested we take this opportunity
                   * to store the decrypted version in the header for future use.
                   */
                  ret = arc_hdr_decrypt(hdr, spa, zb);
                  if (ret != 0)
                          goto error;
          }
  
          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
  
          if (hash_lock != NULL)
                  mutex_exit(hash_lock);
  
          return (0);
  
  error:
          if (hash_lock != NULL)
                  mutex_exit(hash_lock);
  
          return (ret);
  }
  
  /*
   * This function is used by the dbuf code to decrypt bonus buffers in place.
   * The dbuf code itself doesn't have any locking for decrypting a shared dnode
   * block, so we use the hash lock here to protect against concurrent calls to
   * arc_buf_fill().
   */
  static void
  arc_buf_untransform_in_place(arc_buf_t *buf)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          ASSERT(HDR_ENCRYPTED(hdr));
          ASSERT3U(hdr->b_crypt_hdr.b_ot, ==, DMU_OT_DNODE);
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
  
          zio_crypt_copy_dnode_bonus(hdr->b_l1hdr.b_pabd, buf->b_data,
              arc_buf_size(buf));
          buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
          buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
          hdr->b_crypt_hdr.b_ebufcnt -= 1;
  }
  
  /*
   * Given a buf that has a data buffer attached to it, this function will
   * efficiently fill the buf with data of the specified compression setting from
   * the hdr and update the hdr's b_freeze_cksum if necessary. If the buf and hdr
   * are already sharing a data buf, no copy is performed.
   *
   * If the buf is marked as compressed but uncompressed data was requested, this
   * will allocate a new data buffer for the buf, remove that flag, and fill the
   * buf with uncompressed data. You can't request a compressed buf on a hdr with
   * uncompressed data, and (since we haven't added support for it yet) if you
   * want compressed data your buf must already be marked as compressed and have
   * the correct-sized data buffer.
   */
  static int
  arc_buf_fill(arc_buf_t *buf, spa_t *spa, const zbookmark_phys_t *zb,
      arc_fill_flags_t flags)
  {
          int error = 0;
          arc_buf_hdr_t *hdr = buf->b_hdr;
          boolean_t hdr_compressed =
              (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
          boolean_t compressed = (flags & ARC_FILL_COMPRESSED) != 0;
          boolean_t encrypted = (flags & ARC_FILL_ENCRYPTED) != 0;
          dmu_object_byteswap_t bswap = hdr->b_l1hdr.b_byteswap;
          kmutex_t *hash_lock = (flags & ARC_FILL_LOCKED) ? NULL : HDR_LOCK(hdr);
  
          ASSERT3P(buf->b_data, !=, NULL);
          IMPLY(compressed, hdr_compressed || ARC_BUF_ENCRYPTED(buf));
          IMPLY(compressed, ARC_BUF_COMPRESSED(buf));
          IMPLY(encrypted, HDR_ENCRYPTED(hdr));
          IMPLY(encrypted, ARC_BUF_ENCRYPTED(buf));
          IMPLY(encrypted, ARC_BUF_COMPRESSED(buf));
          IMPLY(encrypted, !ARC_BUF_SHARED(buf));
  
          /*
           * If the caller wanted encrypted data we just need to copy it from
           * b_rabd and potentially byteswap it. We won't be able to do any
           * further transforms on it.
           */
          if (encrypted) {
                  ASSERT(HDR_HAS_RABD(hdr));
                  abd_copy_to_buf(buf->b_data, hdr->b_crypt_hdr.b_rabd,
                      HDR_GET_PSIZE(hdr));
                  goto byteswap;
          }
  
          /*
           * Adjust encrypted and authenticated headers to accommodate
           * the request if needed. Dnode blocks (ARC_FILL_IN_PLACE) are
           * allowed to fail decryption due to keys not being loaded
           * without being marked as an IO error.
           */
          if (HDR_PROTECTED(hdr)) {
                  error = arc_fill_hdr_crypt(hdr, hash_lock, spa,
                      zb, !!(flags & ARC_FILL_NOAUTH));
                  if (error == EACCES && (flags & ARC_FILL_IN_PLACE) != 0) {
                          return (error);
                  } else if (error != 0) {
                          if (hash_lock != NULL)
                                  mutex_enter(hash_lock);
                          arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
                          if (hash_lock != NULL)
                                  mutex_exit(hash_lock);
                          return (error);
                  }
          }
  
          /*
           * There is a special case here for dnode blocks which are
           * decrypting their bonus buffers. These blocks may request to
           * be decrypted in-place. This is necessary because there may
           * be many dnodes pointing into this buffer and there is
           * currently no method to synchronize replacing the backing
           * b_data buffer and updating all of the pointers. Here we use
           * the hash lock to ensure there are no races. If the need
           * arises for other types to be decrypted in-place, they must
           * add handling here as well.
           */
          if ((flags & ARC_FILL_IN_PLACE) != 0) {
                  ASSERT(!hdr_compressed);
                  ASSERT(!compressed);
                  ASSERT(!encrypted);
  
                  if (HDR_ENCRYPTED(hdr) && ARC_BUF_ENCRYPTED(buf)) {
                          ASSERT3U(hdr->b_crypt_hdr.b_ot, ==, DMU_OT_DNODE);
  
                          if (hash_lock != NULL)
                                  mutex_enter(hash_lock);
                          arc_buf_untransform_in_place(buf);
                          if (hash_lock != NULL)
                                  mutex_exit(hash_lock);
  
                          /* Compute the hdr's checksum if necessary */
                          arc_cksum_compute(buf);
                  }
  
                  return (0);
          }
  
          if (hdr_compressed == compressed) {
                  if (!arc_buf_is_shared(buf)) {
                          abd_copy_to_buf(buf->b_data, hdr->b_l1hdr.b_pabd,
                              arc_buf_size(buf));
                  }
          } else {
                  ASSERT(hdr_compressed);
                  ASSERT(!compressed);
  
                  /*
                   * If the buf is sharing its data with the hdr, unlink it and
                   * allocate a new data buffer for the buf.
                   */
                  if (arc_buf_is_shared(buf)) {
                          ASSERT(ARC_BUF_COMPRESSED(buf));
  
                          /* We need to give the buf its own b_data */
                          buf->b_flags &= ~ARC_BUF_FLAG_SHARED;
                          buf->b_data =
                              arc_get_data_buf(hdr, HDR_GET_LSIZE(hdr), buf);
                          arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
  
                          /* Previously overhead was 0; just add new overhead */
                          ARCSTAT_INCR(arcstat_overhead_size, HDR_GET_LSIZE(hdr));
                  } else if (ARC_BUF_COMPRESSED(buf)) {
                          /* We need to reallocate the buf's b_data */
                          arc_free_data_buf(hdr, buf->b_data, HDR_GET_PSIZE(hdr),
                              buf);
                          buf->b_data =
                              arc_get_data_buf(hdr, HDR_GET_LSIZE(hdr), buf);
  
                          /* We increased the size of b_data; update overhead */
                          ARCSTAT_INCR(arcstat_overhead_size,
                              HDR_GET_LSIZE(hdr) - HDR_GET_PSIZE(hdr));
                  }
  
                  /*
                   * Regardless of the buf's previous compression settings, it
                   * should not be compressed at the end of this function.
                   */
                  buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
  
                  /*
                   * Try copying the data from another buf which already has a
                   * decompressed version. If that's not possible, it's time to
                   * bite the bullet and decompress the data from the hdr.
                   */
                  if (arc_buf_try_copy_decompressed_data(buf)) {
                          /* Skip byteswapping and checksumming (already done) */
                          return (0);
                  } else {
                          error = zio_decompress_data(HDR_GET_COMPRESS(hdr),
                              hdr->b_l1hdr.b_pabd, buf->b_data,
                              HDR_GET_PSIZE(hdr), HDR_GET_LSIZE(hdr),
                              &hdr->b_complevel);
  
                          /*
                           * Absent hardware errors or software bugs, this should
                           * be impossible, but log it anyway so we can debug it.
                           */
                          if (error != 0) {
                                  zfs_dbgmsg(
                                      "hdr %px, compress %d, psize %d, lsize %d",
                                      hdr, arc_hdr_get_compress(hdr),
                                      HDR_GET_PSIZE(hdr), HDR_GET_LSIZE(hdr));
                                  if (hash_lock != NULL)
                                          mutex_enter(hash_lock);
                                  arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
                                  if (hash_lock != NULL)
                                          mutex_exit(hash_lock);
                                  return (SET_ERROR(EIO));
                          }
                  }
          }
  
  byteswap:
          /* Byteswap the buf's data if necessary */
          if (bswap != DMU_BSWAP_NUMFUNCS) {
                  ASSERT(!HDR_SHARED_DATA(hdr));
                  ASSERT3U(bswap, <, DMU_BSWAP_NUMFUNCS);
                  dmu_ot_byteswap[bswap].ob_func(buf->b_data, HDR_GET_LSIZE(hdr));
          }
  
          /* Compute the hdr's checksum if necessary */
          arc_cksum_compute(buf);
  
          return (0);
  }
  
  /*
   * If this function is being called to decrypt an encrypted buffer or verify an
   * authenticated one, the key must be loaded and a mapping must be made
   * available in the keystore via spa_keystore_create_mapping() or one of its
   * callers.
   */
  int
  arc_untransform(arc_buf_t *buf, spa_t *spa, const zbookmark_phys_t *zb,
      boolean_t in_place)
  {
          int ret;
          arc_fill_flags_t flags = 0;
  
          if (in_place)
                  flags |= ARC_FILL_IN_PLACE;
  
          ret = arc_buf_fill(buf, spa, zb, flags);
          if (ret == ECKSUM) {
                  /*
                   * Convert authentication and decryption errors to EIO
                   * (and generate an ereport) before leaving the ARC.
                   */
                  ret = SET_ERROR(EIO);
                  spa_log_error(spa, zb);
                  (void) zfs_ereport_post(FM_EREPORT_ZFS_AUTHENTICATION,
                      spa, NULL, zb, NULL, 0);
          }
  
          return (ret);
  }
  
  /*
   * Increment the amount of evictable space in the arc_state_t's refcount.
   * We account for the space used by the hdr and the arc buf individually
   * so that we can add and remove them from the refcount individually.
   */
  static void
  arc_evictable_space_increment(arc_buf_hdr_t *hdr, arc_state_t *state)
  {
          arc_buf_contents_t type = arc_buf_type(hdr);
  
          ASSERT(HDR_HAS_L1HDR(hdr));
  
          if (GHOST_STATE(state)) {
                  ASSERT0(hdr->b_l1hdr.b_bufcnt);
                  ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                  ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                  ASSERT(!HDR_HAS_RABD(hdr));
                  (void) zfs_refcount_add_many(&state->arcs_esize[type],
                      HDR_GET_LSIZE(hdr), hdr);
                  return;
          }
  
          if (hdr->b_l1hdr.b_pabd != NULL) {
                  (void) zfs_refcount_add_many(&state->arcs_esize[type],
                      arc_hdr_size(hdr), hdr);
          }
          if (HDR_HAS_RABD(hdr)) {
                  (void) zfs_refcount_add_many(&state->arcs_esize[type],
                      HDR_GET_PSIZE(hdr), hdr);
          }
  
          for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
              buf = buf->b_next) {
                  if (arc_buf_is_shared(buf))
                          continue;
                  (void) zfs_refcount_add_many(&state->arcs_esize[type],
                      arc_buf_size(buf), buf);
          }
  }
  
  /*
   * Decrement the amount of evictable space in the arc_state_t's refcount.
   * We account for the space used by the hdr and the arc buf individually
   * so that we can add and remove them from the refcount individually.
   */
  static void
  arc_evictable_space_decrement(arc_buf_hdr_t *hdr, arc_state_t *state)
  {
          arc_buf_contents_t type = arc_buf_type(hdr);
  
          ASSERT(HDR_HAS_L1HDR(hdr));
  
          if (GHOST_STATE(state)) {
                  ASSERT0(hdr->b_l1hdr.b_bufcnt);
                  ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                  ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                  ASSERT(!HDR_HAS_RABD(hdr));
                  (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                      HDR_GET_LSIZE(hdr), hdr);
                  return;
          }
  
          if (hdr->b_l1hdr.b_pabd != NULL) {
                  (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                      arc_hdr_size(hdr), hdr);
          }
          if (HDR_HAS_RABD(hdr)) {
                  (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                      HDR_GET_PSIZE(hdr), hdr);
          }
  
          for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
              buf = buf->b_next) {
                  if (arc_buf_is_shared(buf))
                          continue;
                  (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                      arc_buf_size(buf), buf);
          }
  }
  
  /*
   * Add a reference to this hdr indicating that someone is actively
   * referencing that memory. When the refcount transitions from 0 to 1,
   * we remove it from the respective arc_state_t list to indicate that
   * it is not evictable.
   */
  static void
  add_reference(arc_buf_hdr_t *hdr, const void *tag)
  {
          arc_state_t *state = hdr->b_l1hdr.b_state;
  
          ASSERT(HDR_HAS_L1HDR(hdr));
          if (!HDR_EMPTY(hdr) && !MUTEX_HELD(HDR_LOCK(hdr))) {
                  ASSERT(state == arc_anon);
                  ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                  ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
          }
  
          if ((zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, tag) == 1) &&
              state != arc_anon && state != arc_l2c_only) {
                  /* We don't use the L2-only state list. */
                  multilist_remove(&state->arcs_list[arc_buf_type(hdr)], hdr);
                  arc_evictable_space_decrement(hdr, state);
          }
  }
  
  /*
   * Remove a reference from this hdr. When the reference transitions from
   * 1 to 0 and we're not anonymous, then we add this hdr to the arc_state_t's
   * list making it eligible for eviction.
   */
  static int
  remove_reference(arc_buf_hdr_t *hdr, const void *tag)
  {
          int cnt;
          arc_state_t *state = hdr->b_l1hdr.b_state;
  
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT(state == arc_anon || MUTEX_HELD(HDR_LOCK(hdr)));
          ASSERT(!GHOST_STATE(state));    /* arc_l2c_only counts as a ghost. */
  
          if ((cnt = zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, tag)) != 0)
                  return (cnt);
  
          if (state == arc_anon) {
                  arc_hdr_destroy(hdr);
                  return (0);
          }
          if (state == arc_uncached && !HDR_PREFETCH(hdr)) {
                  arc_change_state(arc_anon, hdr);
                  arc_hdr_destroy(hdr);
                  return (0);
          }
          multilist_insert(&state->arcs_list[arc_buf_type(hdr)], hdr);
          arc_evictable_space_increment(hdr, state);
          return (0);
  }
  
  /*
   * Returns detailed information about a specific arc buffer.  When the
   * state_index argument is set the function will calculate the arc header
   * list position for its arc state.  Since this requires a linear traversal
   * callers are strongly encourage not to do this.  However, it can be helpful
   * for targeted analysis so the functionality is provided.
   */
  void
  arc_buf_info(arc_buf_t *ab, arc_buf_info_t *abi, int state_index)
  {
          (void) state_index;
          arc_buf_hdr_t *hdr = ab->b_hdr;
          l1arc_buf_hdr_t *l1hdr = NULL;
          l2arc_buf_hdr_t *l2hdr = NULL;
          arc_state_t *state = NULL;
  
          memset(abi, 0, sizeof (arc_buf_info_t));
  
          if (hdr == NULL)
                  return;
  
          abi->abi_flags = hdr->b_flags;
  
          if (HDR_HAS_L1HDR(hdr)) {
                  l1hdr = &hdr->b_l1hdr;
                  state = l1hdr->b_state;
          }
          if (HDR_HAS_L2HDR(hdr))
                  l2hdr = &hdr->b_l2hdr;
  
          if (l1hdr) {
                  abi->abi_bufcnt = l1hdr->b_bufcnt;
                  abi->abi_access = l1hdr->b_arc_access;
                  abi->abi_mru_hits = l1hdr->b_mru_hits;
                  abi->abi_mru_ghost_hits = l1hdr->b_mru_ghost_hits;
                  abi->abi_mfu_hits = l1hdr->b_mfu_hits;
                  abi->abi_mfu_ghost_hits = l1hdr->b_mfu_ghost_hits;
                  abi->abi_holds = zfs_refcount_count(&l1hdr->b_refcnt);
          }
  
          if (l2hdr) {
                  abi->abi_l2arc_dattr = l2hdr->b_daddr;
                  abi->abi_l2arc_hits = l2hdr->b_hits;
          }
  
          abi->abi_state_type = state ? state->arcs_state : ARC_STATE_ANON;
          abi->abi_state_contents = arc_buf_type(hdr);
          abi->abi_size = arc_hdr_size(hdr);
  }
  
  /*
   * Move the supplied buffer to the indicated state. The hash lock
   * for the buffer must be held by the caller.
   */
  static void
  arc_change_state(arc_state_t *new_state, arc_buf_hdr_t *hdr)
  {
          arc_state_t *old_state;
          int64_t refcnt;
          uint32_t bufcnt;
          boolean_t update_old, update_new;
          arc_buf_contents_t buftype = arc_buf_type(hdr);
  
          /*
           * We almost always have an L1 hdr here, since we call arc_hdr_realloc()
           * in arc_read() when bringing a buffer out of the L2ARC.  However, the
           * L1 hdr doesn't always exist when we change state to arc_anon before
           * destroying a header, in which case reallocating to add the L1 hdr is
           * pointless.
           */
          if (HDR_HAS_L1HDR(hdr)) {
                  old_state = hdr->b_l1hdr.b_state;
                  refcnt = zfs_refcount_count(&hdr->b_l1hdr.b_refcnt);
                  bufcnt = hdr->b_l1hdr.b_bufcnt;
                  update_old = (bufcnt > 0 || hdr->b_l1hdr.b_pabd != NULL ||
                      HDR_HAS_RABD(hdr));
  
                  IMPLY(GHOST_STATE(old_state), bufcnt == 0);
                  IMPLY(GHOST_STATE(new_state), bufcnt == 0);
                  IMPLY(GHOST_STATE(old_state), hdr->b_l1hdr.b_buf == NULL);
                  IMPLY(GHOST_STATE(new_state), hdr->b_l1hdr.b_buf == NULL);
                  IMPLY(old_state == arc_anon, bufcnt <= 1);
          } else {
                  old_state = arc_l2c_only;
                  refcnt = 0;
                  bufcnt = 0;
                  update_old = B_FALSE;
          }
          update_new = update_old;
          if (GHOST_STATE(old_state))
                  update_old = B_TRUE;
          if (GHOST_STATE(new_state))
                  update_new = B_TRUE;
  
          ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
          ASSERT3P(new_state, !=, old_state);
  
          /*
           * If this buffer is evictable, transfer it from the
           * old state list to the new state list.
           */
          if (refcnt == 0) {
                  if (old_state != arc_anon && old_state != arc_l2c_only) {
                          ASSERT(HDR_HAS_L1HDR(hdr));
                          /* remove_reference() saves on insert. */
                          if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
                                  multilist_remove(&old_state->arcs_list[buftype],
                                      hdr);
                                  arc_evictable_space_decrement(hdr, old_state);
                          }
                  }
                  if (new_state != arc_anon && new_state != arc_l2c_only) {
                          /*
                           * An L1 header always exists here, since if we're
                           * moving to some L1-cached state (i.e. not l2c_only or
                           * anonymous), we realloc the header to add an L1hdr
                           * beforehand.
                           */
                          ASSERT(HDR_HAS_L1HDR(hdr));
                          multilist_insert(&new_state->arcs_list[buftype], hdr);
                          arc_evictable_space_increment(hdr, new_state);
                  }
          }
  
          ASSERT(!HDR_EMPTY(hdr));
          if (new_state == arc_anon && HDR_IN_HASH_TABLE(hdr))
                  buf_hash_remove(hdr);
  
          /* adjust state sizes (ignore arc_l2c_only) */
  
          if (update_new && new_state != arc_l2c_only) {
                  ASSERT(HDR_HAS_L1HDR(hdr));
                  if (GHOST_STATE(new_state)) {
                          ASSERT0(bufcnt);
  
                          /*
                           * When moving a header to a ghost state, we first
                           * remove all arc buffers. Thus, we'll have a
                           * bufcnt of zero, and no arc buffer to use for
                           * the reference. As a result, we use the arc
                           * header pointer for the reference.
                           */
                          (void) zfs_refcount_add_many(&new_state->arcs_size,
                              HDR_GET_LSIZE(hdr), hdr);
                          ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                          ASSERT(!HDR_HAS_RABD(hdr));
                  } else {
                          uint32_t buffers = 0;
  
                          /*
                           * Each individual buffer holds a unique reference,
                           * thus we must remove each of these references one
                           * at a time.
                           */
                          for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
                              buf = buf->b_next) {
                                  ASSERT3U(bufcnt, !=, 0);
                                  buffers++;
  
                                  /*
                                   * When the arc_buf_t is sharing the data
                                   * block with the hdr, the owner of the
                                   * reference belongs to the hdr. Only
                                   * add to the refcount if the arc_buf_t is
                                   * not shared.
                                   */
                                  if (arc_buf_is_shared(buf))
                                          continue;
  
                                  (void) zfs_refcount_add_many(
                                      &new_state->arcs_size,
                                      arc_buf_size(buf), buf);
                          }
                          ASSERT3U(bufcnt, ==, buffers);
  
                          if (hdr->b_l1hdr.b_pabd != NULL) {
                                  (void) zfs_refcount_add_many(
                                      &new_state->arcs_size,
                                      arc_hdr_size(hdr), hdr);
                          }
  
                          if (HDR_HAS_RABD(hdr)) {
                                  (void) zfs_refcount_add_many(
                                      &new_state->arcs_size,
                                      HDR_GET_PSIZE(hdr), hdr);
                          }
                  }
          }
  
          if (update_old && old_state != arc_l2c_only) {
                  ASSERT(HDR_HAS_L1HDR(hdr));
                  if (GHOST_STATE(old_state)) {
                          ASSERT0(bufcnt);
                          ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                          ASSERT(!HDR_HAS_RABD(hdr));
  
                          /*
                           * When moving a header off of a ghost state,
                           * the header will not contain any arc buffers.
                           * We use the arc header pointer for the reference
                           * which is exactly what we did when we put the
                           * header on the ghost state.
                           */
  
                          (void) zfs_refcount_remove_many(&old_state->arcs_size,
                              HDR_GET_LSIZE(hdr), hdr);
                  } else {
                          uint32_t buffers = 0;
  
                          /*
                           * Each individual buffer holds a unique reference,
                           * thus we must remove each of these references one
                           * at a time.
                           */
                          for (arc_buf_t *buf = hdr->b_l1hdr.b_buf; buf != NULL;
                              buf = buf->b_next) {
                                  ASSERT3U(bufcnt, !=, 0);
                                  buffers++;
  
                                  /*
                                   * When the arc_buf_t is sharing the data
                                   * block with the hdr, the owner of the
                                   * reference belongs to the hdr. Only
                                   * add to the refcount if the arc_buf_t is
                                   * not shared.
                                   */
                                  if (arc_buf_is_shared(buf))
                                          continue;
  
                                  (void) zfs_refcount_remove_many(
                                      &old_state->arcs_size, arc_buf_size(buf),
                                      buf);
                          }
                          ASSERT3U(bufcnt, ==, buffers);
                          ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
                              HDR_HAS_RABD(hdr));
  
                          if (hdr->b_l1hdr.b_pabd != NULL) {
                                  (void) zfs_refcount_remove_many(
                                      &old_state->arcs_size, arc_hdr_size(hdr),
                                      hdr);
                          }
  
                          if (HDR_HAS_RABD(hdr)) {
                                  (void) zfs_refcount_remove_many(
                                      &old_state->arcs_size, HDR_GET_PSIZE(hdr),
                                      hdr);
                          }
                  }
          }
  
          if (HDR_HAS_L1HDR(hdr)) {
                  hdr->b_l1hdr.b_state = new_state;
  
                  if (HDR_HAS_L2HDR(hdr) && new_state != arc_l2c_only) {
                          l2arc_hdr_arcstats_decrement_state(hdr);
                          hdr->b_l2hdr.b_arcs_state = new_state->arcs_state;
                          l2arc_hdr_arcstats_increment_state(hdr);
                  }
          }
  }
  
  void
  arc_space_consume(uint64_t space, arc_space_type_t type)
  {
          ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);
  
          switch (type) {
          default:
                  break;
          case ARC_SPACE_DATA:
                  ARCSTAT_INCR(arcstat_data_size, space);
                  break;
          case ARC_SPACE_META:
                  ARCSTAT_INCR(arcstat_metadata_size, space);
                  break;
          case ARC_SPACE_BONUS:
                  ARCSTAT_INCR(arcstat_bonus_size, space);
                  break;
          case ARC_SPACE_DNODE:
                  aggsum_add(&arc_sums.arcstat_dnode_size, space);
                  break;
          case ARC_SPACE_DBUF:
                  ARCSTAT_INCR(arcstat_dbuf_size, space);
                  break;
          case ARC_SPACE_HDRS:
                  ARCSTAT_INCR(arcstat_hdr_size, space);
                  break;
          case ARC_SPACE_L2HDRS:
                  aggsum_add(&arc_sums.arcstat_l2_hdr_size, space);
                  break;
          case ARC_SPACE_ABD_CHUNK_WASTE:
                  /*
                   * Note: this includes space wasted by all scatter ABD's, not
                   * just those allocated by the ARC.  But the vast majority of
                   * scatter ABD's come from the ARC, because other users are
                   * very short-lived.
                   */
                  ARCSTAT_INCR(arcstat_abd_chunk_waste_size, space);
                  break;
          }
  
          if (type != ARC_SPACE_DATA && type != ARC_SPACE_ABD_CHUNK_WASTE)
                  aggsum_add(&arc_sums.arcstat_meta_used, space);
  
          aggsum_add(&arc_sums.arcstat_size, space);
  }
  
  void
  arc_space_return(uint64_t space, arc_space_type_t type)
  {
          ASSERT(type >= 0 && type < ARC_SPACE_NUMTYPES);
  
          switch (type) {
          default:
                  break;
          case ARC_SPACE_DATA:
                  ARCSTAT_INCR(arcstat_data_size, -space);
                  break;
          case ARC_SPACE_META:
                  ARCSTAT_INCR(arcstat_metadata_size, -space);
                  break;
          case ARC_SPACE_BONUS:
                  ARCSTAT_INCR(arcstat_bonus_size, -space);
                  break;
          case ARC_SPACE_DNODE:
                  aggsum_add(&arc_sums.arcstat_dnode_size, -space);
                  break;
          case ARC_SPACE_DBUF:
                  ARCSTAT_INCR(arcstat_dbuf_size, -space);
                  break;
          case ARC_SPACE_HDRS:
                  ARCSTAT_INCR(arcstat_hdr_size, -space);
                  break;
          case ARC_SPACE_L2HDRS:
                  aggsum_add(&arc_sums.arcstat_l2_hdr_size, -space);
                  break;
          case ARC_SPACE_ABD_CHUNK_WASTE:
                  ARCSTAT_INCR(arcstat_abd_chunk_waste_size, -space);
                  break;
          }
  
          if (type != ARC_SPACE_DATA && type != ARC_SPACE_ABD_CHUNK_WASTE) {
                  ASSERT(aggsum_compare(&arc_sums.arcstat_meta_used,
                      space) >= 0);
                  ARCSTAT_MAX(arcstat_meta_max,
                      aggsum_upper_bound(&arc_sums.arcstat_meta_used));
                  aggsum_add(&arc_sums.arcstat_meta_used, -space);
          }
  
          ASSERT(aggsum_compare(&arc_sums.arcstat_size, space) >= 0);
          aggsum_add(&arc_sums.arcstat_size, -space);
  }
  
  /*
   * Given a hdr and a buf, returns whether that buf can share its b_data buffer
   * with the hdr's b_pabd.
   */
  static boolean_t
  arc_can_share(arc_buf_hdr_t *hdr, arc_buf_t *buf)
  {
          /*
           * The criteria for sharing a hdr's data are:
           * 1. the buffer is not encrypted
           * 2. the hdr's compression matches the buf's compression
           * 3. the hdr doesn't need to be byteswapped
           * 4. the hdr isn't already being shared
           * 5. the buf is either compressed or it is the last buf in the hdr list
           *
           * Criterion #5 maintains the invariant that shared uncompressed
           * bufs must be the final buf in the hdr's b_buf list. Reading this, you
           * might ask, "if a compressed buf is allocated first, won't that be the
           * last thing in the list?", but in that case it's impossible to create
           * a shared uncompressed buf anyway (because the hdr must be compressed
           * to have the compressed buf). You might also think that #3 is
           * sufficient to make this guarantee, however it's possible
           * (specifically in the rare L2ARC write race mentioned in
           * arc_buf_alloc_impl()) there will be an existing uncompressed buf that
           * is shareable, but wasn't at the time of its allocation. Rather than
           * allow a new shared uncompressed buf to be created and then shuffle
           * the list around to make it the last element, this simply disallows
           * sharing if the new buf isn't the first to be added.
           */
          ASSERT3P(buf->b_hdr, ==, hdr);
          boolean_t hdr_compressed =
              arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF;
          boolean_t buf_compressed = ARC_BUF_COMPRESSED(buf) != 0;
          return (!ARC_BUF_ENCRYPTED(buf) &&
              buf_compressed == hdr_compressed &&
              hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS &&
              !HDR_SHARED_DATA(hdr) &&
              (ARC_BUF_LAST(buf) || ARC_BUF_COMPRESSED(buf)));
  }
  
  /*
   * Allocate a buf for this hdr. If you care about the data that's in the hdr,
   * or if you want a compressed buffer, pass those flags in. Returns 0 if the
   * copy was made successfully, or an error code otherwise.
   */
  static int
  arc_buf_alloc_impl(arc_buf_hdr_t *hdr, spa_t *spa, const zbookmark_phys_t *zb,
      const void *tag, boolean_t encrypted, boolean_t compressed,
      boolean_t noauth, boolean_t fill, arc_buf_t **ret)
  {
          arc_buf_t *buf;
          arc_fill_flags_t flags = ARC_FILL_LOCKED;
  
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT3U(HDR_GET_LSIZE(hdr), >, 0);
          VERIFY(hdr->b_type == ARC_BUFC_DATA ||
              hdr->b_type == ARC_BUFC_METADATA);
          ASSERT3P(ret, !=, NULL);
          ASSERT3P(*ret, ==, NULL);
          IMPLY(encrypted, compressed);
  
          buf = *ret = kmem_cache_alloc(buf_cache, KM_PUSHPAGE);
          buf->b_hdr = hdr;
          buf->b_data = NULL;
          buf->b_next = hdr->b_l1hdr.b_buf;
          buf->b_flags = 0;
  
          add_reference(hdr, tag);
  
          /*
           * We're about to change the hdr's b_flags. We must either
           * hold the hash_lock or be undiscoverable.
           */
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
  
          /*
           * Only honor requests for compressed bufs if the hdr is actually
           * compressed. This must be overridden if the buffer is encrypted since
           * encrypted buffers cannot be decompressed.
           */
          if (encrypted) {
                  buf->b_flags |= ARC_BUF_FLAG_COMPRESSED;
                  buf->b_flags |= ARC_BUF_FLAG_ENCRYPTED;
                  flags |= ARC_FILL_COMPRESSED | ARC_FILL_ENCRYPTED;
          } else if (compressed &&
              arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF) {
                  buf->b_flags |= ARC_BUF_FLAG_COMPRESSED;
                  flags |= ARC_FILL_COMPRESSED;
          }
  
          if (noauth) {
                  ASSERT0(encrypted);
                  flags |= ARC_FILL_NOAUTH;
          }
  
          /*
           * If the hdr's data can be shared then we share the data buffer and
           * set the appropriate bit in the hdr's b_flags to indicate the hdr is
           * sharing it's b_pabd with the arc_buf_t. Otherwise, we allocate a new
           * buffer to store the buf's data.
           *
           * There are two additional restrictions here because we're sharing
           * hdr -> buf instead of the usual buf -> hdr. First, the hdr can't be
           * actively involved in an L2ARC write, because if this buf is used by
           * an arc_write() then the hdr's data buffer will be released when the
           * write completes, even though the L2ARC write might still be using it.
           * Second, the hdr's ABD must be linear so that the buf's user doesn't
           * need to be ABD-aware.  It must be allocated via
           * zio_[data_]buf_alloc(), not as a page, because we need to be able
           * to abd_release_ownership_of_buf(), which isn't allowed on "linear
           * page" buffers because the ABD code needs to handle freeing them
           * specially.
           */
          boolean_t can_share = arc_can_share(hdr, buf) &&
              !HDR_L2_WRITING(hdr) &&
              hdr->b_l1hdr.b_pabd != NULL &&
              abd_is_linear(hdr->b_l1hdr.b_pabd) &&
              !abd_is_linear_page(hdr->b_l1hdr.b_pabd);
  
          /* Set up b_data and sharing */
          if (can_share) {
                  buf->b_data = abd_to_buf(hdr->b_l1hdr.b_pabd);
                  buf->b_flags |= ARC_BUF_FLAG_SHARED;
                  arc_hdr_set_flags(hdr, ARC_FLAG_SHARED_DATA);
          } else {
                  buf->b_data =
                      arc_get_data_buf(hdr, arc_buf_size(buf), buf);
                  ARCSTAT_INCR(arcstat_overhead_size, arc_buf_size(buf));
          }
          VERIFY3P(buf->b_data, !=, NULL);
  
          hdr->b_l1hdr.b_buf = buf;
          hdr->b_l1hdr.b_bufcnt += 1;
          if (encrypted)
                  hdr->b_crypt_hdr.b_ebufcnt += 1;
  
          /*
           * If the user wants the data from the hdr, we need to either copy or
           * decompress the data.
           */
          if (fill) {
                  ASSERT3P(zb, !=, NULL);
                  return (arc_buf_fill(buf, spa, zb, flags));
          }
  
          return (0);
  }
  
  static const char *arc_onloan_tag = "onloan";
  
  static inline void
  arc_loaned_bytes_update(int64_t delta)
  {
          atomic_add_64(&arc_loaned_bytes, delta);
  
          /* assert that it did not wrap around */
          ASSERT3S(atomic_add_64_nv(&arc_loaned_bytes, 0), >=, 0);
  }
  
  /*
   * Loan out an anonymous arc buffer. Loaned buffers are not counted as in
   * flight data by arc_tempreserve_space() until they are "returned". Loaned
   * buffers must be returned to the arc before they can be used by the DMU or
   * freed.
   */
  arc_buf_t *
  arc_loan_buf(spa_t *spa, boolean_t is_metadata, int size)
  {
          arc_buf_t *buf = arc_alloc_buf(spa, arc_onloan_tag,
              is_metadata ? ARC_BUFC_METADATA : ARC_BUFC_DATA, size);
  
          arc_loaned_bytes_update(arc_buf_size(buf));
  
          return (buf);
  }
  
  arc_buf_t *
  arc_loan_compressed_buf(spa_t *spa, uint64_t psize, uint64_t lsize,
      enum zio_compress compression_type, uint8_t complevel)
  {
          arc_buf_t *buf = arc_alloc_compressed_buf(spa, arc_onloan_tag,
              psize, lsize, compression_type, complevel);
  
          arc_loaned_bytes_update(arc_buf_size(buf));
  
          return (buf);
  }
  
  arc_buf_t *
  arc_loan_raw_buf(spa_t *spa, uint64_t dsobj, boolean_t byteorder,
      const uint8_t *salt, const uint8_t *iv, const uint8_t *mac,
      dmu_object_type_t ot, uint64_t psize, uint64_t lsize,
      enum zio_compress compression_type, uint8_t complevel)
  {
          arc_buf_t *buf = arc_alloc_raw_buf(spa, arc_onloan_tag, dsobj,
              byteorder, salt, iv, mac, ot, psize, lsize, compression_type,
              complevel);
  
          atomic_add_64(&arc_loaned_bytes, psize);
          return (buf);
  }
  
  
  /*
   * Return a loaned arc buffer to the arc.
   */
  void
  arc_return_buf(arc_buf_t *buf, const void *tag)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          ASSERT3P(buf->b_data, !=, NULL);
          ASSERT(HDR_HAS_L1HDR(hdr));
          (void) zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, tag);
          (void) zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, arc_onloan_tag);
  
          arc_loaned_bytes_update(-arc_buf_size(buf));
  }
  
  /* Detach an arc_buf from a dbuf (tag) */
  void
  arc_loan_inuse_buf(arc_buf_t *buf, const void *tag)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          ASSERT3P(buf->b_data, !=, NULL);
          ASSERT(HDR_HAS_L1HDR(hdr));
          (void) zfs_refcount_add(&hdr->b_l1hdr.b_refcnt, arc_onloan_tag);
          (void) zfs_refcount_remove(&hdr->b_l1hdr.b_refcnt, tag);
  
          arc_loaned_bytes_update(arc_buf_size(buf));
  }
  
  static void
  l2arc_free_abd_on_write(abd_t *abd, size_t size, arc_buf_contents_t type)
  {
          l2arc_data_free_t *df = kmem_alloc(sizeof (*df), KM_SLEEP);
  
          df->l2df_abd = abd;
          df->l2df_size = size;
          df->l2df_type = type;
          mutex_enter(&l2arc_free_on_write_mtx);
          list_insert_head(l2arc_free_on_write, df);
          mutex_exit(&l2arc_free_on_write_mtx);
  }
  
  static void
  arc_hdr_free_on_write(arc_buf_hdr_t *hdr, boolean_t free_rdata)
  {
          arc_state_t *state = hdr->b_l1hdr.b_state;
          arc_buf_contents_t type = arc_buf_type(hdr);
          uint64_t size = (free_rdata) ? HDR_GET_PSIZE(hdr) : arc_hdr_size(hdr);
  
          /* protected by hash lock, if in the hash table */
          if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
                  ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                  ASSERT(state != arc_anon && state != arc_l2c_only);
  
                  (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                      size, hdr);
          }
          (void) zfs_refcount_remove_many(&state->arcs_size, size, hdr);
          if (type == ARC_BUFC_METADATA) {
                  arc_space_return(size, ARC_SPACE_META);
          } else {
                  ASSERT(type == ARC_BUFC_DATA);
                  arc_space_return(size, ARC_SPACE_DATA);
          }
  
          if (free_rdata) {
                  l2arc_free_abd_on_write(hdr->b_crypt_hdr.b_rabd, size, type);
          } else {
                  l2arc_free_abd_on_write(hdr->b_l1hdr.b_pabd, size, type);
          }
  }
  
  /*
   * Share the arc_buf_t's data with the hdr. Whenever we are sharing the
   * data buffer, we transfer the refcount ownership to the hdr and update
   * the appropriate kstats.
   */
  static void
  arc_share_buf(arc_buf_hdr_t *hdr, arc_buf_t *buf)
  {
          ASSERT(arc_can_share(hdr, buf));
          ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
          ASSERT(!ARC_BUF_ENCRYPTED(buf));
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
  
          /*
           * Start sharing the data buffer. We transfer the
           * refcount ownership to the hdr since it always owns
           * the refcount whenever an arc_buf_t is shared.
           */
          zfs_refcount_transfer_ownership_many(&hdr->b_l1hdr.b_state->arcs_size,
              arc_hdr_size(hdr), buf, hdr);
          hdr->b_l1hdr.b_pabd = abd_get_from_buf(buf->b_data, arc_buf_size(buf));
          abd_take_ownership_of_buf(hdr->b_l1hdr.b_pabd,
              HDR_ISTYPE_METADATA(hdr));
          arc_hdr_set_flags(hdr, ARC_FLAG_SHARED_DATA);
          buf->b_flags |= ARC_BUF_FLAG_SHARED;
  
          /*
           * Since we've transferred ownership to the hdr we need
           * to increment its compressed and uncompressed kstats and
           * decrement the overhead size.
           */
          ARCSTAT_INCR(arcstat_compressed_size, arc_hdr_size(hdr));
          ARCSTAT_INCR(arcstat_uncompressed_size, HDR_GET_LSIZE(hdr));
          ARCSTAT_INCR(arcstat_overhead_size, -arc_buf_size(buf));
  }
  
  static void
  arc_unshare_buf(arc_buf_hdr_t *hdr, arc_buf_t *buf)
  {
          ASSERT(arc_buf_is_shared(buf));
          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
  
          /*
           * We are no longer sharing this buffer so we need
           * to transfer its ownership to the rightful owner.
           */
          zfs_refcount_transfer_ownership_many(&hdr->b_l1hdr.b_state->arcs_size,
              arc_hdr_size(hdr), hdr, buf);
          arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
          abd_release_ownership_of_buf(hdr->b_l1hdr.b_pabd);
          abd_free(hdr->b_l1hdr.b_pabd);
          hdr->b_l1hdr.b_pabd = NULL;
          buf->b_flags &= ~ARC_BUF_FLAG_SHARED;
  
          /*
           * Since the buffer is no longer shared between
           * the arc buf and the hdr, count it as overhead.
           */
          ARCSTAT_INCR(arcstat_compressed_size, -arc_hdr_size(hdr));
          ARCSTAT_INCR(arcstat_uncompressed_size, -HDR_GET_LSIZE(hdr));
          ARCSTAT_INCR(arcstat_overhead_size, arc_buf_size(buf));
  }
  
  /*
   * Remove an arc_buf_t from the hdr's buf list and return the last
   * arc_buf_t on the list. If no buffers remain on the list then return
   * NULL.
   */
  static arc_buf_t *
  arc_buf_remove(arc_buf_hdr_t *hdr, arc_buf_t *buf)
  {
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
  
          arc_buf_t **bufp = &hdr->b_l1hdr.b_buf;
          arc_buf_t *lastbuf = NULL;
  
          /*
           * Remove the buf from the hdr list and locate the last
           * remaining buffer on the list.
           */
          while (*bufp != NULL) {
                  if (*bufp == buf)
                          *bufp = buf->b_next;
  
                  /*
                   * If we've removed a buffer in the middle of
                   * the list then update the lastbuf and update
                   * bufp.
                   */
                  if (*bufp != NULL) {
                          lastbuf = *bufp;
                          bufp = &(*bufp)->b_next;
                  }
          }
          buf->b_next = NULL;
          ASSERT3P(lastbuf, !=, buf);
          IMPLY(hdr->b_l1hdr.b_bufcnt > 0, lastbuf != NULL);
          IMPLY(hdr->b_l1hdr.b_bufcnt > 0, hdr->b_l1hdr.b_buf != NULL);
          IMPLY(lastbuf != NULL, ARC_BUF_LAST(lastbuf));
  
          return (lastbuf);
  }
  
  /*
   * Free up buf->b_data and pull the arc_buf_t off of the arc_buf_hdr_t's
   * list and free it.
   */
  static void
  arc_buf_destroy_impl(arc_buf_t *buf)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          /*
           * Free up the data associated with the buf but only if we're not
           * sharing this with the hdr. If we are sharing it with the hdr, the
           * hdr is responsible for doing the free.
           */
          if (buf->b_data != NULL) {
                  /*
                   * We're about to change the hdr's b_flags. We must either
                   * hold the hash_lock or be undiscoverable.
                   */
                  ASSERT(HDR_EMPTY_OR_LOCKED(hdr));
  
                  arc_cksum_verify(buf);
                  arc_buf_unwatch(buf);
  
                  if (arc_buf_is_shared(buf)) {
                          arc_hdr_clear_flags(hdr, ARC_FLAG_SHARED_DATA);
                  } else {
                          uint64_t size = arc_buf_size(buf);
                          arc_free_data_buf(hdr, buf->b_data, size, buf);
                          ARCSTAT_INCR(arcstat_overhead_size, -size);
                  }
                  buf->b_data = NULL;
  
                  ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
                  hdr->b_l1hdr.b_bufcnt -= 1;
  
                  if (ARC_BUF_ENCRYPTED(buf)) {
                          hdr->b_crypt_hdr.b_ebufcnt -= 1;
  
                          /*
                           * If we have no more encrypted buffers and we've
                           * already gotten a copy of the decrypted data we can
                           * free b_rabd to save some space.
                           */
                          if (hdr->b_crypt_hdr.b_ebufcnt == 0 &&
                              HDR_HAS_RABD(hdr) && hdr->b_l1hdr.b_pabd != NULL &&
                              !HDR_IO_IN_PROGRESS(hdr)) {
                                  arc_hdr_free_abd(hdr, B_TRUE);
                          }
                  }
          }
  
          arc_buf_t *lastbuf = arc_buf_remove(hdr, buf);
  
          if (ARC_BUF_SHARED(buf) && !ARC_BUF_COMPRESSED(buf)) {
                  /*
                   * If the current arc_buf_t is sharing its data buffer with the
                   * hdr, then reassign the hdr's b_pabd to share it with the new
                   * buffer at the end of the list. The shared buffer is always
                   * the last one on the hdr's buffer list.
                   *
                   * There is an equivalent case for compressed bufs, but since
                   * they aren't guaranteed to be the last buf in the list and
                   * that is an exceedingly rare case, we just allow that space be
                   * wasted temporarily. We must also be careful not to share
                   * encrypted buffers, since they cannot be shared.
                   */
                  if (lastbuf != NULL && !ARC_BUF_ENCRYPTED(lastbuf)) {
                          /* Only one buf can be shared at once */
                          VERIFY(!arc_buf_is_shared(lastbuf));
                          /* hdr is uncompressed so can't have compressed buf */
                          VERIFY(!ARC_BUF_COMPRESSED(lastbuf));
  
                          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
                          arc_hdr_free_abd(hdr, B_FALSE);
  
                          /*
                           * We must setup a new shared block between the
                           * last buffer and the hdr. The data would have
                           * been allocated by the arc buf so we need to transfer
                           * ownership to the hdr since it's now being shared.
                           */
                          arc_share_buf(hdr, lastbuf);
                  }
          } else if (HDR_SHARED_DATA(hdr)) {
                  /*
                   * Uncompressed shared buffers are always at the end
                   * of the list. Compressed buffers don't have the
                   * same requirements. This makes it hard to
                   * simply assert that the lastbuf is shared so
                   * we rely on the hdr's compression flags to determine
                   * if we have a compressed, shared buffer.
                   */
                  ASSERT3P(lastbuf, !=, NULL);
                  ASSERT(arc_buf_is_shared(lastbuf) ||
                      arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
          }
  
          /*
           * Free the checksum if we're removing the last uncompressed buf from
           * this hdr.
           */
          if (!arc_hdr_has_uncompressed_buf(hdr)) {
                  arc_cksum_free(hdr);
          }
  
          /* clean up the buf */
          buf->b_hdr = NULL;
          kmem_cache_free(buf_cache, buf);
  }
  
  static void
  arc_hdr_alloc_abd(arc_buf_hdr_t *hdr, int alloc_flags)
  {
          uint64_t size;
          boolean_t alloc_rdata = ((alloc_flags & ARC_HDR_ALLOC_RDATA) != 0);
  
          ASSERT3U(HDR_GET_LSIZE(hdr), >, 0);
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT(!HDR_SHARED_DATA(hdr) || alloc_rdata);
          IMPLY(alloc_rdata, HDR_PROTECTED(hdr));
  
          if (alloc_rdata) {
                  size = HDR_GET_PSIZE(hdr);
                  ASSERT3P(hdr->b_crypt_hdr.b_rabd, ==, NULL);
                  hdr->b_crypt_hdr.b_rabd = arc_get_data_abd(hdr, size, hdr,
                      alloc_flags);
                  ASSERT3P(hdr->b_crypt_hdr.b_rabd, !=, NULL);
                  ARCSTAT_INCR(arcstat_raw_size, size);
          } else {
                  size = arc_hdr_size(hdr);
                  ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                  hdr->b_l1hdr.b_pabd = arc_get_data_abd(hdr, size, hdr,
                      alloc_flags);
                  ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
          }
  
          ARCSTAT_INCR(arcstat_compressed_size, size);
          ARCSTAT_INCR(arcstat_uncompressed_size, HDR_GET_LSIZE(hdr));
  }
  
  static void
  arc_hdr_free_abd(arc_buf_hdr_t *hdr, boolean_t free_rdata)
  {
          uint64_t size = (free_rdata) ? HDR_GET_PSIZE(hdr) : arc_hdr_size(hdr);
  
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));
          IMPLY(free_rdata, HDR_HAS_RABD(hdr));
  
          /*
           * If the hdr is currently being written to the l2arc then
           * we defer freeing the data by adding it to the l2arc_free_on_write
           * list. The l2arc will free the data once it's finished
           * writing it to the l2arc device.
           */
          if (HDR_L2_WRITING(hdr)) {
                  arc_hdr_free_on_write(hdr, free_rdata);
                  ARCSTAT_BUMP(arcstat_l2_free_on_write);
          } else if (free_rdata) {
                  arc_free_data_abd(hdr, hdr->b_crypt_hdr.b_rabd, size, hdr);
          } else {
                  arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd, size, hdr);
          }
  
          if (free_rdata) {
                  hdr->b_crypt_hdr.b_rabd = NULL;
                  ARCSTAT_INCR(arcstat_raw_size, -size);
          } else {
                  hdr->b_l1hdr.b_pabd = NULL;
          }
  
          if (hdr->b_l1hdr.b_pabd == NULL && !HDR_HAS_RABD(hdr))
                  hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
  
          ARCSTAT_INCR(arcstat_compressed_size, -size);
          ARCSTAT_INCR(arcstat_uncompressed_size, -HDR_GET_LSIZE(hdr));
  }
  
  /*
   * Allocate empty anonymous ARC header.  The header will get its identity
   * assigned and buffers attached later as part of read or write operations.
   *
   * In case of read arc_read() assigns header its identify (b_dva + b_birth),
   * inserts it into ARC hash to become globally visible and allocates physical
   * (b_pabd) or raw (b_rabd) ABD buffer to read into from disk.  On disk read
   * completion arc_read_done() allocates ARC buffer(s) as needed, potentially
   * sharing one of them with the physical ABD buffer.
   *
   * In case of write arc_alloc_buf() allocates ARC buffer to be filled with
   * data.  Then after compression and/or encryption arc_write_ready() allocates
   * and fills (or potentially shares) physical (b_pabd) or raw (b_rabd) ABD
   * buffer.  On disk write completion arc_write_done() assigns the header its
   * new identity (b_dva + b_birth) and inserts into ARC hash.
   *
   * In case of partial overwrite the old data is read first as described. Then
   * arc_release() either allocates new anonymous ARC header and moves the ARC
   * buffer to it, or reuses the old ARC header by discarding its identity and
   * removing it from ARC hash.  After buffer modification normal write process
   * follows as described.
   */
  static arc_buf_hdr_t *
  arc_hdr_alloc(uint64_t spa, int32_t psize, int32_t lsize,
      boolean_t protected, enum zio_compress compression_type, uint8_t complevel,
      arc_buf_contents_t type)
  {
          arc_buf_hdr_t *hdr;
  
          VERIFY(type == ARC_BUFC_DATA || type == ARC_BUFC_METADATA);
          if (protected) {
                  hdr = kmem_cache_alloc(hdr_full_crypt_cache, KM_PUSHPAGE);
          } else {
                  hdr = kmem_cache_alloc(hdr_full_cache, KM_PUSHPAGE);
          }
  
          ASSERT(HDR_EMPTY(hdr));
  #ifdef ZFS_DEBUG
          ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
  #endif
          HDR_SET_PSIZE(hdr, psize);
          HDR_SET_LSIZE(hdr, lsize);
          hdr->b_spa = spa;
          hdr->b_type = type;
          hdr->b_flags = 0;
          arc_hdr_set_flags(hdr, arc_bufc_to_flags(type) | ARC_FLAG_HAS_L1HDR);
          arc_hdr_set_compress(hdr, compression_type);
          hdr->b_complevel = complevel;
          if (protected)
                  arc_hdr_set_flags(hdr, ARC_FLAG_PROTECTED);
  
          hdr->b_l1hdr.b_state = arc_anon;
          hdr->b_l1hdr.b_arc_access = 0;
          hdr->b_l1hdr.b_mru_hits = 0;
          hdr->b_l1hdr.b_mru_ghost_hits = 0;
          hdr->b_l1hdr.b_mfu_hits = 0;
          hdr->b_l1hdr.b_mfu_ghost_hits = 0;
          hdr->b_l1hdr.b_bufcnt = 0;
          hdr->b_l1hdr.b_buf = NULL;
  
          ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
  
          return (hdr);
  }
  
  /*
   * Transition between the two allocation states for the arc_buf_hdr struct.
   * The arc_buf_hdr struct can be allocated with (hdr_full_cache) or without
   * (hdr_l2only_cache) the fields necessary for the L1 cache - the smaller
   * version is used when a cache buffer is only in the L2ARC in order to reduce
   * memory usage.
   */
  static arc_buf_hdr_t *
  arc_hdr_realloc(arc_buf_hdr_t *hdr, kmem_cache_t *old, kmem_cache_t *new)
  {
          ASSERT(HDR_HAS_L2HDR(hdr));
  
          arc_buf_hdr_t *nhdr;
          l2arc_dev_t *dev = hdr->b_l2hdr.b_dev;
  
          ASSERT((old == hdr_full_cache && new == hdr_l2only_cache) ||
              (old == hdr_l2only_cache && new == hdr_full_cache));
  
          /*
           * if the caller wanted a new full header and the header is to be
           * encrypted we will actually allocate the header from the full crypt
           * cache instead. The same applies to freeing from the old cache.
           */
          if (HDR_PROTECTED(hdr) && new == hdr_full_cache)
                  new = hdr_full_crypt_cache;
          if (HDR_PROTECTED(hdr) && old == hdr_full_cache)
                  old = hdr_full_crypt_cache;
  
          nhdr = kmem_cache_alloc(new, KM_PUSHPAGE);
  
          ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
          buf_hash_remove(hdr);
  
          memcpy(nhdr, hdr, HDR_L2ONLY_SIZE);
  
          if (new == hdr_full_cache || new == hdr_full_crypt_cache) {
                  arc_hdr_set_flags(nhdr, ARC_FLAG_HAS_L1HDR);
                  /*
                   * arc_access and arc_change_state need to be aware that a
                   * header has just come out of L2ARC, so we set its state to
                   * l2c_only even though it's about to change.
                   */
                  nhdr->b_l1hdr.b_state = arc_l2c_only;
  
                  /* Verify previous threads set to NULL before freeing */
                  ASSERT3P(nhdr->b_l1hdr.b_pabd, ==, NULL);
                  ASSERT(!HDR_HAS_RABD(hdr));
          } else {
                  ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
                  ASSERT0(hdr->b_l1hdr.b_bufcnt);
  #ifdef ZFS_DEBUG
                  ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
  #endif
  
                  /*
                   * If we've reached here, We must have been called from
                   * arc_evict_hdr(), as such we should have already been
                   * removed from any ghost list we were previously on
                   * (which protects us from racing with arc_evict_state),
                   * thus no locking is needed during this check.
                   */
                  ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
  
                  /*
                   * A buffer must not be moved into the arc_l2c_only
                   * state if it's not finished being written out to the
                   * l2arc device. Otherwise, the b_l1hdr.b_pabd field
                   * might try to be accessed, even though it was removed.
                   */
                  VERIFY(!HDR_L2_WRITING(hdr));
                  VERIFY3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                  ASSERT(!HDR_HAS_RABD(hdr));
  
                  arc_hdr_clear_flags(nhdr, ARC_FLAG_HAS_L1HDR);
          }
          /*
           * The header has been reallocated so we need to re-insert it into any
           * lists it was on.
           */
          (void) buf_hash_insert(nhdr, NULL);
  
          ASSERT(list_link_active(&hdr->b_l2hdr.b_l2node));
  
          mutex_enter(&dev->l2ad_mtx);
  
          /*
           * We must place the realloc'ed header back into the list at
           * the same spot. Otherwise, if it's placed earlier in the list,
           * l2arc_write_buffers() could find it during the function's
           * write phase, and try to write it out to the l2arc.
           */
          list_insert_after(&dev->l2ad_buflist, hdr, nhdr);
          list_remove(&dev->l2ad_buflist, hdr);
  
          mutex_exit(&dev->l2ad_mtx);
  
          /*
           * Since we're using the pointer address as the tag when
           * incrementing and decrementing the l2ad_alloc refcount, we
           * must remove the old pointer (that we're about to destroy) and
           * add the new pointer to the refcount. Otherwise we'd remove
           * the wrong pointer address when calling arc_hdr_destroy() later.
           */
  
          (void) zfs_refcount_remove_many(&dev->l2ad_alloc,
              arc_hdr_size(hdr), hdr);
          (void) zfs_refcount_add_many(&dev->l2ad_alloc,
              arc_hdr_size(nhdr), nhdr);
  
          buf_discard_identity(hdr);
          kmem_cache_free(old, hdr);
  
          return (nhdr);
  }
  
  /*
   * This function allows an L1 header to be reallocated as a crypt
   * header and vice versa. If we are going to a crypt header, the
   * new fields will be zeroed out.
   */
  static arc_buf_hdr_t *
  arc_hdr_realloc_crypt(arc_buf_hdr_t *hdr, boolean_t need_crypt)
  {
          arc_buf_hdr_t *nhdr;
          arc_buf_t *buf;
          kmem_cache_t *ncache, *ocache;
  
          /*
           * This function requires that hdr is in the arc_anon state.
           * Therefore it won't have any L2ARC data for us to worry
           * about copying.
           */
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT(!HDR_HAS_L2HDR(hdr));
          ASSERT3U(!!HDR_PROTECTED(hdr), !=, need_crypt);
          ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
          ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
          ASSERT(!list_link_active(&hdr->b_l2hdr.b_l2node));
          ASSERT3P(hdr->b_hash_next, ==, NULL);
  
          if (need_crypt) {
                  ncache = hdr_full_crypt_cache;
                  ocache = hdr_full_cache;
          } else {
                  ncache = hdr_full_cache;
                  ocache = hdr_full_crypt_cache;
          }
  
          nhdr = kmem_cache_alloc(ncache, KM_PUSHPAGE);
  
          /*
           * Copy all members that aren't locks or condvars to the new header.
           * No lists are pointing to us (as we asserted above), so we don't
           * need to worry about the list nodes.
           */
          nhdr->b_dva = hdr->b_dva;
          nhdr->b_birth = hdr->b_birth;
          nhdr->b_type = hdr->b_type;
          nhdr->b_flags = hdr->b_flags;
          nhdr->b_psize = hdr->b_psize;
          nhdr->b_lsize = hdr->b_lsize;
          nhdr->b_spa = hdr->b_spa;
  #ifdef ZFS_DEBUG
          nhdr->b_l1hdr.b_freeze_cksum = hdr->b_l1hdr.b_freeze_cksum;
  #endif
          nhdr->b_l1hdr.b_bufcnt = hdr->b_l1hdr.b_bufcnt;
          nhdr->b_l1hdr.b_byteswap = hdr->b_l1hdr.b_byteswap;
          nhdr->b_l1hdr.b_state = hdr->b_l1hdr.b_state;
          nhdr->b_l1hdr.b_arc_access = hdr->b_l1hdr.b_arc_access;
          nhdr->b_l1hdr.b_mru_hits = hdr->b_l1hdr.b_mru_hits;
          nhdr->b_l1hdr.b_mru_ghost_hits = hdr->b_l1hdr.b_mru_ghost_hits;
          nhdr->b_l1hdr.b_mfu_hits = hdr->b_l1hdr.b_mfu_hits;
          nhdr->b_l1hdr.b_mfu_ghost_hits = hdr->b_l1hdr.b_mfu_ghost_hits;
          nhdr->b_l1hdr.b_acb = hdr->b_l1hdr.b_acb;
          nhdr->b_l1hdr.b_pabd = hdr->b_l1hdr.b_pabd;
  
          /*
           * This zfs_refcount_add() exists only to ensure that the individual
           * arc buffers always point to a header that is referenced, avoiding
           * a small race condition that could trigger ASSERTs.
           */
          (void) zfs_refcount_add(&nhdr->b_l1hdr.b_refcnt, FTAG);
          nhdr->b_l1hdr.b_buf = hdr->b_l1hdr.b_buf;
          for (buf = nhdr->b_l1hdr.b_buf; buf != NULL; buf = buf->b_next)
                  buf->b_hdr = nhdr;
  
          zfs_refcount_transfer(&nhdr->b_l1hdr.b_refcnt, &hdr->b_l1hdr.b_refcnt);
          (void) zfs_refcount_remove(&nhdr->b_l1hdr.b_refcnt, FTAG);
          ASSERT0(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt));
  
          if (need_crypt) {
                  arc_hdr_set_flags(nhdr, ARC_FLAG_PROTECTED);
          } else {
                  arc_hdr_clear_flags(nhdr, ARC_FLAG_PROTECTED);
          }
  
          /* unset all members of the original hdr */
          memset(&hdr->b_dva, 0, sizeof (dva_t));
          hdr->b_birth = 0;
          hdr->b_type = ARC_BUFC_INVALID;
          hdr->b_flags = 0;
          hdr->b_psize = 0;
          hdr->b_lsize = 0;
          hdr->b_spa = 0;
  #ifdef ZFS_DEBUG
          hdr->b_l1hdr.b_freeze_cksum = NULL;
  #endif
          hdr->b_l1hdr.b_buf = NULL;
          hdr->b_l1hdr.b_bufcnt = 0;
          hdr->b_l1hdr.b_byteswap = 0;
          hdr->b_l1hdr.b_state = NULL;
          hdr->b_l1hdr.b_arc_access = 0;
          hdr->b_l1hdr.b_mru_hits = 0;
          hdr->b_l1hdr.b_mru_ghost_hits = 0;
          hdr->b_l1hdr.b_mfu_hits = 0;
          hdr->b_l1hdr.b_mfu_ghost_hits = 0;
          hdr->b_l1hdr.b_acb = NULL;
          hdr->b_l1hdr.b_pabd = NULL;
  
          if (ocache == hdr_full_crypt_cache) {
                  ASSERT(!HDR_HAS_RABD(hdr));
                  hdr->b_crypt_hdr.b_ot = DMU_OT_NONE;
                  hdr->b_crypt_hdr.b_ebufcnt = 0;
                  hdr->b_crypt_hdr.b_dsobj = 0;
                  memset(hdr->b_crypt_hdr.b_salt, 0, ZIO_DATA_SALT_LEN);
                  memset(hdr->b_crypt_hdr.b_iv, 0, ZIO_DATA_IV_LEN);
                  memset(hdr->b_crypt_hdr.b_mac, 0, ZIO_DATA_MAC_LEN);
          }
  
          buf_discard_identity(hdr);
          kmem_cache_free(ocache, hdr);
  
          return (nhdr);
  }
  
  /*
   * This function is used by the send / receive code to convert a newly
   * allocated arc_buf_t to one that is suitable for a raw encrypted write. It
   * is also used to allow the root objset block to be updated without altering
   * its embedded MACs. Both block types will always be uncompressed so we do not
   * have to worry about compression type or psize.
   */
  void
  arc_convert_to_raw(arc_buf_t *buf, uint64_t dsobj, boolean_t byteorder,
      dmu_object_type_t ot, const uint8_t *salt, const uint8_t *iv,
      const uint8_t *mac)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          ASSERT(ot == DMU_OT_DNODE || ot == DMU_OT_OBJSET);
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
  
          buf->b_flags |= (ARC_BUF_FLAG_COMPRESSED | ARC_BUF_FLAG_ENCRYPTED);
          if (!HDR_PROTECTED(hdr))
                  hdr = arc_hdr_realloc_crypt(hdr, B_TRUE);
          hdr->b_crypt_hdr.b_dsobj = dsobj;
          hdr->b_crypt_hdr.b_ot = ot;
          hdr->b_l1hdr.b_byteswap = (byteorder == ZFS_HOST_BYTEORDER) ?
              DMU_BSWAP_NUMFUNCS : DMU_OT_BYTESWAP(ot);
          if (!arc_hdr_has_uncompressed_buf(hdr))
                  arc_cksum_free(hdr);
  
          if (salt != NULL)
                  memcpy(hdr->b_crypt_hdr.b_salt, salt, ZIO_DATA_SALT_LEN);
          if (iv != NULL)
                  memcpy(hdr->b_crypt_hdr.b_iv, iv, ZIO_DATA_IV_LEN);
          if (mac != NULL)
                  memcpy(hdr->b_crypt_hdr.b_mac, mac, ZIO_DATA_MAC_LEN);
  }
  
  /*
   * Allocate a new arc_buf_hdr_t and arc_buf_t and return the buf to the caller.
   * The buf is returned thawed since we expect the consumer to modify it.
   */
  arc_buf_t *
  arc_alloc_buf(spa_t *spa, const void *tag, arc_buf_contents_t type,
      int32_t size)
  {
          arc_buf_hdr_t *hdr = arc_hdr_alloc(spa_load_guid(spa), size, size,
              B_FALSE, ZIO_COMPRESS_OFF, 0, type);
  
          arc_buf_t *buf = NULL;
          VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_FALSE, B_FALSE,
              B_FALSE, B_FALSE, &buf));
          arc_buf_thaw(buf);
  
          return (buf);
  }
  
  /*
   * Allocate a compressed buf in the same manner as arc_alloc_buf. Don't use this
   * for bufs containing metadata.
   */
  arc_buf_t *
  arc_alloc_compressed_buf(spa_t *spa, const void *tag, uint64_t psize,
      uint64_t lsize, enum zio_compress compression_type, uint8_t complevel)
  {
          ASSERT3U(lsize, >, 0);
          ASSERT3U(lsize, >=, psize);
          ASSERT3U(compression_type, >, ZIO_COMPRESS_OFF);
          ASSERT3U(compression_type, <, ZIO_COMPRESS_FUNCTIONS);
  
          arc_buf_hdr_t *hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize,
              B_FALSE, compression_type, complevel, ARC_BUFC_DATA);
  
          arc_buf_t *buf = NULL;
          VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_FALSE,
              B_TRUE, B_FALSE, B_FALSE, &buf));
          arc_buf_thaw(buf);
  
          /*
           * To ensure that the hdr has the correct data in it if we call
           * arc_untransform() on this buf before it's been written to disk,
           * it's easiest if we just set up sharing between the buf and the hdr.
           */
          arc_share_buf(hdr, buf);
  
          return (buf);
  }
  
  arc_buf_t *
  arc_alloc_raw_buf(spa_t *spa, const void *tag, uint64_t dsobj,
      boolean_t byteorder, const uint8_t *salt, const uint8_t *iv,
      const uint8_t *mac, dmu_object_type_t ot, uint64_t psize, uint64_t lsize,
      enum zio_compress compression_type, uint8_t complevel)
  {
          arc_buf_hdr_t *hdr;
          arc_buf_t *buf;
          arc_buf_contents_t type = DMU_OT_IS_METADATA(ot) ?
              ARC_BUFC_METADATA : ARC_BUFC_DATA;
  
          ASSERT3U(lsize, >, 0);
          ASSERT3U(lsize, >=, psize);
          ASSERT3U(compression_type, >=, ZIO_COMPRESS_OFF);
          ASSERT3U(compression_type, <, ZIO_COMPRESS_FUNCTIONS);
  
          hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize, B_TRUE,
              compression_type, complevel, type);
  
          hdr->b_crypt_hdr.b_dsobj = dsobj;
          hdr->b_crypt_hdr.b_ot = ot;
          hdr->b_l1hdr.b_byteswap = (byteorder == ZFS_HOST_BYTEORDER) ?
              DMU_BSWAP_NUMFUNCS : DMU_OT_BYTESWAP(ot);
          memcpy(hdr->b_crypt_hdr.b_salt, salt, ZIO_DATA_SALT_LEN);
          memcpy(hdr->b_crypt_hdr.b_iv, iv, ZIO_DATA_IV_LEN);
          memcpy(hdr->b_crypt_hdr.b_mac, mac, ZIO_DATA_MAC_LEN);
  
          /*
           * This buffer will be considered encrypted even if the ot is not an
           * encrypted type. It will become authenticated instead in
           * arc_write_ready().
           */
          buf = NULL;
          VERIFY0(arc_buf_alloc_impl(hdr, spa, NULL, tag, B_TRUE, B_TRUE,
              B_FALSE, B_FALSE, &buf));
          arc_buf_thaw(buf);
  
          return (buf);
  }
  
  static void
  l2arc_hdr_arcstats_update(arc_buf_hdr_t *hdr, boolean_t incr,
      boolean_t state_only)
  {
          l2arc_buf_hdr_t *l2hdr = &hdr->b_l2hdr;
          l2arc_dev_t *dev = l2hdr->b_dev;
          uint64_t lsize = HDR_GET_LSIZE(hdr);
          uint64_t psize = HDR_GET_PSIZE(hdr);
          uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);
          arc_buf_contents_t type = hdr->b_type;
          int64_t lsize_s;
          int64_t psize_s;
          int64_t asize_s;
  
          if (incr) {
                  lsize_s = lsize;
                  psize_s = psize;
                  asize_s = asize;
          } else {
                  lsize_s = -lsize;
                  psize_s = -psize;
                  asize_s = -asize;
          }
  
          /* If the buffer is a prefetch, count it as such. */
          if (HDR_PREFETCH(hdr)) {
                  ARCSTAT_INCR(arcstat_l2_prefetch_asize, asize_s);
          } else {
                  /*
                   * We use the value stored in the L2 header upon initial
                   * caching in L2ARC. This value will be updated in case
                   * an MRU/MRU_ghost buffer transitions to MFU but the L2ARC
                   * metadata (log entry) cannot currently be updated. Having
                   * the ARC state in the L2 header solves the problem of a
                   * possibly absent L1 header (apparent in buffers restored
                   * from persistent L2ARC).
                   */
                  switch (hdr->b_l2hdr.b_arcs_state) {
                          case ARC_STATE_MRU_GHOST:
                          case ARC_STATE_MRU:
                                  ARCSTAT_INCR(arcstat_l2_mru_asize, asize_s);
                                  break;
                          case ARC_STATE_MFU_GHOST:
                          case ARC_STATE_MFU:
                                  ARCSTAT_INCR(arcstat_l2_mfu_asize, asize_s);
                                  break;
                          default:
                                  break;
                  }
          }
  
          if (state_only)
                  return;
  
          ARCSTAT_INCR(arcstat_l2_psize, psize_s);
          ARCSTAT_INCR(arcstat_l2_lsize, lsize_s);
  
          switch (type) {
                  case ARC_BUFC_DATA:
                          ARCSTAT_INCR(arcstat_l2_bufc_data_asize, asize_s);
                          break;
                  case ARC_BUFC_METADATA:
                          ARCSTAT_INCR(arcstat_l2_bufc_metadata_asize, asize_s);
                          break;
                  default:
                          break;
          }
  }
  
  
  static void
  arc_hdr_l2hdr_destroy(arc_buf_hdr_t *hdr)
  {
          l2arc_buf_hdr_t *l2hdr = &hdr->b_l2hdr;
          l2arc_dev_t *dev = l2hdr->b_dev;
          uint64_t psize = HDR_GET_PSIZE(hdr);
          uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);
  
          ASSERT(MUTEX_HELD(&dev->l2ad_mtx));
          ASSERT(HDR_HAS_L2HDR(hdr));
  
          list_remove(&dev->l2ad_buflist, hdr);
  
          l2arc_hdr_arcstats_decrement(hdr);
          vdev_space_update(dev->l2ad_vdev, -asize, 0, 0);
  
          (void) zfs_refcount_remove_many(&dev->l2ad_alloc, arc_hdr_size(hdr),
              hdr);
          arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);
  }
  
  static void
  arc_hdr_destroy(arc_buf_hdr_t *hdr)
  {
          if (HDR_HAS_L1HDR(hdr)) {
                  ASSERT(hdr->b_l1hdr.b_buf == NULL ||
                      hdr->b_l1hdr.b_bufcnt > 0);
                  ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                  ASSERT3P(hdr->b_l1hdr.b_state, ==, arc_anon);
          }
          ASSERT(!HDR_IO_IN_PROGRESS(hdr));
          ASSERT(!HDR_IN_HASH_TABLE(hdr));
  
          if (HDR_HAS_L2HDR(hdr)) {
                  l2arc_dev_t *dev = hdr->b_l2hdr.b_dev;
                  boolean_t buflist_held = MUTEX_HELD(&dev->l2ad_mtx);
  
                  if (!buflist_held)
                          mutex_enter(&dev->l2ad_mtx);
  
                  /*
                   * Even though we checked this conditional above, we
                   * need to check this again now that we have the
                   * l2ad_mtx. This is because we could be racing with
                   * another thread calling l2arc_evict() which might have
                   * destroyed this header's L2 portion as we were waiting
                   * to acquire the l2ad_mtx. If that happens, we don't
                   * want to re-destroy the header's L2 portion.
                   */
                  if (HDR_HAS_L2HDR(hdr)) {
  
                          if (!HDR_EMPTY(hdr))
                                  buf_discard_identity(hdr);
  
                          arc_hdr_l2hdr_destroy(hdr);
                  }
  
                  if (!buflist_held)
                          mutex_exit(&dev->l2ad_mtx);
          }
  
          /*
           * The header's identify can only be safely discarded once it is no
           * longer discoverable.  This requires removing it from the hash table
           * and the l2arc header list.  After this point the hash lock can not
           * be used to protect the header.
           */
          if (!HDR_EMPTY(hdr))
                  buf_discard_identity(hdr);
  
          if (HDR_HAS_L1HDR(hdr)) {
                  arc_cksum_free(hdr);
  
                  while (hdr->b_l1hdr.b_buf != NULL)
                          arc_buf_destroy_impl(hdr->b_l1hdr.b_buf);
  
                  if (hdr->b_l1hdr.b_pabd != NULL)
                          arc_hdr_free_abd(hdr, B_FALSE);
  
                  if (HDR_HAS_RABD(hdr))
                          arc_hdr_free_abd(hdr, B_TRUE);
          }
  
          ASSERT3P(hdr->b_hash_next, ==, NULL);
          if (HDR_HAS_L1HDR(hdr)) {
                  ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
                  ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
  #ifdef ZFS_DEBUG
                  ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
  #endif
  
                  if (!HDR_PROTECTED(hdr)) {
                          kmem_cache_free(hdr_full_cache, hdr);
                  } else {
                          kmem_cache_free(hdr_full_crypt_cache, hdr);
                  }
          } else {
                  kmem_cache_free(hdr_l2only_cache, hdr);
          }
  }
  
  void
  arc_buf_destroy(arc_buf_t *buf, const void *tag)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          if (hdr->b_l1hdr.b_state == arc_anon) {
                  ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
                  ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                  VERIFY0(remove_reference(hdr, tag));
                  return;
          }
  
          kmutex_t *hash_lock = HDR_LOCK(hdr);
          mutex_enter(hash_lock);
  
          ASSERT3P(hdr, ==, buf->b_hdr);
          ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
          ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
          ASSERT3P(hdr->b_l1hdr.b_state, !=, arc_anon);
          ASSERT3P(buf->b_data, !=, NULL);
  
          arc_buf_destroy_impl(buf);
          (void) remove_reference(hdr, tag);
          mutex_exit(hash_lock);
  }
  
  /*
   * Evict the arc_buf_hdr that is provided as a parameter. The resultant
   * state of the header is dependent on its state prior to entering this
   * function. The following transitions are possible:
   *
   *    - arc_mru -> arc_mru_ghost
   *    - arc_mfu -> arc_mfu_ghost
   *    - arc_mru_ghost -> arc_l2c_only
   *    - arc_mru_ghost -> deleted
   *    - arc_mfu_ghost -> arc_l2c_only
   *    - arc_mfu_ghost -> deleted
   *    - arc_uncached -> deleted
   *
   * Return total size of evicted data buffers for eviction progress tracking.
   * When evicting from ghost states return logical buffer size to make eviction
   * progress at the same (or at least comparable) rate as from non-ghost states.
   *
   * Return *real_evicted for actual ARC size reduction to wake up threads
   * waiting for it.  For non-ghost states it includes size of evicted data
   * buffers (the headers are not freed there).  For ghost states it includes
   * only the evicted headers size.
   */
  static int64_t
  arc_evict_hdr(arc_buf_hdr_t *hdr, uint64_t *real_evicted)
  {
          arc_state_t *evicted_state, *state;
          int64_t bytes_evicted = 0;
          uint_t min_lifetime = HDR_PRESCIENT_PREFETCH(hdr) ?
              arc_min_prescient_prefetch_ms : arc_min_prefetch_ms;
  
          ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT(!HDR_IO_IN_PROGRESS(hdr));
          ASSERT0(hdr->b_l1hdr.b_bufcnt);
          ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
          ASSERT0(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt));
  
          *real_evicted = 0;
          state = hdr->b_l1hdr.b_state;
          if (GHOST_STATE(state)) {
  
                  /*
                   * l2arc_write_buffers() relies on a header's L1 portion
                   * (i.e. its b_pabd field) during it's write phase.
                   * Thus, we cannot push a header onto the arc_l2c_only
                   * state (removing its L1 piece) until the header is
                   * done being written to the l2arc.
                   */
                  if (HDR_HAS_L2HDR(hdr) && HDR_L2_WRITING(hdr)) {
                          ARCSTAT_BUMP(arcstat_evict_l2_skip);
                          return (bytes_evicted);
                  }
  
                  ARCSTAT_BUMP(arcstat_deleted);
                  bytes_evicted += HDR_GET_LSIZE(hdr);
  
                  DTRACE_PROBE1(arc__delete, arc_buf_hdr_t *, hdr);
  
                  if (HDR_HAS_L2HDR(hdr)) {
                          ASSERT(hdr->b_l1hdr.b_pabd == NULL);
                          ASSERT(!HDR_HAS_RABD(hdr));
                          /*
                           * This buffer is cached on the 2nd Level ARC;
                           * don't destroy the header.
                           */
                          arc_change_state(arc_l2c_only, hdr);
                          /*
                           * dropping from L1+L2 cached to L2-only,
                           * realloc to remove the L1 header.
                           */
                          (void) arc_hdr_realloc(hdr, hdr_full_cache,
                              hdr_l2only_cache);
                          *real_evicted += HDR_FULL_SIZE - HDR_L2ONLY_SIZE;
                  } else {
                          arc_change_state(arc_anon, hdr);
                          arc_hdr_destroy(hdr);
                          *real_evicted += HDR_FULL_SIZE;
                  }
                  return (bytes_evicted);
          }
  
          ASSERT(state == arc_mru || state == arc_mfu || state == arc_uncached);
          evicted_state = (state == arc_uncached) ? arc_anon :
              ((state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost);
  
          /* prefetch buffers have a minimum lifespan */
          if ((hdr->b_flags & (ARC_FLAG_PREFETCH | ARC_FLAG_INDIRECT)) &&
              ddi_get_lbolt() - hdr->b_l1hdr.b_arc_access <
              MSEC_TO_TICK(min_lifetime)) {
                  ARCSTAT_BUMP(arcstat_evict_skip);
                  return (bytes_evicted);
          }
  
          if (HDR_HAS_L2HDR(hdr)) {
                  ARCSTAT_INCR(arcstat_evict_l2_cached, HDR_GET_LSIZE(hdr));
          } else {
                  if (l2arc_write_eligible(hdr->b_spa, hdr)) {
                          ARCSTAT_INCR(arcstat_evict_l2_eligible,
                              HDR_GET_LSIZE(hdr));
  
                          switch (state->arcs_state) {
                                  case ARC_STATE_MRU:
                                          ARCSTAT_INCR(
                                              arcstat_evict_l2_eligible_mru,
                                              HDR_GET_LSIZE(hdr));
                                          break;
                                  case ARC_STATE_MFU:
                                          ARCSTAT_INCR(
                                              arcstat_evict_l2_eligible_mfu,
                                              HDR_GET_LSIZE(hdr));
                                          break;
                                  default:
                                          break;
                          }
                  } else {
                          ARCSTAT_INCR(arcstat_evict_l2_ineligible,
                              HDR_GET_LSIZE(hdr));
                  }
          }
  
          bytes_evicted += arc_hdr_size(hdr);
          *real_evicted += arc_hdr_size(hdr);
  
          /*
           * If this hdr is being evicted and has a compressed buffer then we
           * discard it here before we change states.  This ensures that the
           * accounting is updated correctly in arc_free_data_impl().
           */
          if (hdr->b_l1hdr.b_pabd != NULL)
                  arc_hdr_free_abd(hdr, B_FALSE);
  
          if (HDR_HAS_RABD(hdr))
                  arc_hdr_free_abd(hdr, B_TRUE);
  
          arc_change_state(evicted_state, hdr);
          DTRACE_PROBE1(arc__evict, arc_buf_hdr_t *, hdr);
          if (evicted_state == arc_anon) {
                  arc_hdr_destroy(hdr);
                  *real_evicted += HDR_FULL_SIZE;
          } else {
                  ASSERT(HDR_IN_HASH_TABLE(hdr));
          }
  
          return (bytes_evicted);
  }
  
  static void
  arc_set_need_free(void)
  {
          ASSERT(MUTEX_HELD(&arc_evict_lock));
          int64_t remaining = arc_free_memory() - arc_sys_free / 2;
          arc_evict_waiter_t *aw = list_tail(&arc_evict_waiters);
          if (aw == NULL) {
                  arc_need_free = MAX(-remaining, 0);
          } else {
                  arc_need_free =
                      MAX(-remaining, (int64_t)(aw->aew_count - arc_evict_count));
          }
  }
  
  static uint64_t
  arc_evict_state_impl(multilist_t *ml, int idx, arc_buf_hdr_t *marker,
      uint64_t spa, uint64_t bytes)
  {
          multilist_sublist_t *mls;
          uint64_t bytes_evicted = 0, real_evicted = 0;
          arc_buf_hdr_t *hdr;
          kmutex_t *hash_lock;
          uint_t evict_count = zfs_arc_evict_batch_limit;
  
          ASSERT3P(marker, !=, NULL);
  
          mls = multilist_sublist_lock(ml, idx);
  
          for (hdr = multilist_sublist_prev(mls, marker); likely(hdr != NULL);
              hdr = multilist_sublist_prev(mls, marker)) {
                  if ((evict_count == 0) || (bytes_evicted >= bytes))
                          break;
  
                  /*
                   * To keep our iteration location, move the marker
                   * forward. Since we're not holding hdr's hash lock, we
                   * must be very careful and not remove 'hdr' from the
                   * sublist. Otherwise, other consumers might mistake the
                   * 'hdr' as not being on a sublist when they call the
                   * multilist_link_active() function (they all rely on
                   * the hash lock protecting concurrent insertions and
                   * removals). multilist_sublist_move_forward() was
                   * specifically implemented to ensure this is the case
                   * (only 'marker' will be removed and re-inserted).
                   */
                  multilist_sublist_move_forward(mls, marker);
  
                  /*
                   * The only case where the b_spa field should ever be
                   * zero, is the marker headers inserted by
                   * arc_evict_state(). It's possible for multiple threads
                   * to be calling arc_evict_state() concurrently (e.g.
                   * dsl_pool_close() and zio_inject_fault()), so we must
                   * skip any markers we see from these other threads.
                   */
                  if (hdr->b_spa == 0)
                          continue;
  
                  /* we're only interested in evicting buffers of a certain spa */
                  if (spa != 0 && hdr->b_spa != spa) {
                          ARCSTAT_BUMP(arcstat_evict_skip);
                          continue;
                  }
  
                  hash_lock = HDR_LOCK(hdr);
  
                  /*
                   * We aren't calling this function from any code path
                   * that would already be holding a hash lock, so we're
                   * asserting on this assumption to be defensive in case
                   * this ever changes. Without this check, it would be
                   * possible to incorrectly increment arcstat_mutex_miss
                   * below (e.g. if the code changed such that we called
                   * this function with a hash lock held).
                   */
                  ASSERT(!MUTEX_HELD(hash_lock));
  
                  if (mutex_tryenter(hash_lock)) {
                          uint64_t revicted;
                          uint64_t evicted = arc_evict_hdr(hdr, &revicted);
                          mutex_exit(hash_lock);
  
                          bytes_evicted += evicted;
                          real_evicted += revicted;
  
                          /*
                           * If evicted is zero, arc_evict_hdr() must have
                           * decided to skip this header, don't increment
                           * evict_count in this case.
                           */
                          if (evicted != 0)
                                  evict_count--;
  
                  } else {
                          ARCSTAT_BUMP(arcstat_mutex_miss);
                  }
          }
  
          multilist_sublist_unlock(mls);
  
          /*
           * Increment the count of evicted bytes, and wake up any threads that
           * are waiting for the count to reach this value.  Since the list is
           * ordered by ascending aew_count, we pop off the beginning of the
           * list until we reach the end, or a waiter that's past the current
           * "count".  Doing this outside the loop reduces the number of times
           * we need to acquire the global arc_evict_lock.
           *
           * Only wake when there's sufficient free memory in the system
           * (specifically, arc_sys_free/2, which by default is a bit more than
           * 1/64th of RAM).  See the comments in arc_wait_for_eviction().
           */
          mutex_enter(&arc_evict_lock);
          arc_evict_count += real_evicted;
  
          if (arc_free_memory() > arc_sys_free / 2) {
                  arc_evict_waiter_t *aw;
                  while ((aw = list_head(&arc_evict_waiters)) != NULL &&
                      aw->aew_count <= arc_evict_count) {
                          list_remove(&arc_evict_waiters, aw);
                          cv_broadcast(&aw->aew_cv);
                  }
          }
          arc_set_need_free();
          mutex_exit(&arc_evict_lock);
  
          /*
           * If the ARC size is reduced from arc_c_max to arc_c_min (especially
           * if the average cached block is small), eviction can be on-CPU for
           * many seconds.  To ensure that other threads that may be bound to
           * this CPU are able to make progress, make a voluntary preemption
           * call here.
           */
          kpreempt(KPREEMPT_SYNC);
  
          return (bytes_evicted);
  }
  
  /*
   * Allocate an array of buffer headers used as placeholders during arc state
   * eviction.
   */
  static arc_buf_hdr_t **
  arc_state_alloc_markers(int count)
  {
          arc_buf_hdr_t **markers;
  
          markers = kmem_zalloc(sizeof (*markers) * count, KM_SLEEP);
          for (int i = 0; i < count; i++) {
                  markers[i] = kmem_cache_alloc(hdr_full_cache, KM_SLEEP);
  
                  /*
                   * A b_spa of 0 is used to indicate that this header is
                   * a marker. This fact is used in arc_evict_type() and
                   * arc_evict_state_impl().
                   */
                  markers[i]->b_spa = 0;
  
          }
          return (markers);
  }
  
  static void
  arc_state_free_markers(arc_buf_hdr_t **markers, int count)
  {
          for (int i = 0; i < count; i++)
                  kmem_cache_free(hdr_full_cache, markers[i]);
          kmem_free(markers, sizeof (*markers) * count);
  }
  
  /*
   * Evict buffers from the given arc state, until we've removed the
   * specified number of bytes. Move the removed buffers to the
   * appropriate evict state.
   *
   * This function makes a "best effort". It skips over any buffers
   * it can't get a hash_lock on, and so, may not catch all candidates.
   * It may also return without evicting as much space as requested.
   *
   * If bytes is specified using the special value ARC_EVICT_ALL, this
   * will evict all available (i.e. unlocked and evictable) buffers from
   * the given arc state; which is used by arc_flush().
   */
  static uint64_t
  arc_evict_state(arc_state_t *state, uint64_t spa, uint64_t bytes,
      arc_buf_contents_t type)
  {
          uint64_t total_evicted = 0;
          multilist_t *ml = &state->arcs_list[type];
          int num_sublists;
          arc_buf_hdr_t **markers;
  
          num_sublists = multilist_get_num_sublists(ml);
  
          /*
           * If we've tried to evict from each sublist, made some
           * progress, but still have not hit the target number of bytes
           * to evict, we want to keep trying. The markers allow us to
           * pick up where we left off for each individual sublist, rather
           * than starting from the tail each time.
           */
          if (zthr_iscurthread(arc_evict_zthr)) {
                  markers = arc_state_evict_markers;
                  ASSERT3S(num_sublists, <=, arc_state_evict_marker_count);
          } else {
                  markers = arc_state_alloc_markers(num_sublists);
          }
          for (int i = 0; i < num_sublists; i++) {
                  multilist_sublist_t *mls;
  
                  mls = multilist_sublist_lock(ml, i);
                  multilist_sublist_insert_tail(mls, markers[i]);
                  multilist_sublist_unlock(mls);
          }
  
          /*
           * While we haven't hit our target number of bytes to evict, or
           * we're evicting all available buffers.
           */
          while (total_evicted < bytes) {
                  int sublist_idx = multilist_get_random_index(ml);
                  uint64_t scan_evicted = 0;
  
                  /*
                   * Try to reduce pinned dnodes with a floor of arc_dnode_limit.
                   * Request that 10% of the LRUs be scanned by the superblock
                   * shrinker.
                   */
                  if (type == ARC_BUFC_DATA && aggsum_compare(
                      &arc_sums.arcstat_dnode_size, arc_dnode_size_limit) > 0) {
                          arc_prune_async((aggsum_upper_bound(
                              &arc_sums.arcstat_dnode_size) -
                              arc_dnode_size_limit) / sizeof (dnode_t) /
                              zfs_arc_dnode_reduce_percent);
                  }
  
                  /*
                   * Start eviction using a randomly selected sublist,
                   * this is to try and evenly balance eviction across all
                   * sublists. Always starting at the same sublist
                   * (e.g. index 0) would cause evictions to favor certain
                   * sublists over others.
                   */
                  for (int i = 0; i < num_sublists; i++) {
                          uint64_t bytes_remaining;
                          uint64_t bytes_evicted;
  
                          if (total_evicted < bytes)
                                  bytes_remaining = bytes - total_evicted;
                          else
                                  break;
  
                          bytes_evicted = arc_evict_state_impl(ml, sublist_idx,
                              markers[sublist_idx], spa, bytes_remaining);
  
                          scan_evicted += bytes_evicted;
                          total_evicted += bytes_evicted;
  
                          /* we've reached the end, wrap to the beginning */
                          if (++sublist_idx >= num_sublists)
                                  sublist_idx = 0;
                  }
  
                  /*
                   * If we didn't evict anything during this scan, we have
                   * no reason to believe we'll evict more during another
                   * scan, so break the loop.
                   */
                  if (scan_evicted == 0) {
                          /* This isn't possible, let's make that obvious */
                          ASSERT3S(bytes, !=, 0);
  
                          /*
                           * When bytes is ARC_EVICT_ALL, the only way to
                           * break the loop is when scan_evicted is zero.
                           * In that case, we actually have evicted enough,
                           * so we don't want to increment the kstat.
                           */
                          if (bytes != ARC_EVICT_ALL) {
                                  ASSERT3S(total_evicted, <, bytes);
                                  ARCSTAT_BUMP(arcstat_evict_not_enough);
                          }
  
                          break;
                  }
          }
  
          for (int i = 0; i < num_sublists; i++) {
                  multilist_sublist_t *mls = multilist_sublist_lock(ml, i);
                  multilist_sublist_remove(mls, markers[i]);
                  multilist_sublist_unlock(mls);
          }
          if (markers != arc_state_evict_markers)
                  arc_state_free_markers(markers, num_sublists);
  
          return (total_evicted);
  }
  
  /*
   * Flush all "evictable" data of the given type from the arc state
   * specified. This will not evict any "active" buffers (i.e. referenced).
   *
   * When 'retry' is set to B_FALSE, the function will make a single pass
   * over the state and evict any buffers that it can. Since it doesn't
   * continually retry the eviction, it might end up leaving some buffers
   * in the ARC due to lock misses.
   *
   * When 'retry' is set to B_TRUE, the function will continually retry the
   * eviction until *all* evictable buffers have been removed from the
   * state. As a result, if concurrent insertions into the state are
   * allowed (e.g. if the ARC isn't shutting down), this function might
   * wind up in an infinite loop, continually trying to evict buffers.
   */
  static uint64_t
  arc_flush_state(arc_state_t *state, uint64_t spa, arc_buf_contents_t type,
      boolean_t retry)
  {
          uint64_t evicted = 0;
  
          while (zfs_refcount_count(&state->arcs_esize[type]) != 0) {
                  evicted += arc_evict_state(state, spa, ARC_EVICT_ALL, type);
  
                  if (!retry)
                          break;
          }
  
          return (evicted);
  }
  
  /*
   * Evict the specified number of bytes from the state specified,
   * restricting eviction to the spa and type given. This function
   * prevents us from trying to evict more from a state's list than
   * is "evictable", and to skip evicting altogether when passed a
   * negative value for "bytes". In contrast, arc_evict_state() will
   * evict everything it can, when passed a negative value for "bytes".
   */
  static uint64_t
  arc_evict_impl(arc_state_t *state, uint64_t spa, int64_t bytes,
      arc_buf_contents_t type)
  {
          uint64_t delta;
  
          if (bytes > 0 && zfs_refcount_count(&state->arcs_esize[type]) > 0) {
                  delta = MIN(zfs_refcount_count(&state->arcs_esize[type]),
                      bytes);
                  return (arc_evict_state(state, spa, delta, type));
          }
  
          return (0);
  }
  
  /*
   * The goal of this function is to evict enough meta data buffers from the
   * ARC in order to enforce the arc_meta_limit.  Achieving this is slightly
   * more complicated than it appears because it is common for data buffers
   * to have holds on meta data buffers.  In addition, dnode meta data buffers
   * will be held by the dnodes in the block preventing them from being freed.
   * This means we can't simply traverse the ARC and expect to always find
   * enough unheld meta data buffer to release.
   *
   * Therefore, this function has been updated to make alternating passes
   * over the ARC releasing data buffers and then newly unheld meta data
   * buffers.  This ensures forward progress is maintained and meta_used
   * will decrease.  Normally this is sufficient, but if required the ARC
   * will call the registered prune callbacks causing dentry and inodes to
   * be dropped from the VFS cache.  This will make dnode meta data buffers
   * available for reclaim.
   */
  static uint64_t
  arc_evict_meta_balanced(uint64_t meta_used)
  {
          int64_t delta, adjustmnt;
          uint64_t total_evicted = 0, prune = 0;
          arc_buf_contents_t type = ARC_BUFC_DATA;
          uint_t restarts = zfs_arc_meta_adjust_restarts;
  
  restart:
          /*
           * This slightly differs than the way we evict from the mru in
           * arc_evict because we don't have a "target" value (i.e. no
           * "meta" arc_p). As a result, I think we can completely
           * cannibalize the metadata in the MRU before we evict the
           * metadata from the MFU. I think we probably need to implement a
           * "metadata arc_p" value to do this properly.
           */
          adjustmnt = meta_used - arc_meta_limit;
  
          if (adjustmnt > 0 &&
              zfs_refcount_count(&arc_mru->arcs_esize[type]) > 0) {
                  delta = MIN(zfs_refcount_count(&arc_mru->arcs_esize[type]),
                      adjustmnt);
                  total_evicted += arc_evict_impl(arc_mru, 0, delta, type);
                  adjustmnt -= delta;
          }
  
          /*
           * We can't afford to recalculate adjustmnt here. If we do,
           * new metadata buffers can sneak into the MRU or ANON lists,
           * thus penalize the MFU metadata. Although the fudge factor is
           * small, it has been empirically shown to be significant for
           * certain workloads (e.g. creating many empty directories). As
           * such, we use the original calculation for adjustmnt, and
           * simply decrement the amount of data evicted from the MRU.
           */
  
          if (adjustmnt > 0 &&
              zfs_refcount_count(&arc_mfu->arcs_esize[type]) > 0) {
                  delta = MIN(zfs_refcount_count(&arc_mfu->arcs_esize[type]),
                      adjustmnt);
                  total_evicted += arc_evict_impl(arc_mfu, 0, delta, type);
          }
  
          adjustmnt = meta_used - arc_meta_limit;
  
          if (adjustmnt > 0 &&
              zfs_refcount_count(&arc_mru_ghost->arcs_esize[type]) > 0) {
                  delta = MIN(adjustmnt,
                      zfs_refcount_count(&arc_mru_ghost->arcs_esize[type]));
                  total_evicted += arc_evict_impl(arc_mru_ghost, 0, delta, type);
                  adjustmnt -= delta;
          }
  
          if (adjustmnt > 0 &&
              zfs_refcount_count(&arc_mfu_ghost->arcs_esize[type]) > 0) {
                  delta = MIN(adjustmnt,
                      zfs_refcount_count(&arc_mfu_ghost->arcs_esize[type]));
                  total_evicted += arc_evict_impl(arc_mfu_ghost, 0, delta, type);
          }
  
          /*
           * If after attempting to make the requested adjustment to the ARC
           * the meta limit is still being exceeded then request that the
           * higher layers drop some cached objects which have holds on ARC
           * meta buffers.  Requests to the upper layers will be made with
           * increasingly large scan sizes until the ARC is below the limit.
           */
          if (meta_used > arc_meta_limit || arc_available_memory() < 0) {
                  if (type == ARC_BUFC_DATA) {
                          type = ARC_BUFC_METADATA;
                  } else {
                          type = ARC_BUFC_DATA;
  
                          if (zfs_arc_meta_prune) {
                                  prune += zfs_arc_meta_prune;
                                  arc_prune_async(prune);
                          }
                  }
  
                  if (restarts > 0) {
                          restarts--;
                          goto restart;
                  }
          }
          return (total_evicted);
  }
  
  /*
   * Evict metadata buffers from the cache, such that arcstat_meta_used is
   * capped by the arc_meta_limit tunable.
   */
  static uint64_t
  arc_evict_meta_only(uint64_t meta_used)
  {
          uint64_t total_evicted = 0;
          int64_t target;
  
          /*
           * If we're over the meta limit, we want to evict enough
           * metadata to get back under the meta limit. We don't want to
           * evict so much that we drop the MRU below arc_p, though. If
           * we're over the meta limit more than we're over arc_p, we
           * evict some from the MRU here, and some from the MFU below.
           */
          target = MIN((int64_t)(meta_used - arc_meta_limit),
              (int64_t)(zfs_refcount_count(&arc_anon->arcs_size) +
              zfs_refcount_count(&arc_mru->arcs_size) - arc_p));
  
          total_evicted += arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
  
          /*
           * Similar to the above, we want to evict enough bytes to get us
           * below the meta limit, but not so much as to drop us below the
           * space allotted to the MFU (which is defined as arc_c - arc_p).
           */
          target = MIN((int64_t)(meta_used - arc_meta_limit),
              (int64_t)(zfs_refcount_count(&arc_mfu->arcs_size) -
              (arc_c - arc_p)));
  
          total_evicted += arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
  
          return (total_evicted);
  }
  
  static uint64_t
  arc_evict_meta(uint64_t meta_used)
  {
          if (zfs_arc_meta_strategy == ARC_STRATEGY_META_ONLY)
                  return (arc_evict_meta_only(meta_used));
          else
                  return (arc_evict_meta_balanced(meta_used));
  }
  
  /*
   * Return the type of the oldest buffer in the given arc state
   *
   * This function will select a random sublist of type ARC_BUFC_DATA and
   * a random sublist of type ARC_BUFC_METADATA. The tail of each sublist
   * is compared, and the type which contains the "older" buffer will be
   * returned.
   */
  static arc_buf_contents_t
  arc_evict_type(arc_state_t *state)
  {
          multilist_t *data_ml = &state->arcs_list[ARC_BUFC_DATA];
          multilist_t *meta_ml = &state->arcs_list[ARC_BUFC_METADATA];
          int data_idx = multilist_get_random_index(data_ml);
          int meta_idx = multilist_get_random_index(meta_ml);
          multilist_sublist_t *data_mls;
          multilist_sublist_t *meta_mls;
          arc_buf_contents_t type;
          arc_buf_hdr_t *data_hdr;
          arc_buf_hdr_t *meta_hdr;
  
          /*
           * We keep the sublist lock until we're finished, to prevent
           * the headers from being destroyed via arc_evict_state().
           */
          data_mls = multilist_sublist_lock(data_ml, data_idx);
          meta_mls = multilist_sublist_lock(meta_ml, meta_idx);
  
          /*
           * These two loops are to ensure we skip any markers that
           * might be at the tail of the lists due to arc_evict_state().
           */
  
          for (data_hdr = multilist_sublist_tail(data_mls); data_hdr != NULL;
              data_hdr = multilist_sublist_prev(data_mls, data_hdr)) {
                  if (data_hdr->b_spa != 0)
                          break;
          }
  
          for (meta_hdr = multilist_sublist_tail(meta_mls); meta_hdr != NULL;
              meta_hdr = multilist_sublist_prev(meta_mls, meta_hdr)) {
                  if (meta_hdr->b_spa != 0)
                          break;
          }
  
          if (data_hdr == NULL && meta_hdr == NULL) {
                  type = ARC_BUFC_DATA;
          } else if (data_hdr == NULL) {
                  ASSERT3P(meta_hdr, !=, NULL);
                  type = ARC_BUFC_METADATA;
          } else if (meta_hdr == NULL) {
                  ASSERT3P(data_hdr, !=, NULL);
                  type = ARC_BUFC_DATA;
          } else {
                  ASSERT3P(data_hdr, !=, NULL);
                  ASSERT3P(meta_hdr, !=, NULL);
  
                  /* The headers can't be on the sublist without an L1 header */
                  ASSERT(HDR_HAS_L1HDR(data_hdr));
                  ASSERT(HDR_HAS_L1HDR(meta_hdr));
  
                  if (data_hdr->b_l1hdr.b_arc_access <
                      meta_hdr->b_l1hdr.b_arc_access) {
                          type = ARC_BUFC_DATA;
                  } else {
                          type = ARC_BUFC_METADATA;
                  }
          }
  
          multilist_sublist_unlock(meta_mls);
          multilist_sublist_unlock(data_mls);
  
          return (type);
  }
  
  /*
   * Evict buffers from the cache, such that arcstat_size is capped by arc_c.
   */
  static uint64_t
  arc_evict(void)
  {
          uint64_t total_evicted = 0;
          uint64_t bytes;
          int64_t target;
          uint64_t asize = aggsum_value(&arc_sums.arcstat_size);
          uint64_t ameta = aggsum_value(&arc_sums.arcstat_meta_used);
  
          /*
           * If we're over arc_meta_limit, we want to correct that before
           * potentially evicting data buffers below.
           */
          total_evicted += arc_evict_meta(ameta);
  
          /*
           * Adjust MRU size
           *
           * If we're over the target cache size, we want to evict enough
           * from the list to get back to our target size. We don't want
           * to evict too much from the MRU, such that it drops below
           * arc_p. So, if we're over our target cache size more than
           * the MRU is over arc_p, we'll evict enough to get back to
           * arc_p here, and then evict more from the MFU below.
           */
          target = MIN((int64_t)(asize - arc_c),
              (int64_t)(zfs_refcount_count(&arc_anon->arcs_size) +
              zfs_refcount_count(&arc_mru->arcs_size) + ameta - arc_p));
  
          /*
           * If we're below arc_meta_min, always prefer to evict data.
           * Otherwise, try to satisfy the requested number of bytes to
           * evict from the type which contains older buffers; in an
           * effort to keep newer buffers in the cache regardless of their
           * type. If we cannot satisfy the number of bytes from this
           * type, spill over into the next type.
           */
          if (arc_evict_type(arc_mru) == ARC_BUFC_METADATA &&
              ameta > arc_meta_min) {
                  bytes = arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
                  total_evicted += bytes;
  
                  /*
                   * If we couldn't evict our target number of bytes from
                   * metadata, we try to get the rest from data.
                   */
                  target -= bytes;
  
                  total_evicted +=
                      arc_evict_impl(arc_mru, 0, target, ARC_BUFC_DATA);
          } else {
                  bytes = arc_evict_impl(arc_mru, 0, target, ARC_BUFC_DATA);
                  total_evicted += bytes;
  
                  /*
                   * If we couldn't evict our target number of bytes from
                   * data, we try to get the rest from metadata.
                   */
                  target -= bytes;
  
                  total_evicted +=
                      arc_evict_impl(arc_mru, 0, target, ARC_BUFC_METADATA);
          }
  
          /*
           * Re-sum ARC stats after the first round of evictions.
           */
          asize = aggsum_value(&arc_sums.arcstat_size);
          ameta = aggsum_value(&arc_sums.arcstat_meta_used);
  
  
          /*
           * Adjust MFU size
           *
           * Now that we've tried to evict enough from the MRU to get its
           * size back to arc_p, if we're still above the target cache
           * size, we evict the rest from the MFU.
           */
          target = asize - arc_c;
  
          if (arc_evict_type(arc_mfu) == ARC_BUFC_METADATA &&
              ameta > arc_meta_min) {
                  bytes = arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
                  total_evicted += bytes;
  
                  /*
                   * If we couldn't evict our target number of bytes from
                   * metadata, we try to get the rest from data.
                   */
                  target -= bytes;
  
                  total_evicted +=
                      arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_DATA);
          } else {
                  bytes = arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_DATA);
                  total_evicted += bytes;
  
                  /*
                   * If we couldn't evict our target number of bytes from
                   * data, we try to get the rest from data.
                   */
                  target -= bytes;
  
                  total_evicted +=
                      arc_evict_impl(arc_mfu, 0, target, ARC_BUFC_METADATA);
          }
  
          /*
           * Adjust ghost lists
           *
           * In addition to the above, the ARC also defines target values
           * for the ghost lists. The sum of the mru list and mru ghost
           * list should never exceed the target size of the cache, and
           * the sum of the mru list, mfu list, mru ghost list, and mfu
           * ghost list should never exceed twice the target size of the
           * cache. The following logic enforces these limits on the ghost
           * caches, and evicts from them as needed.
           */
          target = zfs_refcount_count(&arc_mru->arcs_size) +
              zfs_refcount_count(&arc_mru_ghost->arcs_size) - arc_c;
  
          bytes = arc_evict_impl(arc_mru_ghost, 0, target, ARC_BUFC_DATA);
          total_evicted += bytes;
  
          target -= bytes;
  
          total_evicted +=
              arc_evict_impl(arc_mru_ghost, 0, target, ARC_BUFC_METADATA);
  
          /*
           * We assume the sum of the mru list and mfu list is less than
           * or equal to arc_c (we enforced this above), which means we
           * can use the simpler of the two equations below:
           *
           *      mru + mfu + mru ghost + mfu ghost <= 2 * arc_c
           *                  mru ghost + mfu ghost <= arc_c
           */
          target = zfs_refcount_count(&arc_mru_ghost->arcs_size) +
              zfs_refcount_count(&arc_mfu_ghost->arcs_size) - arc_c;
  
          bytes = arc_evict_impl(arc_mfu_ghost, 0, target, ARC_BUFC_DATA);
          total_evicted += bytes;
  
          target -= bytes;
  
          total_evicted +=
              arc_evict_impl(arc_mfu_ghost, 0, target, ARC_BUFC_METADATA);
  
          return (total_evicted);
  }
  
  void
  arc_flush(spa_t *spa, boolean_t retry)
  {
          uint64_t guid = 0;
  
          /*
           * If retry is B_TRUE, a spa must not be specified since we have
           * no good way to determine if all of a spa's buffers have been
           * evicted from an arc state.
           */
          ASSERT(!retry || spa == NULL);
  
          if (spa != NULL)
                  guid = spa_load_guid(spa);
  
          (void) arc_flush_state(arc_mru, guid, ARC_BUFC_DATA, retry);
          (void) arc_flush_state(arc_mru, guid, ARC_BUFC_METADATA, retry);
  
          (void) arc_flush_state(arc_mfu, guid, ARC_BUFC_DATA, retry);
          (void) arc_flush_state(arc_mfu, guid, ARC_BUFC_METADATA, retry);
  
          (void) arc_flush_state(arc_mru_ghost, guid, ARC_BUFC_DATA, retry);
          (void) arc_flush_state(arc_mru_ghost, guid, ARC_BUFC_METADATA, retry);
  
          (void) arc_flush_state(arc_mfu_ghost, guid, ARC_BUFC_DATA, retry);
          (void) arc_flush_state(arc_mfu_ghost, guid, ARC_BUFC_METADATA, retry);
  
          (void) arc_flush_state(arc_uncached, guid, ARC_BUFC_DATA, retry);
          (void) arc_flush_state(arc_uncached, guid, ARC_BUFC_METADATA, retry);
  }
  
  void
  arc_reduce_target_size(int64_t to_free)
  {
          uint64_t asize = aggsum_value(&arc_sums.arcstat_size);
  
          /*
           * All callers want the ARC to actually evict (at least) this much
           * memory.  Therefore we reduce from the lower of the current size and
           * the target size.  This way, even if arc_c is much higher than
           * arc_size (as can be the case after many calls to arc_freed(), we will
           * immediately have arc_c < arc_size and therefore the arc_evict_zthr
           * will evict.
           */
          uint64_t c = MIN(arc_c, asize);
  
          if (c > to_free && c - to_free > arc_c_min) {
                  arc_c = c - to_free;
                  atomic_add_64(&arc_p, -(arc_p >> arc_shrink_shift));
                  if (arc_p > arc_c)
                          arc_p = (arc_c >> 1);
                  ASSERT(arc_c >= arc_c_min);
                  ASSERT((int64_t)arc_p >= 0);
          } else {
                  arc_c = arc_c_min;
          }
  
          if (asize > arc_c) {
                  /* See comment in arc_evict_cb_check() on why lock+flag */
                  mutex_enter(&arc_evict_lock);
                  arc_evict_needed = B_TRUE;
                  mutex_exit(&arc_evict_lock);
                  zthr_wakeup(arc_evict_zthr);
          }
  }
  
  /*
   * Determine if the system is under memory pressure and is asking
   * to reclaim memory. A return value of B_TRUE indicates that the system
   * is under memory pressure and that the arc should adjust accordingly.
   */
  boolean_t
  arc_reclaim_needed(void)
  {
          return (arc_available_memory() < 0);
  }
  
  void
  arc_kmem_reap_soon(void)
  {
          size_t                  i;
          kmem_cache_t            *prev_cache = NULL;
          kmem_cache_t            *prev_data_cache = NULL;
  
  #ifdef _KERNEL
          if ((aggsum_compare(&arc_sums.arcstat_meta_used,
              arc_meta_limit) >= 0) && zfs_arc_meta_prune) {
                  /*
                   * We are exceeding our meta-data cache limit.
                   * Prune some entries to release holds on meta-data.
                   */
                  arc_prune_async(zfs_arc_meta_prune);
          }
  #if defined(_ILP32)
          /*
           * Reclaim unused memory from all kmem caches.
           */
          kmem_reap();
  #endif
  #endif
  
          for (i = 0; i < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; i++) {
  #if defined(_ILP32)
                  /* reach upper limit of cache size on 32-bit */
                  if (zio_buf_cache[i] == NULL)
                          break;
  #endif
                  if (zio_buf_cache[i] != prev_cache) {
                          prev_cache = zio_buf_cache[i];
                          kmem_cache_reap_now(zio_buf_cache[i]);
                  }
                  if (zio_data_buf_cache[i] != prev_data_cache) {
                          prev_data_cache = zio_data_buf_cache[i];
                          kmem_cache_reap_now(zio_data_buf_cache[i]);
                  }
          }
          kmem_cache_reap_now(buf_cache);
          kmem_cache_reap_now(hdr_full_cache);
          kmem_cache_reap_now(hdr_l2only_cache);
          kmem_cache_reap_now(zfs_btree_leaf_cache);
          abd_cache_reap_now();
  }
  
  static boolean_t
  arc_evict_cb_check(void *arg, zthr_t *zthr)
  {
          (void) arg, (void) zthr;
  
  #ifdef ZFS_DEBUG
          /*
           * This is necessary in order to keep the kstat information
           * up to date for tools that display kstat data such as the
           * mdb ::arc dcmd and the Linux crash utility.  These tools
           * typically do not call kstat's update function, but simply
           * dump out stats from the most recent update.  Without
           * this call, these commands may show stale stats for the
           * anon, mru, mru_ghost, mfu, and mfu_ghost lists.  Even
           * with this call, the data might be out of date if the
           * evict thread hasn't been woken recently; but that should
           * suffice.  The arc_state_t structures can be queried
           * directly if more accurate information is needed.
           */
          if (arc_ksp != NULL)
                  arc_ksp->ks_update(arc_ksp, KSTAT_READ);
  #endif
  
          /*
           * We have to rely on arc_wait_for_eviction() to tell us when to
           * evict, rather than checking if we are overflowing here, so that we
           * are sure to not leave arc_wait_for_eviction() waiting on aew_cv.
           * If we have become "not overflowing" since arc_wait_for_eviction()
           * checked, we need to wake it up.  We could broadcast the CV here,
           * but arc_wait_for_eviction() may have not yet gone to sleep.  We
           * would need to use a mutex to ensure that this function doesn't
           * broadcast until arc_wait_for_eviction() has gone to sleep (e.g.
           * the arc_evict_lock).  However, the lock ordering of such a lock
           * would necessarily be incorrect with respect to the zthr_lock,
           * which is held before this function is called, and is held by
           * arc_wait_for_eviction() when it calls zthr_wakeup().
           */
          if (arc_evict_needed)
                  return (B_TRUE);
  
          /*
           * If we have buffers in uncached state, evict them periodically.
           */
          return ((zfs_refcount_count(&arc_uncached->arcs_esize[ARC_BUFC_DATA]) +
              zfs_refcount_count(&arc_uncached->arcs_esize[ARC_BUFC_METADATA]) &&
              ddi_get_lbolt() - arc_last_uncached_flush >
              MSEC_TO_TICK(arc_min_prefetch_ms / 2)));
  }
  
  /*
   * Keep arc_size under arc_c by running arc_evict which evicts data
   * from the ARC.
   */
  static void
  arc_evict_cb(void *arg, zthr_t *zthr)
  {
          (void) arg, (void) zthr;
  
          uint64_t evicted = 0;
          fstrans_cookie_t cookie = spl_fstrans_mark();
  
          /* Always try to evict from uncached state. */
          arc_last_uncached_flush = ddi_get_lbolt();
          evicted += arc_flush_state(arc_uncached, 0, ARC_BUFC_DATA, B_FALSE);
          evicted += arc_flush_state(arc_uncached, 0, ARC_BUFC_METADATA, B_FALSE);
  
          /* Evict from other states only if told to. */
          if (arc_evict_needed)
                  evicted += arc_evict();
  
          /*
           * If evicted is zero, we couldn't evict anything
           * via arc_evict(). This could be due to hash lock
           * collisions, but more likely due to the majority of
           * arc buffers being unevictable. Therefore, even if
           * arc_size is above arc_c, another pass is unlikely to
           * be helpful and could potentially cause us to enter an
           * infinite loop.  Additionally, zthr_iscancelled() is
           * checked here so that if the arc is shutting down, the
           * broadcast will wake any remaining arc evict waiters.
           */
          mutex_enter(&arc_evict_lock);
          arc_evict_needed = !zthr_iscancelled(arc_evict_zthr) &&
              evicted > 0 && aggsum_compare(&arc_sums.arcstat_size, arc_c) > 0;
          if (!arc_evict_needed) {
                  /*
                   * We're either no longer overflowing, or we
                   * can't evict anything more, so we should wake
                   * arc_get_data_impl() sooner.
                   */
                  arc_evict_waiter_t *aw;
                  while ((aw = list_remove_head(&arc_evict_waiters)) != NULL) {
                          cv_broadcast(&aw->aew_cv);
                  }
                  arc_set_need_free();
          }
          mutex_exit(&arc_evict_lock);
          spl_fstrans_unmark(cookie);
  }
  
  static boolean_t
  arc_reap_cb_check(void *arg, zthr_t *zthr)
  {
          (void) arg, (void) zthr;
  
          int64_t free_memory = arc_available_memory();
          static int reap_cb_check_counter = 0;
  
          /*
           * If a kmem reap is already active, don't schedule more.  We must
           * check for this because kmem_cache_reap_soon() won't actually
           * block on the cache being reaped (this is to prevent callers from
           * becoming implicitly blocked by a system-wide kmem reap -- which,
           * on a system with many, many full magazines, can take minutes).
           */
          if (!kmem_cache_reap_active() && free_memory < 0) {
  
                  arc_no_grow = B_TRUE;
                  arc_warm = B_TRUE;
                  /*
                   * Wait at least zfs_grow_retry (default 5) seconds
                   * before considering growing.
                   */
                  arc_growtime = gethrtime() + SEC2NSEC(arc_grow_retry);
                  return (B_TRUE);
          } else if (free_memory < arc_c >> arc_no_grow_shift) {
                  arc_no_grow = B_TRUE;
          } else if (gethrtime() >= arc_growtime) {
                  arc_no_grow = B_FALSE;
          }
  
          /*
           * Called unconditionally every 60 seconds to reclaim unused
           * zstd compression and decompression context. This is done
           * here to avoid the need for an independent thread.
           */
          if (!((reap_cb_check_counter++) % 60))
                  zfs_zstd_cache_reap_now();
  
          return (B_FALSE);
  }
  
  /*
   * Keep enough free memory in the system by reaping the ARC's kmem
   * caches.  To cause more slabs to be reapable, we may reduce the
   * target size of the cache (arc_c), causing the arc_evict_cb()
   * to free more buffers.
   */
  static void
  arc_reap_cb(void *arg, zthr_t *zthr)
  {
          (void) arg, (void) zthr;
  
          int64_t free_memory;
          fstrans_cookie_t cookie = spl_fstrans_mark();
  
          /*
           * Kick off asynchronous kmem_reap()'s of all our caches.
           */
          arc_kmem_reap_soon();
  
          /*
           * Wait at least arc_kmem_cache_reap_retry_ms between
           * arc_kmem_reap_soon() calls. Without this check it is possible to
           * end up in a situation where we spend lots of time reaping
           * caches, while we're near arc_c_min.  Waiting here also gives the
           * subsequent free memory check a chance of finding that the
           * asynchronous reap has already freed enough memory, and we don't
           * need to call arc_reduce_target_size().
           */
          delay((hz * arc_kmem_cache_reap_retry_ms + 999) / 1000);
  
          /*
           * Reduce the target size as needed to maintain the amount of free
           * memory in the system at a fraction of the arc_size (1/128th by
           * default).  If oversubscribed (free_memory < 0) then reduce the
           * target arc_size by the deficit amount plus the fractional
           * amount.  If free memory is positive but less than the fractional
           * amount, reduce by what is needed to hit the fractional amount.
           */
          free_memory = arc_available_memory();
  
          int64_t can_free = arc_c - arc_c_min;
          if (can_free > 0) {
                  int64_t to_free = (can_free >> arc_shrink_shift) - free_memory;
                  if (to_free > 0)
                          arc_reduce_target_size(to_free);
          }
          spl_fstrans_unmark(cookie);
  }
  
  #ifdef _KERNEL
  /*
   * Determine the amount of memory eligible for eviction contained in the
   * ARC. All clean data reported by the ghost lists can always be safely
   * evicted. Due to arc_c_min, the same does not hold for all clean data
   * contained by the regular mru and mfu lists.
   *
   * In the case of the regular mru and mfu lists, we need to report as
   * much clean data as possible, such that evicting that same reported
   * data will not bring arc_size below arc_c_min. Thus, in certain
   * circumstances, the total amount of clean data in the mru and mfu
   * lists might not actually be evictable.
   *
   * The following two distinct cases are accounted for:
   *
   * 1. The sum of the amount of dirty data contained by both the mru and
   *    mfu lists, plus the ARC's other accounting (e.g. the anon list),
   *    is greater than or equal to arc_c_min.
   *    (i.e. amount of dirty data >= arc_c_min)
   *
   *    This is the easy case; all clean data contained by the mru and mfu
   *    lists is evictable. Evicting all clean data can only drop arc_size
   *    to the amount of dirty data, which is greater than arc_c_min.
   *
   * 2. The sum of the amount of dirty data contained by both the mru and
   *    mfu lists, plus the ARC's other accounting (e.g. the anon list),
   *    is less than arc_c_min.
   *    (i.e. arc_c_min > amount of dirty data)
   *
   *    2.1. arc_size is greater than or equal arc_c_min.
   *         (i.e. arc_size >= arc_c_min > amount of dirty data)
   *
   *         In this case, not all clean data from the regular mru and mfu
   *         lists is actually evictable; we must leave enough clean data
   *         to keep arc_size above arc_c_min. Thus, the maximum amount of
   *         evictable data from the two lists combined, is exactly the
   *         difference between arc_size and arc_c_min.
   *
   *    2.2. arc_size is less than arc_c_min
   *         (i.e. arc_c_min > arc_size > amount of dirty data)
   *
   *         In this case, none of the data contained in the mru and mfu
   *         lists is evictable, even if it's clean. Since arc_size is
   *         already below arc_c_min, evicting any more would only
   *         increase this negative difference.
   */
  
  #endif /* _KERNEL */
  
  /*
   * Adapt arc info given the number of bytes we are trying to add and
   * the state that we are coming from.  This function is only called
   * when we are adding new content to the cache.
   */
  static void
  arc_adapt(int bytes, arc_state_t *state)
  {
          int mult;
          uint64_t arc_p_min = (arc_c >> arc_p_min_shift);
          int64_t mrug_size = zfs_refcount_count(&arc_mru_ghost->arcs_size);
          int64_t mfug_size = zfs_refcount_count(&arc_mfu_ghost->arcs_size);
  
          ASSERT(bytes > 0);
          /*
           * Adapt the target size of the MRU list:
           *      - if we just hit in the MRU ghost list, then increase
           *        the target size of the MRU list.
           *      - if we just hit in the MFU ghost list, then increase
           *        the target size of the MFU list by decreasing the
           *        target size of the MRU list.
           */
          if (state == arc_mru_ghost) {
                  mult = (mrug_size >= mfug_size) ? 1 : (mfug_size / mrug_size);
                  if (!zfs_arc_p_dampener_disable)
                          mult = MIN(mult, 10); /* avoid wild arc_p adjustment */
  
                  arc_p = MIN(arc_c - arc_p_min, arc_p + (uint64_t)bytes * mult);
          } else if (state == arc_mfu_ghost) {
                  uint64_t delta;
  
                  mult = (mfug_size >= mrug_size) ? 1 : (mrug_size / mfug_size);
                  if (!zfs_arc_p_dampener_disable)
                          mult = MIN(mult, 10);
  
                  delta = MIN(bytes * mult, arc_p);
                  arc_p = MAX(arc_p_min, arc_p - delta);
          }
          ASSERT((int64_t)arc_p >= 0);
  
          /*
           * Wake reap thread if we do not have any available memory
           */
          if (arc_reclaim_needed()) {
                  zthr_wakeup(arc_reap_zthr);
                  return;
          }
  
          if (arc_no_grow)
                  return;
  
          if (arc_c >= arc_c_max)
                  return;
  
          /*
           * If we're within (2 * maxblocksize) bytes of the target
           * cache size, increment the target cache size
           */
          ASSERT3U(arc_c, >=, 2ULL << SPA_MAXBLOCKSHIFT);
          if (aggsum_upper_bound(&arc_sums.arcstat_size) >=
              arc_c - (2ULL << SPA_MAXBLOCKSHIFT)) {
                  atomic_add_64(&arc_c, (int64_t)bytes);
                  if (arc_c > arc_c_max)
                          arc_c = arc_c_max;
                  else if (state == arc_anon && arc_p < arc_c >> 1)
                          atomic_add_64(&arc_p, (int64_t)bytes);
                  if (arc_p > arc_c)
                          arc_p = arc_c;
          }
          ASSERT((int64_t)arc_p >= 0);
  }
  
  /*
   * Check if arc_size has grown past our upper threshold, determined by
   * zfs_arc_overflow_shift.
   */
  static arc_ovf_level_t
  arc_is_overflowing(boolean_t use_reserve)
  {
          /* Always allow at least one block of overflow */
          int64_t overflow = MAX(SPA_MAXBLOCKSIZE,
              arc_c >> zfs_arc_overflow_shift);
  
          /*
           * We just compare the lower bound here for performance reasons. Our
           * primary goals are to make sure that the arc never grows without
           * bound, and that it can reach its maximum size. This check
           * accomplishes both goals. The maximum amount we could run over by is
           * 2 * aggsum_borrow_multiplier * NUM_CPUS * the average size of a block
           * in the ARC. In practice, that's in the tens of MB, which is low
           * enough to be safe.
           */
          int64_t over = aggsum_lower_bound(&arc_sums.arcstat_size) -
              arc_c - overflow / 2;
          if (!use_reserve)
                  overflow /= 2;
          return (over < 0 ? ARC_OVF_NONE :
              over < overflow ? ARC_OVF_SOME : ARC_OVF_SEVERE);
  }
  
  static abd_t *
  arc_get_data_abd(arc_buf_hdr_t *hdr, uint64_t size, const void *tag,
      int alloc_flags)
  {
          arc_buf_contents_t type = arc_buf_type(hdr);
  
          arc_get_data_impl(hdr, size, tag, alloc_flags);
          if (alloc_flags & ARC_HDR_ALLOC_LINEAR)
                  return (abd_alloc_linear(size, type == ARC_BUFC_METADATA));
          else
                  return (abd_alloc(size, type == ARC_BUFC_METADATA));
  }
  
  static void *
  arc_get_data_buf(arc_buf_hdr_t *hdr, uint64_t size, const void *tag)
  {
          arc_buf_contents_t type = arc_buf_type(hdr);
  
          arc_get_data_impl(hdr, size, tag, ARC_HDR_DO_ADAPT);
          if (type == ARC_BUFC_METADATA) {
                  return (zio_buf_alloc(size));
          } else {
                  ASSERT(type == ARC_BUFC_DATA);
                  return (zio_data_buf_alloc(size));
          }
  }
  
  /*
   * Wait for the specified amount of data (in bytes) to be evicted from the
   * ARC, and for there to be sufficient free memory in the system.  Waiting for
   * eviction ensures that the memory used by the ARC decreases.  Waiting for
   * free memory ensures that the system won't run out of free pages, regardless
   * of ARC behavior and settings.  See arc_lowmem_init().
   */
  void
  arc_wait_for_eviction(uint64_t amount, boolean_t use_reserve)
  {
          switch (arc_is_overflowing(use_reserve)) {
          case ARC_OVF_NONE:
                  return;
          case ARC_OVF_SOME:
                  /*
                   * This is a bit racy without taking arc_evict_lock, but the
                   * worst that can happen is we either call zthr_wakeup() extra
                   * time due to race with other thread here, or the set flag
                   * get cleared by arc_evict_cb(), which is unlikely due to
                   * big hysteresis, but also not important since at this level
                   * of overflow the eviction is purely advisory.  Same time
                   * taking the global lock here every time without waiting for
                   * the actual eviction creates a significant lock contention.
                   */
                  if (!arc_evict_needed) {
                          arc_evict_needed = B_TRUE;
                          zthr_wakeup(arc_evict_zthr);
                  }
                  return;
          case ARC_OVF_SEVERE:
          default:
          {
                  arc_evict_waiter_t aw;
                  list_link_init(&aw.aew_node);
                  cv_init(&aw.aew_cv, NULL, CV_DEFAULT, NULL);
  
                  uint64_t last_count = 0;
                  mutex_enter(&arc_evict_lock);
                  if (!list_is_empty(&arc_evict_waiters)) {
                          arc_evict_waiter_t *last =
                              list_tail(&arc_evict_waiters);
                          last_count = last->aew_count;
                  } else if (!arc_evict_needed) {
                          arc_evict_needed = B_TRUE;
                          zthr_wakeup(arc_evict_zthr);
                  }
                  /*
                   * Note, the last waiter's count may be less than
                   * arc_evict_count if we are low on memory in which
                   * case arc_evict_state_impl() may have deferred
                   * wakeups (but still incremented arc_evict_count).
                   */
                  aw.aew_count = MAX(last_count, arc_evict_count) + amount;
  
                  list_insert_tail(&arc_evict_waiters, &aw);
  
                  arc_set_need_free();
  
                  DTRACE_PROBE3(arc__wait__for__eviction,
                      uint64_t, amount,
                      uint64_t, arc_evict_count,
                      uint64_t, aw.aew_count);
  
                  /*
                   * We will be woken up either when arc_evict_count reaches
                   * aew_count, or when the ARC is no longer overflowing and
                   * eviction completes.
                   * In case of "false" wakeup, we will still be on the list.
                   */
                  do {
                          cv_wait(&aw.aew_cv, &arc_evict_lock);
                  } while (list_link_active(&aw.aew_node));
                  mutex_exit(&arc_evict_lock);
  
                  cv_destroy(&aw.aew_cv);
          }
          }
  }
  
  /*
   * Allocate a block and return it to the caller. If we are hitting the
   * hard limit for the cache size, we must sleep, waiting for the eviction
   * thread to catch up. If we're past the target size but below the hard
   * limit, we'll only signal the reclaim thread and continue on.
   */
  static void
  arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, const void *tag,
      int alloc_flags)
  {
          arc_state_t *state = hdr->b_l1hdr.b_state;
          arc_buf_contents_t type = arc_buf_type(hdr);
  
          if (alloc_flags & ARC_HDR_DO_ADAPT)
                  arc_adapt(size, state);
  
          /*
           * If arc_size is currently overflowing, we must be adding data
           * faster than we are evicting.  To ensure we don't compound the
           * problem by adding more data and forcing arc_size to grow even
           * further past it's target size, we wait for the eviction thread to
           * make some progress.  We also wait for there to be sufficient free
           * memory in the system, as measured by arc_free_memory().
           *
           * Specifically, we wait for zfs_arc_eviction_pct percent of the
           * requested size to be evicted.  This should be more than 100%, to
           * ensure that that progress is also made towards getting arc_size
           * under arc_c.  See the comment above zfs_arc_eviction_pct.
           */
          arc_wait_for_eviction(size * zfs_arc_eviction_pct / 100,
              alloc_flags & ARC_HDR_USE_RESERVE);
  
          VERIFY3U(hdr->b_type, ==, type);
          if (type == ARC_BUFC_METADATA) {
                  arc_space_consume(size, ARC_SPACE_META);
          } else {
                  arc_space_consume(size, ARC_SPACE_DATA);
          }
  
          /*
           * Update the state size.  Note that ghost states have a
           * "ghost size" and so don't need to be updated.
           */
          if (!GHOST_STATE(state)) {
  
                  (void) zfs_refcount_add_many(&state->arcs_size, size, tag);
  
                  /*
                   * If this is reached via arc_read, the link is
                   * protected by the hash lock. If reached via
                   * arc_buf_alloc, the header should not be accessed by
                   * any other thread. And, if reached via arc_read_done,
                   * the hash lock will protect it if it's found in the
                   * hash table; otherwise no other thread should be
                   * trying to [add|remove]_reference it.
                   */
                  if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
                          ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                          (void) zfs_refcount_add_many(&state->arcs_esize[type],
                              size, tag);
                  }
  
                  /*
                   * If we are growing the cache, and we are adding anonymous
                   * data, and we have outgrown arc_p, update arc_p
                   */
                  if (aggsum_upper_bound(&arc_sums.arcstat_size) < arc_c &&
                      hdr->b_l1hdr.b_state == arc_anon &&
                      (zfs_refcount_count(&arc_anon->arcs_size) +
                      zfs_refcount_count(&arc_mru->arcs_size) > arc_p &&
                      arc_p < arc_c >> 1))
                          arc_p = MIN(arc_c, arc_p + size);
          }
  }
  
  static void
  arc_free_data_abd(arc_buf_hdr_t *hdr, abd_t *abd, uint64_t size,
      const void *tag)
  {
          arc_free_data_impl(hdr, size, tag);
          abd_free(abd);
  }
  
  static void
  arc_free_data_buf(arc_buf_hdr_t *hdr, void *buf, uint64_t size, const void *tag)
  {
          arc_buf_contents_t type = arc_buf_type(hdr);
  
          arc_free_data_impl(hdr, size, tag);
          if (type == ARC_BUFC_METADATA) {
                  zio_buf_free(buf, size);
          } else {
                  ASSERT(type == ARC_BUFC_DATA);
                  zio_data_buf_free(buf, size);
          }
  }
  
  /*
   * Free the arc data buffer.
   */
  static void
  arc_free_data_impl(arc_buf_hdr_t *hdr, uint64_t size, const void *tag)
  {
          arc_state_t *state = hdr->b_l1hdr.b_state;
          arc_buf_contents_t type = arc_buf_type(hdr);
  
          /* protected by hash lock, if in the hash table */
          if (multilist_link_active(&hdr->b_l1hdr.b_arc_node)) {
                  ASSERT(zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt));
                  ASSERT(state != arc_anon && state != arc_l2c_only);
  
                  (void) zfs_refcount_remove_many(&state->arcs_esize[type],
                      size, tag);
          }
          (void) zfs_refcount_remove_many(&state->arcs_size, size, tag);
  
          VERIFY3U(hdr->b_type, ==, type);
          if (type == ARC_BUFC_METADATA) {
                  arc_space_return(size, ARC_SPACE_META);
          } else {
                  ASSERT(type == ARC_BUFC_DATA);
                  arc_space_return(size, ARC_SPACE_DATA);
          }
  }
  
  /*
   * This routine is called whenever a buffer is accessed.
   */
  static void
  arc_access(arc_buf_hdr_t *hdr, arc_flags_t arc_flags, boolean_t hit)
  {
          ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
          ASSERT(HDR_HAS_L1HDR(hdr));
  
          /*
           * Update buffer prefetch status.
           */
          boolean_t was_prefetch = HDR_PREFETCH(hdr);
          boolean_t now_prefetch = arc_flags & ARC_FLAG_PREFETCH;
          if (was_prefetch != now_prefetch) {
                  if (was_prefetch) {
                          ARCSTAT_CONDSTAT(hit, demand_hit, demand_iohit,
                              HDR_PRESCIENT_PREFETCH(hdr), prescient, predictive,
                              prefetch);
                  }
                  if (HDR_HAS_L2HDR(hdr))
                          l2arc_hdr_arcstats_decrement_state(hdr);
                  if (was_prefetch) {
                          arc_hdr_clear_flags(hdr,
                              ARC_FLAG_PREFETCH | ARC_FLAG_PRESCIENT_PREFETCH);
                  } else {
                          arc_hdr_set_flags(hdr, ARC_FLAG_PREFETCH);
                  }
                  if (HDR_HAS_L2HDR(hdr))
                          l2arc_hdr_arcstats_increment_state(hdr);
          }
          if (now_prefetch) {
                  if (arc_flags & ARC_FLAG_PRESCIENT_PREFETCH) {
                          arc_hdr_set_flags(hdr, ARC_FLAG_PRESCIENT_PREFETCH);
                          ARCSTAT_BUMP(arcstat_prescient_prefetch);
                  } else {
                          ARCSTAT_BUMP(arcstat_predictive_prefetch);
                  }
          }
          if (arc_flags & ARC_FLAG_L2CACHE)
                  arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
  
          clock_t now = ddi_get_lbolt();
          if (hdr->b_l1hdr.b_state == arc_anon) {
                  arc_state_t     *new_state;
                  /*
                   * This buffer is not in the cache, and does not appear in
                   * our "ghost" lists.  Add it to the MRU or uncached state.
                   */
                  ASSERT0(hdr->b_l1hdr.b_arc_access);
                  hdr->b_l1hdr.b_arc_access = now;
                  if (HDR_UNCACHED(hdr)) {
                          new_state = arc_uncached;
                          DTRACE_PROBE1(new_state__uncached, arc_buf_hdr_t *,
                              hdr);
                  } else {
                          new_state = arc_mru;
                          DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
                  }
                  arc_change_state(new_state, hdr);
          } else if (hdr->b_l1hdr.b_state == arc_mru) {
                  /*
                   * This buffer has been accessed once recently and either
                   * its read is still in progress or it is in the cache.
                   */
                  if (HDR_IO_IN_PROGRESS(hdr)) {
                          hdr->b_l1hdr.b_arc_access = now;
                          return;
                  }
                  hdr->b_l1hdr.b_mru_hits++;
                  ARCSTAT_BUMP(arcstat_mru_hits);
  
                  /*
                   * If the previous access was a prefetch, then it already
                   * handled possible promotion, so nothing more to do for now.
                   */
                  if (was_prefetch) {
                          hdr->b_l1hdr.b_arc_access = now;
                          return;
                  }
  
                  /*
                   * If more than ARC_MINTIME have passed from the previous
                   * hit, promote the buffer to the MFU state.
                   */
                  if (ddi_time_after(now, hdr->b_l1hdr.b_arc_access +
                      ARC_MINTIME)) {
                          hdr->b_l1hdr.b_arc_access = now;
                          DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
                          arc_change_state(arc_mfu, hdr);
                  }
          } else if (hdr->b_l1hdr.b_state == arc_mru_ghost) {
                  arc_state_t     *new_state;
                  /*
                   * This buffer has been accessed once recently, but was
                   * evicted from the cache.  Would we have bigger MRU, it
                   * would be an MRU hit, so handle it the same way, except
                   * we don't need to check the previous access time.
                   */
                  hdr->b_l1hdr.b_mru_ghost_hits++;
                  ARCSTAT_BUMP(arcstat_mru_ghost_hits);
                  hdr->b_l1hdr.b_arc_access = now;
                  if (was_prefetch) {
                          new_state = arc_mru;
                          DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
                  } else {
                          new_state = arc_mfu;
                          DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
                  }
                  arc_change_state(new_state, hdr);
          } else if (hdr->b_l1hdr.b_state == arc_mfu) {
                  /*
                   * This buffer has been accessed more than once and either
                   * still in the cache or being restored from one of ghosts.
                   */
                  if (!HDR_IO_IN_PROGRESS(hdr)) {
                          hdr->b_l1hdr.b_mfu_hits++;
                          ARCSTAT_BUMP(arcstat_mfu_hits);
                  }
                  hdr->b_l1hdr.b_arc_access = now;
          } else if (hdr->b_l1hdr.b_state == arc_mfu_ghost) {
                  /*
                   * This buffer has been accessed more than once recently, but
                   * has been evicted from the cache.  Would we have bigger MFU
                   * it would stay in cache, so move it back to MFU state.
                   */
                  hdr->b_l1hdr.b_mfu_ghost_hits++;
                  ARCSTAT_BUMP(arcstat_mfu_ghost_hits);
                  hdr->b_l1hdr.b_arc_access = now;
                  DTRACE_PROBE1(new_state__mfu, arc_buf_hdr_t *, hdr);
                  arc_change_state(arc_mfu, hdr);
          } else if (hdr->b_l1hdr.b_state == arc_uncached) {
                  /*
                   * This buffer is uncacheable, but we got a hit.  Probably
                   * a demand read after prefetch.  Nothing more to do here.
                   */
                  if (!HDR_IO_IN_PROGRESS(hdr))
                          ARCSTAT_BUMP(arcstat_uncached_hits);
                  hdr->b_l1hdr.b_arc_access = now;
          } else if (hdr->b_l1hdr.b_state == arc_l2c_only) {
                  /*
                   * This buffer is on the 2nd Level ARC and was not accessed
                   * for a long time, so treat it as new and put into MRU.
                   */
                  hdr->b_l1hdr.b_arc_access = now;
                  DTRACE_PROBE1(new_state__mru, arc_buf_hdr_t *, hdr);
                  arc_change_state(arc_mru, hdr);
          } else {
                  cmn_err(CE_PANIC, "invalid arc state 0x%p",
                      hdr->b_l1hdr.b_state);
          }
  }
  
  /*
   * This routine is called by dbuf_hold() to update the arc_access() state
   * which otherwise would be skipped for entries in the dbuf cache.
   */
  void
  arc_buf_access(arc_buf_t *buf)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          /*
           * Avoid taking the hash_lock when possible as an optimization.
           * The header must be checked again under the hash_lock in order
           * to handle the case where it is concurrently being released.
           */
          if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr))
                  return;
  
          kmutex_t *hash_lock = HDR_LOCK(hdr);
          mutex_enter(hash_lock);
  
          if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
                  mutex_exit(hash_lock);
                  ARCSTAT_BUMP(arcstat_access_skip);
                  return;
          }
  
          ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
              hdr->b_l1hdr.b_state == arc_mfu ||
              hdr->b_l1hdr.b_state == arc_uncached);
  
          DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
          arc_access(hdr, 0, B_TRUE);
          mutex_exit(hash_lock);
  
          ARCSTAT_BUMP(arcstat_hits);
          ARCSTAT_CONDSTAT(B_TRUE /* demand */, demand, prefetch,
              !HDR_ISTYPE_METADATA(hdr), data, metadata, hits);
  }
  
  /* a generic arc_read_done_func_t which you can use */
  void
  arc_bcopy_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
      arc_buf_t *buf, void *arg)
  {
          (void) zio, (void) zb, (void) bp;
  
          if (buf == NULL)
                  return;
  
          memcpy(arg, buf->b_data, arc_buf_size(buf));
          arc_buf_destroy(buf, arg);
  }
  
  /* a generic arc_read_done_func_t */
  void
  arc_getbuf_func(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
      arc_buf_t *buf, void *arg)
  {
          (void) zb, (void) bp;
          arc_buf_t **bufp = arg;
  
          if (buf == NULL) {
                  ASSERT(zio == NULL || zio->io_error != 0);
                  *bufp = NULL;
          } else {
                  ASSERT(zio == NULL || zio->io_error == 0);
                  *bufp = buf;
                  ASSERT(buf->b_data != NULL);
          }
  }
  
  static void
  arc_hdr_verify(arc_buf_hdr_t *hdr, blkptr_t *bp)
  {
          if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp)) {
                  ASSERT3U(HDR_GET_PSIZE(hdr), ==, 0);
                  ASSERT3U(arc_hdr_get_compress(hdr), ==, ZIO_COMPRESS_OFF);
          } else {
                  if (HDR_COMPRESSION_ENABLED(hdr)) {
                          ASSERT3U(arc_hdr_get_compress(hdr), ==,
                              BP_GET_COMPRESS(bp));
                  }
                  ASSERT3U(HDR_GET_LSIZE(hdr), ==, BP_GET_LSIZE(bp));
                  ASSERT3U(HDR_GET_PSIZE(hdr), ==, BP_GET_PSIZE(bp));
                  ASSERT3U(!!HDR_PROTECTED(hdr), ==, BP_IS_PROTECTED(bp));
          }
  }
  
  static void
  arc_read_done(zio_t *zio)
  {
          blkptr_t        *bp = zio->io_bp;
          arc_buf_hdr_t   *hdr = zio->io_private;
          kmutex_t        *hash_lock = NULL;
          arc_callback_t  *callback_list;
          arc_callback_t  *acb;
  
          /*
           * The hdr was inserted into hash-table and removed from lists
           * prior to starting I/O.  We should find this header, since
           * it's in the hash table, and it should be legit since it's
           * not possible to evict it during the I/O.  The only possible
           * reason for it not to be found is if we were freed during the
           * read.
           */
          if (HDR_IN_HASH_TABLE(hdr)) {
                  arc_buf_hdr_t *found;
  
                  ASSERT3U(hdr->b_birth, ==, BP_PHYSICAL_BIRTH(zio->io_bp));
                  ASSERT3U(hdr->b_dva.dva_word[0], ==,
                      BP_IDENTITY(zio->io_bp)->dva_word[0]);
                  ASSERT3U(hdr->b_dva.dva_word[1], ==,
                      BP_IDENTITY(zio->io_bp)->dva_word[1]);
  
                  found = buf_hash_find(hdr->b_spa, zio->io_bp, &hash_lock);
  
                  ASSERT((found == hdr &&
                      DVA_EQUAL(&hdr->b_dva, BP_IDENTITY(zio->io_bp))) ||
                      (found == hdr && HDR_L2_READING(hdr)));
                  ASSERT3P(hash_lock, !=, NULL);
          }
  
          if (BP_IS_PROTECTED(bp)) {
                  hdr->b_crypt_hdr.b_ot = BP_GET_TYPE(bp);
                  hdr->b_crypt_hdr.b_dsobj = zio->io_bookmark.zb_objset;
                  zio_crypt_decode_params_bp(bp, hdr->b_crypt_hdr.b_salt,
                      hdr->b_crypt_hdr.b_iv);
  
                  if (zio->io_error == 0) {
                          if (BP_GET_TYPE(bp) == DMU_OT_INTENT_LOG) {
                                  void *tmpbuf;
  
                                  tmpbuf = abd_borrow_buf_copy(zio->io_abd,
                                      sizeof (zil_chain_t));
                                  zio_crypt_decode_mac_zil(tmpbuf,
                                      hdr->b_crypt_hdr.b_mac);
                                  abd_return_buf(zio->io_abd, tmpbuf,
                                      sizeof (zil_chain_t));
                          } else {
                                  zio_crypt_decode_mac_bp(bp,
                                      hdr->b_crypt_hdr.b_mac);
                          }
                  }
          }
  
          if (zio->io_error == 0) {
                  /* byteswap if necessary */
                  if (BP_SHOULD_BYTESWAP(zio->io_bp)) {
                          if (BP_GET_LEVEL(zio->io_bp) > 0) {
                                  hdr->b_l1hdr.b_byteswap = DMU_BSWAP_UINT64;
                          } else {
                                  hdr->b_l1hdr.b_byteswap =
                                      DMU_OT_BYTESWAP(BP_GET_TYPE(zio->io_bp));
                          }
                  } else {
                          hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
                  }
                  if (!HDR_L2_READING(hdr)) {
                          hdr->b_complevel = zio->io_prop.zp_complevel;
                  }
          }
  
          arc_hdr_clear_flags(hdr, ARC_FLAG_L2_EVICTED);
          if (l2arc_noprefetch && HDR_PREFETCH(hdr))
                  arc_hdr_clear_flags(hdr, ARC_FLAG_L2CACHE);
  
          callback_list = hdr->b_l1hdr.b_acb;
          ASSERT3P(callback_list, !=, NULL);
          hdr->b_l1hdr.b_acb = NULL;
  
          /*
           * If a read request has a callback (i.e. acb_done is not NULL), then we
           * make a buf containing the data according to the parameters which were
           * passed in. The implementation of arc_buf_alloc_impl() ensures that we
           * aren't needlessly decompressing the data multiple times.
           */
          int callback_cnt = 0;
          for (acb = callback_list; acb != NULL; acb = acb->acb_next) {
  
                  /* We need the last one to call below in original order. */
                  callback_list = acb;
  
                  if (!acb->acb_done || acb->acb_nobuf)
                          continue;
  
                  callback_cnt++;
  
                  if (zio->io_error != 0)
                          continue;
  
                  int error = arc_buf_alloc_impl(hdr, zio->io_spa,
                      &acb->acb_zb, acb->acb_private, acb->acb_encrypted,
                      acb->acb_compressed, acb->acb_noauth, B_TRUE,
                      &acb->acb_buf);
  
                  /*
                   * Assert non-speculative zios didn't fail because an
                   * encryption key wasn't loaded
                   */
                  ASSERT((zio->io_flags & ZIO_FLAG_SPECULATIVE) ||
                      error != EACCES);
  
                  /*
                   * If we failed to decrypt, report an error now (as the zio
                   * layer would have done if it had done the transforms).
                   */
                  if (error == ECKSUM) {
                          ASSERT(BP_IS_PROTECTED(bp));
                          error = SET_ERROR(EIO);
                          if ((zio->io_flags & ZIO_FLAG_SPECULATIVE) == 0) {
                                  spa_log_error(zio->io_spa, &acb->acb_zb);
                                  (void) zfs_ereport_post(
                                      FM_EREPORT_ZFS_AUTHENTICATION,
                                      zio->io_spa, NULL, &acb->acb_zb, zio, 0);
                          }
                  }
  
                  if (error != 0) {
                          /*
                           * Decompression or decryption failed.  Set
                           * io_error so that when we call acb_done
                           * (below), we will indicate that the read
                           * failed. Note that in the unusual case
                           * where one callback is compressed and another
                           * uncompressed, we will mark all of them
                           * as failed, even though the uncompressed
                           * one can't actually fail.  In this case,
                           * the hdr will not be anonymous, because
                           * if there are multiple callbacks, it's
                           * because multiple threads found the same
                           * arc buf in the hash table.
                           */
                          zio->io_error = error;
                  }
          }
  
          /*
           * If there are multiple callbacks, we must have the hash lock,
           * because the only way for multiple threads to find this hdr is
           * in the hash table.  This ensures that if there are multiple
           * callbacks, the hdr is not anonymous.  If it were anonymous,
           * we couldn't use arc_buf_destroy() in the error case below.
           */
          ASSERT(callback_cnt < 2 || hash_lock != NULL);
  
          if (zio->io_error == 0) {
                  arc_hdr_verify(hdr, zio->io_bp);
          } else {
                  arc_hdr_set_flags(hdr, ARC_FLAG_IO_ERROR);
                  if (hdr->b_l1hdr.b_state != arc_anon)
                          arc_change_state(arc_anon, hdr);
                  if (HDR_IN_HASH_TABLE(hdr))
                          buf_hash_remove(hdr);
          }
  
          /*
           * Broadcast before we drop the hash_lock to avoid the possibility
           * that the hdr (and hence the cv) might be freed before we get to
           * the cv_broadcast().
           */
          cv_broadcast(&hdr->b_l1hdr.b_cv);
  
          arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
          (void) remove_reference(hdr, hdr);
  
          if (hash_lock != NULL)
                  mutex_exit(hash_lock);
  
          /* execute each callback and free its structure */
          while ((acb = callback_list) != NULL) {
                  if (acb->acb_done != NULL) {
                          if (zio->io_error != 0 && acb->acb_buf != NULL) {
                                  /*
                                   * If arc_buf_alloc_impl() fails during
                                   * decompression, the buf will still be
                                   * allocated, and needs to be freed here.
                                   */
                                  arc_buf_destroy(acb->acb_buf,
                                      acb->acb_private);
                                  acb->acb_buf = NULL;
                          }
                          acb->acb_done(zio, &zio->io_bookmark, zio->io_bp,
                              acb->acb_buf, acb->acb_private);
                  }
  
                  if (acb->acb_zio_dummy != NULL) {
                          acb->acb_zio_dummy->io_error = zio->io_error;
                          zio_nowait(acb->acb_zio_dummy);
                  }
  
                  callback_list = acb->acb_prev;
                  if (acb->acb_wait) {
                          mutex_enter(&acb->acb_wait_lock);
                          acb->acb_wait_error = zio->io_error;
                          acb->acb_wait = B_FALSE;
                          cv_signal(&acb->acb_wait_cv);
                          mutex_exit(&acb->acb_wait_lock);
                          /* acb will be freed by the waiting thread. */
                  } else {
                          kmem_free(acb, sizeof (arc_callback_t));
                  }
          }
  }
  
  /*
   * "Read" the block at the specified DVA (in bp) via the
   * cache.  If the block is found in the cache, invoke the provided
   * callback immediately and return.  Note that the `zio' parameter
   * in the callback will be NULL in this case, since no IO was
   * required.  If the block is not in the cache pass the read request
   * on to the spa with a substitute callback function, so that the
   * requested block will be added to the cache.
   *
   * If a read request arrives for a block that has a read in-progress,
   * either wait for the in-progress read to complete (and return the
   * results); or, if this is a read with a "done" func, add a record
   * to the read to invoke the "done" func when the read completes,
   * and return; or just return.
   *
   * arc_read_done() will invoke all the requested "done" functions
   * for readers of this block.
   */
  int
  arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,
      arc_read_done_func_t *done, void *private, zio_priority_t priority,
      int zio_flags, arc_flags_t *arc_flags, const zbookmark_phys_t *zb)
  {
          arc_buf_hdr_t *hdr = NULL;
          kmutex_t *hash_lock = NULL;
          zio_t *rzio;
          uint64_t guid = spa_load_guid(spa);
          boolean_t compressed_read = (zio_flags & ZIO_FLAG_RAW_COMPRESS) != 0;
          boolean_t encrypted_read = BP_IS_ENCRYPTED(bp) &&
              (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
          boolean_t noauth_read = BP_IS_AUTHENTICATED(bp) &&
              (zio_flags & ZIO_FLAG_RAW_ENCRYPT) != 0;
          boolean_t embedded_bp = !!BP_IS_EMBEDDED(bp);
          boolean_t no_buf = *arc_flags & ARC_FLAG_NO_BUF;
          int rc = 0;
  
          ASSERT(!embedded_bp ||
              BPE_GET_ETYPE(bp) == BP_EMBEDDED_TYPE_DATA);
          ASSERT(!BP_IS_HOLE(bp));
          ASSERT(!BP_IS_REDACTED(bp));
  
          /*
           * Normally SPL_FSTRANS will already be set since kernel threads which
           * expect to call the DMU interfaces will set it when created.  System
           * calls are similarly handled by setting/cleaning the bit in the
           * registered callback (module/os/.../zfs/zpl_*).
           *
           * External consumers such as Lustre which call the exported DMU
           * interfaces may not have set SPL_FSTRANS.  To avoid a deadlock
           * on the hash_lock always set and clear the bit.
           */
          fstrans_cookie_t cookie = spl_fstrans_mark();
  top:
          /*
           * Verify the block pointer contents are reasonable.  This should
           * always be the case since the blkptr is protected by a checksum.
           * However, if there is damage it's desirable to detect this early
           * and treat it as a checksum error.  This allows an alternate blkptr
           * to be tried when one is available (e.g. ditto blocks).
           */
          if (!zfs_blkptr_verify(spa, bp, zio_flags & ZIO_FLAG_CONFIG_WRITER,
              BLK_VERIFY_LOG)) {
                  rc = SET_ERROR(ECKSUM);
                  goto out;
          }
  
          if (!embedded_bp) {
                  /*
                   * Embedded BP's have no DVA and require no I/O to "read".
                   * Create an anonymous arc buf to back it.
                   */
                  hdr = buf_hash_find(guid, bp, &hash_lock);
          }
  
          /*
           * Determine if we have an L1 cache hit or a cache miss. For simplicity
           * we maintain encrypted data separately from compressed / uncompressed
           * data. If the user is requesting raw encrypted data and we don't have
           * that in the header we will read from disk to guarantee that we can
           * get it even if the encryption keys aren't loaded.
           */
          if (hdr != NULL && HDR_HAS_L1HDR(hdr) && (HDR_HAS_RABD(hdr) ||
              (hdr->b_l1hdr.b_pabd != NULL && !encrypted_read))) {
                  boolean_t is_data = !HDR_ISTYPE_METADATA(hdr);
                  arc_buf_t *buf = NULL;
  
                  if (HDR_IO_IN_PROGRESS(hdr)) {
                          if (*arc_flags & ARC_FLAG_CACHED_ONLY) {
                                  mutex_exit(hash_lock);
                                  ARCSTAT_BUMP(arcstat_cached_only_in_progress);
                                  rc = SET_ERROR(ENOENT);
                                  goto out;
                          }
  
                          zio_t *head_zio = hdr->b_l1hdr.b_acb->acb_zio_head;
                          ASSERT3P(head_zio, !=, NULL);
                          if ((hdr->b_flags & ARC_FLAG_PRIO_ASYNC_READ) &&
                              priority == ZIO_PRIORITY_SYNC_READ) {
                                  /*
                                   * This is a sync read that needs to wait for
                                   * an in-flight async read. Request that the
                                   * zio have its priority upgraded.
                                   */
                                  zio_change_priority(head_zio, priority);
                                  DTRACE_PROBE1(arc__async__upgrade__sync,
                                      arc_buf_hdr_t *, hdr);
                                  ARCSTAT_BUMP(arcstat_async_upgrade_sync);
                          }
  
                          DTRACE_PROBE1(arc__iohit, arc_buf_hdr_t *, hdr);
                          arc_access(hdr, *arc_flags, B_FALSE);
  
                          /*
                           * If there are multiple threads reading the same block
                           * and that block is not yet in the ARC, then only one
                           * thread will do the physical I/O and all other
                           * threads will wait until that I/O completes.
                           * Synchronous reads use the acb_wait_cv whereas nowait
                           * reads register a callback. Both are signalled/called
                           * in arc_read_done.
                           *
                           * Errors of the physical I/O may need to be propagated.
                           * Synchronous read errors are returned here from
                           * arc_read_done via acb_wait_error.  Nowait reads
                           * attach the acb_zio_dummy zio to pio and
                           * arc_read_done propagates the physical I/O's io_error
                           * to acb_zio_dummy, and thereby to pio.
                           */
                          arc_callback_t *acb = NULL;
                          if (done || pio || *arc_flags & ARC_FLAG_WAIT) {
                                  acb = kmem_zalloc(sizeof (arc_callback_t),
                                      KM_SLEEP);
                                  acb->acb_done = done;
                                  acb->acb_private = private;
                                  acb->acb_compressed = compressed_read;
                                  acb->acb_encrypted = encrypted_read;
                                  acb->acb_noauth = noauth_read;
                                  acb->acb_nobuf = no_buf;
                                  if (*arc_flags & ARC_FLAG_WAIT) {
                                          acb->acb_wait = B_TRUE;
                                          mutex_init(&acb->acb_wait_lock, NULL,
                                              MUTEX_DEFAULT, NULL);
                                          cv_init(&acb->acb_wait_cv, NULL,
                                              CV_DEFAULT, NULL);
                                  }
                                  acb->acb_zb = *zb;
                                  if (pio != NULL) {
                                          acb->acb_zio_dummy = zio_null(pio,
                                              spa, NULL, NULL, NULL, zio_flags);
                                  }
                                  acb->acb_zio_head = head_zio;
                                  acb->acb_next = hdr->b_l1hdr.b_acb;
                                  if (hdr->b_l1hdr.b_acb)
                                          hdr->b_l1hdr.b_acb->acb_prev = acb;
                                  hdr->b_l1hdr.b_acb = acb;
                          }
                          mutex_exit(hash_lock);
  
                          ARCSTAT_BUMP(arcstat_iohits);
                          ARCSTAT_CONDSTAT(!(*arc_flags & ARC_FLAG_PREFETCH),
                              demand, prefetch, is_data, data, metadata, iohits);
  
                          if (*arc_flags & ARC_FLAG_WAIT) {
                                  mutex_enter(&acb->acb_wait_lock);
                                  while (acb->acb_wait) {
                                          cv_wait(&acb->acb_wait_cv,
                                              &acb->acb_wait_lock);
                                  }
                                  rc = acb->acb_wait_error;
                                  mutex_exit(&acb->acb_wait_lock);
                                  mutex_destroy(&acb->acb_wait_lock);
                                  cv_destroy(&acb->acb_wait_cv);
                                  kmem_free(acb, sizeof (arc_callback_t));
                          }
                          goto out;
                  }
  
                  ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
                      hdr->b_l1hdr.b_state == arc_mfu ||
                      hdr->b_l1hdr.b_state == arc_uncached);
  
                  DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
                  arc_access(hdr, *arc_flags, B_TRUE);
  
                  if (done && !no_buf) {
                          ASSERT(!embedded_bp || !BP_IS_HOLE(bp));
  
                          /* Get a buf with the desired data in it. */
                          rc = arc_buf_alloc_impl(hdr, spa, zb, private,
                              encrypted_read, compressed_read, noauth_read,
                              B_TRUE, &buf);
                          if (rc == ECKSUM) {
                                  /*
                                   * Convert authentication and decryption errors
                                   * to EIO (and generate an ereport if needed)
                                   * before leaving the ARC.
                                   */
                                  rc = SET_ERROR(EIO);
                                  if ((zio_flags & ZIO_FLAG_SPECULATIVE) == 0) {
                                          spa_log_error(spa, zb);
                                          (void) zfs_ereport_post(
                                              FM_EREPORT_ZFS_AUTHENTICATION,
                                              spa, NULL, zb, NULL, 0);
                                  }
                          }
                          if (rc != 0) {
                                  arc_buf_destroy_impl(buf);
                                  buf = NULL;
                                  (void) remove_reference(hdr, private);
                          }
  
                          /* assert any errors weren't due to unloaded keys */
                          ASSERT((zio_flags & ZIO_FLAG_SPECULATIVE) ||
                              rc != EACCES);
                  }
                  mutex_exit(hash_lock);
                  ARCSTAT_BUMP(arcstat_hits);
                  ARCSTAT_CONDSTAT(!(*arc_flags & ARC_FLAG_PREFETCH),
                      demand, prefetch, is_data, data, metadata, hits);
                  *arc_flags |= ARC_FLAG_CACHED;
  
                  if (done)
                          done(NULL, zb, bp, buf, private);
          } else {
                  uint64_t lsize = BP_GET_LSIZE(bp);
                  uint64_t psize = BP_GET_PSIZE(bp);
                  arc_callback_t *acb;
                  vdev_t *vd = NULL;
                  uint64_t addr = 0;
                  boolean_t devw = B_FALSE;
                  uint64_t size;
                  abd_t *hdr_abd;
                  int alloc_flags = encrypted_read ? ARC_HDR_ALLOC_RDATA : 0;
  
                  if (*arc_flags & ARC_FLAG_CACHED_ONLY) {
                          rc = SET_ERROR(ENOENT);
                          if (hash_lock != NULL)
                                  mutex_exit(hash_lock);
                          goto out;
                  }
  
                  if (hdr == NULL) {
                          /*
                           * This block is not in the cache or it has
                           * embedded data.
                           */
                          arc_buf_hdr_t *exists = NULL;
                          arc_buf_contents_t type = BP_GET_BUFC_TYPE(bp);
                          hdr = arc_hdr_alloc(spa_load_guid(spa), psize, lsize,
                              BP_IS_PROTECTED(bp), BP_GET_COMPRESS(bp), 0, type);
  
                          if (!embedded_bp) {
                                  hdr->b_dva = *BP_IDENTITY(bp);
                                  hdr->b_birth = BP_PHYSICAL_BIRTH(bp);
                                  exists = buf_hash_insert(hdr, &hash_lock);
                          }
                          if (exists != NULL) {
                                  /* somebody beat us to the hash insert */
                                  mutex_exit(hash_lock);
                                  buf_discard_identity(hdr);
                                  arc_hdr_destroy(hdr);
                                  goto top; /* restart the IO request */
                          }
                  } else {
                          /*
                           * This block is in the ghost cache or encrypted data
                           * was requested and we didn't have it. If it was
                           * L2-only (and thus didn't have an L1 hdr),
                           * we realloc the header to add an L1 hdr.
                           */
                          if (!HDR_HAS_L1HDR(hdr)) {
                                  hdr = arc_hdr_realloc(hdr, hdr_l2only_cache,
                                      hdr_full_cache);
                          }
  
                          if (GHOST_STATE(hdr->b_l1hdr.b_state)) {
                                  ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
                                  ASSERT(!HDR_HAS_RABD(hdr));
                                  ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                                  ASSERT0(zfs_refcount_count(
                                      &hdr->b_l1hdr.b_refcnt));
                                  ASSERT3P(hdr->b_l1hdr.b_buf, ==, NULL);
  #ifdef ZFS_DEBUG
                                  ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, ==, NULL);
  #endif
                          } else if (HDR_IO_IN_PROGRESS(hdr)) {
                                  /*
                                   * If this header already had an IO in progress
                                   * and we are performing another IO to fetch
                                   * encrypted data we must wait until the first
                                   * IO completes so as not to confuse
                                   * arc_read_done(). This should be very rare
                                   * and so the performance impact shouldn't
                                   * matter.
                                   */
                                  cv_wait(&hdr->b_l1hdr.b_cv, hash_lock);
                                  mutex_exit(hash_lock);
                                  goto top;
                          }
                  }
                  if (*arc_flags & ARC_FLAG_UNCACHED) {
                          arc_hdr_set_flags(hdr, ARC_FLAG_UNCACHED);
                          if (!encrypted_read)
                                  alloc_flags |= ARC_HDR_ALLOC_LINEAR;
                  }
  
                  /*
                   * Call arc_adapt() explicitly before arc_access() to allow
                   * its logic to balance MRU/MFU based on the original state.
                   */
                  arc_adapt(arc_hdr_size(hdr), hdr->b_l1hdr.b_state);
                  /*
                   * Take additional reference for IO_IN_PROGRESS.  It stops
                   * arc_access() from putting this header without any buffers
                   * and so other references but obviously nonevictable onto
                   * the evictable list of MRU or MFU state.
                   */
                  add_reference(hdr, hdr);
                  if (!embedded_bp)
                          arc_access(hdr, *arc_flags, B_FALSE);
                  arc_hdr_set_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
                  arc_hdr_alloc_abd(hdr, alloc_flags);
                  if (encrypted_read) {
                          ASSERT(HDR_HAS_RABD(hdr));
                          size = HDR_GET_PSIZE(hdr);
                          hdr_abd = hdr->b_crypt_hdr.b_rabd;
                          zio_flags |= ZIO_FLAG_RAW;
                  } else {
                          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
                          size = arc_hdr_size(hdr);
                          hdr_abd = hdr->b_l1hdr.b_pabd;
  
                          if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF) {
                                  zio_flags |= ZIO_FLAG_RAW_COMPRESS;
                          }
  
                          /*
                           * For authenticated bp's, we do not ask the ZIO layer
                           * to authenticate them since this will cause the entire
                           * IO to fail if the key isn't loaded. Instead, we
                           * defer authentication until arc_buf_fill(), which will
                           * verify the data when the key is available.
                           */
                          if (BP_IS_AUTHENTICATED(bp))
                                  zio_flags |= ZIO_FLAG_RAW_ENCRYPT;
                  }
  
                  if (BP_IS_AUTHENTICATED(bp))
                          arc_hdr_set_flags(hdr, ARC_FLAG_NOAUTH);
                  if (BP_GET_LEVEL(bp) > 0)
                          arc_hdr_set_flags(hdr, ARC_FLAG_INDIRECT);
                  ASSERT(!GHOST_STATE(hdr->b_l1hdr.b_state));
  
                  acb = kmem_zalloc(sizeof (arc_callback_t), KM_SLEEP);
                  acb->acb_done = done;
                  acb->acb_private = private;
                  acb->acb_compressed = compressed_read;
                  acb->acb_encrypted = encrypted_read;
                  acb->acb_noauth = noauth_read;
                  acb->acb_zb = *zb;
  
                  ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
                  hdr->b_l1hdr.b_acb = acb;
  
                  if (HDR_HAS_L2HDR(hdr) &&
                      (vd = hdr->b_l2hdr.b_dev->l2ad_vdev) != NULL) {
                          devw = hdr->b_l2hdr.b_dev->l2ad_writing;
                          addr = hdr->b_l2hdr.b_daddr;
                          /*
                           * Lock out L2ARC device removal.
                           */
                          if (vdev_is_dead(vd) ||
                              !spa_config_tryenter(spa, SCL_L2ARC, vd, RW_READER))
                                  vd = NULL;
                  }
  
                  /*
                   * We count both async reads and scrub IOs as asynchronous so
                   * that both can be upgraded in the event of a cache hit while
                   * the read IO is still in-flight.
                   */
                  if (priority == ZIO_PRIORITY_ASYNC_READ ||
                      priority == ZIO_PRIORITY_SCRUB)
                          arc_hdr_set_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
                  else
                          arc_hdr_clear_flags(hdr, ARC_FLAG_PRIO_ASYNC_READ);
  
                  /*
                   * At this point, we have a level 1 cache miss or a blkptr
                   * with embedded data.  Try again in L2ARC if possible.
                   */
                  ASSERT3U(HDR_GET_LSIZE(hdr), ==, lsize);
  
                  /*
                   * Skip ARC stat bump for block pointers with embedded
                   * data. The data are read from the blkptr itself via
                   * decode_embedded_bp_compressed().
                   */
                  if (!embedded_bp) {
                          DTRACE_PROBE4(arc__miss, arc_buf_hdr_t *, hdr,
                              blkptr_t *, bp, uint64_t, lsize,
                              zbookmark_phys_t *, zb);
                          ARCSTAT_BUMP(arcstat_misses);
                          ARCSTAT_CONDSTAT(!(*arc_flags & ARC_FLAG_PREFETCH),
                              demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data,
                              metadata, misses);
                          zfs_racct_read(size, 1);
                  }
  
                  /* Check if the spa even has l2 configured */
                  const boolean_t spa_has_l2 = l2arc_ndev != 0 &&
                      spa->spa_l2cache.sav_count > 0;
  
                  if (vd != NULL && spa_has_l2 && !(l2arc_norw && devw)) {
                          /*
                           * Read from the L2ARC if the following are true:
                           * 1. The L2ARC vdev was previously cached.
                           * 2. This buffer still has L2ARC metadata.
                           * 3. This buffer isn't currently writing to the L2ARC.
                           * 4. The L2ARC entry wasn't evicted, which may
                           *    also have invalidated the vdev.
                           * 5. This isn't prefetch or l2arc_noprefetch is 0.
                           */
                          if (HDR_HAS_L2HDR(hdr) &&
                              !HDR_L2_WRITING(hdr) && !HDR_L2_EVICTED(hdr) &&
                              !(l2arc_noprefetch &&
                              (*arc_flags & ARC_FLAG_PREFETCH))) {
                                  l2arc_read_callback_t *cb;
                                  abd_t *abd;
                                  uint64_t asize;
  
                                  DTRACE_PROBE1(l2arc__hit, arc_buf_hdr_t *, hdr);
                                  ARCSTAT_BUMP(arcstat_l2_hits);
                                  hdr->b_l2hdr.b_hits++;
  
                                  cb = kmem_zalloc(sizeof (l2arc_read_callback_t),
                                      KM_SLEEP);
                                  cb->l2rcb_hdr = hdr;
                                  cb->l2rcb_bp = *bp;
                                  cb->l2rcb_zb = *zb;
                                  cb->l2rcb_flags = zio_flags;
  
                                  /*
                                   * When Compressed ARC is disabled, but the
                                   * L2ARC block is compressed, arc_hdr_size()
                                   * will have returned LSIZE rather than PSIZE.
                                   */
                                  if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
                                      !HDR_COMPRESSION_ENABLED(hdr) &&
                                      HDR_GET_PSIZE(hdr) != 0) {
                                          size = HDR_GET_PSIZE(hdr);
                                  }
  
                                  asize = vdev_psize_to_asize(vd, size);
                                  if (asize != size) {
                                          abd = abd_alloc_for_io(asize,
                                              HDR_ISTYPE_METADATA(hdr));
                                          cb->l2rcb_abd = abd;
                                  } else {
                                          abd = hdr_abd;
                                  }
  
                                  ASSERT(addr >= VDEV_LABEL_START_SIZE &&
                                      addr + asize <= vd->vdev_psize -
                                      VDEV_LABEL_END_SIZE);
  
                                  /*
                                   * l2arc read.  The SCL_L2ARC lock will be
                                   * released by l2arc_read_done().
                                   * Issue a null zio if the underlying buffer
                                   * was squashed to zero size by compression.
                                   */
                                  ASSERT3U(arc_hdr_get_compress(hdr), !=,
                                      ZIO_COMPRESS_EMPTY);
                                  rzio = zio_read_phys(pio, vd, addr,
                                      asize, abd,
                                      ZIO_CHECKSUM_OFF,
                                      l2arc_read_done, cb, priority,
                                      zio_flags | ZIO_FLAG_DONT_CACHE |
                                      ZIO_FLAG_CANFAIL |
                                      ZIO_FLAG_DONT_PROPAGATE |
                                      ZIO_FLAG_DONT_RETRY, B_FALSE);
                                  acb->acb_zio_head = rzio;
  
                                  if (hash_lock != NULL)
                                          mutex_exit(hash_lock);
  
                                  DTRACE_PROBE2(l2arc__read, vdev_t *, vd,
                                      zio_t *, rzio);
                                  ARCSTAT_INCR(arcstat_l2_read_bytes,
                                      HDR_GET_PSIZE(hdr));
  
                                  if (*arc_flags & ARC_FLAG_NOWAIT) {
                                          zio_nowait(rzio);
                                          goto out;
                                  }
  
                                  ASSERT(*arc_flags & ARC_FLAG_WAIT);
                                  if (zio_wait(rzio) == 0)
                                          goto out;
  
                                  /* l2arc read error; goto zio_read() */
                                  if (hash_lock != NULL)
                                          mutex_enter(hash_lock);
                          } else {
                                  DTRACE_PROBE1(l2arc__miss,
                                      arc_buf_hdr_t *, hdr);
                                  ARCSTAT_BUMP(arcstat_l2_misses);
                                  if (HDR_L2_WRITING(hdr))
                                          ARCSTAT_BUMP(arcstat_l2_rw_clash);
                                  spa_config_exit(spa, SCL_L2ARC, vd);
                          }
                  } else {
                          if (vd != NULL)
                                  spa_config_exit(spa, SCL_L2ARC, vd);
  
                          /*
                           * Only a spa with l2 should contribute to l2
                           * miss stats.  (Including the case of having a
                           * faulted cache device - that's also a miss.)
                           */
                          if (spa_has_l2) {
                                  /*
                                   * Skip ARC stat bump for block pointers with
                                   * embedded data. The data are read from the
                                   * blkptr itself via
                                   * decode_embedded_bp_compressed().
                                   */
                                  if (!embedded_bp) {
                                          DTRACE_PROBE1(l2arc__miss,
                                              arc_buf_hdr_t *, hdr);
                                          ARCSTAT_BUMP(arcstat_l2_misses);
                                  }
                          }
                  }
  
                  rzio = zio_read(pio, spa, bp, hdr_abd, size,
                      arc_read_done, hdr, priority, zio_flags, zb);
                  acb->acb_zio_head = rzio;
  
                  if (hash_lock != NULL)
                          mutex_exit(hash_lock);
  
                  if (*arc_flags & ARC_FLAG_WAIT) {
                          rc = zio_wait(rzio);
                          goto out;
                  }
  
                  ASSERT(*arc_flags & ARC_FLAG_NOWAIT);
                  zio_nowait(rzio);
          }
  
  out:
          /* embedded bps don't actually go to disk */
          if (!embedded_bp)
                  spa_read_history_add(spa, zb, *arc_flags);
          spl_fstrans_unmark(cookie);
          return (rc);
  }
  
  arc_prune_t *
  arc_add_prune_callback(arc_prune_func_t *func, void *private)
  {
          arc_prune_t *p;
  
          p = kmem_alloc(sizeof (*p), KM_SLEEP);
          p->p_pfunc = func;
          p->p_private = private;
          list_link_init(&p->p_node);
          zfs_refcount_create(&p->p_refcnt);
  
          mutex_enter(&arc_prune_mtx);
          zfs_refcount_add(&p->p_refcnt, &arc_prune_list);
          list_insert_head(&arc_prune_list, p);
          mutex_exit(&arc_prune_mtx);
  
          return (p);
  }
  
  void
  arc_remove_prune_callback(arc_prune_t *p)
  {
          boolean_t wait = B_FALSE;
          mutex_enter(&arc_prune_mtx);
          list_remove(&arc_prune_list, p);
          if (zfs_refcount_remove(&p->p_refcnt, &arc_prune_list) > 0)
                  wait = B_TRUE;
          mutex_exit(&arc_prune_mtx);
  
          /* wait for arc_prune_task to finish */
          if (wait)
                  taskq_wait_outstanding(arc_prune_taskq, 0);
          ASSERT0(zfs_refcount_count(&p->p_refcnt));
          zfs_refcount_destroy(&p->p_refcnt);
          kmem_free(p, sizeof (*p));
  }
  
  /*
   * Notify the arc that a block was freed, and thus will never be used again.
   */
  void
  arc_freed(spa_t *spa, const blkptr_t *bp)
  {
          arc_buf_hdr_t *hdr;
          kmutex_t *hash_lock;
          uint64_t guid = spa_load_guid(spa);
  
          ASSERT(!BP_IS_EMBEDDED(bp));
  
          hdr = buf_hash_find(guid, bp, &hash_lock);
          if (hdr == NULL)
                  return;
  
          /*
           * We might be trying to free a block that is still doing I/O
           * (i.e. prefetch) or has some other reference (i.e. a dedup-ed,
           * dmu_sync-ed block). A block may also have a reference if it is
           * part of a dedup-ed, dmu_synced write. The dmu_sync() function would
           * have written the new block to its final resting place on disk but
           * without the dedup flag set. This would have left the hdr in the MRU
           * state and discoverable. When the txg finally syncs it detects that
           * the block was overridden in open context and issues an override I/O.
           * Since this is a dedup block, the override I/O will determine if the
           * block is already in the DDT. If so, then it will replace the io_bp
           * with the bp from the DDT and allow the I/O to finish. When the I/O
           * reaches the done callback, dbuf_write_override_done, it will
           * check to see if the io_bp and io_bp_override are identical.
           * If they are not, then it indicates that the bp was replaced with
           * the bp in the DDT and the override bp is freed. This allows
           * us to arrive here with a reference on a block that is being
           * freed. So if we have an I/O in progress, or a reference to
           * this hdr, then we don't destroy the hdr.
           */
          if (!HDR_HAS_L1HDR(hdr) ||
              zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt)) {
                  arc_change_state(arc_anon, hdr);
                  arc_hdr_destroy(hdr);
                  mutex_exit(hash_lock);
          } else {
                  mutex_exit(hash_lock);
          }
  
  }
  
  /*
   * Release this buffer from the cache, making it an anonymous buffer.  This
   * must be done after a read and prior to modifying the buffer contents.
   * If the buffer has more than one reference, we must make
   * a new hdr for the buffer.
   */
  void
  arc_release(arc_buf_t *buf, const void *tag)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          /*
           * It would be nice to assert that if its DMU metadata (level >
           * 0 || it's the dnode file), then it must be syncing context.
           * But we don't know that information at this level.
           */
  
          ASSERT(HDR_HAS_L1HDR(hdr));
  
          /*
           * We don't grab the hash lock prior to this check, because if
           * the buffer's header is in the arc_anon state, it won't be
           * linked into the hash table.
           */
          if (hdr->b_l1hdr.b_state == arc_anon) {
                  ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                  ASSERT(!HDR_IN_HASH_TABLE(hdr));
                  ASSERT(!HDR_HAS_L2HDR(hdr));
  
                  ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
                  ASSERT3S(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt), ==, 1);
                  ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
  
                  hdr->b_l1hdr.b_arc_access = 0;
  
                  /*
                   * If the buf is being overridden then it may already
                   * have a hdr that is not empty.
                   */
                  buf_discard_identity(hdr);
                  arc_buf_thaw(buf);
  
                  return;
          }
  
          kmutex_t *hash_lock = HDR_LOCK(hdr);
          mutex_enter(hash_lock);
  
          /*
           * This assignment is only valid as long as the hash_lock is
           * held, we must be careful not to reference state or the
           * b_state field after dropping the lock.
           */
          arc_state_t *state = hdr->b_l1hdr.b_state;
          ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
          ASSERT3P(state, !=, arc_anon);
  
          /* this buffer is not on any list */
          ASSERT3S(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt), >, 0);
  
          if (HDR_HAS_L2HDR(hdr)) {
                  mutex_enter(&hdr->b_l2hdr.b_dev->l2ad_mtx);
  
                  /*
                   * We have to recheck this conditional again now that
                   * we're holding the l2ad_mtx to prevent a race with
                   * another thread which might be concurrently calling
                   * l2arc_evict(). In that case, l2arc_evict() might have
                   * destroyed the header's L2 portion as we were waiting
                   * to acquire the l2ad_mtx.
                   */
                  if (HDR_HAS_L2HDR(hdr))
                          arc_hdr_l2hdr_destroy(hdr);
  
                  mutex_exit(&hdr->b_l2hdr.b_dev->l2ad_mtx);
          }
  
          /*
           * Do we have more than one buf?
           */
          if (hdr->b_l1hdr.b_bufcnt > 1) {
                  arc_buf_hdr_t *nhdr;
                  uint64_t spa = hdr->b_spa;
                  uint64_t psize = HDR_GET_PSIZE(hdr);
                  uint64_t lsize = HDR_GET_LSIZE(hdr);
                  boolean_t protected = HDR_PROTECTED(hdr);
                  enum zio_compress compress = arc_hdr_get_compress(hdr);
                  arc_buf_contents_t type = arc_buf_type(hdr);
                  VERIFY3U(hdr->b_type, ==, type);
  
                  ASSERT(hdr->b_l1hdr.b_buf != buf || buf->b_next != NULL);
                  VERIFY3S(remove_reference(hdr, tag), >, 0);
  
                  if (arc_buf_is_shared(buf) && !ARC_BUF_COMPRESSED(buf)) {
                          ASSERT3P(hdr->b_l1hdr.b_buf, !=, buf);
                          ASSERT(ARC_BUF_LAST(buf));
                  }
  
                  /*
                   * Pull the data off of this hdr and attach it to
                   * a new anonymous hdr. Also find the last buffer
                   * in the hdr's buffer list.
                   */
                  arc_buf_t *lastbuf = arc_buf_remove(hdr, buf);
                  ASSERT3P(lastbuf, !=, NULL);
  
                  /*
                   * If the current arc_buf_t and the hdr are sharing their data
                   * buffer, then we must stop sharing that block.
                   */
                  if (arc_buf_is_shared(buf)) {
                          ASSERT3P(hdr->b_l1hdr.b_buf, !=, buf);
                          VERIFY(!arc_buf_is_shared(lastbuf));
  
                          /*
                           * First, sever the block sharing relationship between
                           * buf and the arc_buf_hdr_t.
                           */
                          arc_unshare_buf(hdr, buf);
  
                          /*
                           * Now we need to recreate the hdr's b_pabd. Since we
                           * have lastbuf handy, we try to share with it, but if
                           * we can't then we allocate a new b_pabd and copy the
                           * data from buf into it.
                           */
                          if (arc_can_share(hdr, lastbuf)) {
                                  arc_share_buf(hdr, lastbuf);
                          } else {
                                  arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT);
                                  abd_copy_from_buf(hdr->b_l1hdr.b_pabd,
                                      buf->b_data, psize);
                          }
                          VERIFY3P(lastbuf->b_data, !=, NULL);
                  } else if (HDR_SHARED_DATA(hdr)) {
                          /*
                           * Uncompressed shared buffers are always at the end
                           * of the list. Compressed buffers don't have the
                           * same requirements. This makes it hard to
                           * simply assert that the lastbuf is shared so
                           * we rely on the hdr's compression flags to determine
                           * if we have a compressed, shared buffer.
                           */
                          ASSERT(arc_buf_is_shared(lastbuf) ||
                              arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF);
                          ASSERT(!ARC_BUF_SHARED(buf));
                  }
  
                  ASSERT(hdr->b_l1hdr.b_pabd != NULL || HDR_HAS_RABD(hdr));
                  ASSERT3P(state, !=, arc_l2c_only);
  
                  (void) zfs_refcount_remove_many(&state->arcs_size,
                      arc_buf_size(buf), buf);
  
                  if (zfs_refcount_is_zero(&hdr->b_l1hdr.b_refcnt)) {
                          ASSERT3P(state, !=, arc_l2c_only);
                          (void) zfs_refcount_remove_many(
                              &state->arcs_esize[type],
                              arc_buf_size(buf), buf);
                  }
  
                  hdr->b_l1hdr.b_bufcnt -= 1;
                  if (ARC_BUF_ENCRYPTED(buf))
                          hdr->b_crypt_hdr.b_ebufcnt -= 1;
  
                  arc_cksum_verify(buf);
                  arc_buf_unwatch(buf);
  
                  /* if this is the last uncompressed buf free the checksum */
                  if (!arc_hdr_has_uncompressed_buf(hdr))
                          arc_cksum_free(hdr);
  
                  mutex_exit(hash_lock);
  
                  nhdr = arc_hdr_alloc(spa, psize, lsize, protected,
                      compress, hdr->b_complevel, type);
                  ASSERT3P(nhdr->b_l1hdr.b_buf, ==, NULL);
                  ASSERT0(nhdr->b_l1hdr.b_bufcnt);
                  ASSERT0(zfs_refcount_count(&nhdr->b_l1hdr.b_refcnt));
                  VERIFY3U(nhdr->b_type, ==, type);
                  ASSERT(!HDR_SHARED_DATA(nhdr));
  
                  nhdr->b_l1hdr.b_buf = buf;
                  nhdr->b_l1hdr.b_bufcnt = 1;
                  if (ARC_BUF_ENCRYPTED(buf))
                          nhdr->b_crypt_hdr.b_ebufcnt = 1;
                  (void) zfs_refcount_add(&nhdr->b_l1hdr.b_refcnt, tag);
                  buf->b_hdr = nhdr;
  
                  (void) zfs_refcount_add_many(&arc_anon->arcs_size,
                      arc_buf_size(buf), buf);
          } else {
                  ASSERT(zfs_refcount_count(&hdr->b_l1hdr.b_refcnt) == 1);
                  /* protected by hash lock, or hdr is on arc_anon */
                  ASSERT(!multilist_link_active(&hdr->b_l1hdr.b_arc_node));
                  ASSERT(!HDR_IO_IN_PROGRESS(hdr));
                  hdr->b_l1hdr.b_mru_hits = 0;
                  hdr->b_l1hdr.b_mru_ghost_hits = 0;
                  hdr->b_l1hdr.b_mfu_hits = 0;
                  hdr->b_l1hdr.b_mfu_ghost_hits = 0;
                  arc_change_state(arc_anon, hdr);
                  hdr->b_l1hdr.b_arc_access = 0;
  
                  mutex_exit(hash_lock);
                  buf_discard_identity(hdr);
                  arc_buf_thaw(buf);
          }
  }
  
  int
  arc_released(arc_buf_t *buf)
  {
          return (buf->b_data != NULL &&
              buf->b_hdr->b_l1hdr.b_state == arc_anon);
  }
  
  #ifdef ZFS_DEBUG
  int
  arc_referenced(arc_buf_t *buf)
  {
          return (zfs_refcount_count(&buf->b_hdr->b_l1hdr.b_refcnt));
  }
  #endif
  
  static void
  arc_write_ready(zio_t *zio)
  {
          arc_write_callback_t *callback = zio->io_private;
          arc_buf_t *buf = callback->awcb_buf;
          arc_buf_hdr_t *hdr = buf->b_hdr;
          blkptr_t *bp = zio->io_bp;
          uint64_t psize = BP_IS_HOLE(bp) ? 0 : BP_GET_PSIZE(bp);
          fstrans_cookie_t cookie = spl_fstrans_mark();
  
          ASSERT(HDR_HAS_L1HDR(hdr));
          ASSERT(!zfs_refcount_is_zero(&buf->b_hdr->b_l1hdr.b_refcnt));
          ASSERT(hdr->b_l1hdr.b_bufcnt > 0);
  
          /*
           * If we're reexecuting this zio because the pool suspended, then
           * cleanup any state that was previously set the first time the
           * callback was invoked.
           */
          if (zio->io_flags & ZIO_FLAG_REEXECUTED) {
                  arc_cksum_free(hdr);
                  arc_buf_unwatch(buf);
                  if (hdr->b_l1hdr.b_pabd != NULL) {
                          if (arc_buf_is_shared(buf)) {
                                  arc_unshare_buf(hdr, buf);
                          } else {
                                  arc_hdr_free_abd(hdr, B_FALSE);
                          }
                  }
  
                  if (HDR_HAS_RABD(hdr))
                          arc_hdr_free_abd(hdr, B_TRUE);
          }
          ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
          ASSERT(!HDR_HAS_RABD(hdr));
          ASSERT(!HDR_SHARED_DATA(hdr));
          ASSERT(!arc_buf_is_shared(buf));
  
          callback->awcb_ready(zio, buf, callback->awcb_private);
  
          if (HDR_IO_IN_PROGRESS(hdr)) {
                  ASSERT(zio->io_flags & ZIO_FLAG_REEXECUTED);
          } else {
                  arc_hdr_set_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
                  add_reference(hdr, hdr); /* For IO_IN_PROGRESS. */
          }
  
          if (BP_IS_PROTECTED(bp) != !!HDR_PROTECTED(hdr))
                  hdr = arc_hdr_realloc_crypt(hdr, BP_IS_PROTECTED(bp));
  
          if (BP_IS_PROTECTED(bp)) {
                  /* ZIL blocks are written through zio_rewrite */
                  ASSERT3U(BP_GET_TYPE(bp), !=, DMU_OT_INTENT_LOG);
                  ASSERT(HDR_PROTECTED(hdr));
  
                  if (BP_SHOULD_BYTESWAP(bp)) {
                          if (BP_GET_LEVEL(bp) > 0) {
                                  hdr->b_l1hdr.b_byteswap = DMU_BSWAP_UINT64;
                          } else {
                                  hdr->b_l1hdr.b_byteswap =
                                      DMU_OT_BYTESWAP(BP_GET_TYPE(bp));
                          }
                  } else {
                          hdr->b_l1hdr.b_byteswap = DMU_BSWAP_NUMFUNCS;
                  }
  
                  hdr->b_crypt_hdr.b_ot = BP_GET_TYPE(bp);
                  hdr->b_crypt_hdr.b_dsobj = zio->io_bookmark.zb_objset;
                  zio_crypt_decode_params_bp(bp, hdr->b_crypt_hdr.b_salt,
                      hdr->b_crypt_hdr.b_iv);
                  zio_crypt_decode_mac_bp(bp, hdr->b_crypt_hdr.b_mac);
          }
  
          /*
           * If this block was written for raw encryption but the zio layer
           * ended up only authenticating it, adjust the buffer flags now.
           */
          if (BP_IS_AUTHENTICATED(bp) && ARC_BUF_ENCRYPTED(buf)) {
                  arc_hdr_set_flags(hdr, ARC_FLAG_NOAUTH);
                  buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
                  if (BP_GET_COMPRESS(bp) == ZIO_COMPRESS_OFF)
                          buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
          } else if (BP_IS_HOLE(bp) && ARC_BUF_ENCRYPTED(buf)) {
                  buf->b_flags &= ~ARC_BUF_FLAG_ENCRYPTED;
                  buf->b_flags &= ~ARC_BUF_FLAG_COMPRESSED;
          }
  
          /* this must be done after the buffer flags are adjusted */
          arc_cksum_compute(buf);
  
          enum zio_compress compress;
          if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp)) {
                  compress = ZIO_COMPRESS_OFF;
          } else {
                  ASSERT3U(HDR_GET_LSIZE(hdr), ==, BP_GET_LSIZE(bp));
                  compress = BP_GET_COMPRESS(bp);
          }
          HDR_SET_PSIZE(hdr, psize);
          arc_hdr_set_compress(hdr, compress);
          hdr->b_complevel = zio->io_prop.zp_complevel;
  
          if (zio->io_error != 0 || psize == 0)
                  goto out;
  
          /*
           * Fill the hdr with data. If the buffer is encrypted we have no choice
           * but to copy the data into b_radb. If the hdr is compressed, the data
           * we want is available from the zio, otherwise we can take it from
           * the buf.
           *
           * We might be able to share the buf's data with the hdr here. However,
           * doing so would cause the ARC to be full of linear ABDs if we write a
           * lot of shareable data. As a compromise, we check whether scattered
           * ABDs are allowed, and assume that if they are then the user wants
           * the ARC to be primarily filled with them regardless of the data being
           * written. Therefore, if they're allowed then we allocate one and copy
           * the data into it; otherwise, we share the data directly if we can.
           */
          if (ARC_BUF_ENCRYPTED(buf)) {
                  ASSERT3U(psize, >, 0);
                  ASSERT(ARC_BUF_COMPRESSED(buf));
                  arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT | ARC_HDR_ALLOC_RDATA |
                      ARC_HDR_USE_RESERVE);
                  abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
          } else if (!(HDR_UNCACHED(hdr) ||
              abd_size_alloc_linear(arc_buf_size(buf))) ||
              !arc_can_share(hdr, buf)) {
                  /*
                   * Ideally, we would always copy the io_abd into b_pabd, but the
                   * user may have disabled compressed ARC, thus we must check the
                   * hdr's compression setting rather than the io_bp's.
                   */
                  if (BP_IS_ENCRYPTED(bp)) {
                          ASSERT3U(psize, >, 0);
                          arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT |
                              ARC_HDR_ALLOC_RDATA | ARC_HDR_USE_RESERVE);
                          abd_copy(hdr->b_crypt_hdr.b_rabd, zio->io_abd, psize);
                  } else if (arc_hdr_get_compress(hdr) != ZIO_COMPRESS_OFF &&
                      !ARC_BUF_COMPRESSED(buf)) {
                          ASSERT3U(psize, >, 0);
                          arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT |
                              ARC_HDR_USE_RESERVE);
                          abd_copy(hdr->b_l1hdr.b_pabd, zio->io_abd, psize);
                  } else {
                          ASSERT3U(zio->io_orig_size, ==, arc_hdr_size(hdr));
                          arc_hdr_alloc_abd(hdr, ARC_HDR_DO_ADAPT |
                              ARC_HDR_USE_RESERVE);
                          abd_copy_from_buf(hdr->b_l1hdr.b_pabd, buf->b_data,
                              arc_buf_size(buf));
                  }
          } else {
                  ASSERT3P(buf->b_data, ==, abd_to_buf(zio->io_orig_abd));
                  ASSERT3U(zio->io_orig_size, ==, arc_buf_size(buf));
                  ASSERT3U(hdr->b_l1hdr.b_bufcnt, ==, 1);
  
                  arc_share_buf(hdr, buf);
          }
  
  out:
          arc_hdr_verify(hdr, bp);
          spl_fstrans_unmark(cookie);
  }
  
  static void
  arc_write_children_ready(zio_t *zio)
  {
          arc_write_callback_t *callback = zio->io_private;
          arc_buf_t *buf = callback->awcb_buf;
  
          callback->awcb_children_ready(zio, buf, callback->awcb_private);
  }
  
  /*
   * The SPA calls this callback for each physical write that happens on behalf
   * of a logical write.  See the comment in dbuf_write_physdone() for details.
   */
  static void
  arc_write_physdone(zio_t *zio)
  {
          arc_write_callback_t *cb = zio->io_private;
          if (cb->awcb_physdone != NULL)
                  cb->awcb_physdone(zio, cb->awcb_buf, cb->awcb_private);
  }
  
  static void
  arc_write_done(zio_t *zio)
  {
          arc_write_callback_t *callback = zio->io_private;
          arc_buf_t *buf = callback->awcb_buf;
          arc_buf_hdr_t *hdr = buf->b_hdr;
  
          ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
  
          if (zio->io_error == 0) {
                  arc_hdr_verify(hdr, zio->io_bp);
  
                  if (BP_IS_HOLE(zio->io_bp) || BP_IS_EMBEDDED(zio->io_bp)) {
                          buf_discard_identity(hdr);
                  } else {
                          hdr->b_dva = *BP_IDENTITY(zio->io_bp);
                          hdr->b_birth = BP_PHYSICAL_BIRTH(zio->io_bp);
                  }
          } else {
                  ASSERT(HDR_EMPTY(hdr));
          }
  
          /*
           * If the block to be written was all-zero or compressed enough to be
           * embedded in the BP, no write was performed so there will be no
           * dva/birth/checksum.  The buffer must therefore remain anonymous
           * (and uncached).
           */
          if (!HDR_EMPTY(hdr)) {
                  arc_buf_hdr_t *exists;
                  kmutex_t *hash_lock;
  
                  ASSERT3U(zio->io_error, ==, 0);
  
                  arc_cksum_verify(buf);
  
                  exists = buf_hash_insert(hdr, &hash_lock);
                  if (exists != NULL) {
                          /*
                           * This can only happen if we overwrite for
                           * sync-to-convergence, because we remove
                           * buffers from the hash table when we arc_free().
                           */
                          if (zio->io_flags & ZIO_FLAG_IO_REWRITE) {
                                  if (!BP_EQUAL(&zio->io_bp_orig, zio->io_bp))
                                          panic("bad overwrite, hdr=%p exists=%p",
                                              (void *)hdr, (void *)exists);
                                  ASSERT(zfs_refcount_is_zero(
                                      &exists->b_l1hdr.b_refcnt));
                                  arc_change_state(arc_anon, exists);
                                  arc_hdr_destroy(exists);
                                  mutex_exit(hash_lock);
                                  exists = buf_hash_insert(hdr, &hash_lock);
                                  ASSERT3P(exists, ==, NULL);
                          } else if (zio->io_flags & ZIO_FLAG_NOPWRITE) {
                                  /* nopwrite */
                                  ASSERT(zio->io_prop.zp_nopwrite);
                                  if (!BP_EQUAL(&zio->io_bp_orig, zio->io_bp))
                                          panic("bad nopwrite, hdr=%p exists=%p",
                                              (void *)hdr, (void *)exists);
                          } else {
                                  /* Dedup */
                                  ASSERT(hdr->b_l1hdr.b_bufcnt == 1);
                                  ASSERT(hdr->b_l1hdr.b_state == arc_anon);
                                  ASSERT(BP_GET_DEDUP(zio->io_bp));
                                  ASSERT(BP_GET_LEVEL(zio->io_bp) == 0);
                          }
                  }
                  arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
                  VERIFY3S(remove_reference(hdr, hdr), >, 0);
                  /* if it's not anon, we are doing a scrub */
                  if (exists == NULL && hdr->b_l1hdr.b_state == arc_anon)
                          arc_access(hdr, 0, B_FALSE);
                  mutex_exit(hash_lock);
          } else {
                  arc_hdr_clear_flags(hdr, ARC_FLAG_IO_IN_PROGRESS);
                  VERIFY3S(remove_reference(hdr, hdr), >, 0);
          }
  
          callback->awcb_done(zio, buf, callback->awcb_private);
  
          abd_free(zio->io_abd);
          kmem_free(callback, sizeof (arc_write_callback_t));
  }
  
  zio_t *
  arc_write(zio_t *pio, spa_t *spa, uint64_t txg,
      blkptr_t *bp, arc_buf_t *buf, boolean_t uncached, boolean_t l2arc,
      const zio_prop_t *zp, arc_write_done_func_t *ready,
      arc_write_done_func_t *children_ready, arc_write_done_func_t *physdone,
      arc_write_done_func_t *done, void *private, zio_priority_t priority,
      int zio_flags, const zbookmark_phys_t *zb)
  {
          arc_buf_hdr_t *hdr = buf->b_hdr;
          arc_write_callback_t *callback;
          zio_t *zio;
          zio_prop_t localprop = *zp;
  
          ASSERT3P(ready, !=, NULL);
          ASSERT3P(done, !=, NULL);
          ASSERT(!HDR_IO_ERROR(hdr));
          ASSERT(!HDR_IO_IN_PROGRESS(hdr));
          ASSERT3P(hdr->b_l1hdr.b_acb, ==, NULL);
          ASSERT3U(hdr->b_l1hdr.b_bufcnt, >, 0);
          if (uncached)
                  arc_hdr_set_flags(hdr, ARC_FLAG_UNCACHED);
          else if (l2arc)
                  arc_hdr_set_flags(hdr, ARC_FLAG_L2CACHE);
  
          if (ARC_BUF_ENCRYPTED(buf)) {
                  ASSERT(ARC_BUF_COMPRESSED(buf));
                  localprop.zp_encrypt = B_TRUE;
                  localprop.zp_compress = HDR_GET_COMPRESS(hdr);
                  localprop.zp_complevel = hdr->b_complevel;
                  localprop.zp_byteorder =
                      (hdr->b_l1hdr.b_byteswap == DMU_BSWAP_NUMFUNCS) ?
                      ZFS_HOST_BYTEORDER : !ZFS_HOST_BYTEORDER;
                  memcpy(localprop.zp_salt, hdr->b_crypt_hdr.b_salt,
                      ZIO_DATA_SALT_LEN);
                  memcpy(localprop.zp_iv, hdr->b_crypt_hdr.b_iv,
                      ZIO_DATA_IV_LEN);
                  memcpy(localprop.zp_mac, hdr->b_crypt_hdr.b_mac,
                      ZIO_DATA_MAC_LEN);
                  if (DMU_OT_IS_ENCRYPTED(localprop.zp_type)) {
                          localprop.zp_nopwrite = B_FALSE;
                          localprop.zp_copies =
                              MIN(localprop.zp_copies, SPA_DVAS_PER_BP - 1);
                  }
                  zio_flags |= ZIO_FLAG_RAW;
          } else if (ARC_BUF_COMPRESSED(buf)) {
                  ASSERT3U(HDR_GET_LSIZE(hdr), !=, arc_buf_size(buf));
                  localprop.zp_compress = HDR_GET_COMPRESS(hdr);
                  localprop.zp_complevel = hdr->b_complevel;
                  zio_flags |= ZIO_FLAG_RAW_COMPRESS;
          }
          callback = kmem_zalloc(sizeof (arc_write_callback_t), KM_SLEEP);
          callback->awcb_ready = ready;
          callback->awcb_children_ready = children_ready;
          callback->awcb_physdone = physdone;
          callback->awcb_done = done;
          callback->awcb_private = private;
          callback->awcb_buf = buf;
  
          /*
           * The hdr's b_pabd is now stale, free it now. A new data block
           * will be allocated when the zio pipeline calls arc_write_ready().
           */
          if (hdr->b_l1hdr.b_pabd != NULL) {
                  /*
                   * If the buf is currently sharing the data block with
                   * the hdr then we need to break that relationship here.
                   * The hdr will remain with a NULL data pointer and the
                   * buf will take sole ownership of the block.
                   */
                  if (arc_buf_is_shared(buf)) {
                          arc_unshare_buf(hdr, buf);
                  } else {
                          arc_hdr_free_abd(hdr, B_FALSE);
                  }
                  VERIFY3P(buf->b_data, !=, NULL);
          }
  
          if (HDR_HAS_RABD(hdr))
                  arc_hdr_free_abd(hdr, B_TRUE);
  
          if (!(zio_flags & ZIO_FLAG_RAW))
                  arc_hdr_set_compress(hdr, ZIO_COMPRESS_OFF);
  
          ASSERT(!arc_buf_is_shared(buf));
          ASSERT3P(hdr->b_l1hdr.b_pabd, ==, NULL);
  
          zio = zio_write(pio, spa, txg, bp,
              abd_get_from_buf(buf->b_data, HDR_GET_LSIZE(hdr)),
              HDR_GET_LSIZE(hdr), arc_buf_size(buf), &localprop, arc_write_ready,
              (children_ready != NULL) ? arc_write_children_ready : NULL,
              arc_write_physdone, arc_write_done, callback,
              priority, zio_flags, zb);
  
          return (zio);
  }
  
  void
  arc_tempreserve_clear(uint64_t reserve)
  {
          atomic_add_64(&arc_tempreserve, -reserve);
          ASSERT((int64_t)arc_tempreserve >= 0);
  }
  
  int
  arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg)
  {
          int error;
          uint64_t anon_size;
  
          if (!arc_no_grow &&
              reserve > arc_c/4 &&
              reserve * 4 > (2ULL << SPA_MAXBLOCKSHIFT))
                  arc_c = MIN(arc_c_max, reserve * 4);
  
          /*
           * Throttle when the calculated memory footprint for the TXG
           * exceeds the target ARC size.
           */
          if (reserve > arc_c) {
                  DMU_TX_STAT_BUMP(dmu_tx_memory_reserve);
                  return (SET_ERROR(ERESTART));
          }
  
          /*
           * Don't count loaned bufs as in flight dirty data to prevent long
           * network delays from blocking transactions that are ready to be
           * assigned to a txg.
           */
  
          /* assert that it has not wrapped around */
          ASSERT3S(atomic_add_64_nv(&arc_loaned_bytes, 0), >=, 0);
  
          anon_size = MAX((int64_t)(zfs_refcount_count(&arc_anon->arcs_size) -
              arc_loaned_bytes), 0);
  
          /*
           * Writes will, almost always, require additional memory allocations
           * in order to compress/encrypt/etc the data.  We therefore need to
           * make sure that there is sufficient available memory for this.
           */
          error = arc_memory_throttle(spa, reserve, txg);
          if (error != 0)
                  return (error);
  
          /*
           * Throttle writes when the amount of dirty data in the cache
           * gets too large.  We try to keep the cache less than half full
           * of dirty blocks so that our sync times don't grow too large.
           *
           * In the case of one pool being built on another pool, we want
           * to make sure we don't end up throttling the lower (backing)
           * pool when the upper pool is the majority contributor to dirty
           * data. To insure we make forward progress during throttling, we
           * also check the current pool's net dirty data and only throttle
           * if it exceeds zfs_arc_pool_dirty_percent of the anonymous dirty
           * data in the cache.
           *
           * Note: if two requests come in concurrently, we might let them
           * both succeed, when one of them should fail.  Not a huge deal.
           */
          uint64_t total_dirty = reserve + arc_tempreserve + anon_size;
          uint64_t spa_dirty_anon = spa_dirty_data(spa);
          uint64_t rarc_c = arc_warm ? arc_c : arc_c_max;
          if (total_dirty > rarc_c * zfs_arc_dirty_limit_percent / 100 &&
              anon_size > rarc_c * zfs_arc_anon_limit_percent / 100 &&
              spa_dirty_anon > anon_size * zfs_arc_pool_dirty_percent / 100) {
  #ifdef ZFS_DEBUG
                  uint64_t meta_esize = zfs_refcount_count(
                      &arc_anon->arcs_esize[ARC_BUFC_METADATA]);
                  uint64_t data_esize =
                      zfs_refcount_count(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
                  dprintf("failing, arc_tempreserve=%lluK anon_meta=%lluK "
                      "anon_data=%lluK tempreserve=%lluK rarc_c=%lluK\n",
                      (u_longlong_t)arc_tempreserve >> 10,
                      (u_longlong_t)meta_esize >> 10,
                      (u_longlong_t)data_esize >> 10,
                      (u_longlong_t)reserve >> 10,
                      (u_longlong_t)rarc_c >> 10);
  #endif
                  DMU_TX_STAT_BUMP(dmu_tx_dirty_throttle);
                  return (SET_ERROR(ERESTART));
          }
          atomic_add_64(&arc_tempreserve, reserve);
          return (0);
  }
  
  static void
  arc_kstat_update_state(arc_state_t *state, kstat_named_t *size,
      kstat_named_t *evict_data, kstat_named_t *evict_metadata)
  {
          size->value.ui64 = zfs_refcount_count(&state->arcs_size);
          evict_data->value.ui64 =
              zfs_refcount_count(&state->arcs_esize[ARC_BUFC_DATA]);
          evict_metadata->value.ui64 =
              zfs_refcount_count(&state->arcs_esize[ARC_BUFC_METADATA]);
  }
  
  static int
  arc_kstat_update(kstat_t *ksp, int rw)
  {
          arc_stats_t *as = ksp->ks_data;
  
          if (rw == KSTAT_WRITE)
                  return (SET_ERROR(EACCES));
  
          as->arcstat_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_hits);
          as->arcstat_iohits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_iohits);
          as->arcstat_misses.value.ui64 =
              wmsum_value(&arc_sums.arcstat_misses);
          as->arcstat_demand_data_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_data_hits);
          as->arcstat_demand_data_iohits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_data_iohits);
          as->arcstat_demand_data_misses.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_data_misses);
          as->arcstat_demand_metadata_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_metadata_hits);
          as->arcstat_demand_metadata_iohits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_metadata_iohits);
          as->arcstat_demand_metadata_misses.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_metadata_misses);
          as->arcstat_prefetch_data_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prefetch_data_hits);
          as->arcstat_prefetch_data_iohits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prefetch_data_iohits);
          as->arcstat_prefetch_data_misses.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prefetch_data_misses);
          as->arcstat_prefetch_metadata_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prefetch_metadata_hits);
          as->arcstat_prefetch_metadata_iohits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prefetch_metadata_iohits);
          as->arcstat_prefetch_metadata_misses.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prefetch_metadata_misses);
          as->arcstat_mru_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_mru_hits);
          as->arcstat_mru_ghost_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_mru_ghost_hits);
          as->arcstat_mfu_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_mfu_hits);
          as->arcstat_mfu_ghost_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_mfu_ghost_hits);
          as->arcstat_uncached_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_uncached_hits);
          as->arcstat_deleted.value.ui64 =
              wmsum_value(&arc_sums.arcstat_deleted);
          as->arcstat_mutex_miss.value.ui64 =
              wmsum_value(&arc_sums.arcstat_mutex_miss);
          as->arcstat_access_skip.value.ui64 =
              wmsum_value(&arc_sums.arcstat_access_skip);
          as->arcstat_evict_skip.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_skip);
          as->arcstat_evict_not_enough.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_not_enough);
          as->arcstat_evict_l2_cached.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_l2_cached);
          as->arcstat_evict_l2_eligible.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_l2_eligible);
          as->arcstat_evict_l2_eligible_mfu.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_l2_eligible_mfu);
          as->arcstat_evict_l2_eligible_mru.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_l2_eligible_mru);
          as->arcstat_evict_l2_ineligible.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_l2_ineligible);
          as->arcstat_evict_l2_skip.value.ui64 =
              wmsum_value(&arc_sums.arcstat_evict_l2_skip);
          as->arcstat_hash_collisions.value.ui64 =
              wmsum_value(&arc_sums.arcstat_hash_collisions);
          as->arcstat_hash_chains.value.ui64 =
              wmsum_value(&arc_sums.arcstat_hash_chains);
          as->arcstat_size.value.ui64 =
              aggsum_value(&arc_sums.arcstat_size);
          as->arcstat_compressed_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_compressed_size);
          as->arcstat_uncompressed_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_uncompressed_size);
          as->arcstat_overhead_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_overhead_size);
          as->arcstat_hdr_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_hdr_size);
          as->arcstat_data_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_data_size);
          as->arcstat_metadata_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_metadata_size);
          as->arcstat_dbuf_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_dbuf_size);
  #if defined(COMPAT_FREEBSD11)
          as->arcstat_other_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_bonus_size) +
              aggsum_value(&arc_sums.arcstat_dnode_size) +
              wmsum_value(&arc_sums.arcstat_dbuf_size);
  #endif
  
          arc_kstat_update_state(arc_anon,
              &as->arcstat_anon_size,
              &as->arcstat_anon_evictable_data,
              &as->arcstat_anon_evictable_metadata);
          arc_kstat_update_state(arc_mru,
              &as->arcstat_mru_size,
              &as->arcstat_mru_evictable_data,
              &as->arcstat_mru_evictable_metadata);
          arc_kstat_update_state(arc_mru_ghost,
              &as->arcstat_mru_ghost_size,
              &as->arcstat_mru_ghost_evictable_data,
              &as->arcstat_mru_ghost_evictable_metadata);
          arc_kstat_update_state(arc_mfu,
              &as->arcstat_mfu_size,
              &as->arcstat_mfu_evictable_data,
              &as->arcstat_mfu_evictable_metadata);
          arc_kstat_update_state(arc_mfu_ghost,
              &as->arcstat_mfu_ghost_size,
              &as->arcstat_mfu_ghost_evictable_data,
              &as->arcstat_mfu_ghost_evictable_metadata);
          arc_kstat_update_state(arc_uncached,
              &as->arcstat_uncached_size,
              &as->arcstat_uncached_evictable_data,
              &as->arcstat_uncached_evictable_metadata);
  
          as->arcstat_dnode_size.value.ui64 =
              aggsum_value(&arc_sums.arcstat_dnode_size);
          as->arcstat_bonus_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_bonus_size);
          as->arcstat_l2_hits.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_hits);
          as->arcstat_l2_misses.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_misses);
          as->arcstat_l2_prefetch_asize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_prefetch_asize);
          as->arcstat_l2_mru_asize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_mru_asize);
          as->arcstat_l2_mfu_asize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_mfu_asize);
          as->arcstat_l2_bufc_data_asize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_bufc_data_asize);
          as->arcstat_l2_bufc_metadata_asize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_bufc_metadata_asize);
          as->arcstat_l2_feeds.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_feeds);
          as->arcstat_l2_rw_clash.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rw_clash);
          as->arcstat_l2_read_bytes.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_read_bytes);
          as->arcstat_l2_write_bytes.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_write_bytes);
          as->arcstat_l2_writes_sent.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_writes_sent);
          as->arcstat_l2_writes_done.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_writes_done);
          as->arcstat_l2_writes_error.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_writes_error);
          as->arcstat_l2_writes_lock_retry.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_writes_lock_retry);
          as->arcstat_l2_evict_lock_retry.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_evict_lock_retry);
          as->arcstat_l2_evict_reading.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_evict_reading);
          as->arcstat_l2_evict_l1cached.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_evict_l1cached);
          as->arcstat_l2_free_on_write.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_free_on_write);
          as->arcstat_l2_abort_lowmem.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_abort_lowmem);
          as->arcstat_l2_cksum_bad.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_cksum_bad);
          as->arcstat_l2_io_error.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_io_error);
          as->arcstat_l2_lsize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_lsize);
          as->arcstat_l2_psize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_psize);
          as->arcstat_l2_hdr_size.value.ui64 =
              aggsum_value(&arc_sums.arcstat_l2_hdr_size);
          as->arcstat_l2_log_blk_writes.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_log_blk_writes);
          as->arcstat_l2_log_blk_asize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_log_blk_asize);
          as->arcstat_l2_log_blk_count.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_log_blk_count);
          as->arcstat_l2_rebuild_success.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_success);
          as->arcstat_l2_rebuild_abort_unsupported.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_abort_unsupported);
          as->arcstat_l2_rebuild_abort_io_errors.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_abort_io_errors);
          as->arcstat_l2_rebuild_abort_dh_errors.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_abort_dh_errors);
          as->arcstat_l2_rebuild_abort_cksum_lb_errors.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_abort_cksum_lb_errors);
          as->arcstat_l2_rebuild_abort_lowmem.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_abort_lowmem);
          as->arcstat_l2_rebuild_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_size);
          as->arcstat_l2_rebuild_asize.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_asize);
          as->arcstat_l2_rebuild_bufs.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_bufs);
          as->arcstat_l2_rebuild_bufs_precached.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_bufs_precached);
          as->arcstat_l2_rebuild_log_blks.value.ui64 =
              wmsum_value(&arc_sums.arcstat_l2_rebuild_log_blks);
          as->arcstat_memory_throttle_count.value.ui64 =
              wmsum_value(&arc_sums.arcstat_memory_throttle_count);
          as->arcstat_memory_direct_count.value.ui64 =
              wmsum_value(&arc_sums.arcstat_memory_direct_count);
          as->arcstat_memory_indirect_count.value.ui64 =
              wmsum_value(&arc_sums.arcstat_memory_indirect_count);
  
          as->arcstat_memory_all_bytes.value.ui64 =
              arc_all_memory();
          as->arcstat_memory_free_bytes.value.ui64 =
              arc_free_memory();
          as->arcstat_memory_available_bytes.value.i64 =
              arc_available_memory();
  
          as->arcstat_prune.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prune);
          as->arcstat_meta_used.value.ui64 =
              aggsum_value(&arc_sums.arcstat_meta_used);
          as->arcstat_async_upgrade_sync.value.ui64 =
              wmsum_value(&arc_sums.arcstat_async_upgrade_sync);
          as->arcstat_predictive_prefetch.value.ui64 =
              wmsum_value(&arc_sums.arcstat_predictive_prefetch);
          as->arcstat_demand_hit_predictive_prefetch.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_hit_predictive_prefetch);
          as->arcstat_demand_iohit_predictive_prefetch.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_iohit_predictive_prefetch);
          as->arcstat_prescient_prefetch.value.ui64 =
              wmsum_value(&arc_sums.arcstat_prescient_prefetch);
          as->arcstat_demand_hit_prescient_prefetch.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_hit_prescient_prefetch);
          as->arcstat_demand_iohit_prescient_prefetch.value.ui64 =
              wmsum_value(&arc_sums.arcstat_demand_iohit_prescient_prefetch);
          as->arcstat_raw_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_raw_size);
          as->arcstat_cached_only_in_progress.value.ui64 =
              wmsum_value(&arc_sums.arcstat_cached_only_in_progress);
          as->arcstat_abd_chunk_waste_size.value.ui64 =
              wmsum_value(&arc_sums.arcstat_abd_chunk_waste_size);
  
          return (0);
  }
  
  /*
   * This function *must* return indices evenly distributed between all
   * sublists of the multilist. This is needed due to how the ARC eviction
   * code is laid out; arc_evict_state() assumes ARC buffers are evenly
   * distributed between all sublists and uses this assumption when
   * deciding which sublist to evict from and how much to evict from it.
   */
  static unsigned int
  arc_state_multilist_index_func(multilist_t *ml, void *obj)
  {
          arc_buf_hdr_t *hdr = obj;
  
          /*
           * We rely on b_dva to generate evenly distributed index
           * numbers using buf_hash below. So, as an added precaution,
           * let's make sure we never add empty buffers to the arc lists.
           */
          ASSERT(!HDR_EMPTY(hdr));
  
          /*
           * The assumption here, is the hash value for a given
           * arc_buf_hdr_t will remain constant throughout its lifetime
           * (i.e. its b_spa, b_dva, and b_birth fields don't change).
           * Thus, we don't need to store the header's sublist index
           * on insertion, as this index can be recalculated on removal.
           *
           * Also, the low order bits of the hash value are thought to be
           * distributed evenly. Otherwise, in the case that the multilist
           * has a power of two number of sublists, each sublists' usage
           * would not be evenly distributed. In this context full 64bit
           * division would be a waste of time, so limit it to 32 bits.
           */
          return ((unsigned int)buf_hash(hdr->b_spa, &hdr->b_dva, hdr->b_birth) %
              multilist_get_num_sublists(ml));
  }
  
  static unsigned int
  arc_state_l2c_multilist_index_func(multilist_t *ml, void *obj)
  {
          panic("Header %p insert into arc_l2c_only %p", obj, ml);
  }
  
  #define WARN_IF_TUNING_IGNORED(tuning, value, do_warn) do {     \
          if ((do_warn) && (tuning) && ((tuning) != (value))) {   \
                  cmn_err(CE_WARN,                                \
                      "ignoring tunable %s (using %llu instead)", \
                      (#tuning), (u_longlong_t)(value));  \
          }                                                       \
  } while (0)
  
  /*
   * Called during module initialization and periodically thereafter to
   * apply reasonable changes to the exposed performance tunings.  Can also be
   * called explicitly by param_set_arc_*() functions when ARC tunables are
   * updated manually.  Non-zero zfs_* values which differ from the currently set
   * values will be applied.
   */
  void
  arc_tuning_update(boolean_t verbose)
  {
          uint64_t allmem = arc_all_memory();
          unsigned long limit;
  
          /* Valid range: 32M - <arc_c_max> */
          if ((zfs_arc_min) && (zfs_arc_min != arc_c_min) &&
              (zfs_arc_min >= 2ULL << SPA_MAXBLOCKSHIFT) &&
              (zfs_arc_min <= arc_c_max)) {
                  arc_c_min = zfs_arc_min;
                  arc_c = MAX(arc_c, arc_c_min);
          }
          WARN_IF_TUNING_IGNORED(zfs_arc_min, arc_c_min, verbose);
  
          /* Valid range: 64M - <all physical memory> */
          if ((zfs_arc_max) && (zfs_arc_max != arc_c_max) &&
              (zfs_arc_max >= MIN_ARC_MAX) && (zfs_arc_max < allmem) &&
              (zfs_arc_max > arc_c_min)) {
                  arc_c_max = zfs_arc_max;
                  arc_c = MIN(arc_c, arc_c_max);
                  arc_p = (arc_c >> 1);
                  if (arc_meta_limit > arc_c_max)
                          arc_meta_limit = arc_c_max;
                  if (arc_dnode_size_limit > arc_meta_limit)
                          arc_dnode_size_limit = arc_meta_limit;
          }
          WARN_IF_TUNING_IGNORED(zfs_arc_max, arc_c_max, verbose);
  
          /* Valid range: 16M - <arc_c_max> */
          if ((zfs_arc_meta_min) && (zfs_arc_meta_min != arc_meta_min) &&
              (zfs_arc_meta_min >= 1ULL << SPA_MAXBLOCKSHIFT) &&
              (zfs_arc_meta_min <= arc_c_max)) {
                  arc_meta_min = zfs_arc_meta_min;
                  if (arc_meta_limit < arc_meta_min)
                          arc_meta_limit = arc_meta_min;
                  if (arc_dnode_size_limit < arc_meta_min)
                          arc_dnode_size_limit = arc_meta_min;
          }
          WARN_IF_TUNING_IGNORED(zfs_arc_meta_min, arc_meta_min, verbose);
  
          /* Valid range: <arc_meta_min> - <arc_c_max> */
          limit = zfs_arc_meta_limit ? zfs_arc_meta_limit :
              MIN(zfs_arc_meta_limit_percent, 100) * arc_c_max / 100;
          if ((limit != arc_meta_limit) &&
              (limit >= arc_meta_min) &&
              (limit <= arc_c_max))
                  arc_meta_limit = limit;
          WARN_IF_TUNING_IGNORED(zfs_arc_meta_limit, arc_meta_limit, verbose);
  
          /* Valid range: <arc_meta_min> - <arc_meta_limit> */
          limit = zfs_arc_dnode_limit ? zfs_arc_dnode_limit :
              MIN(zfs_arc_dnode_limit_percent, 100) * arc_meta_limit / 100;
          if ((limit != arc_dnode_size_limit) &&
              (limit >= arc_meta_min) &&
              (limit <= arc_meta_limit))
                  arc_dnode_size_limit = limit;
          WARN_IF_TUNING_IGNORED(zfs_arc_dnode_limit, arc_dnode_size_limit,
              verbose);
  
          /* Valid range: 1 - N */
          if (zfs_arc_grow_retry)
                  arc_grow_retry = zfs_arc_grow_retry;
  
          /* Valid range: 1 - N */
          if (zfs_arc_shrink_shift) {
                  arc_shrink_shift = zfs_arc_shrink_shift;
                  arc_no_grow_shift = MIN(arc_no_grow_shift, arc_shrink_shift -1);
          }
  
          /* Valid range: 1 - N */
          if (zfs_arc_p_min_shift)
                  arc_p_min_shift = zfs_arc_p_min_shift;
  
          /* Valid range: 1 - N ms */
          if (zfs_arc_min_prefetch_ms)
                  arc_min_prefetch_ms = zfs_arc_min_prefetch_ms;
  
          /* Valid range: 1 - N ms */
          if (zfs_arc_min_prescient_prefetch_ms) {
                  arc_min_prescient_prefetch_ms =
                      zfs_arc_min_prescient_prefetch_ms;
          }
  
          /* Valid range: 0 - 100 */
          if (zfs_arc_lotsfree_percent <= 100)
                  arc_lotsfree_percent = zfs_arc_lotsfree_percent;
          WARN_IF_TUNING_IGNORED(zfs_arc_lotsfree_percent, arc_lotsfree_percent,
              verbose);
  
          /* Valid range: 0 - <all physical memory> */
          if ((zfs_arc_sys_free) && (zfs_arc_sys_free != arc_sys_free))
                  arc_sys_free = MIN(zfs_arc_sys_free, allmem);
          WARN_IF_TUNING_IGNORED(zfs_arc_sys_free, arc_sys_free, verbose);
  }
  
  static void
  arc_state_multilist_init(multilist_t *ml,
      multilist_sublist_index_func_t *index_func, int *maxcountp)
  {
          multilist_create(ml, sizeof (arc_buf_hdr_t),
              offsetof(arc_buf_hdr_t, b_l1hdr.b_arc_node), index_func);
          *maxcountp = MAX(*maxcountp, multilist_get_num_sublists(ml));
  }
  
  static void
  arc_state_init(void)
  {
          int num_sublists = 0;
  
          arc_state_multilist_init(&arc_mru->arcs_list[ARC_BUFC_METADATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_mru->arcs_list[ARC_BUFC_DATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_mru_ghost->arcs_list[ARC_BUFC_METADATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_mru_ghost->arcs_list[ARC_BUFC_DATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_mfu->arcs_list[ARC_BUFC_METADATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_mfu->arcs_list[ARC_BUFC_DATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_mfu_ghost->arcs_list[ARC_BUFC_DATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_uncached->arcs_list[ARC_BUFC_METADATA],
              arc_state_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_uncached->arcs_list[ARC_BUFC_DATA],
              arc_state_multilist_index_func, &num_sublists);
  
          /*
           * L2 headers should never be on the L2 state list since they don't
           * have L1 headers allocated.  Special index function asserts that.
           */
          arc_state_multilist_init(&arc_l2c_only->arcs_list[ARC_BUFC_METADATA],
              arc_state_l2c_multilist_index_func, &num_sublists);
          arc_state_multilist_init(&arc_l2c_only->arcs_list[ARC_BUFC_DATA],
              arc_state_l2c_multilist_index_func, &num_sublists);
  
          /*
           * Keep track of the number of markers needed to reclaim buffers from
           * any ARC state.  The markers will be pre-allocated so as to minimize
           * the number of memory allocations performed by the eviction thread.
           */
          arc_state_evict_marker_count = num_sublists;
  
          zfs_refcount_create(&arc_anon->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_create(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_create(&arc_mru->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_create(&arc_mru->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_create(&arc_mru_ghost->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_create(&arc_mru_ghost->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_create(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_create(&arc_mfu->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_create(&arc_mfu_ghost->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_create(&arc_mfu_ghost->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_create(&arc_l2c_only->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_create(&arc_l2c_only->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_create(&arc_uncached->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_create(&arc_uncached->arcs_esize[ARC_BUFC_DATA]);
  
          zfs_refcount_create(&arc_anon->arcs_size);
          zfs_refcount_create(&arc_mru->arcs_size);
          zfs_refcount_create(&arc_mru_ghost->arcs_size);
          zfs_refcount_create(&arc_mfu->arcs_size);
          zfs_refcount_create(&arc_mfu_ghost->arcs_size);
          zfs_refcount_create(&arc_l2c_only->arcs_size);
          zfs_refcount_create(&arc_uncached->arcs_size);
  
          wmsum_init(&arc_sums.arcstat_hits, 0);
          wmsum_init(&arc_sums.arcstat_iohits, 0);
          wmsum_init(&arc_sums.arcstat_misses, 0);
          wmsum_init(&arc_sums.arcstat_demand_data_hits, 0);
          wmsum_init(&arc_sums.arcstat_demand_data_iohits, 0);
          wmsum_init(&arc_sums.arcstat_demand_data_misses, 0);
          wmsum_init(&arc_sums.arcstat_demand_metadata_hits, 0);
          wmsum_init(&arc_sums.arcstat_demand_metadata_iohits, 0);
          wmsum_init(&arc_sums.arcstat_demand_metadata_misses, 0);
          wmsum_init(&arc_sums.arcstat_prefetch_data_hits, 0);
          wmsum_init(&arc_sums.arcstat_prefetch_data_iohits, 0);
          wmsum_init(&arc_sums.arcstat_prefetch_data_misses, 0);
          wmsum_init(&arc_sums.arcstat_prefetch_metadata_hits, 0);
          wmsum_init(&arc_sums.arcstat_prefetch_metadata_iohits, 0);
          wmsum_init(&arc_sums.arcstat_prefetch_metadata_misses, 0);
          wmsum_init(&arc_sums.arcstat_mru_hits, 0);
          wmsum_init(&arc_sums.arcstat_mru_ghost_hits, 0);
          wmsum_init(&arc_sums.arcstat_mfu_hits, 0);
          wmsum_init(&arc_sums.arcstat_mfu_ghost_hits, 0);
          wmsum_init(&arc_sums.arcstat_uncached_hits, 0);
          wmsum_init(&arc_sums.arcstat_deleted, 0);
          wmsum_init(&arc_sums.arcstat_mutex_miss, 0);
          wmsum_init(&arc_sums.arcstat_access_skip, 0);
          wmsum_init(&arc_sums.arcstat_evict_skip, 0);
          wmsum_init(&arc_sums.arcstat_evict_not_enough, 0);
          wmsum_init(&arc_sums.arcstat_evict_l2_cached, 0);
          wmsum_init(&arc_sums.arcstat_evict_l2_eligible, 0);
          wmsum_init(&arc_sums.arcstat_evict_l2_eligible_mfu, 0);
          wmsum_init(&arc_sums.arcstat_evict_l2_eligible_mru, 0);
          wmsum_init(&arc_sums.arcstat_evict_l2_ineligible, 0);
          wmsum_init(&arc_sums.arcstat_evict_l2_skip, 0);
          wmsum_init(&arc_sums.arcstat_hash_collisions, 0);
          wmsum_init(&arc_sums.arcstat_hash_chains, 0);
          aggsum_init(&arc_sums.arcstat_size, 0);
          wmsum_init(&arc_sums.arcstat_compressed_size, 0);
          wmsum_init(&arc_sums.arcstat_uncompressed_size, 0);
          wmsum_init(&arc_sums.arcstat_overhead_size, 0);
          wmsum_init(&arc_sums.arcstat_hdr_size, 0);
          wmsum_init(&arc_sums.arcstat_data_size, 0);
          wmsum_init(&arc_sums.arcstat_metadata_size, 0);
          wmsum_init(&arc_sums.arcstat_dbuf_size, 0);
          aggsum_init(&arc_sums.arcstat_dnode_size, 0);
          wmsum_init(&arc_sums.arcstat_bonus_size, 0);
          wmsum_init(&arc_sums.arcstat_l2_hits, 0);
          wmsum_init(&arc_sums.arcstat_l2_misses, 0);
          wmsum_init(&arc_sums.arcstat_l2_prefetch_asize, 0);
          wmsum_init(&arc_sums.arcstat_l2_mru_asize, 0);
          wmsum_init(&arc_sums.arcstat_l2_mfu_asize, 0);
          wmsum_init(&arc_sums.arcstat_l2_bufc_data_asize, 0);
          wmsum_init(&arc_sums.arcstat_l2_bufc_metadata_asize, 0);
          wmsum_init(&arc_sums.arcstat_l2_feeds, 0);
          wmsum_init(&arc_sums.arcstat_l2_rw_clash, 0);
          wmsum_init(&arc_sums.arcstat_l2_read_bytes, 0);
          wmsum_init(&arc_sums.arcstat_l2_write_bytes, 0);
          wmsum_init(&arc_sums.arcstat_l2_writes_sent, 0);
          wmsum_init(&arc_sums.arcstat_l2_writes_done, 0);
          wmsum_init(&arc_sums.arcstat_l2_writes_error, 0);
          wmsum_init(&arc_sums.arcstat_l2_writes_lock_retry, 0);
          wmsum_init(&arc_sums.arcstat_l2_evict_lock_retry, 0);
          wmsum_init(&arc_sums.arcstat_l2_evict_reading, 0);
          wmsum_init(&arc_sums.arcstat_l2_evict_l1cached, 0);
          wmsum_init(&arc_sums.arcstat_l2_free_on_write, 0);
          wmsum_init(&arc_sums.arcstat_l2_abort_lowmem, 0);
          wmsum_init(&arc_sums.arcstat_l2_cksum_bad, 0);
          wmsum_init(&arc_sums.arcstat_l2_io_error, 0);
          wmsum_init(&arc_sums.arcstat_l2_lsize, 0);
          wmsum_init(&arc_sums.arcstat_l2_psize, 0);
          aggsum_init(&arc_sums.arcstat_l2_hdr_size, 0);
          wmsum_init(&arc_sums.arcstat_l2_log_blk_writes, 0);
          wmsum_init(&arc_sums.arcstat_l2_log_blk_asize, 0);
          wmsum_init(&arc_sums.arcstat_l2_log_blk_count, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_success, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_abort_unsupported, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_abort_io_errors, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_abort_dh_errors, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_abort_cksum_lb_errors, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_abort_lowmem, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_size, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_asize, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_bufs, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_bufs_precached, 0);
          wmsum_init(&arc_sums.arcstat_l2_rebuild_log_blks, 0);
          wmsum_init(&arc_sums.arcstat_memory_throttle_count, 0);
          wmsum_init(&arc_sums.arcstat_memory_direct_count, 0);
          wmsum_init(&arc_sums.arcstat_memory_indirect_count, 0);
          wmsum_init(&arc_sums.arcstat_prune, 0);
          aggsum_init(&arc_sums.arcstat_meta_used, 0);
          wmsum_init(&arc_sums.arcstat_async_upgrade_sync, 0);
          wmsum_init(&arc_sums.arcstat_predictive_prefetch, 0);
          wmsum_init(&arc_sums.arcstat_demand_hit_predictive_prefetch, 0);
          wmsum_init(&arc_sums.arcstat_demand_iohit_predictive_prefetch, 0);
          wmsum_init(&arc_sums.arcstat_prescient_prefetch, 0);
          wmsum_init(&arc_sums.arcstat_demand_hit_prescient_prefetch, 0);
          wmsum_init(&arc_sums.arcstat_demand_iohit_prescient_prefetch, 0);
          wmsum_init(&arc_sums.arcstat_raw_size, 0);
          wmsum_init(&arc_sums.arcstat_cached_only_in_progress, 0);
          wmsum_init(&arc_sums.arcstat_abd_chunk_waste_size, 0);
  
          arc_anon->arcs_state = ARC_STATE_ANON;
          arc_mru->arcs_state = ARC_STATE_MRU;
          arc_mru_ghost->arcs_state = ARC_STATE_MRU_GHOST;
          arc_mfu->arcs_state = ARC_STATE_MFU;
          arc_mfu_ghost->arcs_state = ARC_STATE_MFU_GHOST;
          arc_l2c_only->arcs_state = ARC_STATE_L2C_ONLY;
          arc_uncached->arcs_state = ARC_STATE_UNCACHED;
  }
  
  static void
  arc_state_fini(void)
  {
          zfs_refcount_destroy(&arc_anon->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_destroy(&arc_anon->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_destroy(&arc_mru->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_destroy(&arc_mru->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_destroy(&arc_mru_ghost->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_destroy(&arc_mru_ghost->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_destroy(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_destroy(&arc_mfu->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_destroy(&arc_mfu_ghost->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_destroy(&arc_mfu_ghost->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_destroy(&arc_l2c_only->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_destroy(&arc_l2c_only->arcs_esize[ARC_BUFC_DATA]);
          zfs_refcount_destroy(&arc_uncached->arcs_esize[ARC_BUFC_METADATA]);
          zfs_refcount_destroy(&arc_uncached->arcs_esize[ARC_BUFC_DATA]);
  
          zfs_refcount_destroy(&arc_anon->arcs_size);
          zfs_refcount_destroy(&arc_mru->arcs_size);
          zfs_refcount_destroy(&arc_mru_ghost->arcs_size);
          zfs_refcount_destroy(&arc_mfu->arcs_size);
          zfs_refcount_destroy(&arc_mfu_ghost->arcs_size);
          zfs_refcount_destroy(&arc_l2c_only->arcs_size);
          zfs_refcount_destroy(&arc_uncached->arcs_size);
  
          multilist_destroy(&arc_mru->arcs_list[ARC_BUFC_METADATA]);
          multilist_destroy(&arc_mru_ghost->arcs_list[ARC_BUFC_METADATA]);
          multilist_destroy(&arc_mfu->arcs_list[ARC_BUFC_METADATA]);
          multilist_destroy(&arc_mfu_ghost->arcs_list[ARC_BUFC_METADATA]);
          multilist_destroy(&arc_mru->arcs_list[ARC_BUFC_DATA]);
          multilist_destroy(&arc_mru_ghost->arcs_list[ARC_BUFC_DATA]);
          multilist_destroy(&arc_mfu->arcs_list[ARC_BUFC_DATA]);
          multilist_destroy(&arc_mfu_ghost->arcs_list[ARC_BUFC_DATA]);
          multilist_destroy(&arc_l2c_only->arcs_list[ARC_BUFC_METADATA]);
          multilist_destroy(&arc_l2c_only->arcs_list[ARC_BUFC_DATA]);
          multilist_destroy(&arc_uncached->arcs_list[ARC_BUFC_METADATA]);
          multilist_destroy(&arc_uncached->arcs_list[ARC_BUFC_DATA]);
  
          wmsum_fini(&arc_sums.arcstat_hits);
          wmsum_fini(&arc_sums.arcstat_iohits);
          wmsum_fini(&arc_sums.arcstat_misses);
          wmsum_fini(&arc_sums.arcstat_demand_data_hits);
          wmsum_fini(&arc_sums.arcstat_demand_data_iohits);
          wmsum_fini(&arc_sums.arcstat_demand_data_misses);
          wmsum_fini(&arc_sums.arcstat_demand_metadata_hits);
          wmsum_fini(&arc_sums.arcstat_demand_metadata_iohits);
          wmsum_fini(&arc_sums.arcstat_demand_metadata_misses);
          wmsum_fini(&arc_sums.arcstat_prefetch_data_hits);
          wmsum_fini(&arc_sums.arcstat_prefetch_data_iohits);
          wmsum_fini(&arc_sums.arcstat_prefetch_data_misses);
          wmsum_fini(&arc_sums.arcstat_prefetch_metadata_hits);
          wmsum_fini(&arc_sums.arcstat_prefetch_metadata_iohits);
          wmsum_fini(&arc_sums.arcstat_prefetch_metadata_misses);
          wmsum_fini(&arc_sums.arcstat_mru_hits);
          wmsum_fini(&arc_sums.arcstat_mru_ghost_hits);
          wmsum_fini(&arc_sums.arcstat_mfu_hits);
          wmsum_fini(&arc_sums.arcstat_mfu_ghost_hits);
          wmsum_fini(&arc_sums.arcstat_uncached_hits);
          wmsum_fini(&arc_sums.arcstat_deleted);
          wmsum_fini(&arc_sums.arcstat_mutex_miss);
          wmsum_fini(&arc_sums.arcstat_access_skip);
          wmsum_fini(&arc_sums.arcstat_evict_skip);
          wmsum_fini(&arc_sums.arcstat_evict_not_enough);
          wmsum_fini(&arc_sums.arcstat_evict_l2_cached);
          wmsum_fini(&arc_sums.arcstat_evict_l2_eligible);
          wmsum_fini(&arc_sums.arcstat_evict_l2_eligible_mfu);
          wmsum_fini(&arc_sums.arcstat_evict_l2_eligible_mru);
          wmsum_fini(&arc_sums.arcstat_evict_l2_ineligible);
          wmsum_fini(&arc_sums.arcstat_evict_l2_skip);
          wmsum_fini(&arc_sums.arcstat_hash_collisions);
          wmsum_fini(&arc_sums.arcstat_hash_chains);
          aggsum_fini(&arc_sums.arcstat_size);
          wmsum_fini(&arc_sums.arcstat_compressed_size);
          wmsum_fini(&arc_sums.arcstat_uncompressed_size);
          wmsum_fini(&arc_sums.arcstat_overhead_size);
          wmsum_fini(&arc_sums.arcstat_hdr_size);
          wmsum_fini(&arc_sums.arcstat_data_size);
          wmsum_fini(&arc_sums.arcstat_metadata_size);
          wmsum_fini(&arc_sums.arcstat_dbuf_size);
          aggsum_fini(&arc_sums.arcstat_dnode_size);
          wmsum_fini(&arc_sums.arcstat_bonus_size);
          wmsum_fini(&arc_sums.arcstat_l2_hits);
          wmsum_fini(&arc_sums.arcstat_l2_misses);
          wmsum_fini(&arc_sums.arcstat_l2_prefetch_asize);
          wmsum_fini(&arc_sums.arcstat_l2_mru_asize);
          wmsum_fini(&arc_sums.arcstat_l2_mfu_asize);
          wmsum_fini(&arc_sums.arcstat_l2_bufc_data_asize);
          wmsum_fini(&arc_sums.arcstat_l2_bufc_metadata_asize);
          wmsum_fini(&arc_sums.arcstat_l2_feeds);
          wmsum_fini(&arc_sums.arcstat_l2_rw_clash);
          wmsum_fini(&arc_sums.arcstat_l2_read_bytes);
          wmsum_fini(&arc_sums.arcstat_l2_write_bytes);
          wmsum_fini(&arc_sums.arcstat_l2_writes_sent);
          wmsum_fini(&arc_sums.arcstat_l2_writes_done);
          wmsum_fini(&arc_sums.arcstat_l2_writes_error);
          wmsum_fini(&arc_sums.arcstat_l2_writes_lock_retry);
          wmsum_fini(&arc_sums.arcstat_l2_evict_lock_retry);
          wmsum_fini(&arc_sums.arcstat_l2_evict_reading);
          wmsum_fini(&arc_sums.arcstat_l2_evict_l1cached);
          wmsum_fini(&arc_sums.arcstat_l2_free_on_write);
          wmsum_fini(&arc_sums.arcstat_l2_abort_lowmem);
          wmsum_fini(&arc_sums.arcstat_l2_cksum_bad);
          wmsum_fini(&arc_sums.arcstat_l2_io_error);
          wmsum_fini(&arc_sums.arcstat_l2_lsize);
          wmsum_fini(&arc_sums.arcstat_l2_psize);
          aggsum_fini(&arc_sums.arcstat_l2_hdr_size);
          wmsum_fini(&arc_sums.arcstat_l2_log_blk_writes);
          wmsum_fini(&arc_sums.arcstat_l2_log_blk_asize);
          wmsum_fini(&arc_sums.arcstat_l2_log_blk_count);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_success);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_abort_unsupported);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_abort_io_errors);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_abort_dh_errors);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_abort_cksum_lb_errors);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_abort_lowmem);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_size);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_asize);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_bufs);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_bufs_precached);
          wmsum_fini(&arc_sums.arcstat_l2_rebuild_log_blks);
          wmsum_fini(&arc_sums.arcstat_memory_throttle_count);
          wmsum_fini(&arc_sums.arcstat_memory_direct_count);
          wmsum_fini(&arc_sums.arcstat_memory_indirect_count);
          wmsum_fini(&arc_sums.arcstat_prune);
          aggsum_fini(&arc_sums.arcstat_meta_used);
          wmsum_fini(&arc_sums.arcstat_async_upgrade_sync);
          wmsum_fini(&arc_sums.arcstat_predictive_prefetch);
          wmsum_fini(&arc_sums.arcstat_demand_hit_predictive_prefetch);
          wmsum_fini(&arc_sums.arcstat_demand_iohit_predictive_prefetch);
          wmsum_fini(&arc_sums.arcstat_prescient_prefetch);
          wmsum_fini(&arc_sums.arcstat_demand_hit_prescient_prefetch);
          wmsum_fini(&arc_sums.arcstat_demand_iohit_prescient_prefetch);
          wmsum_fini(&arc_sums.arcstat_raw_size);
          wmsum_fini(&arc_sums.arcstat_cached_only_in_progress);
          wmsum_fini(&arc_sums.arcstat_abd_chunk_waste_size);
  }
  
  uint64_t
  arc_target_bytes(void)
  {
          return (arc_c);
  }
  
  void
  arc_set_limits(uint64_t allmem)
  {
          /* Set min cache to 1/32 of all memory, or 32MB, whichever is more. */
          arc_c_min = MAX(allmem / 32, 2ULL << SPA_MAXBLOCKSHIFT);
  
          /* How to set default max varies by platform. */
          arc_c_max = arc_default_max(arc_c_min, allmem);
  }
  void
  arc_init(void)
  {
          uint64_t percent, allmem = arc_all_memory();
          mutex_init(&arc_evict_lock, NULL, MUTEX_DEFAULT, NULL);
          list_create(&arc_evict_waiters, sizeof (arc_evict_waiter_t),
              offsetof(arc_evict_waiter_t, aew_node));
  
          arc_min_prefetch_ms = 1000;
          arc_min_prescient_prefetch_ms = 6000;
  
  #if defined(_KERNEL)
          arc_lowmem_init();
  #endif
  
          arc_set_limits(allmem);
  
  #ifdef _KERNEL
          /*
           * If zfs_arc_max is non-zero at init, meaning it was set in the kernel
           * environment before the module was loaded, don't block setting the
           * maximum because it is less than arc_c_min, instead, reset arc_c_min
           * to a lower value.
           * zfs_arc_min will be handled by arc_tuning_update().
           */
          if (zfs_arc_max != 0 && zfs_arc_max >= MIN_ARC_MAX &&
              zfs_arc_max < allmem) {
                  arc_c_max = zfs_arc_max;
                  if (arc_c_min >= arc_c_max) {
                          arc_c_min = MAX(zfs_arc_max / 2,
                              2ULL << SPA_MAXBLOCKSHIFT);
                  }
          }
  #else
          /*
           * In userland, there's only the memory pressure that we artificially
           * create (see arc_available_memory()).  Don't let arc_c get too
           * small, because it can cause transactions to be larger than
           * arc_c, causing arc_tempreserve_space() to fail.
           */
          arc_c_min = MAX(arc_c_max / 2, 2ULL << SPA_MAXBLOCKSHIFT);
  #endif
  
          arc_c = arc_c_min;
          arc_p = (arc_c >> 1);
  
          /* Set min to 1/2 of arc_c_min */
          arc_meta_min = 1ULL << SPA_MAXBLOCKSHIFT;
          /*
           * Set arc_meta_limit to a percent of arc_c_max with a floor of
           * arc_meta_min, and a ceiling of arc_c_max.
           */
          percent = MIN(zfs_arc_meta_limit_percent, 100);
          arc_meta_limit = MAX(arc_meta_min, (percent * arc_c_max) / 100);
          percent = MIN(zfs_arc_dnode_limit_percent, 100);
          arc_dnode_size_limit = (percent * arc_meta_limit) / 100;
  
          /* Apply user specified tunings */
          arc_tuning_update(B_TRUE);
  
          /* if kmem_flags are set, lets try to use less memory */
          if (kmem_debugging())
                  arc_c = arc_c / 2;
          if (arc_c < arc_c_min)
                  arc_c = arc_c_min;
  
          arc_register_hotplug();
  
          arc_state_init();
  
          buf_init();
  
          list_create(&arc_prune_list, sizeof (arc_prune_t),
              offsetof(arc_prune_t, p_node));
          mutex_init(&arc_prune_mtx, NULL, MUTEX_DEFAULT, NULL);
  
          arc_prune_taskq = taskq_create("arc_prune", zfs_arc_prune_task_threads,
              defclsyspri, 100, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
  
          arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
              sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
  
          if (arc_ksp != NULL) {
                  arc_ksp->ks_data = &arc_stats;
                  arc_ksp->ks_update = arc_kstat_update;
                  kstat_install(arc_ksp);
          }
  
          arc_state_evict_markers =
              arc_state_alloc_markers(arc_state_evict_marker_count);
          arc_evict_zthr = zthr_create_timer("arc_evict",
              arc_evict_cb_check, arc_evict_cb, NULL, SEC2NSEC(1), defclsyspri);
          arc_reap_zthr = zthr_create_timer("arc_reap",
              arc_reap_cb_check, arc_reap_cb, NULL, SEC2NSEC(1), minclsyspri);
  
          arc_warm = B_FALSE;
  
          /*
           * Calculate maximum amount of dirty data per pool.
           *
           * If it has been set by a module parameter, take that.
           * Otherwise, use a percentage of physical memory defined by
           * zfs_dirty_data_max_percent (default 10%) with a cap at
           * zfs_dirty_data_max_max (default 4G or 25% of physical memory).
           */
  #ifdef __LP64__
          if (zfs_dirty_data_max_max == 0)
                  zfs_dirty_data_max_max = MIN(4ULL * 1024 * 1024 * 1024,
                      allmem * zfs_dirty_data_max_max_percent / 100);
  #else
          if (zfs_dirty_data_max_max == 0)
                  zfs_dirty_data_max_max = MIN(1ULL * 1024 * 1024 * 1024,
                      allmem * zfs_dirty_data_max_max_percent / 100);
  #endif
  
          if (zfs_dirty_data_max == 0) {
                  zfs_dirty_data_max = allmem *
                      zfs_dirty_data_max_percent / 100;
                  zfs_dirty_data_max = MIN(zfs_dirty_data_max,
                      zfs_dirty_data_max_max);
          }
  
          if (zfs_wrlog_data_max == 0) {
  
                  /*
                   * dp_wrlog_total is reduced for each txg at the end of
                   * spa_sync(). However, dp_dirty_total is reduced every time
                   * a block is written out. Thus under normal operation,
                   * dp_wrlog_total could grow 2 times as big as
                   * zfs_dirty_data_max.
                   */
                  zfs_wrlog_data_max = zfs_dirty_data_max * 2;
          }
  }
  
  void
  arc_fini(void)
  {
          arc_prune_t *p;
  
  #ifdef _KERNEL
          arc_lowmem_fini();
  #endif /* _KERNEL */
  
          /* Use B_TRUE to ensure *all* buffers are evicted */
          arc_flush(NULL, B_TRUE);
  
          if (arc_ksp != NULL) {
                  kstat_delete(arc_ksp);
                  arc_ksp = NULL;
          }
  
          taskq_wait(arc_prune_taskq);
          taskq_destroy(arc_prune_taskq);
  
          mutex_enter(&arc_prune_mtx);
          while ((p = list_head(&arc_prune_list)) != NULL) {
                  list_remove(&arc_prune_list, p);
                  zfs_refcount_remove(&p->p_refcnt, &arc_prune_list);
                  zfs_refcount_destroy(&p->p_refcnt);
                  kmem_free(p, sizeof (*p));
          }
          mutex_exit(&arc_prune_mtx);
  
          list_destroy(&arc_prune_list);
          mutex_destroy(&arc_prune_mtx);
  
          (void) zthr_cancel(arc_evict_zthr);
          (void) zthr_cancel(arc_reap_zthr);
          arc_state_free_markers(arc_state_evict_markers,
              arc_state_evict_marker_count);
  
          mutex_destroy(&arc_evict_lock);
          list_destroy(&arc_evict_waiters);
  
          /*
           * Free any buffers that were tagged for destruction.  This needs
           * to occur before arc_state_fini() runs and destroys the aggsum
           * values which are updated when freeing scatter ABDs.
           */
          l2arc_do_free_on_write();
  
          /*
           * buf_fini() must proceed arc_state_fini() because buf_fin() may
           * trigger the release of kmem magazines, which can callback to
           * arc_space_return() which accesses aggsums freed in act_state_fini().
           */
          buf_fini();
          arc_state_fini();
  
          arc_unregister_hotplug();
  
          /*
           * We destroy the zthrs after all the ARC state has been
           * torn down to avoid the case of them receiving any
           * wakeup() signals after they are destroyed.
           */
          zthr_destroy(arc_evict_zthr);
          zthr_destroy(arc_reap_zthr);
  
          ASSERT0(arc_loaned_bytes);
  }
  
  /*
   * Level 2 ARC
   *
   * The level 2 ARC (L2ARC) is a cache layer in-between main memory and disk.
   * It uses dedicated storage devices to hold cached data, which are populated
   * using large infrequent writes.  The main role of this cache is to boost
   * the performance of random read workloads.  The intended L2ARC devices
   * include short-stroked disks, solid state disks, and other media with
   * substantially faster read latency than disk.
   *
   *                 +-----------------------+
   *                 |         ARC           |
   *                 +-----------------------+
   *                    |         ^     ^
   *                    |         |     |
   *      l2arc_feed_thread()    arc_read()
   *                    |         |     |
   *                    |  l2arc read   |
   *                    V         |     |
   *               +---------------+    |
   *               |     L2ARC     |    |
   *               +---------------+    |
   *                   |    ^           |
   *          l2arc_write() |           |
   *                   |    |           |
   *                   V    |           |
   *                 +-------+      +-------+
   *                 | vdev  |      | vdev  |
   *                 | cache |      | cache |
   *                 +-------+      +-------+
   *                 +=========+     .-----.
   *                 :  L2ARC  :    |-_____-|
   *                 : devices :    | Disks |
   *                 +=========+    `-_____-'
   *
   * Read requests are satisfied from the following sources, in order:
   *
   *      1) ARC
   *      2) vdev cache of L2ARC devices
   *      3) L2ARC devices
   *      4) vdev cache of disks
   *      5) disks
   *
   * Some L2ARC device types exhibit extremely slow write performance.
   * To accommodate for this there are some significant differences between
   * the L2ARC and traditional cache design:
   *
   * 1. There is no eviction path from the ARC to the L2ARC.  Evictions from
   * the ARC behave as usual, freeing buffers and placing headers on ghost
   * lists.  The ARC does not send buffers to the L2ARC during eviction as
   * this would add inflated write latencies for all ARC memory pressure.
   *
   * 2. The L2ARC attempts to cache data from the ARC before it is evicted.
   * It does this by periodically scanning buffers from the eviction-end of
   * the MFU and MRU ARC lists, copying them to the L2ARC devices if they are
   * not already there. It scans until a headroom of buffers is satisfied,
   * which itself is a buffer for ARC eviction. If a compressible buffer is
   * found during scanning and selected for writing to an L2ARC device, we
   * temporarily boost scanning headroom during the next scan cycle to make
   * sure we adapt to compression effects (which might significantly reduce
   * the data volume we write to L2ARC). The thread that does this is
   * l2arc_feed_thread(), illustrated below; example sizes are included to
   * provide a better sense of ratio than this diagram:
   *
   *             head -->                        tail
   *              +---------------------+----------+
   *      ARC_mfu |:::::#:::::::::::::::|o#o###o###|-->.   # already on L2ARC
   *              +---------------------+----------+   |   o L2ARC eligible
   *      ARC_mru |:#:::::::::::::::::::|#o#ooo####|-->|   : ARC buffer
   *              +---------------------+----------+   |
   *                   15.9 Gbytes      ^ 32 Mbytes    |
   *                                 headroom          |
   *                                            l2arc_feed_thread()
   *                                                   |
   *                       l2arc write hand <--[oooo]--'
   *                               |           8 Mbyte
   *                               |          write max
   *                               V
   *                +==============================+
   *      L2ARC dev |####|#|###|###|    |####| ... |
   *                +==============================+
   *                           32 Gbytes
   *
   * 3. If an ARC buffer is copied to the L2ARC but then hit instead of
   * evicted, then the L2ARC has cached a buffer much sooner than it probably
   * needed to, potentially wasting L2ARC device bandwidth and storage.  It is
   * safe to say that this is an uncommon case, since buffers at the end of
   * the ARC lists have moved there due to inactivity.
   *
   * 4. If the ARC evicts faster than the L2ARC can maintain a headroom,
   * then the L2ARC simply misses copying some buffers.  This serves as a
   * pressure valve to prevent heavy read workloads from both stalling the ARC
   * with waits and clogging the L2ARC with writes.  This also helps prevent
   * the potential for the L2ARC to churn if it attempts to cache content too
   * quickly, such as during backups of the entire pool.
   *
   * 5. After system boot and before the ARC has filled main memory, there are
   * no evictions from the ARC and so the tails of the ARC_mfu and ARC_mru
   * lists can remain mostly static.  Instead of searching from tail of these
   * lists as pictured, the l2arc_feed_thread() will search from the list heads
   * for eligible buffers, greatly increasing its chance of finding them.
   *
   * The L2ARC device write speed is also boosted during this time so that
   * the L2ARC warms up faster.  Since there have been no ARC evictions yet,
   * there are no L2ARC reads, and no fear of degrading read performance
   * through increased writes.
   *
   * 6. Writes to the L2ARC devices are grouped and sent in-sequence, so that
   * the vdev queue can aggregate them into larger and fewer writes.  Each
   * device is written to in a rotor fashion, sweeping writes through
   * available space then repeating.
   *
   * 7. The L2ARC does not store dirty content.  It never needs to flush
   * write buffers back to disk based storage.
   *
   * 8. If an ARC buffer is written (and dirtied) which also exists in the
   * L2ARC, the now stale L2ARC buffer is immediately dropped.
   *
   * The performance of the L2ARC can be tweaked by a number of tunables, which
   * may be necessary for different workloads:
   *
   *      l2arc_write_max         max write bytes per interval
   *      l2arc_write_boost       extra write bytes during device warmup
   *      l2arc_noprefetch        skip caching prefetched buffers
   *      l2arc_headroom          number of max device writes to precache
   *      l2arc_headroom_boost    when we find compressed buffers during ARC
   *                              scanning, we multiply headroom by this
   *                              percentage factor for the next scan cycle,
   *                              since more compressed buffers are likely to
   *                              be present
   *      l2arc_feed_secs         seconds between L2ARC writing
   *
   * Tunables may be removed or added as future performance improvements are
   * integrated, and also may become zpool properties.
   *
   * There are three key functions that control how the L2ARC warms up:
   *
   *      l2arc_write_eligible()  check if a buffer is eligible to cache
   *      l2arc_write_size()      calculate how much to write
   *      l2arc_write_interval()  calculate sleep delay between writes
   *
   * These three functions determine what to write, how much, and how quickly
   * to send writes.
   *
   * L2ARC persistence:
   *
   * When writing buffers to L2ARC, we periodically add some metadata to
   * make sure we can pick them up after reboot, thus dramatically reducing
   * the impact that any downtime has on the performance of storage systems
   * with large caches.
   *
   * The implementation works fairly simply by integrating the following two
   * modifications:
   *
   * *) When writing to the L2ARC, we occasionally write a "l2arc log block",
   *    which is an additional piece of metadata which describes what's been
   *    written. This allows us to rebuild the arc_buf_hdr_t structures of the
   *    main ARC buffers. There are 2 linked-lists of log blocks headed by
   *    dh_start_lbps[2]. We alternate which chain we append to, so they are
   *    time-wise and offset-wise interleaved, but that is an optimization rather
   *    than for correctness. The log block also includes a pointer to the
   *    previous block in its chain.
   *
   * *) We reserve SPA_MINBLOCKSIZE of space at the start of each L2ARC device
   *    for our header bookkeeping purposes. This contains a device header,
   *    which contains our top-level reference structures. We update it each
   *    time we write a new log block, so that we're able to locate it in the
   *    L2ARC device. If this write results in an inconsistent device header
   *    (e.g. due to power failure), we detect this by verifying the header's
   *    checksum and simply fail to reconstruct the L2ARC after reboot.
   *
   * Implementation diagram:
   *
   * +=== L2ARC device (not to scale) ======================================+
   * |       ___two newest log block pointers__.__________                  |
   * |      /                                   \dh_start_lbps[1]           |
   * |     /                                     \         \dh_start_lbps[0]|
   * |.___/__.                                    V         V               |
   * ||L2 dev|....|lb |bufs |lb |bufs |lb |bufs |lb |bufs |lb |---(empty)---|
   * ||   hdr|      ^         /^       /^        /         /                |
   * |+------+  ...--\-------/  \-----/--\------/         /                 |
   * |                \--------------/    \--------------/                  |
   * +======================================================================+
   *
   * As can be seen on the diagram, rather than using a simple linked list,
   * we use a pair of linked lists with alternating elements. This is a
   * performance enhancement due to the fact that we only find out the
   * address of the next log block access once the current block has been
   * completely read in. Obviously, this hurts performance, because we'd be
   * keeping the device's I/O queue at only a 1 operation deep, thus
   * incurring a large amount of I/O round-trip latency. Having two lists
   * allows us to fetch two log blocks ahead of where we are currently
   * rebuilding L2ARC buffers.
   *
   * On-device data structures:
   *
   * L2ARC device header: l2arc_dev_hdr_phys_t
   * L2ARC log block:     l2arc_log_blk_phys_t
   *
   * L2ARC reconstruction:
   *
   * When writing data, we simply write in the standard rotary fashion,
   * evicting buffers as we go and simply writing new data over them (writing
   * a new log block every now and then). This obviously means that once we
   * loop around the end of the device, we will start cutting into an already
   * committed log block (and its referenced data buffers), like so:
   *
   *    current write head__       __old tail
   *                        \     /
   *                        V    V
   * <--|bufs |lb |bufs |lb |    |bufs |lb |bufs |lb |-->
   *                         ^    ^^^^^^^^^___________________________________
   *                         |                                                \
   *                   <<nextwrite>> may overwrite this blk and/or its bufs --'
   *
   * When importing the pool, we detect this situation and use it to stop
   * our scanning process (see l2arc_rebuild).
   *
   * There is one significant caveat to consider when rebuilding ARC contents
   * from an L2ARC device: what about invalidated buffers? Given the above
   * construction, we cannot update blocks which we've already written to amend
   * them to remove buffers which were invalidated. Thus, during reconstruction,
   * we might be populating the cache with buffers for data that's not on the
   * main pool anymore, or may have been overwritten!
   *
   * As it turns out, this isn't a problem. Every arc_read request includes
   * both the DVA and, crucially, the birth TXG of the BP the caller is
   * looking for. So even if the cache were populated by completely rotten
   * blocks for data that had been long deleted and/or overwritten, we'll
   * never actually return bad data from the cache, since the DVA with the
   * birth TXG uniquely identify a block in space and time - once created,
   * a block is immutable on disk. The worst thing we have done is wasted
   * some time and memory at l2arc rebuild to reconstruct outdated ARC
   * entries that will get dropped from the l2arc as it is being updated
   * with new blocks.
   *
   * L2ARC buffers that have been evicted by l2arc_evict() ahead of the write
   * hand are not restored. This is done by saving the offset (in bytes)
   * l2arc_evict() has evicted to in the L2ARC device header and taking it
   * into account when restoring buffers.
   */
  
  static boolean_t
  l2arc_write_eligible(uint64_t spa_guid, arc_buf_hdr_t *hdr)
  {
          /*
           * A buffer is *not* eligible for the L2ARC if it:
           * 1. belongs to a different spa.
           * 2. is already cached on the L2ARC.
           * 3. has an I/O in progress (it may be an incomplete read).
           * 4. is flagged not eligible (zfs property).
           */
          if (hdr->b_spa != spa_guid || HDR_HAS_L2HDR(hdr) ||
              HDR_IO_IN_PROGRESS(hdr) || !HDR_L2CACHE(hdr))
                  return (B_FALSE);
  
          return (B_TRUE);
  }
  
  static uint64_t
  l2arc_write_size(l2arc_dev_t *dev)
  {
          uint64_t size, dev_size, tsize;
  
          /*
           * Make sure our globals have meaningful values in case the user
           * altered them.
           */
          size = l2arc_write_max;
          if (size == 0) {
                  cmn_err(CE_NOTE, "Bad value for l2arc_write_max, value must "
                      "be greater than zero, resetting it to the default (%d)",
                      L2ARC_WRITE_SIZE);
                  size = l2arc_write_max = L2ARC_WRITE_SIZE;
          }
  
          if (arc_warm == B_FALSE)
                  size += l2arc_write_boost;
  
          /*
           * Make sure the write size does not exceed the size of the cache
           * device. This is important in l2arc_evict(), otherwise infinite
           * iteration can occur.
           */
          dev_size = dev->l2ad_end - dev->l2ad_start;
          tsize = size + l2arc_log_blk_overhead(size, dev);
          if (dev->l2ad_vdev->vdev_has_trim && l2arc_trim_ahead > 0)
                  tsize += MAX(64 * 1024 * 1024,
                      (tsize * l2arc_trim_ahead) / 100);
  
          if (tsize >= dev_size) {
                  cmn_err(CE_NOTE, "l2arc_write_max or l2arc_write_boost "
                      "plus the overhead of log blocks (persistent L2ARC, "
                      "%llu bytes) exceeds the size of the cache device "
                      "(guid %llu), resetting them to the default (%d)",
                      (u_longlong_t)l2arc_log_blk_overhead(size, dev),
                      (u_longlong_t)dev->l2ad_vdev->vdev_guid, L2ARC_WRITE_SIZE);
                  size = l2arc_write_max = l2arc_write_boost = L2ARC_WRITE_SIZE;
  
                  if (arc_warm == B_FALSE)
                          size += l2arc_write_boost;
          }
  
          return (size);
  
  }
  
  static clock_t
  l2arc_write_interval(clock_t began, uint64_t wanted, uint64_t wrote)
  {
          clock_t interval, next, now;
  
          /*
           * If the ARC lists are busy, increase our write rate; if the
           * lists are stale, idle back.  This is achieved by checking
           * how much we previously wrote - if it was more than half of
           * what we wanted, schedule the next write much sooner.
           */
          if (l2arc_feed_again && wrote > (wanted / 2))
                  interval = (hz * l2arc_feed_min_ms) / 1000;
          else
                  interval = hz * l2arc_feed_secs;
  
          now = ddi_get_lbolt();
          next = MAX(now, MIN(now + interval, began + interval));
  
          return (next);
  }
  
  /*
   * Cycle through L2ARC devices.  This is how L2ARC load balances.
   * If a device is returned, this also returns holding the spa config lock.
   */
  static l2arc_dev_t *
  l2arc_dev_get_next(void)
  {
          l2arc_dev_t *first, *next = NULL;
  
          /*
           * Lock out the removal of spas (spa_namespace_lock), then removal
           * of cache devices (l2arc_dev_mtx).  Once a device has been selected,
           * both locks will be dropped and a spa config lock held instead.
           */
          mutex_enter(&spa_namespace_lock);
          mutex_enter(&l2arc_dev_mtx);
  
          /* if there are no vdevs, there is nothing to do */
          if (l2arc_ndev == 0)
                  goto out;
  
          first = NULL;
          next = l2arc_dev_last;
          do {
                  /* loop around the list looking for a non-faulted vdev */
                  if (next == NULL) {
                          next = list_head(l2arc_dev_list);
                  } else {
                          next = list_next(l2arc_dev_list, next);
                          if (next == NULL)
                                  next = list_head(l2arc_dev_list);
                  }
  
                  /* if we have come back to the start, bail out */
                  if (first == NULL)
                          first = next;
                  else if (next == first)
                          break;
  
                  ASSERT3P(next, !=, NULL);
          } while (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
              next->l2ad_trim_all);
  
          /* if we were unable to find any usable vdevs, return NULL */
          if (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
              next->l2ad_trim_all)
                  next = NULL;
  
          l2arc_dev_last = next;
  
  out:
          mutex_exit(&l2arc_dev_mtx);
  
          /*
           * Grab the config lock to prevent the 'next' device from being
           * removed while we are writing to it.
           */
          if (next != NULL)
                  spa_config_enter(next->l2ad_spa, SCL_L2ARC, next, RW_READER);
          mutex_exit(&spa_namespace_lock);
  
          return (next);
  }
  
  /*
   * Free buffers that were tagged for destruction.
   */
  static void
  l2arc_do_free_on_write(void)
  {
          list_t *buflist;
          l2arc_data_free_t *df, *df_prev;
  
          mutex_enter(&l2arc_free_on_write_mtx);
          buflist = l2arc_free_on_write;
  
          for (df = list_tail(buflist); df; df = df_prev) {
                  df_prev = list_prev(buflist, df);
                  ASSERT3P(df->l2df_abd, !=, NULL);
                  abd_free(df->l2df_abd);
                  list_remove(buflist, df);
                  kmem_free(df, sizeof (l2arc_data_free_t));
          }
  
          mutex_exit(&l2arc_free_on_write_mtx);
  }
  
  /*
   * A write to a cache device has completed.  Update all headers to allow
   * reads from these buffers to begin.
   */
  static void
  l2arc_write_done(zio_t *zio)
  {
          l2arc_write_callback_t  *cb;
          l2arc_lb_abd_buf_t      *abd_buf;
          l2arc_lb_ptr_buf_t      *lb_ptr_buf;
          l2arc_dev_t             *dev;
          l2arc_dev_hdr_phys_t    *l2dhdr;
          list_t                  *buflist;
          arc_buf_hdr_t           *head, *hdr, *hdr_prev;
          kmutex_t                *hash_lock;
          int64_t                 bytes_dropped = 0;
  
          cb = zio->io_private;
          ASSERT3P(cb, !=, NULL);
          dev = cb->l2wcb_dev;
          l2dhdr = dev->l2ad_dev_hdr;
          ASSERT3P(dev, !=, NULL);
          head = cb->l2wcb_head;
          ASSERT3P(head, !=, NULL);
          buflist = &dev->l2ad_buflist;
          ASSERT3P(buflist, !=, NULL);
          DTRACE_PROBE2(l2arc__iodone, zio_t *, zio,
              l2arc_write_callback_t *, cb);
  
          /*
           * All writes completed, or an error was hit.
           */
  top:
          mutex_enter(&dev->l2ad_mtx);
          for (hdr = list_prev(buflist, head); hdr; hdr = hdr_prev) {
                  hdr_prev = list_prev(buflist, hdr);
  
                  hash_lock = HDR_LOCK(hdr);
  
                  /*
                   * We cannot use mutex_enter or else we can deadlock
                   * with l2arc_write_buffers (due to swapping the order
                   * the hash lock and l2ad_mtx are taken).
                   */
                  if (!mutex_tryenter(hash_lock)) {
                          /*
                           * Missed the hash lock. We must retry so we
                           * don't leave the ARC_FLAG_L2_WRITING bit set.
                           */
                          ARCSTAT_BUMP(arcstat_l2_writes_lock_retry);
  
                          /*
                           * We don't want to rescan the headers we've
                           * already marked as having been written out, so
                           * we reinsert the head node so we can pick up
                           * where we left off.
                           */
                          list_remove(buflist, head);
                          list_insert_after(buflist, hdr, head);
  
                          mutex_exit(&dev->l2ad_mtx);
  
                          /*
                           * We wait for the hash lock to become available
                           * to try and prevent busy waiting, and increase
                           * the chance we'll be able to acquire the lock
                           * the next time around.
                           */
                          mutex_enter(hash_lock);
                          mutex_exit(hash_lock);
                          goto top;
                  }
  
                  /*
                   * We could not have been moved into the arc_l2c_only
                   * state while in-flight due to our ARC_FLAG_L2_WRITING
                   * bit being set. Let's just ensure that's being enforced.
                   */
                  ASSERT(HDR_HAS_L1HDR(hdr));
  
                  /*
                   * Skipped - drop L2ARC entry and mark the header as no
                   * longer L2 eligibile.
                   */
                  if (zio->io_error != 0) {
                          /*
                           * Error - drop L2ARC entry.
                           */
                          list_remove(buflist, hdr);
                          arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);
  
                          uint64_t psize = HDR_GET_PSIZE(hdr);
                          l2arc_hdr_arcstats_decrement(hdr);
  
                          bytes_dropped +=
                              vdev_psize_to_asize(dev->l2ad_vdev, psize);
                          (void) zfs_refcount_remove_many(&dev->l2ad_alloc,
                              arc_hdr_size(hdr), hdr);
                  }
  
                  /*
                   * Allow ARC to begin reads and ghost list evictions to
                   * this L2ARC entry.
                   */
                  arc_hdr_clear_flags(hdr, ARC_FLAG_L2_WRITING);
  
                  mutex_exit(hash_lock);
          }
  
          /*
           * Free the allocated abd buffers for writing the log blocks.
           * If the zio failed reclaim the allocated space and remove the
           * pointers to these log blocks from the log block pointer list
           * of the L2ARC device.
           */
          while ((abd_buf = list_remove_tail(&cb->l2wcb_abd_list)) != NULL) {
                  abd_free(abd_buf->abd);
                  zio_buf_free(abd_buf, sizeof (*abd_buf));
                  if (zio->io_error != 0) {
                          lb_ptr_buf = list_remove_head(&dev->l2ad_lbptr_list);
                          /*
                           * L2BLK_GET_PSIZE returns aligned size for log
                           * blocks.
                           */
                          uint64_t asize =
                              L2BLK_GET_PSIZE((lb_ptr_buf->lb_ptr)->lbp_prop);
                          bytes_dropped += asize;
                          ARCSTAT_INCR(arcstat_l2_log_blk_asize, -asize);
                          ARCSTAT_BUMPDOWN(arcstat_l2_log_blk_count);
                          zfs_refcount_remove_many(&dev->l2ad_lb_asize, asize,
                              lb_ptr_buf);
                          zfs_refcount_remove(&dev->l2ad_lb_count, lb_ptr_buf);
                          kmem_free(lb_ptr_buf->lb_ptr,
                              sizeof (l2arc_log_blkptr_t));
                          kmem_free(lb_ptr_buf, sizeof (l2arc_lb_ptr_buf_t));
                  }
          }
          list_destroy(&cb->l2wcb_abd_list);
  
          if (zio->io_error != 0) {
                  ARCSTAT_BUMP(arcstat_l2_writes_error);
  
                  /*
                   * Restore the lbps array in the header to its previous state.
                   * If the list of log block pointers is empty, zero out the
                   * log block pointers in the device header.
                   */
                  lb_ptr_buf = list_head(&dev->l2ad_lbptr_list);
                  for (int i = 0; i < 2; i++) {
                          if (lb_ptr_buf == NULL) {
                                  /*
                                   * If the list is empty zero out the device
                                   * header. Otherwise zero out the second log
                                   * block pointer in the header.
                                   */
                                  if (i == 0) {
                                          memset(l2dhdr, 0,
                                              dev->l2ad_dev_hdr_asize);
                                  } else {
                                          memset(&l2dhdr->dh_start_lbps[i], 0,
                                              sizeof (l2arc_log_blkptr_t));
                                  }
                                  break;
                          }
                          memcpy(&l2dhdr->dh_start_lbps[i], lb_ptr_buf->lb_ptr,
                              sizeof (l2arc_log_blkptr_t));
                          lb_ptr_buf = list_next(&dev->l2ad_lbptr_list,
                              lb_ptr_buf);
                  }
          }
  
          ARCSTAT_BUMP(arcstat_l2_writes_done);
          list_remove(buflist, head);
          ASSERT(!HDR_HAS_L1HDR(head));
          kmem_cache_free(hdr_l2only_cache, head);
          mutex_exit(&dev->l2ad_mtx);
  
          ASSERT(dev->l2ad_vdev != NULL);
          vdev_space_update(dev->l2ad_vdev, -bytes_dropped, 0, 0);
  
          l2arc_do_free_on_write();
  
          kmem_free(cb, sizeof (l2arc_write_callback_t));
  }
  
  static int
  l2arc_untransform(zio_t *zio, l2arc_read_callback_t *cb)
  {
          int ret;
          spa_t *spa = zio->io_spa;
          arc_buf_hdr_t *hdr = cb->l2rcb_hdr;
          blkptr_t *bp = zio->io_bp;
          uint8_t salt[ZIO_DATA_SALT_LEN];
          uint8_t iv[ZIO_DATA_IV_LEN];
          uint8_t mac[ZIO_DATA_MAC_LEN];
          boolean_t no_crypt = B_FALSE;
  
          /*
           * ZIL data is never be written to the L2ARC, so we don't need
           * special handling for its unique MAC storage.
           */
          ASSERT3U(BP_GET_TYPE(bp), !=, DMU_OT_INTENT_LOG);
          ASSERT(MUTEX_HELD(HDR_LOCK(hdr)));
          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
  
          /*
           * If the data was encrypted, decrypt it now. Note that
           * we must check the bp here and not the hdr, since the
           * hdr does not have its encryption parameters updated
           * until arc_read_done().
           */
          if (BP_IS_ENCRYPTED(bp)) {
                  abd_t *eabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr,
                      ARC_HDR_DO_ADAPT | ARC_HDR_USE_RESERVE);
  
                  zio_crypt_decode_params_bp(bp, salt, iv);
                  zio_crypt_decode_mac_bp(bp, mac);
  
                  ret = spa_do_crypt_abd(B_FALSE, spa, &cb->l2rcb_zb,
                      BP_GET_TYPE(bp), BP_GET_DEDUP(bp), BP_SHOULD_BYTESWAP(bp),
                      salt, iv, mac, HDR_GET_PSIZE(hdr), eabd,
                      hdr->b_l1hdr.b_pabd, &no_crypt);
                  if (ret != 0) {
                          arc_free_data_abd(hdr, eabd, arc_hdr_size(hdr), hdr);
                          goto error;
                  }
  
                  /*
                   * If we actually performed decryption, replace b_pabd
                   * with the decrypted data. Otherwise we can just throw
                   * our decryption buffer away.
                   */
                  if (!no_crypt) {
                          arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
                              arc_hdr_size(hdr), hdr);
                          hdr->b_l1hdr.b_pabd = eabd;
                          zio->io_abd = eabd;
                  } else {
                          arc_free_data_abd(hdr, eabd, arc_hdr_size(hdr), hdr);
                  }
          }
  
          /*
           * If the L2ARC block was compressed, but ARC compression
           * is disabled we decompress the data into a new buffer and
           * replace the existing data.
           */
          if (HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
              !HDR_COMPRESSION_ENABLED(hdr)) {
                  abd_t *cabd = arc_get_data_abd(hdr, arc_hdr_size(hdr), hdr,
                      ARC_HDR_DO_ADAPT | ARC_HDR_USE_RESERVE);
                  void *tmp = abd_borrow_buf(cabd, arc_hdr_size(hdr));
  
                  ret = zio_decompress_data(HDR_GET_COMPRESS(hdr),
                      hdr->b_l1hdr.b_pabd, tmp, HDR_GET_PSIZE(hdr),
                      HDR_GET_LSIZE(hdr), &hdr->b_complevel);
                  if (ret != 0) {
                          abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
                          arc_free_data_abd(hdr, cabd, arc_hdr_size(hdr), hdr);
                          goto error;
                  }
  
                  abd_return_buf_copy(cabd, tmp, arc_hdr_size(hdr));
                  arc_free_data_abd(hdr, hdr->b_l1hdr.b_pabd,
                      arc_hdr_size(hdr), hdr);
                  hdr->b_l1hdr.b_pabd = cabd;
                  zio->io_abd = cabd;
                  zio->io_size = HDR_GET_LSIZE(hdr);
          }
  
          return (0);
  
  error:
          return (ret);
  }
  
  
  /*
   * A read to a cache device completed.  Validate buffer contents before
   * handing over to the regular ARC routines.
   */
  static void
  l2arc_read_done(zio_t *zio)
  {
          int tfm_error = 0;
          l2arc_read_callback_t *cb = zio->io_private;
          arc_buf_hdr_t *hdr;
          kmutex_t *hash_lock;
          boolean_t valid_cksum;
          boolean_t using_rdata = (BP_IS_ENCRYPTED(&cb->l2rcb_bp) &&
              (cb->l2rcb_flags & ZIO_FLAG_RAW_ENCRYPT));
  
          ASSERT3P(zio->io_vd, !=, NULL);
          ASSERT(zio->io_flags & ZIO_FLAG_DONT_PROPAGATE);
  
          spa_config_exit(zio->io_spa, SCL_L2ARC, zio->io_vd);
  
          ASSERT3P(cb, !=, NULL);
          hdr = cb->l2rcb_hdr;
          ASSERT3P(hdr, !=, NULL);
  
          hash_lock = HDR_LOCK(hdr);
          mutex_enter(hash_lock);
          ASSERT3P(hash_lock, ==, HDR_LOCK(hdr));
  
          /*
           * If the data was read into a temporary buffer,
           * move it and free the buffer.
           */
          if (cb->l2rcb_abd != NULL) {
                  ASSERT3U(arc_hdr_size(hdr), <, zio->io_size);
                  if (zio->io_error == 0) {
                          if (using_rdata) {
                                  abd_copy(hdr->b_crypt_hdr.b_rabd,
                                      cb->l2rcb_abd, arc_hdr_size(hdr));
                          } else {
                                  abd_copy(hdr->b_l1hdr.b_pabd,
                                      cb->l2rcb_abd, arc_hdr_size(hdr));
                          }
                  }
  
                  /*
                   * The following must be done regardless of whether
                   * there was an error:
                   * - free the temporary buffer
                   * - point zio to the real ARC buffer
                   * - set zio size accordingly
                   * These are required because zio is either re-used for
                   * an I/O of the block in the case of the error
                   * or the zio is passed to arc_read_done() and it
                   * needs real data.
                   */
                  abd_free(cb->l2rcb_abd);
                  zio->io_size = zio->io_orig_size = arc_hdr_size(hdr);
  
                  if (using_rdata) {
                          ASSERT(HDR_HAS_RABD(hdr));
                          zio->io_abd = zio->io_orig_abd =
                              hdr->b_crypt_hdr.b_rabd;
                  } else {
                          ASSERT3P(hdr->b_l1hdr.b_pabd, !=, NULL);
                          zio->io_abd = zio->io_orig_abd = hdr->b_l1hdr.b_pabd;
                  }
          }
  
          ASSERT3P(zio->io_abd, !=, NULL);
  
          /*
           * Check this survived the L2ARC journey.
           */
          ASSERT(zio->io_abd == hdr->b_l1hdr.b_pabd ||
              (HDR_HAS_RABD(hdr) && zio->io_abd == hdr->b_crypt_hdr.b_rabd));
          zio->io_bp_copy = cb->l2rcb_bp; /* XXX fix in L2ARC 2.0 */
          zio->io_bp = &zio->io_bp_copy;  /* XXX fix in L2ARC 2.0 */
          zio->io_prop.zp_complevel = hdr->b_complevel;
  
          valid_cksum = arc_cksum_is_equal(hdr, zio);
  
          /*
           * b_rabd will always match the data as it exists on disk if it is
           * being used. Therefore if we are reading into b_rabd we do not
           * attempt to untransform the data.
           */
          if (valid_cksum && !using_rdata)
                  tfm_error = l2arc_untransform(zio, cb);
  
          if (valid_cksum && tfm_error == 0 && zio->io_error == 0 &&
              !HDR_L2_EVICTED(hdr)) {
                  mutex_exit(hash_lock);
                  zio->io_private = hdr;
                  arc_read_done(zio);
          } else {
                  /*
                   * Buffer didn't survive caching.  Increment stats and
                   * reissue to the original storage device.
                   */
                  if (zio->io_error != 0) {
                          ARCSTAT_BUMP(arcstat_l2_io_error);
                  } else {
                          zio->io_error = SET_ERROR(EIO);
                  }
                  if (!valid_cksum || tfm_error != 0)
                          ARCSTAT_BUMP(arcstat_l2_cksum_bad);
  
                  /*
                   * If there's no waiter, issue an async i/o to the primary
                   * storage now.  If there *is* a waiter, the caller must
                   * issue the i/o in a context where it's OK to block.
                   */
                  if (zio->io_waiter == NULL) {
                          zio_t *pio = zio_unique_parent(zio);
                          void *abd = (using_rdata) ?
                              hdr->b_crypt_hdr.b_rabd : hdr->b_l1hdr.b_pabd;
  
                          ASSERT(!pio || pio->io_child_type == ZIO_CHILD_LOGICAL);
  
                          zio = zio_read(pio, zio->io_spa, zio->io_bp,
                              abd, zio->io_size, arc_read_done,
                              hdr, zio->io_priority, cb->l2rcb_flags,
                              &cb->l2rcb_zb);
  
                          /*
                           * Original ZIO will be freed, so we need to update
                           * ARC header with the new ZIO pointer to be used
                           * by zio_change_priority() in arc_read().
                           */
                          for (struct arc_callback *acb = hdr->b_l1hdr.b_acb;
                              acb != NULL; acb = acb->acb_next)
                                  acb->acb_zio_head = zio;
  
                          mutex_exit(hash_lock);
                          zio_nowait(zio);
                  } else {
                          mutex_exit(hash_lock);
                  }
          }
  
          kmem_free(cb, sizeof (l2arc_read_callback_t));
  }
  
  /*
   * This is the list priority from which the L2ARC will search for pages to
   * cache.  This is used within loops (0..3) to cycle through lists in the
   * desired order.  This order can have a significant effect on cache
   * performance.
   *
   * Currently the metadata lists are hit first, MFU then MRU, followed by
   * the data lists.  This function returns a locked list, and also returns
   * the lock pointer.
   */
  static multilist_sublist_t *
  l2arc_sublist_lock(int list_num)
  {
          multilist_t *ml = NULL;
          unsigned int idx;
  
          ASSERT(list_num >= 0 && list_num < L2ARC_FEED_TYPES);
  
          switch (list_num) {
          case 0:
                  ml = &arc_mfu->arcs_list[ARC_BUFC_METADATA];
                  break;
          case 1:
                  ml = &arc_mru->arcs_list[ARC_BUFC_METADATA];
                  break;
          case 2:
                  ml = &arc_mfu->arcs_list[ARC_BUFC_DATA];
                  break;
          case 3:
                  ml = &arc_mru->arcs_list[ARC_BUFC_DATA];
                  break;
          default:
                  return (NULL);
          }
  
          /*
           * Return a randomly-selected sublist. This is acceptable
           * because the caller feeds only a little bit of data for each
           * call (8MB). Subsequent calls will result in different
           * sublists being selected.
           */
          idx = multilist_get_random_index(ml);
          return (multilist_sublist_lock(ml, idx));
  }
  
  /*
   * Calculates the maximum overhead of L2ARC metadata log blocks for a given
   * L2ARC write size. l2arc_evict and l2arc_write_size need to include this
   * overhead in processing to make sure there is enough headroom available
   * when writing buffers.
   */
  static inline uint64_t
  l2arc_log_blk_overhead(uint64_t write_sz, l2arc_dev_t *dev)
  {
          if (dev->l2ad_log_entries == 0) {
                  return (0);
          } else {
                  uint64_t log_entries = write_sz >> SPA_MINBLOCKSHIFT;
  
                  uint64_t log_blocks = (log_entries +
                      dev->l2ad_log_entries - 1) /
                      dev->l2ad_log_entries;
  
                  return (vdev_psize_to_asize(dev->l2ad_vdev,
                      sizeof (l2arc_log_blk_phys_t)) * log_blocks);
          }
  }
  
  /*
   * Evict buffers from the device write hand to the distance specified in
   * bytes. This distance may span populated buffers, it may span nothing.
   * This is clearing a region on the L2ARC device ready for writing.
   * If the 'all' boolean is set, every buffer is evicted.
   */
  static void
  l2arc_evict(l2arc_dev_t *dev, uint64_t distance, boolean_t all)
  {
          list_t *buflist;
          arc_buf_hdr_t *hdr, *hdr_prev;
          kmutex_t *hash_lock;
          uint64_t taddr;
          l2arc_lb_ptr_buf_t *lb_ptr_buf, *lb_ptr_buf_prev;
          vdev_t *vd = dev->l2ad_vdev;
          boolean_t rerun;
  
          buflist = &dev->l2ad_buflist;
  
          /*
           * We need to add in the worst case scenario of log block overhead.
           */
          distance += l2arc_log_blk_overhead(distance, dev);
          if (vd->vdev_has_trim && l2arc_trim_ahead > 0) {
                  /*
                   * Trim ahead of the write size 64MB or (l2arc_trim_ahead/100)
                   * times the write size, whichever is greater.
                   */
                  distance += MAX(64 * 1024 * 1024,
                      (distance * l2arc_trim_ahead) / 100);
          }
  
  top:
          rerun = B_FALSE;
          if (dev->l2ad_hand >= (dev->l2ad_end - distance)) {
                  /*
                   * When there is no space to accommodate upcoming writes,
                   * evict to the end. Then bump the write and evict hands
                   * to the start and iterate. This iteration does not
                   * happen indefinitely as we make sure in
                   * l2arc_write_size() that when the write hand is reset,
                   * the write size does not exceed the end of the device.
                   */
                  rerun = B_TRUE;
                  taddr = dev->l2ad_end;
          } else {
                  taddr = dev->l2ad_hand + distance;
          }
          DTRACE_PROBE4(l2arc__evict, l2arc_dev_t *, dev, list_t *, buflist,
              uint64_t, taddr, boolean_t, all);
  
          if (!all) {
                  /*
                   * This check has to be placed after deciding whether to
                   * iterate (rerun).
                   */
                  if (dev->l2ad_first) {
                          /*
                           * This is the first sweep through the device. There is
                           * nothing to evict. We have already trimmmed the
                           * whole device.
                           */
                          goto out;
                  } else {
                          /*
                           * Trim the space to be evicted.
                           */
                          if (vd->vdev_has_trim && dev->l2ad_evict < taddr &&
                              l2arc_trim_ahead > 0) {
                                  /*
                                   * We have to drop the spa_config lock because
                                   * vdev_trim_range() will acquire it.
                                   * l2ad_evict already accounts for the label
                                   * size. To prevent vdev_trim_ranges() from
                                   * adding it again, we subtract it from
                                   * l2ad_evict.
                                   */
                                  spa_config_exit(dev->l2ad_spa, SCL_L2ARC, dev);
                                  vdev_trim_simple(vd,
                                      dev->l2ad_evict - VDEV_LABEL_START_SIZE,
                                      taddr - dev->l2ad_evict);
                                  spa_config_enter(dev->l2ad_spa, SCL_L2ARC, dev,
                                      RW_READER);
                          }
  
                          /*
                           * When rebuilding L2ARC we retrieve the evict hand
                           * from the header of the device. Of note, l2arc_evict()
                           * does not actually delete buffers from the cache
                           * device, but trimming may do so depending on the
                           * hardware implementation. Thus keeping track of the
                           * evict hand is useful.
                           */
                          dev->l2ad_evict = MAX(dev->l2ad_evict, taddr);
                  }
          }
  
  retry:
          mutex_enter(&dev->l2ad_mtx);
          /*
           * We have to account for evicted log blocks. Run vdev_space_update()
           * on log blocks whose offset (in bytes) is before the evicted offset
           * (in bytes) by searching in the list of pointers to log blocks
           * present in the L2ARC device.
           */
          for (lb_ptr_buf = list_tail(&dev->l2ad_lbptr_list); lb_ptr_buf;
              lb_ptr_buf = lb_ptr_buf_prev) {
  
                  lb_ptr_buf_prev = list_prev(&dev->l2ad_lbptr_list, lb_ptr_buf);
  
                  /* L2BLK_GET_PSIZE returns aligned size for log blocks */
                  uint64_t asize = L2BLK_GET_PSIZE(
                      (lb_ptr_buf->lb_ptr)->lbp_prop);
  
                  /*
                   * We don't worry about log blocks left behind (ie
                   * lbp_payload_start < l2ad_hand) because l2arc_write_buffers()
                   * will never write more than l2arc_evict() evicts.
                   */
                  if (!all && l2arc_log_blkptr_valid(dev, lb_ptr_buf->lb_ptr)) {
                          break;
                  } else {
                          vdev_space_update(vd, -asize, 0, 0);
                          ARCSTAT_INCR(arcstat_l2_log_blk_asize, -asize);
                          ARCSTAT_BUMPDOWN(arcstat_l2_log_blk_count);
                          zfs_refcount_remove_many(&dev->l2ad_lb_asize, asize,
                              lb_ptr_buf);
                          zfs_refcount_remove(&dev->l2ad_lb_count, lb_ptr_buf);
                          list_remove(&dev->l2ad_lbptr_list, lb_ptr_buf);
                          kmem_free(lb_ptr_buf->lb_ptr,
                              sizeof (l2arc_log_blkptr_t));
                          kmem_free(lb_ptr_buf, sizeof (l2arc_lb_ptr_buf_t));
                  }
          }
  
          for (hdr = list_tail(buflist); hdr; hdr = hdr_prev) {
                  hdr_prev = list_prev(buflist, hdr);
  
                  ASSERT(!HDR_EMPTY(hdr));
                  hash_lock = HDR_LOCK(hdr);
  
                  /*
                   * We cannot use mutex_enter or else we can deadlock
                   * with l2arc_write_buffers (due to swapping the order
                   * the hash lock and l2ad_mtx are taken).
                   */
                  if (!mutex_tryenter(hash_lock)) {
                          /*
                           * Missed the hash lock.  Retry.
                           */
                          ARCSTAT_BUMP(arcstat_l2_evict_lock_retry);
                          mutex_exit(&dev->l2ad_mtx);
                          mutex_enter(hash_lock);
                          mutex_exit(hash_lock);
                          goto retry;
                  }
  
                  /*
                   * A header can't be on this list if it doesn't have L2 header.
                   */
                  ASSERT(HDR_HAS_L2HDR(hdr));
  
                  /* Ensure this header has finished being written. */
                  ASSERT(!HDR_L2_WRITING(hdr));
                  ASSERT(!HDR_L2_WRITE_HEAD(hdr));
  
                  if (!all && (hdr->b_l2hdr.b_daddr >= dev->l2ad_evict ||
                      hdr->b_l2hdr.b_daddr < dev->l2ad_hand)) {
                          /*
                           * We've evicted to the target address,
                           * or the end of the device.
                           */
                          mutex_exit(hash_lock);
                          break;
                  }
  
                  if (!HDR_HAS_L1HDR(hdr)) {
                          ASSERT(!HDR_L2_READING(hdr));
                          /*
                           * This doesn't exist in the ARC.  Destroy.
                           * arc_hdr_destroy() will call list_remove()
                           * and decrement arcstat_l2_lsize.
                           */
                          arc_change_state(arc_anon, hdr);
                          arc_hdr_destroy(hdr);
                  } else {
                          ASSERT(hdr->b_l1hdr.b_state != arc_l2c_only);
                          ARCSTAT_BUMP(arcstat_l2_evict_l1cached);
                          /*
                           * Invalidate issued or about to be issued
                           * reads, since we may be about to write
                           * over this location.
                           */
                          if (HDR_L2_READING(hdr)) {
                                  ARCSTAT_BUMP(arcstat_l2_evict_reading);
                                  arc_hdr_set_flags(hdr, ARC_FLAG_L2_EVICTED);
                          }
  
                          arc_hdr_l2hdr_destroy(hdr);
                  }
                  mutex_exit(hash_lock);
          }
          mutex_exit(&dev->l2ad_mtx);
  
  out:
          /*
           * We need to check if we evict all buffers, otherwise we may iterate
           * unnecessarily.
           */
          if (!all && rerun) {
                  /*
                   * Bump device hand to the device start if it is approaching the
                   * end. l2arc_evict() has already evicted ahead for this case.
                   */
                  dev->l2ad_hand = dev->l2ad_start;
                  dev->l2ad_evict = dev->l2ad_start;
                  dev->l2ad_first = B_FALSE;
                  goto top;
          }
  
          if (!all) {
                  /*
                   * In case of cache device removal (all) the following
                   * assertions may be violated without functional consequences
                   * as the device is about to be removed.
                   */
                  ASSERT3U(dev->l2ad_hand + distance, <, dev->l2ad_end);
                  if (!dev->l2ad_first)
                          ASSERT3U(dev->l2ad_hand, <, dev->l2ad_evict);
          }
  }
  
  /*
   * Handle any abd transforms that might be required for writing to the L2ARC.
   * If successful, this function will always return an abd with the data
   * transformed as it is on disk in a new abd of asize bytes.
   */
  static int
  l2arc_apply_transforms(spa_t *spa, arc_buf_hdr_t *hdr, uint64_t asize,
      abd_t **abd_out)
  {
          int ret;
          void *tmp = NULL;
          abd_t *cabd = NULL, *eabd = NULL, *to_write = hdr->b_l1hdr.b_pabd;
          enum zio_compress compress = HDR_GET_COMPRESS(hdr);
          uint64_t psize = HDR_GET_PSIZE(hdr);
          uint64_t size = arc_hdr_size(hdr);
          boolean_t ismd = HDR_ISTYPE_METADATA(hdr);
          boolean_t bswap = (hdr->b_l1hdr.b_byteswap != DMU_BSWAP_NUMFUNCS);
          dsl_crypto_key_t *dck = NULL;
          uint8_t mac[ZIO_DATA_MAC_LEN] = { 0 };
          boolean_t no_crypt = B_FALSE;
  
          ASSERT((HDR_GET_COMPRESS(hdr) != ZIO_COMPRESS_OFF &&
              !HDR_COMPRESSION_ENABLED(hdr)) ||
              HDR_ENCRYPTED(hdr) || HDR_SHARED_DATA(hdr) || psize != asize);
          ASSERT3U(psize, <=, asize);
  
          /*
           * If this data simply needs its own buffer, we simply allocate it
           * and copy the data. This may be done to eliminate a dependency on a
           * shared buffer or to reallocate the buffer to match asize.
           */
          if (HDR_HAS_RABD(hdr) && asize != psize) {
                  ASSERT3U(asize, >=, psize);
                  to_write = abd_alloc_for_io(asize, ismd);
                  abd_copy(to_write, hdr->b_crypt_hdr.b_rabd, psize);
                  if (psize != asize)
                          abd_zero_off(to_write, psize, asize - psize);
                  goto out;
          }
  
          if ((compress == ZIO_COMPRESS_OFF || HDR_COMPRESSION_ENABLED(hdr)) &&
              !HDR_ENCRYPTED(hdr)) {
                  ASSERT3U(size, ==, psize);
                  to_write = abd_alloc_for_io(asize, ismd);
                  abd_copy(to_write, hdr->b_l1hdr.b_pabd, size);
                  if (size != asize)
                          abd_zero_off(to_write, size, asize - size);
                  goto out;
          }
  
          if (compress != ZIO_COMPRESS_OFF && !HDR_COMPRESSION_ENABLED(hdr)) {
                  /*
                   * In some cases, we can wind up with size > asize, so
                   * we need to opt for the larger allocation option here.
                   *
                   * (We also need abd_return_buf_copy in all cases because
                   * it's an ASSERT() to modify the buffer before returning it
                   * with arc_return_buf(), and all the compressors
                   * write things before deciding to fail compression in nearly
                   * every case.)
                   */
                  cabd = abd_alloc_for_io(size, ismd);
                  tmp = abd_borrow_buf(cabd, size);
  
                  psize = zio_compress_data(compress, to_write, tmp, size,
                      hdr->b_complevel);
  
                  if (psize >= asize) {
                          psize = HDR_GET_PSIZE(hdr);
                          abd_return_buf_copy(cabd, tmp, size);
                          HDR_SET_COMPRESS(hdr, ZIO_COMPRESS_OFF);
                          to_write = cabd;
                          abd_copy(to_write, hdr->b_l1hdr.b_pabd, psize);
                          if (psize != asize)
                                  abd_zero_off(to_write, psize, asize - psize);
                          goto encrypt;
                  }
                  ASSERT3U(psize, <=, HDR_GET_PSIZE(hdr));
                  if (psize < asize)
                          memset((char *)tmp + psize, 0, asize - psize);
                  psize = HDR_GET_PSIZE(hdr);
                  abd_return_buf_copy(cabd, tmp, size);
                  to_write = cabd;
          }
  
  encrypt:
          if (HDR_ENCRYPTED(hdr)) {
                  eabd = abd_alloc_for_io(asize, ismd);
  
                  /*
                   * If the dataset was disowned before the buffer
                   * made it to this point, the key to re-encrypt
                   * it won't be available. In this case we simply
                   * won't write the buffer to the L2ARC.
                   */
                  ret = spa_keystore_lookup_key(spa, hdr->b_crypt_hdr.b_dsobj,
                      FTAG, &dck);
                  if (ret != 0)
                          goto error;
  
                  ret = zio_do_crypt_abd(B_TRUE, &dck->dck_key,
                      hdr->b_crypt_hdr.b_ot, bswap, hdr->b_crypt_hdr.b_salt,
                      hdr->b_crypt_hdr.b_iv, mac, psize, to_write, eabd,
                      &no_crypt);
                  if (ret != 0)
                          goto error;
  
                  if (no_crypt)
                          abd_copy(eabd, to_write, psize);
  
                  if (psize != asize)
                          abd_zero_off(eabd, psize, asize - psize);
  
                  /* assert that the MAC we got here matches the one we saved */
                  ASSERT0(memcmp(mac, hdr->b_crypt_hdr.b_mac, ZIO_DATA_MAC_LEN));
                  spa_keystore_dsl_key_rele(spa, dck, FTAG);
  
                  if (to_write == cabd)
                          abd_free(cabd);
  
                  to_write = eabd;
          }
  
  out:
          ASSERT3P(to_write, !=, hdr->b_l1hdr.b_pabd);
          *abd_out = to_write;
          return (0);
  
  error:
          if (dck != NULL)
                  spa_keystore_dsl_key_rele(spa, dck, FTAG);
          if (cabd != NULL)
                  abd_free(cabd);
          if (eabd != NULL)
                  abd_free(eabd);
  
          *abd_out = NULL;
          return (ret);
  }
  
  static void
  l2arc_blk_fetch_done(zio_t *zio)
  {
          l2arc_read_callback_t *cb;
  
          cb = zio->io_private;
          if (cb->l2rcb_abd != NULL)
                  abd_free(cb->l2rcb_abd);
          kmem_free(cb, sizeof (l2arc_read_callback_t));
  }
  
  /*
   * Find and write ARC buffers to the L2ARC device.
   *
   * An ARC_FLAG_L2_WRITING flag is set so that the L2ARC buffers are not valid
   * for reading until they have completed writing.
   * The headroom_boost is an in-out parameter used to maintain headroom boost
   * state between calls to this function.
   *
   * Returns the number of bytes actually written (which may be smaller than
   * the delta by which the device hand has changed due to alignment and the
   * writing of log blocks).
   */
  static uint64_t
  l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
  {
          arc_buf_hdr_t           *hdr, *hdr_prev, *head;
          uint64_t                write_asize, write_psize, write_lsize, headroom;
          boolean_t               full;
          l2arc_write_callback_t  *cb = NULL;
          zio_t                   *pio, *wzio;
          uint64_t                guid = spa_load_guid(spa);
          l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
  
          ASSERT3P(dev->l2ad_vdev, !=, NULL);
  
          pio = NULL;
          write_lsize = write_asize = write_psize = 0;
          full = B_FALSE;
          head = kmem_cache_alloc(hdr_l2only_cache, KM_PUSHPAGE);
          arc_hdr_set_flags(head, ARC_FLAG_L2_WRITE_HEAD | ARC_FLAG_HAS_L2HDR);
  
          /*
           * Copy buffers for L2ARC writing.
           */
          for (int pass = 0; pass < L2ARC_FEED_TYPES; pass++) {
                  /*
                   * If pass == 1 or 3, we cache MRU metadata and data
                   * respectively.
                   */
                  if (l2arc_mfuonly) {
                          if (pass == 1 || pass == 3)
                                  continue;
                  }
  
                  multilist_sublist_t *mls = l2arc_sublist_lock(pass);
                  uint64_t passed_sz = 0;
  
                  VERIFY3P(mls, !=, NULL);
  
                  /*
                   * L2ARC fast warmup.
                   *
                   * Until the ARC is warm and starts to evict, read from the
                   * head of the ARC lists rather than the tail.
                   */
                  if (arc_warm == B_FALSE)
                          hdr = multilist_sublist_head(mls);
                  else
                          hdr = multilist_sublist_tail(mls);
  
                  headroom = target_sz * l2arc_headroom;
                  if (zfs_compressed_arc_enabled)
                          headroom = (headroom * l2arc_headroom_boost) / 100;
  
                  for (; hdr; hdr = hdr_prev) {
                          kmutex_t *hash_lock;
                          abd_t *to_write = NULL;
  
                          if (arc_warm == B_FALSE)
                                  hdr_prev = multilist_sublist_next(mls, hdr);
                          else
                                  hdr_prev = multilist_sublist_prev(mls, hdr);
  
                          hash_lock = HDR_LOCK(hdr);
                          if (!mutex_tryenter(hash_lock)) {
                                  /*
                                   * Skip this buffer rather than waiting.
                                   */
                                  continue;
                          }
  
                          passed_sz += HDR_GET_LSIZE(hdr);
                          if (l2arc_headroom != 0 && passed_sz > headroom) {
                                  /*
                                   * Searched too far.
                                   */
                                  mutex_exit(hash_lock);
                                  break;
                          }
  
                          if (!l2arc_write_eligible(guid, hdr)) {
                                  mutex_exit(hash_lock);
                                  continue;
                          }
  
                          ASSERT(HDR_HAS_L1HDR(hdr));
  
                          ASSERT3U(HDR_GET_PSIZE(hdr), >, 0);
                          ASSERT3U(arc_hdr_size(hdr), >, 0);
                          ASSERT(hdr->b_l1hdr.b_pabd != NULL ||
                              HDR_HAS_RABD(hdr));
                          uint64_t psize = HDR_GET_PSIZE(hdr);
                          uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev,
                              psize);
  
                          if ((write_asize + asize) > target_sz) {
                                  full = B_TRUE;
                                  mutex_exit(hash_lock);
                                  break;
                          }
  
                          /*
                           * We rely on the L1 portion of the header below, so
                           * it's invalid for this header to have been evicted out
                           * of the ghost cache, prior to being written out. The
                           * ARC_FLAG_L2_WRITING bit ensures this won't happen.
                           */
                          arc_hdr_set_flags(hdr, ARC_FLAG_L2_WRITING);
  
                          /*
                           * If this header has b_rabd, we can use this since it
                           * must always match the data exactly as it exists on
                           * disk. Otherwise, the L2ARC can normally use the
                           * hdr's data, but if we're sharing data between the
                           * hdr and one of its bufs, L2ARC needs its own copy of
                           * the data so that the ZIO below can't race with the
                           * buf consumer. To ensure that this copy will be
                           * available for the lifetime of the ZIO and be cleaned
                           * up afterwards, we add it to the l2arc_free_on_write
                           * queue. If we need to apply any transforms to the
                           * data (compression, encryption) we will also need the
                           * extra buffer.
                           */
                          if (HDR_HAS_RABD(hdr) && psize == asize) {
                                  to_write = hdr->b_crypt_hdr.b_rabd;
                          } else if ((HDR_COMPRESSION_ENABLED(hdr) ||
                              HDR_GET_COMPRESS(hdr) == ZIO_COMPRESS_OFF) &&
                              !HDR_ENCRYPTED(hdr) && !HDR_SHARED_DATA(hdr) &&
                              psize == asize) {
                                  to_write = hdr->b_l1hdr.b_pabd;
                          } else {
                                  int ret;
                                  arc_buf_contents_t type = arc_buf_type(hdr);
  
                                  ret = l2arc_apply_transforms(spa, hdr, asize,
                                      &to_write);
                                  if (ret != 0) {
                                          arc_hdr_clear_flags(hdr,
                                              ARC_FLAG_L2_WRITING);
                                          mutex_exit(hash_lock);
                                          continue;
                                  }
  
                                  l2arc_free_abd_on_write(to_write, asize, type);
                          }
  
                          if (pio == NULL) {
                                  /*
                                   * Insert a dummy header on the buflist so
                                   * l2arc_write_done() can find where the
                                   * write buffers begin without searching.
                                   */
                                  mutex_enter(&dev->l2ad_mtx);
                                  list_insert_head(&dev->l2ad_buflist, head);
                                  mutex_exit(&dev->l2ad_mtx);
  
                                  cb = kmem_alloc(
                                      sizeof (l2arc_write_callback_t), KM_SLEEP);
                                  cb->l2wcb_dev = dev;
                                  cb->l2wcb_head = head;
                                  /*
                                   * Create a list to save allocated abd buffers
                                   * for l2arc_log_blk_commit().
                                   */
                                  list_create(&cb->l2wcb_abd_list,
                                      sizeof (l2arc_lb_abd_buf_t),
                                      offsetof(l2arc_lb_abd_buf_t, node));
                                  pio = zio_root(spa, l2arc_write_done, cb,
                                      ZIO_FLAG_CANFAIL);
                          }
  
                          hdr->b_l2hdr.b_dev = dev;
                          hdr->b_l2hdr.b_hits = 0;
  
                          hdr->b_l2hdr.b_daddr = dev->l2ad_hand;
                          hdr->b_l2hdr.b_arcs_state =
                              hdr->b_l1hdr.b_state->arcs_state;
                          arc_hdr_set_flags(hdr, ARC_FLAG_HAS_L2HDR);
  
                          mutex_enter(&dev->l2ad_mtx);
                          list_insert_head(&dev->l2ad_buflist, hdr);
                          mutex_exit(&dev->l2ad_mtx);
  
                          (void) zfs_refcount_add_many(&dev->l2ad_alloc,
                              arc_hdr_size(hdr), hdr);
  
                          wzio = zio_write_phys(pio, dev->l2ad_vdev,
                              hdr->b_l2hdr.b_daddr, asize, to_write,
                              ZIO_CHECKSUM_OFF, NULL, hdr,
                              ZIO_PRIORITY_ASYNC_WRITE,
                              ZIO_FLAG_CANFAIL, B_FALSE);
  
                          write_lsize += HDR_GET_LSIZE(hdr);
                          DTRACE_PROBE2(l2arc__write, vdev_t *, dev->l2ad_vdev,
                              zio_t *, wzio);
  
                          write_psize += psize;
                          write_asize += asize;
                          dev->l2ad_hand += asize;
                          l2arc_hdr_arcstats_increment(hdr);
                          vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
  
                          mutex_exit(hash_lock);
  
                          /*
                           * Append buf info to current log and commit if full.
                           * arcstat_l2_{size,asize} kstats are updated
                           * internally.
                           */
                          if (l2arc_log_blk_insert(dev, hdr))
                                  l2arc_log_blk_commit(dev, pio, cb);
  
                          zio_nowait(wzio);
                  }
  
                  multilist_sublist_unlock(mls);
  
                  if (full == B_TRUE)
                          break;
          }
  
          /* No buffers selected for writing? */
          if (pio == NULL) {
                  ASSERT0(write_lsize);
                  ASSERT(!HDR_HAS_L1HDR(head));
                  kmem_cache_free(hdr_l2only_cache, head);
  
                  /*
                   * Although we did not write any buffers l2ad_evict may
                   * have advanced.
                   */
                  if (dev->l2ad_evict != l2dhdr->dh_evict)
                          l2arc_dev_hdr_update(dev);
  
                  return (0);
          }
  
          if (!dev->l2ad_first)
                  ASSERT3U(dev->l2ad_hand, <=, dev->l2ad_evict);
  
          ASSERT3U(write_asize, <=, target_sz);
          ARCSTAT_BUMP(arcstat_l2_writes_sent);
          ARCSTAT_INCR(arcstat_l2_write_bytes, write_psize);
  
          dev->l2ad_writing = B_TRUE;
          (void) zio_wait(pio);
          dev->l2ad_writing = B_FALSE;
  
          /*
           * Update the device header after the zio completes as
           * l2arc_write_done() may have updated the memory holding the log block
           * pointers in the device header.
           */
          l2arc_dev_hdr_update(dev);
  
          return (write_asize);
  }
  
  static boolean_t
  l2arc_hdr_limit_reached(void)
  {
          int64_t s = aggsum_upper_bound(&arc_sums.arcstat_l2_hdr_size);
  
          return (arc_reclaim_needed() || (s > arc_meta_limit * 3 / 4) ||
              (s > (arc_warm ? arc_c : arc_c_max) * l2arc_meta_percent / 100));
  }
  
  /*
   * This thread feeds the L2ARC at regular intervals.  This is the beating
   * heart of the L2ARC.
   */
  static  __attribute__((noreturn)) void
  l2arc_feed_thread(void *unused)
  {
          (void) unused;
          callb_cpr_t cpr;
          l2arc_dev_t *dev;
          spa_t *spa;
          uint64_t size, wrote;
          clock_t begin, next = ddi_get_lbolt();
          fstrans_cookie_t cookie;
  
          CALLB_CPR_INIT(&cpr, &l2arc_feed_thr_lock, callb_generic_cpr, FTAG);
  
          mutex_enter(&l2arc_feed_thr_lock);
  
          cookie = spl_fstrans_mark();
          while (l2arc_thread_exit == 0) {
                  CALLB_CPR_SAFE_BEGIN(&cpr);
                  (void) cv_timedwait_idle(&l2arc_feed_thr_cv,
                      &l2arc_feed_thr_lock, next);
                  CALLB_CPR_SAFE_END(&cpr, &l2arc_feed_thr_lock);
                  next = ddi_get_lbolt() + hz;
  
                  /*
                   * Quick check for L2ARC devices.
                   */
                  mutex_enter(&l2arc_dev_mtx);
                  if (l2arc_ndev == 0) {
                          mutex_exit(&l2arc_dev_mtx);
                          continue;
                  }
                  mutex_exit(&l2arc_dev_mtx);
                  begin = ddi_get_lbolt();
  
                  /*
                   * This selects the next l2arc device to write to, and in
                   * doing so the next spa to feed from: dev->l2ad_spa.   This
                   * will return NULL if there are now no l2arc devices or if
                   * they are all faulted.
                   *
                   * If a device is returned, its spa's config lock is also
                   * held to prevent device removal.  l2arc_dev_get_next()
                   * will grab and release l2arc_dev_mtx.
                   */
                  if ((dev = l2arc_dev_get_next()) == NULL)
                          continue;
  
                  spa = dev->l2ad_spa;
                  ASSERT3P(spa, !=, NULL);
  
                  /*
                   * If the pool is read-only then force the feed thread to
                   * sleep a little longer.
                   */
                  if (!spa_writeable(spa)) {
                          next = ddi_get_lbolt() + 5 * l2arc_feed_secs * hz;
                          spa_config_exit(spa, SCL_L2ARC, dev);
                          continue;
                  }
  
                  /*
                   * Avoid contributing to memory pressure.
                   */
                  if (l2arc_hdr_limit_reached()) {
                          ARCSTAT_BUMP(arcstat_l2_abort_lowmem);
                          spa_config_exit(spa, SCL_L2ARC, dev);
                          continue;
                  }
  
                  ARCSTAT_BUMP(arcstat_l2_feeds);
  
                  size = l2arc_write_size(dev);
  
                  /*
                   * Evict L2ARC buffers that will be overwritten.
                   */
                  l2arc_evict(dev, size, B_FALSE);
  
                  /*
                   * Write ARC buffers.
                   */
                  wrote = l2arc_write_buffers(spa, dev, size);
  
                  /*
                   * Calculate interval between writes.
                   */
                  next = l2arc_write_interval(begin, size, wrote);
                  spa_config_exit(spa, SCL_L2ARC, dev);
          }
          spl_fstrans_unmark(cookie);
  
          l2arc_thread_exit = 0;
          cv_broadcast(&l2arc_feed_thr_cv);
          CALLB_CPR_EXIT(&cpr);           /* drops l2arc_feed_thr_lock */
          thread_exit();
  }
  
  boolean_t
  l2arc_vdev_present(vdev_t *vd)
  {
          return (l2arc_vdev_get(vd) != NULL);
  }
  
  /*
   * Returns the l2arc_dev_t associated with a particular vdev_t or NULL if
   * the vdev_t isn't an L2ARC device.
   */
  l2arc_dev_t *
  l2arc_vdev_get(vdev_t *vd)
  {
          l2arc_dev_t     *dev;
  
          mutex_enter(&l2arc_dev_mtx);
          for (dev = list_head(l2arc_dev_list); dev != NULL;
              dev = list_next(l2arc_dev_list, dev)) {
                  if (dev->l2ad_vdev == vd)
                          break;
          }
          mutex_exit(&l2arc_dev_mtx);
  
          return (dev);
  }
  
  static void
  l2arc_rebuild_dev(l2arc_dev_t *dev, boolean_t reopen)
  {
          l2arc_dev_hdr_phys_t *l2dhdr = dev->l2ad_dev_hdr;
          uint64_t l2dhdr_asize = dev->l2ad_dev_hdr_asize;
          spa_t *spa = dev->l2ad_spa;
  
          /*
           * The L2ARC has to hold at least the payload of one log block for
           * them to be restored (persistent L2ARC). The payload of a log block
           * depends on the amount of its log entries. We always write log blocks
           * with 1022 entries. How many of them are committed or restored depends
           * on the size of the L2ARC device. Thus the maximum payload of
           * one log block is 1022 * SPA_MAXBLOCKSIZE = 16GB. If the L2ARC device
           * is less than that, we reduce the amount of committed and restored
           * log entries per block so as to enable persistence.
           */
          if (dev->l2ad_end < l2arc_rebuild_blocks_min_l2size) {
                  dev->l2ad_log_entries = 0;
          } else {
                  dev->l2ad_log_entries = MIN((dev->l2ad_end -
                      dev->l2ad_start) >> SPA_MAXBLOCKSHIFT,
                      L2ARC_LOG_BLK_MAX_ENTRIES);
          }
  
          /*
           * Read the device header, if an error is returned do not rebuild L2ARC.
           */
          if (l2arc_dev_hdr_read(dev) == 0 && dev->l2ad_log_entries > 0) {
                  /*
                   * If we are onlining a cache device (vdev_reopen) that was
                   * still present (l2arc_vdev_present()) and rebuild is enabled,
                   * we should evict all ARC buffers and pointers to log blocks
                   * and reclaim their space before restoring its contents to
                   * L2ARC.
                   */
                  if (reopen) {
                          if (!l2arc_rebuild_enabled) {
                                  return;
                          } else {
                                  l2arc_evict(dev, 0, B_TRUE);
                                  /* start a new log block */
                                  dev->l2ad_log_ent_idx = 0;
                                  dev->l2ad_log_blk_payload_asize = 0;
                                  dev->l2ad_log_blk_payload_start = 0;
                          }
                  }
                  /*
                   * Just mark the device as pending for a rebuild. We won't
                   * be starting a rebuild in line here as it would block pool
                   * import. Instead spa_load_impl will hand that off to an
                   * async task which will call l2arc_spa_rebuild_start.
                   */
                  dev->l2ad_rebuild = B_TRUE;
          } else if (spa_writeable(spa)) {
                  /*
                   * In this case TRIM the whole device if l2arc_trim_ahead > 0,
                   * otherwise create a new header. We zero out the memory holding
                   * the header to reset dh_start_lbps. If we TRIM the whole
                   * device the new header will be written by
                   * vdev_trim_l2arc_thread() at the end of the TRIM to update the
                   * trim_state in the header too. When reading the header, if
                   * trim_state is not VDEV_TRIM_COMPLETE and l2arc_trim_ahead > 0
                   * we opt to TRIM the whole device again.
                   */
                  if (l2arc_trim_ahead > 0) {
                          dev->l2ad_trim_all = B_TRUE;
                  } else {
                          memset(l2dhdr, 0, l2dhdr_asize);
                          l2arc_dev_hdr_update(dev);
                  }
          }
  }
  
  /*
   * Add a vdev for use by the L2ARC.  By this point the spa has already
   * validated the vdev and opened it.
   */
  void
  l2arc_add_vdev(spa_t *spa, vdev_t *vd)
  {
          l2arc_dev_t             *adddev;
          uint64_t                l2dhdr_asize;
  
          ASSERT(!l2arc_vdev_present(vd));
  
          /*
           * Create a new l2arc device entry.
           */
          adddev = vmem_zalloc(sizeof (l2arc_dev_t), KM_SLEEP);
          adddev->l2ad_spa = spa;
          adddev->l2ad_vdev = vd;
          /* leave extra size for an l2arc device header */
          l2dhdr_asize = adddev->l2ad_dev_hdr_asize =
              MAX(sizeof (*adddev->l2ad_dev_hdr), 1 << vd->vdev_ashift);
          adddev->l2ad_start = VDEV_LABEL_START_SIZE + l2dhdr_asize;
          adddev->l2ad_end = VDEV_LABEL_START_SIZE + vdev_get_min_asize(vd);
          ASSERT3U(adddev->l2ad_start, <, adddev->l2ad_end);
          adddev->l2ad_hand = adddev->l2ad_start;
          adddev->l2ad_evict = adddev->l2ad_start;
          adddev->l2ad_first = B_TRUE;
          adddev->l2ad_writing = B_FALSE;
          adddev->l2ad_trim_all = B_FALSE;
          list_link_init(&adddev->l2ad_node);
          adddev->l2ad_dev_hdr = kmem_zalloc(l2dhdr_asize, KM_SLEEP);
  
          mutex_init(&adddev->l2ad_mtx, NULL, MUTEX_DEFAULT, NULL);
          /*
           * This is a list of all ARC buffers that are still valid on the
           * device.
           */
          list_create(&adddev->l2ad_buflist, sizeof (arc_buf_hdr_t),
             offsetof(arc_buf_hdr_t, b_l2hdr.b_l2node));
 
         /*
          * This is a list of pointers to log blocks that are still present
          * on the device.
          */
         list_create(&adddev->l2ad_lbptr_list, sizeof (l2arc_lb_ptr_buf_t),
             offsetof(l2arc_lb_ptr_buf_t, node));
 
         vdev_space_update(vd, 0, 0, adddev->l2ad_end - adddev->l2ad_hand);
         zfs_refcount_create(&adddev->l2ad_alloc);
         zfs_refcount_create(&adddev->l2ad_lb_asize);
         zfs_refcount_create(&adddev->l2ad_lb_count);
 
         /*
          * Decide if dev is eligible for L2ARC rebuild or whole device
          * trimming. This has to happen before the device is added in the
          * cache device list and l2arc_dev_mtx is released. Otherwise
          * l2arc_feed_thread() might already start writing on the
          * device.
          */
         l2arc_rebuild_dev(adddev, B_FALSE);
 
         /*
          * Add device to global list
          */
         mutex_enter(&l2arc_dev_mtx);
         list_insert_head(l2arc_dev_list, adddev);
         atomic_inc_64(&l2arc_ndev);
         mutex_exit(&l2arc_dev_mtx);
 }
 
 /*
  * Decide if a vdev is eligible for L2ARC rebuild, called from vdev_reopen()
  * in case of onlining a cache device.
  */
 void
 l2arc_rebuild_vdev(vdev_t *vd, boolean_t reopen)
 {
         l2arc_dev_t             *dev = NULL;
 
         dev = l2arc_vdev_get(vd);
         ASSERT3P(dev, !=, NULL);
 
         /*
          * In contrast to l2arc_add_vdev() we do not have to worry about
          * l2arc_feed_thread() invalidating previous content when onlining a
          * cache device. The device parameters (l2ad*) are not cleared when
          * offlining the device and writing new buffers will not invalidate
          * all previous content. In worst case only buffers that have not had
          * their log block written to the device will be lost.
          * When onlining the cache device (ie offline->online without exporting
          * the pool in between) this happens:
          * vdev_reopen() -> vdev_open() -> l2arc_rebuild_vdev()
          *                      |                       |
          *              vdev_is_dead() = B_FALSE        l2ad_rebuild = B_TRUE
          * During the time where vdev_is_dead = B_FALSE and until l2ad_rebuild
          * is set to B_TRUE we might write additional buffers to the device.
          */
         l2arc_rebuild_dev(dev, reopen);
 }
 
 /*
  * Remove a vdev from the L2ARC.
  */
 void
 l2arc_remove_vdev(vdev_t *vd)
 {
         l2arc_dev_t *remdev = NULL;
 
         /*
          * Find the device by vdev
          */
         remdev = l2arc_vdev_get(vd);
         ASSERT3P(remdev, !=, NULL);
 
         /*
          * Cancel any ongoing or scheduled rebuild.
          */
         mutex_enter(&l2arc_rebuild_thr_lock);
         if (remdev->l2ad_rebuild_began == B_TRUE) {
                 remdev->l2ad_rebuild_cancel = B_TRUE;
                 while (remdev->l2ad_rebuild == B_TRUE)
                         cv_wait(&l2arc_rebuild_thr_cv, &l2arc_rebuild_thr_lock);
         }
         mutex_exit(&l2arc_rebuild_thr_lock);
 
         /*
          * Remove device from global list
          */
         mutex_enter(&l2arc_dev_mtx);
         list_remove(l2arc_dev_list, remdev);
         l2arc_dev_last = NULL;          /* may have been invalidated */
         atomic_dec_64(&l2arc_ndev);
         mutex_exit(&l2arc_dev_mtx);
 
         /*
          * Clear all buflists and ARC references.  L2ARC device flush.
          */
         l2arc_evict(remdev, 0, B_TRUE);
         list_destroy(&remdev->l2ad_buflist);
         ASSERT(list_is_empty(&remdev->l2ad_lbptr_list));
         list_destroy(&remdev->l2ad_lbptr_list);
         mutex_destroy(&remdev->l2ad_mtx);
         zfs_refcount_destroy(&remdev->l2ad_alloc);
         zfs_refcount_destroy(&remdev->l2ad_lb_asize);
         zfs_refcount_destroy(&remdev->l2ad_lb_count);
         kmem_free(remdev->l2ad_dev_hdr, remdev->l2ad_dev_hdr_asize);
         vmem_free(remdev, sizeof (l2arc_dev_t));
 }
 
 void
 l2arc_init(void)
 {
         l2arc_thread_exit = 0;
         l2arc_ndev = 0;
 
         mutex_init(&l2arc_feed_thr_lock, NULL, MUTEX_DEFAULT, NULL);
         cv_init(&l2arc_feed_thr_cv, NULL, CV_DEFAULT, NULL);
         mutex_init(&l2arc_rebuild_thr_lock, NULL, MUTEX_DEFAULT, NULL);
         cv_init(&l2arc_rebuild_thr_cv, NULL, CV_DEFAULT, NULL);
         mutex_init(&l2arc_dev_mtx, NULL, MUTEX_DEFAULT, NULL);
         mutex_init(&l2arc_free_on_write_mtx, NULL, MUTEX_DEFAULT, NULL);
 
         l2arc_dev_list = &L2ARC_dev_list;
         l2arc_free_on_write = &L2ARC_free_on_write;
         list_create(l2arc_dev_list, sizeof (l2arc_dev_t),
             offsetof(l2arc_dev_t, l2ad_node));
         list_create(l2arc_free_on_write, sizeof (l2arc_data_free_t),
             offsetof(l2arc_data_free_t, l2df_list_node));
 }
 
 void
 l2arc_fini(void)
 {
         mutex_destroy(&l2arc_feed_thr_lock);
         cv_destroy(&l2arc_feed_thr_cv);
         mutex_destroy(&l2arc_rebuild_thr_lock);
         cv_destroy(&l2arc_rebuild_thr_cv);
         mutex_destroy(&l2arc_dev_mtx);
         mutex_destroy(&l2arc_free_on_write_mtx);
 
         list_destroy(l2arc_dev_list);
         list_destroy(l2arc_free_on_write);
 }
 
 void
 l2arc_start(void)
 {
         if (!(spa_mode_global & SPA_MODE_WRITE))
                 return;
 
         (void) thread_create(NULL, 0, l2arc_feed_thread, NULL, 0, &p0,
             TS_RUN, defclsyspri);
 }
 
 void
 l2arc_stop(void)
 {
         if (!(spa_mode_global & SPA_MODE_WRITE))
                 return;
 
         mutex_enter(&l2arc_feed_thr_lock);
         cv_signal(&l2arc_feed_thr_cv);  /* kick thread out of startup */
         l2arc_thread_exit = 1;
         while (l2arc_thread_exit != 0)
                 cv_wait(&l2arc_feed_thr_cv, &l2arc_feed_thr_lock);
         mutex_exit(&l2arc_feed_thr_lock);
 }
 
 /*
  * Punches out rebuild threads for the L2ARC devices in a spa. This should
  * be called after pool import from the spa async thread, since starting
  * these threads directly from spa_import() will make them part of the
  * "zpool import" context and delay process exit (and thus pool import).
  */
 void
 l2arc_spa_rebuild_start(spa_t *spa)
 {
         ASSERT(MUTEX_HELD(&spa_namespace_lock));
 
         /*
          * Locate the spa's l2arc devices and kick off rebuild threads.
          */
         for (int i = 0; i < spa->spa_l2cache.sav_count; i++) {
                 l2arc_dev_t *dev =
                     l2arc_vdev_get(spa->spa_l2cache.sav_vdevs[i]);
                 if (dev == NULL) {
                         /* Don't attempt a rebuild if the vdev is UNAVAIL */
                         continue;
                 }
                 mutex_enter(&l2arc_rebuild_thr_lock);
                 if (dev->l2ad_rebuild && !dev->l2ad_rebuild_cancel) {
                         dev->l2ad_rebuild_began = B_TRUE;
                         (void) thread_create(NULL, 0, l2arc_dev_rebuild_thread,
                             dev, 0, &p0, TS_RUN, minclsyspri);
                 }
                 mutex_exit(&l2arc_rebuild_thr_lock);
         }
 }
 
 /*
  * Main entry point for L2ARC rebuilding.
  */
 static __attribute__((noreturn)) void
 l2arc_dev_rebuild_thread(void *arg)
 {
         l2arc_dev_t *dev = arg;
 
         VERIFY(!dev->l2ad_rebuild_cancel);
         VERIFY(dev->l2ad_rebuild);
         (void) l2arc_rebuild(dev);
         mutex_enter(&l2arc_rebuild_thr_lock);
         dev->l2ad_rebuild_began = B_FALSE;
         dev->l2ad_rebuild = B_FALSE;
         mutex_exit(&l2arc_rebuild_thr_lock);
 
         thread_exit();
 }
 
 /*
  * This function implements the actual L2ARC metadata rebuild. It:
  * starts reading the log block chain and restores each block's contents
  * to memory (reconstructing arc_buf_hdr_t's).
  *
  * Operation stops under any of the following conditions:
  *
  * 1) We reach the end of the log block chain.
  * 2) We encounter *any* error condition (cksum errors, io errors)
  */
 static int
 l2arc_rebuild(l2arc_dev_t *dev)
 {
         vdev_t                  *vd = dev->l2ad_vdev;
         spa_t                   *spa = vd->vdev_spa;
         int                     err = 0;
         l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
         l2arc_log_blk_phys_t    *this_lb, *next_lb;
         zio_t                   *this_io = NULL, *next_io = NULL;
         l2arc_log_blkptr_t      lbps[2];
         l2arc_lb_ptr_buf_t      *lb_ptr_buf;
         boolean_t               lock_held;
 
         this_lb = vmem_zalloc(sizeof (*this_lb), KM_SLEEP);
         next_lb = vmem_zalloc(sizeof (*next_lb), KM_SLEEP);
 
         /*
          * We prevent device removal while issuing reads to the device,
          * then during the rebuilding phases we drop this lock again so
          * that a spa_unload or device remove can be initiated - this is
          * safe, because the spa will signal us to stop before removing
          * our device and wait for us to stop.
          */
         spa_config_enter(spa, SCL_L2ARC, vd, RW_READER);
         lock_held = B_TRUE;
 
         /*
          * Retrieve the persistent L2ARC device state.
          * L2BLK_GET_PSIZE returns aligned size for log blocks.
          */
         dev->l2ad_evict = MAX(l2dhdr->dh_evict, dev->l2ad_start);
         dev->l2ad_hand = MAX(l2dhdr->dh_start_lbps[0].lbp_daddr +
             L2BLK_GET_PSIZE((&l2dhdr->dh_start_lbps[0])->lbp_prop),
             dev->l2ad_start);
         dev->l2ad_first = !!(l2dhdr->dh_flags & L2ARC_DEV_HDR_EVICT_FIRST);
 
         vd->vdev_trim_action_time = l2dhdr->dh_trim_action_time;
         vd->vdev_trim_state = l2dhdr->dh_trim_state;
 
         /*
          * In case the zfs module parameter l2arc_rebuild_enabled is false
          * we do not start the rebuild process.
          */
         if (!l2arc_rebuild_enabled)
                 goto out;
 
         /* Prepare the rebuild process */
         memcpy(lbps, l2dhdr->dh_start_lbps, sizeof (lbps));
 
         /* Start the rebuild process */
         for (;;) {
                 if (!l2arc_log_blkptr_valid(dev, &lbps[0]))
                         break;
 
                 if ((err = l2arc_log_blk_read(dev, &lbps[0], &lbps[1],
                     this_lb, next_lb, this_io, &next_io)) != 0)
                         goto out;
 
                 /*
                  * Our memory pressure valve. If the system is running low
                  * on memory, rather than swamping memory with new ARC buf
                  * hdrs, we opt not to rebuild the L2ARC. At this point,
                  * however, we have already set up our L2ARC dev to chain in
                  * new metadata log blocks, so the user may choose to offline/
                  * online the L2ARC dev at a later time (or re-import the pool)
                  * to reconstruct it (when there's less memory pressure).
                  */
                 if (l2arc_hdr_limit_reached()) {
                         ARCSTAT_BUMP(arcstat_l2_rebuild_abort_lowmem);
                         cmn_err(CE_NOTE, "System running low on memory, "
                             "aborting L2ARC rebuild.");
                         err = SET_ERROR(ENOMEM);
                         goto out;
                 }
 
                 spa_config_exit(spa, SCL_L2ARC, vd);
                 lock_held = B_FALSE;
 
                 /*
                  * Now that we know that the next_lb checks out alright, we
                  * can start reconstruction from this log block.
                  * L2BLK_GET_PSIZE returns aligned size for log blocks.
                  */
                 uint64_t asize = L2BLK_GET_PSIZE((&lbps[0])->lbp_prop);
                 l2arc_log_blk_restore(dev, this_lb, asize);
 
                 /*
                  * log block restored, include its pointer in the list of
                  * pointers to log blocks present in the L2ARC device.
                  */
                 lb_ptr_buf = kmem_zalloc(sizeof (l2arc_lb_ptr_buf_t), KM_SLEEP);
                 lb_ptr_buf->lb_ptr = kmem_zalloc(sizeof (l2arc_log_blkptr_t),
                     KM_SLEEP);
                 memcpy(lb_ptr_buf->lb_ptr, &lbps[0],
                     sizeof (l2arc_log_blkptr_t));
                 mutex_enter(&dev->l2ad_mtx);
                 list_insert_tail(&dev->l2ad_lbptr_list, lb_ptr_buf);
                 ARCSTAT_INCR(arcstat_l2_log_blk_asize, asize);
                 ARCSTAT_BUMP(arcstat_l2_log_blk_count);
                 zfs_refcount_add_many(&dev->l2ad_lb_asize, asize, lb_ptr_buf);
                 zfs_refcount_add(&dev->l2ad_lb_count, lb_ptr_buf);
                 mutex_exit(&dev->l2ad_mtx);
                 vdev_space_update(vd, asize, 0, 0);
 
                 /*
                  * Protection against loops of log blocks:
                  *
                  *                                     l2ad_hand  l2ad_evict
                  *                                         V          V
                  * l2ad_start |=======================================| l2ad_end
                  *             -----|||----|||---|||----|||
                  *                  (3)    (2)   (1)    (0)
                  *             ---|||---|||----|||---|||
                  *                (7)   (6)    (5)   (4)
                  *
                  * In this situation the pointer of log block (4) passes
                  * l2arc_log_blkptr_valid() but the log block should not be
                  * restored as it is overwritten by the payload of log block
                  * (0). Only log blocks (0)-(3) should be restored. We check
                  * whether l2ad_evict lies in between the payload starting
                  * offset of the next log block (lbps[1].lbp_payload_start)
                  * and the payload starting offset of the present log block
                  * (lbps[0].lbp_payload_start). If true and this isn't the
                  * first pass, we are looping from the beginning and we should
                  * stop.
                  */
                 if (l2arc_range_check_overlap(lbps[1].lbp_payload_start,
                     lbps[0].lbp_payload_start, dev->l2ad_evict) &&
                     !dev->l2ad_first)
                         goto out;
 
                 kpreempt(KPREEMPT_SYNC);
                 for (;;) {
                         mutex_enter(&l2arc_rebuild_thr_lock);
                         if (dev->l2ad_rebuild_cancel) {
                                 dev->l2ad_rebuild = B_FALSE;
                                 cv_signal(&l2arc_rebuild_thr_cv);
                                 mutex_exit(&l2arc_rebuild_thr_lock);
                                 err = SET_ERROR(ECANCELED);
                                 goto out;
                         }
                         mutex_exit(&l2arc_rebuild_thr_lock);
                         if (spa_config_tryenter(spa, SCL_L2ARC, vd,
                             RW_READER)) {
                                 lock_held = B_TRUE;
                                 break;
                         }
                         /*
                          * L2ARC config lock held by somebody in writer,
                          * possibly due to them trying to remove us. They'll
                          * likely to want us to shut down, so after a little
                          * delay, we check l2ad_rebuild_cancel and retry
                          * the lock again.
                          */
                         delay(1);
                 }
 
                 /*
                  * Continue with the next log block.
                  */
                 lbps[0] = lbps[1];
                 lbps[1] = this_lb->lb_prev_lbp;
                 PTR_SWAP(this_lb, next_lb);
                 this_io = next_io;
                 next_io = NULL;
         }
 
         if (this_io != NULL)
                 l2arc_log_blk_fetch_abort(this_io);
 out:
         if (next_io != NULL)
                 l2arc_log_blk_fetch_abort(next_io);
         vmem_free(this_lb, sizeof (*this_lb));
         vmem_free(next_lb, sizeof (*next_lb));
 
         if (!l2arc_rebuild_enabled) {
                 spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                     "disabled");
         } else if (err == 0 && zfs_refcount_count(&dev->l2ad_lb_count) > 0) {
                 ARCSTAT_BUMP(arcstat_l2_rebuild_success);
                 spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                     "successful, restored %llu blocks",
                     (u_longlong_t)zfs_refcount_count(&dev->l2ad_lb_count));
         } else if (err == 0 && zfs_refcount_count(&dev->l2ad_lb_count) == 0) {
                 /*
                  * No error but also nothing restored, meaning the lbps array
                  * in the device header points to invalid/non-present log
                  * blocks. Reset the header.
                  */
                 spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                     "no valid log blocks");
                 memset(l2dhdr, 0, dev->l2ad_dev_hdr_asize);
                 l2arc_dev_hdr_update(dev);
         } else if (err == ECANCELED) {
                 /*
                  * In case the rebuild was canceled do not log to spa history
                  * log as the pool may be in the process of being removed.
                  */
                 zfs_dbgmsg("L2ARC rebuild aborted, restored %llu blocks",
                     (u_longlong_t)zfs_refcount_count(&dev->l2ad_lb_count));
         } else if (err != 0) {
                 spa_history_log_internal(spa, "L2ARC rebuild", NULL,
                     "aborted, restored %llu blocks",
                     (u_longlong_t)zfs_refcount_count(&dev->l2ad_lb_count));
         }
 
         if (lock_held)
                 spa_config_exit(spa, SCL_L2ARC, vd);
 
         return (err);
 }
 
 /*
  * Attempts to read the device header on the provided L2ARC device and writes
  * it to `hdr'. On success, this function returns 0, otherwise the appropriate
  * error code is returned.
  */
 static int
 l2arc_dev_hdr_read(l2arc_dev_t *dev)
 {
         int                     err;
         uint64_t                guid;
         l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
         const uint64_t          l2dhdr_asize = dev->l2ad_dev_hdr_asize;
         abd_t                   *abd;
 
         guid = spa_guid(dev->l2ad_vdev->vdev_spa);
 
         abd = abd_get_from_buf(l2dhdr, l2dhdr_asize);
 
         err = zio_wait(zio_read_phys(NULL, dev->l2ad_vdev,
             VDEV_LABEL_START_SIZE, l2dhdr_asize, abd,
             ZIO_CHECKSUM_LABEL, NULL, NULL, ZIO_PRIORITY_SYNC_READ,
             ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
             ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY |
             ZIO_FLAG_SPECULATIVE, B_FALSE));
 
         abd_free(abd);
 
         if (err != 0) {
                 ARCSTAT_BUMP(arcstat_l2_rebuild_abort_dh_errors);
                 zfs_dbgmsg("L2ARC IO error (%d) while reading device header, "
                     "vdev guid: %llu", err,
                     (u_longlong_t)dev->l2ad_vdev->vdev_guid);
                 return (err);
         }
 
         if (l2dhdr->dh_magic == BSWAP_64(L2ARC_DEV_HDR_MAGIC))
                 byteswap_uint64_array(l2dhdr, sizeof (*l2dhdr));
 
         if (l2dhdr->dh_magic != L2ARC_DEV_HDR_MAGIC ||
             l2dhdr->dh_spa_guid != guid ||
             l2dhdr->dh_vdev_guid != dev->l2ad_vdev->vdev_guid ||
             l2dhdr->dh_version != L2ARC_PERSISTENT_VERSION ||
             l2dhdr->dh_log_entries != dev->l2ad_log_entries ||
             l2dhdr->dh_end != dev->l2ad_end ||
             !l2arc_range_check_overlap(dev->l2ad_start, dev->l2ad_end,
             l2dhdr->dh_evict) ||
             (l2dhdr->dh_trim_state != VDEV_TRIM_COMPLETE &&
             l2arc_trim_ahead > 0)) {
                 /*
                  * Attempt to rebuild a device containing no actual dev hdr
                  * or containing a header from some other pool or from another
                  * version of persistent L2ARC.
                  */
                 ARCSTAT_BUMP(arcstat_l2_rebuild_abort_unsupported);
                 return (SET_ERROR(ENOTSUP));
         }
 
         return (0);
 }
 
 /*
  * Reads L2ARC log blocks from storage and validates their contents.
  *
  * This function implements a simple fetcher to make sure that while
  * we're processing one buffer the L2ARC is already fetching the next
  * one in the chain.
  *
  * The arguments this_lp and next_lp point to the current and next log block
  * address in the block chain. Similarly, this_lb and next_lb hold the
  * l2arc_log_blk_phys_t's of the current and next L2ARC blk.
  *
  * The `this_io' and `next_io' arguments are used for block fetching.
  * When issuing the first blk IO during rebuild, you should pass NULL for
  * `this_io'. This function will then issue a sync IO to read the block and
  * also issue an async IO to fetch the next block in the block chain. The
  * fetched IO is returned in `next_io'. On subsequent calls to this
  * function, pass the value returned in `next_io' from the previous call
  * as `this_io' and a fresh `next_io' pointer to hold the next fetch IO.
  * Prior to the call, you should initialize your `next_io' pointer to be
  * NULL. If no fetch IO was issued, the pointer is left set at NULL.
  *
  * On success, this function returns 0, otherwise it returns an appropriate
  * error code. On error the fetching IO is aborted and cleared before
  * returning from this function. Therefore, if we return `success', the
  * caller can assume that we have taken care of cleanup of fetch IOs.
  */
 static int
 l2arc_log_blk_read(l2arc_dev_t *dev,
     const l2arc_log_blkptr_t *this_lbp, const l2arc_log_blkptr_t *next_lbp,
     l2arc_log_blk_phys_t *this_lb, l2arc_log_blk_phys_t *next_lb,
     zio_t *this_io, zio_t **next_io)
 {
         int             err = 0;
         zio_cksum_t     cksum;
         abd_t           *abd = NULL;
         uint64_t        asize;
 
         ASSERT(this_lbp != NULL && next_lbp != NULL);
         ASSERT(this_lb != NULL && next_lb != NULL);
         ASSERT(next_io != NULL && *next_io == NULL);
         ASSERT(l2arc_log_blkptr_valid(dev, this_lbp));
 
         /*
          * Check to see if we have issued the IO for this log block in a
          * previous run. If not, this is the first call, so issue it now.
          */
         if (this_io == NULL) {
                 this_io = l2arc_log_blk_fetch(dev->l2ad_vdev, this_lbp,
                     this_lb);
         }
 
         /*
          * Peek to see if we can start issuing the next IO immediately.
          */
         if (l2arc_log_blkptr_valid(dev, next_lbp)) {
                 /*
                  * Start issuing IO for the next log block early - this
                  * should help keep the L2ARC device busy while we
                  * decompress and restore this log block.
                  */
                 *next_io = l2arc_log_blk_fetch(dev->l2ad_vdev, next_lbp,
                     next_lb);
         }
 
         /* Wait for the IO to read this log block to complete */
         if ((err = zio_wait(this_io)) != 0) {
                 ARCSTAT_BUMP(arcstat_l2_rebuild_abort_io_errors);
                 zfs_dbgmsg("L2ARC IO error (%d) while reading log block, "
                     "offset: %llu, vdev guid: %llu", err,
                     (u_longlong_t)this_lbp->lbp_daddr,
                     (u_longlong_t)dev->l2ad_vdev->vdev_guid);
                 goto cleanup;
         }
 
         /*
          * Make sure the buffer checks out.
          * L2BLK_GET_PSIZE returns aligned size for log blocks.
          */
         asize = L2BLK_GET_PSIZE((this_lbp)->lbp_prop);
         fletcher_4_native(this_lb, asize, NULL, &cksum);
         if (!ZIO_CHECKSUM_EQUAL(cksum, this_lbp->lbp_cksum)) {
                 ARCSTAT_BUMP(arcstat_l2_rebuild_abort_cksum_lb_errors);
                 zfs_dbgmsg("L2ARC log block cksum failed, offset: %llu, "
                     "vdev guid: %llu, l2ad_hand: %llu, l2ad_evict: %llu",
                     (u_longlong_t)this_lbp->lbp_daddr,
                     (u_longlong_t)dev->l2ad_vdev->vdev_guid,
                     (u_longlong_t)dev->l2ad_hand,
                     (u_longlong_t)dev->l2ad_evict);
                 err = SET_ERROR(ECKSUM);
                 goto cleanup;
         }
 
         /* Now we can take our time decoding this buffer */
         switch (L2BLK_GET_COMPRESS((this_lbp)->lbp_prop)) {
         case ZIO_COMPRESS_OFF:
                 break;
         case ZIO_COMPRESS_LZ4:
                 abd = abd_alloc_for_io(asize, B_TRUE);
                 abd_copy_from_buf_off(abd, this_lb, 0, asize);
                 if ((err = zio_decompress_data(
                     L2BLK_GET_COMPRESS((this_lbp)->lbp_prop),
                     abd, this_lb, asize, sizeof (*this_lb), NULL)) != 0) {
                         err = SET_ERROR(EINVAL);
                         goto cleanup;
                 }
                 break;
         default:
                 err = SET_ERROR(EINVAL);
                 goto cleanup;
         }
         if (this_lb->lb_magic == BSWAP_64(L2ARC_LOG_BLK_MAGIC))
                 byteswap_uint64_array(this_lb, sizeof (*this_lb));
         if (this_lb->lb_magic != L2ARC_LOG_BLK_MAGIC) {
                 err = SET_ERROR(EINVAL);
                 goto cleanup;
         }
 cleanup:
         /* Abort an in-flight fetch I/O in case of error */
         if (err != 0 && *next_io != NULL) {
                 l2arc_log_blk_fetch_abort(*next_io);
                 *next_io = NULL;
         }
         if (abd != NULL)
                 abd_free(abd);
         return (err);
 }
 
 /*
  * Restores the payload of a log block to ARC. This creates empty ARC hdr
  * entries which only contain an l2arc hdr, essentially restoring the
  * buffers to their L2ARC evicted state. This function also updates space
  * usage on the L2ARC vdev to make sure it tracks restored buffers.
  */
 static void
 l2arc_log_blk_restore(l2arc_dev_t *dev, const l2arc_log_blk_phys_t *lb,
     uint64_t lb_asize)
 {
         uint64_t        size = 0, asize = 0;
         uint64_t        log_entries = dev->l2ad_log_entries;
 
         /*
          * Usually arc_adapt() is called only for data, not headers, but
          * since we may allocate significant amount of memory here, let ARC
          * grow its arc_c.
          */
         arc_adapt(log_entries * HDR_L2ONLY_SIZE, arc_l2c_only);
 
         for (int i = log_entries - 1; i >= 0; i--) {
                 /*
                  * Restore goes in the reverse temporal direction to preserve
                  * correct temporal ordering of buffers in the l2ad_buflist.
                  * l2arc_hdr_restore also does a list_insert_tail instead of
                  * list_insert_head on the l2ad_buflist:
                  *
                  *              LIST    l2ad_buflist            LIST
                  *              HEAD  <------ (time) ------     TAIL
                  * direction    +-----+-----+-----+-----+-----+    direction
                  * of l2arc <== | buf | buf | buf | buf | buf | ===> of rebuild
                  * fill         +-----+-----+-----+-----+-----+
                  *              ^                               ^
                  *              |                               |
                  *              |                               |
                  *      l2arc_feed_thread               l2arc_rebuild
                  *      will place new bufs here        restores bufs here
                  *
                  * During l2arc_rebuild() the device is not used by
                  * l2arc_feed_thread() as dev->l2ad_rebuild is set to true.
                  */
                 size += L2BLK_GET_LSIZE((&lb->lb_entries[i])->le_prop);
                 asize += vdev_psize_to_asize(dev->l2ad_vdev,
                     L2BLK_GET_PSIZE((&lb->lb_entries[i])->le_prop));
                 l2arc_hdr_restore(&lb->lb_entries[i], dev);
         }
 
         /*
          * Record rebuild stats:
          *      size            Logical size of restored buffers in the L2ARC
          *      asize           Aligned size of restored buffers in the L2ARC
          */
         ARCSTAT_INCR(arcstat_l2_rebuild_size, size);
         ARCSTAT_INCR(arcstat_l2_rebuild_asize, asize);
         ARCSTAT_INCR(arcstat_l2_rebuild_bufs, log_entries);
         ARCSTAT_F_AVG(arcstat_l2_log_blk_avg_asize, lb_asize);
         ARCSTAT_F_AVG(arcstat_l2_data_to_meta_ratio, asize / lb_asize);
         ARCSTAT_BUMP(arcstat_l2_rebuild_log_blks);
 }
 
 /*
  * Restores a single ARC buf hdr from a log entry. The ARC buffer is put
  * into a state indicating that it has been evicted to L2ARC.
  */
 static void
 l2arc_hdr_restore(const l2arc_log_ent_phys_t *le, l2arc_dev_t *dev)
 {
         arc_buf_hdr_t           *hdr, *exists;
         kmutex_t                *hash_lock;
         arc_buf_contents_t      type = L2BLK_GET_TYPE((le)->le_prop);
         uint64_t                asize;
 
         /*
          * Do all the allocation before grabbing any locks, this lets us
          * sleep if memory is full and we don't have to deal with failed
          * allocations.
          */
         hdr = arc_buf_alloc_l2only(L2BLK_GET_LSIZE((le)->le_prop), type,
             dev, le->le_dva, le->le_daddr,
             L2BLK_GET_PSIZE((le)->le_prop), le->le_birth,
             L2BLK_GET_COMPRESS((le)->le_prop), le->le_complevel,
             L2BLK_GET_PROTECTED((le)->le_prop),
             L2BLK_GET_PREFETCH((le)->le_prop),
             L2BLK_GET_STATE((le)->le_prop));
         asize = vdev_psize_to_asize(dev->l2ad_vdev,
             L2BLK_GET_PSIZE((le)->le_prop));
 
         /*
          * vdev_space_update() has to be called before arc_hdr_destroy() to
          * avoid underflow since the latter also calls vdev_space_update().
          */
         l2arc_hdr_arcstats_increment(hdr);
         vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
 
         mutex_enter(&dev->l2ad_mtx);
         list_insert_tail(&dev->l2ad_buflist, hdr);
         (void) zfs_refcount_add_many(&dev->l2ad_alloc, arc_hdr_size(hdr), hdr);
         mutex_exit(&dev->l2ad_mtx);
 
         exists = buf_hash_insert(hdr, &hash_lock);
         if (exists) {
                 /* Buffer was already cached, no need to restore it. */
                 arc_hdr_destroy(hdr);
                 /*
                  * If the buffer is already cached, check whether it has
                  * L2ARC metadata. If not, enter them and update the flag.
                  * This is important is case of onlining a cache device, since
                  * we previously evicted all L2ARC metadata from ARC.
                  */
                 if (!HDR_HAS_L2HDR(exists)) {
                         arc_hdr_set_flags(exists, ARC_FLAG_HAS_L2HDR);
                         exists->b_l2hdr.b_dev = dev;
                         exists->b_l2hdr.b_daddr = le->le_daddr;
                         exists->b_l2hdr.b_arcs_state =
                             L2BLK_GET_STATE((le)->le_prop);
                         mutex_enter(&dev->l2ad_mtx);
                         list_insert_tail(&dev->l2ad_buflist, exists);
                         (void) zfs_refcount_add_many(&dev->l2ad_alloc,
                             arc_hdr_size(exists), exists);
                         mutex_exit(&dev->l2ad_mtx);
                         l2arc_hdr_arcstats_increment(exists);
                         vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
                 }
                 ARCSTAT_BUMP(arcstat_l2_rebuild_bufs_precached);
         }
 
         mutex_exit(hash_lock);
 }
 
 /*
  * Starts an asynchronous read IO to read a log block. This is used in log
  * block reconstruction to start reading the next block before we are done
  * decoding and reconstructing the current block, to keep the l2arc device
  * nice and hot with read IO to process.
  * The returned zio will contain a newly allocated memory buffers for the IO
  * data which should then be freed by the caller once the zio is no longer
  * needed (i.e. due to it having completed). If you wish to abort this
  * zio, you should do so using l2arc_log_blk_fetch_abort, which takes
  * care of disposing of the allocated buffers correctly.
  */
 static zio_t *
 l2arc_log_blk_fetch(vdev_t *vd, const l2arc_log_blkptr_t *lbp,
     l2arc_log_blk_phys_t *lb)
 {
         uint32_t                asize;
         zio_t                   *pio;
         l2arc_read_callback_t   *cb;
 
         /* L2BLK_GET_PSIZE returns aligned size for log blocks */
         asize = L2BLK_GET_PSIZE((lbp)->lbp_prop);
         ASSERT(asize <= sizeof (l2arc_log_blk_phys_t));
 
         cb = kmem_zalloc(sizeof (l2arc_read_callback_t), KM_SLEEP);
         cb->l2rcb_abd = abd_get_from_buf(lb, asize);
         pio = zio_root(vd->vdev_spa, l2arc_blk_fetch_done, cb,
             ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL | ZIO_FLAG_DONT_PROPAGATE |
             ZIO_FLAG_DONT_RETRY);
         (void) zio_nowait(zio_read_phys(pio, vd, lbp->lbp_daddr, asize,
             cb->l2rcb_abd, ZIO_CHECKSUM_OFF, NULL, NULL,
             ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_DONT_CACHE | ZIO_FLAG_CANFAIL |
             ZIO_FLAG_DONT_PROPAGATE | ZIO_FLAG_DONT_RETRY, B_FALSE));
 
         return (pio);
 }
 
 /*
  * Aborts a zio returned from l2arc_log_blk_fetch and frees the data
  * buffers allocated for it.
  */
 static void
 l2arc_log_blk_fetch_abort(zio_t *zio)
 {
         (void) zio_wait(zio);
 }
 
 /*
  * Creates a zio to update the device header on an l2arc device.
  */
 void
 l2arc_dev_hdr_update(l2arc_dev_t *dev)
 {
         l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
         const uint64_t          l2dhdr_asize = dev->l2ad_dev_hdr_asize;
         abd_t                   *abd;
         int                     err;
 
         VERIFY(spa_config_held(dev->l2ad_spa, SCL_STATE_ALL, RW_READER));
 
         l2dhdr->dh_magic = L2ARC_DEV_HDR_MAGIC;
         l2dhdr->dh_version = L2ARC_PERSISTENT_VERSION;
         l2dhdr->dh_spa_guid = spa_guid(dev->l2ad_vdev->vdev_spa);
         l2dhdr->dh_vdev_guid = dev->l2ad_vdev->vdev_guid;
         l2dhdr->dh_log_entries = dev->l2ad_log_entries;
         l2dhdr->dh_evict = dev->l2ad_evict;
         l2dhdr->dh_start = dev->l2ad_start;
         l2dhdr->dh_end = dev->l2ad_end;
         l2dhdr->dh_lb_asize = zfs_refcount_count(&dev->l2ad_lb_asize);
         l2dhdr->dh_lb_count = zfs_refcount_count(&dev->l2ad_lb_count);
         l2dhdr->dh_flags = 0;
         l2dhdr->dh_trim_action_time = dev->l2ad_vdev->vdev_trim_action_time;
         l2dhdr->dh_trim_state = dev->l2ad_vdev->vdev_trim_state;
         if (dev->l2ad_first)
                 l2dhdr->dh_flags |= L2ARC_DEV_HDR_EVICT_FIRST;
 
         abd = abd_get_from_buf(l2dhdr, l2dhdr_asize);
 
         err = zio_wait(zio_write_phys(NULL, dev->l2ad_vdev,
             VDEV_LABEL_START_SIZE, l2dhdr_asize, abd, ZIO_CHECKSUM_LABEL, NULL,
             NULL, ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_CANFAIL, B_FALSE));
 
         abd_free(abd);
 
         if (err != 0) {
                 zfs_dbgmsg("L2ARC IO error (%d) while writing device header, "
                     "vdev guid: %llu", err,
                     (u_longlong_t)dev->l2ad_vdev->vdev_guid);
         }
 }
 
 /*
  * Commits a log block to the L2ARC device. This routine is invoked from
  * l2arc_write_buffers when the log block fills up.
  * This function allocates some memory to temporarily hold the serialized
  * buffer to be written. This is then released in l2arc_write_done.
  */
 static void
 l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio, l2arc_write_callback_t *cb)
 {
         l2arc_log_blk_phys_t    *lb = &dev->l2ad_log_blk;
         l2arc_dev_hdr_phys_t    *l2dhdr = dev->l2ad_dev_hdr;
         uint64_t                psize, asize;
         zio_t                   *wzio;
         l2arc_lb_abd_buf_t      *abd_buf;
         uint8_t                 *tmpbuf;
         l2arc_lb_ptr_buf_t      *lb_ptr_buf;
 
         VERIFY3S(dev->l2ad_log_ent_idx, ==, dev->l2ad_log_entries);
 
         tmpbuf = zio_buf_alloc(sizeof (*lb));
         abd_buf = zio_buf_alloc(sizeof (*abd_buf));
         abd_buf->abd = abd_get_from_buf(lb, sizeof (*lb));
         lb_ptr_buf = kmem_zalloc(sizeof (l2arc_lb_ptr_buf_t), KM_SLEEP);
         lb_ptr_buf->lb_ptr = kmem_zalloc(sizeof (l2arc_log_blkptr_t), KM_SLEEP);
 
         /* link the buffer into the block chain */
         lb->lb_prev_lbp = l2dhdr->dh_start_lbps[1];
         lb->lb_magic = L2ARC_LOG_BLK_MAGIC;
 
         /*
          * l2arc_log_blk_commit() may be called multiple times during a single
          * l2arc_write_buffers() call. Save the allocated abd buffers in a list
          * so we can free them in l2arc_write_done() later on.
          */
         list_insert_tail(&cb->l2wcb_abd_list, abd_buf);
 
         /* try to compress the buffer */
         psize = zio_compress_data(ZIO_COMPRESS_LZ4,
             abd_buf->abd, tmpbuf, sizeof (*lb), 0);
 
         /* a log block is never entirely zero */
         ASSERT(psize != 0);
         asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);
         ASSERT(asize <= sizeof (*lb));
 
         /*
          * Update the start log block pointer in the device header to point
          * to the log block we're about to write.
          */
         l2dhdr->dh_start_lbps[1] = l2dhdr->dh_start_lbps[0];
         l2dhdr->dh_start_lbps[0].lbp_daddr = dev->l2ad_hand;
         l2dhdr->dh_start_lbps[0].lbp_payload_asize =
             dev->l2ad_log_blk_payload_asize;
         l2dhdr->dh_start_lbps[0].lbp_payload_start =
             dev->l2ad_log_blk_payload_start;
         L2BLK_SET_LSIZE(
             (&l2dhdr->dh_start_lbps[0])->lbp_prop, sizeof (*lb));
         L2BLK_SET_PSIZE(
             (&l2dhdr->dh_start_lbps[0])->lbp_prop, asize);
         L2BLK_SET_CHECKSUM(
             (&l2dhdr->dh_start_lbps[0])->lbp_prop,
             ZIO_CHECKSUM_FLETCHER_4);
         if (asize < sizeof (*lb)) {
                 /* compression succeeded */
                 memset(tmpbuf + psize, 0, asize - psize);
                 L2BLK_SET_COMPRESS(
                     (&l2dhdr->dh_start_lbps[0])->lbp_prop,
                     ZIO_COMPRESS_LZ4);
         } else {
                 /* compression failed */
                 memcpy(tmpbuf, lb, sizeof (*lb));
                 L2BLK_SET_COMPRESS(
                     (&l2dhdr->dh_start_lbps[0])->lbp_prop,
                     ZIO_COMPRESS_OFF);
         }
 
         /* checksum what we're about to write */
         fletcher_4_native(tmpbuf, asize, NULL,
             &l2dhdr->dh_start_lbps[0].lbp_cksum);
 
         abd_free(abd_buf->abd);
 
         /* perform the write itself */
         abd_buf->abd = abd_get_from_buf(tmpbuf, sizeof (*lb));
         abd_take_ownership_of_buf(abd_buf->abd, B_TRUE);
         wzio = zio_write_phys(pio, dev->l2ad_vdev, dev->l2ad_hand,
             asize, abd_buf->abd, ZIO_CHECKSUM_OFF, NULL, NULL,
             ZIO_PRIORITY_ASYNC_WRITE, ZIO_FLAG_CANFAIL, B_FALSE);
         DTRACE_PROBE2(l2arc__write, vdev_t *, dev->l2ad_vdev, zio_t *, wzio);
         (void) zio_nowait(wzio);
 
         dev->l2ad_hand += asize;
         /*
          * Include the committed log block's pointer  in the list of pointers
          * to log blocks present in the L2ARC device.
          */
         memcpy(lb_ptr_buf->lb_ptr, &l2dhdr->dh_start_lbps[0],
             sizeof (l2arc_log_blkptr_t));
         mutex_enter(&dev->l2ad_mtx);
         list_insert_head(&dev->l2ad_lbptr_list, lb_ptr_buf);
         ARCSTAT_INCR(arcstat_l2_log_blk_asize, asize);
         ARCSTAT_BUMP(arcstat_l2_log_blk_count);
         zfs_refcount_add_many(&dev->l2ad_lb_asize, asize, lb_ptr_buf);
         zfs_refcount_add(&dev->l2ad_lb_count, lb_ptr_buf);
         mutex_exit(&dev->l2ad_mtx);
         vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
 
         /* bump the kstats */
         ARCSTAT_INCR(arcstat_l2_write_bytes, asize);
         ARCSTAT_BUMP(arcstat_l2_log_blk_writes);
         ARCSTAT_F_AVG(arcstat_l2_log_blk_avg_asize, asize);
         ARCSTAT_F_AVG(arcstat_l2_data_to_meta_ratio,
             dev->l2ad_log_blk_payload_asize / asize);
 
         /* start a new log block */
         dev->l2ad_log_ent_idx = 0;
         dev->l2ad_log_blk_payload_asize = 0;
         dev->l2ad_log_blk_payload_start = 0;
 }
 
 /*
  * Validates an L2ARC log block address to make sure that it can be read
  * from the provided L2ARC device.
  */
 boolean_t
 l2arc_log_blkptr_valid(l2arc_dev_t *dev, const l2arc_log_blkptr_t *lbp)
 {
         /* L2BLK_GET_PSIZE returns aligned size for log blocks */
         uint64_t asize = L2BLK_GET_PSIZE((lbp)->lbp_prop);
         uint64_t end = lbp->lbp_daddr + asize - 1;
         uint64_t start = lbp->lbp_payload_start;
         boolean_t evicted = B_FALSE;
 
         /*
          * A log block is valid if all of the following conditions are true:
          * - it fits entirely (including its payload) between l2ad_start and
          *   l2ad_end
          * - it has a valid size
          * - neither the log block itself nor part of its payload was evicted
          *   by l2arc_evict():
          *
          *              l2ad_hand          l2ad_evict
          *              |                        |      lbp_daddr
          *              |     start              |      |  end
          *              |     |                  |      |  |
          *              V     V                  V      V  V
          *   l2ad_start ============================================ l2ad_end
          *                    --------------------------||||
          *                              ^                ^
          *                              |               log block
          *                              payload
          */
 
         evicted =
             l2arc_range_check_overlap(start, end, dev->l2ad_hand) ||
             l2arc_range_check_overlap(start, end, dev->l2ad_evict) ||
             l2arc_range_check_overlap(dev->l2ad_hand, dev->l2ad_evict, start) ||
             l2arc_range_check_overlap(dev->l2ad_hand, dev->l2ad_evict, end);
 
         return (start >= dev->l2ad_start && end <= dev->l2ad_end &&
             asize > 0 && asize <= sizeof (l2arc_log_blk_phys_t) &&
             (!evicted || dev->l2ad_first));
 }
 
 /*
  * Inserts ARC buffer header `hdr' into the current L2ARC log block on
  * the device. The buffer being inserted must be present in L2ARC.
  * Returns B_TRUE if the L2ARC log block is full and needs to be committed
  * to L2ARC, or B_FALSE if it still has room for more ARC buffers.
  */
 static boolean_t
 l2arc_log_blk_insert(l2arc_dev_t *dev, const arc_buf_hdr_t *hdr)
 {
         l2arc_log_blk_phys_t    *lb = &dev->l2ad_log_blk;
         l2arc_log_ent_phys_t    *le;
 
         if (dev->l2ad_log_entries == 0)
                 return (B_FALSE);
 
         int index = dev->l2ad_log_ent_idx++;
 
         ASSERT3S(index, <, dev->l2ad_log_entries);
         ASSERT(HDR_HAS_L2HDR(hdr));
 
         le = &lb->lb_entries[index];
         memset(le, 0, sizeof (*le));
         le->le_dva = hdr->b_dva;
         le->le_birth = hdr->b_birth;
         le->le_daddr = hdr->b_l2hdr.b_daddr;
         if (index == 0)
                 dev->l2ad_log_blk_payload_start = le->le_daddr;
         L2BLK_SET_LSIZE((le)->le_prop, HDR_GET_LSIZE(hdr));
         L2BLK_SET_PSIZE((le)->le_prop, HDR_GET_PSIZE(hdr));
         L2BLK_SET_COMPRESS((le)->le_prop, HDR_GET_COMPRESS(hdr));
         le->le_complevel = hdr->b_complevel;
         L2BLK_SET_TYPE((le)->le_prop, hdr->b_type);
         L2BLK_SET_PROTECTED((le)->le_prop, !!(HDR_PROTECTED(hdr)));
         L2BLK_SET_PREFETCH((le)->le_prop, !!(HDR_PREFETCH(hdr)));
         L2BLK_SET_STATE((le)->le_prop, hdr->b_l1hdr.b_state->arcs_state);
 
         dev->l2ad_log_blk_payload_asize += vdev_psize_to_asize(dev->l2ad_vdev,
             HDR_GET_PSIZE(hdr));
 
         return (dev->l2ad_log_ent_idx == dev->l2ad_log_entries);
 }
 
 /*
  * Checks whether a given L2ARC device address sits in a time-sequential
  * range. The trick here is that the L2ARC is a rotary buffer, so we can't
  * just do a range comparison, we need to handle the situation in which the
  * range wraps around the end of the L2ARC device. Arguments:
  *      bottom -- Lower end of the range to check (written to earlier).
  *      top    -- Upper end of the range to check (written to later).
  *      check  -- The address for which we want to determine if it sits in
  *                between the top and bottom.
  *
  * The 3-way conditional below represents the following cases:
  *
  *      bottom < top : Sequentially ordered case:
  *        <check>--------+-------------------+
  *                       |  (overlap here?)  |
  *       L2ARC dev       V                   V
  *       |---------------<bottom>============<top>--------------|
  *
  *      bottom > top: Looped-around case:
  *                            <check>--------+------------------+
  *                                           |  (overlap here?) |
  *       L2ARC dev                           V                  V
  *       |===============<top>---------------<bottom>===========|
  *       ^               ^
  *       |  (or here?)   |
  *       +---------------+---------<check>
  *
  *      top == bottom : Just a single address comparison.
  */
 boolean_t
 l2arc_range_check_overlap(uint64_t bottom, uint64_t top, uint64_t check)
 {
         if (bottom < top)
                 return (bottom <= check && check <= top);
         else if (bottom > top)
                 return (check <= top || bottom <= check);
         else
                 return (check == top);
 }
 
 EXPORT_SYMBOL(arc_buf_size);
 EXPORT_SYMBOL(arc_write);
 EXPORT_SYMBOL(arc_read);
 EXPORT_SYMBOL(arc_buf_info);
 EXPORT_SYMBOL(arc_getbuf_func);
 EXPORT_SYMBOL(arc_add_prune_callback);
 EXPORT_SYMBOL(arc_remove_prune_callback);
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min, param_set_arc_min,
         spl_param_get_u64, ZMOD_RW, "Minimum ARC size in bytes");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, max, param_set_arc_max,
         spl_param_get_u64, ZMOD_RW, "Maximum ARC size in bytes");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_limit, param_set_arc_u64,
         spl_param_get_u64, ZMOD_RW, "Metadata limit for ARC size in bytes");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_limit_percent,
     param_set_arc_int, param_get_uint, ZMOD_RW,
         "Percent of ARC size for ARC meta limit");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, meta_min, param_set_arc_u64,
         spl_param_get_u64, ZMOD_RW, "Minimum ARC metadata size in bytes");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_prune, INT, ZMOD_RW,
         "Meta objects to scan for prune");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_adjust_restarts, UINT, ZMOD_RW,
         "Limit number of restarts in arc_evict_meta");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, meta_strategy, UINT, ZMOD_RW,
         "Meta reclaim strategy");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, grow_retry, param_set_arc_int,
         param_get_uint, ZMOD_RW, "Seconds before growing ARC size");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, p_dampener_disable, INT, ZMOD_RW,
         "Disable arc_p adapt dampener");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, shrink_shift, param_set_arc_int,
         param_get_uint, ZMOD_RW, "log2(fraction of ARC to reclaim)");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, pc_percent, UINT, ZMOD_RW,
         "Percent of pagecache to reclaim ARC to");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, p_min_shift, param_set_arc_int,
         param_get_uint, ZMOD_RW, "arc_c shift to calc min/max arc_p");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, average_blocksize, UINT, ZMOD_RD,
         "Target average block size");
 
 ZFS_MODULE_PARAM(zfs, zfs_, compressed_arc_enabled, INT, ZMOD_RW,
         "Disable compressed ARC buffers");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min_prefetch_ms, param_set_arc_int,
         param_get_uint, ZMOD_RW, "Min life of prefetch block in ms");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, min_prescient_prefetch_ms,
     param_set_arc_int, param_get_uint, ZMOD_RW,
         "Min life of prescient prefetched block in ms");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, write_max, U64, ZMOD_RW,
         "Max write bytes per interval");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, write_boost, U64, ZMOD_RW,
         "Extra write bytes during device warmup");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, headroom, U64, ZMOD_RW,
         "Number of max device writes to precache");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, headroom_boost, U64, ZMOD_RW,
         "Compressed l2arc_headroom multiplier");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, trim_ahead, U64, ZMOD_RW,
         "TRIM ahead L2ARC write size multiplier");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_secs, U64, ZMOD_RW,
         "Seconds between L2ARC writing");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_min_ms, U64, ZMOD_RW,
         "Min feed interval in milliseconds");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, noprefetch, INT, ZMOD_RW,
         "Skip caching prefetched buffers");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, feed_again, INT, ZMOD_RW,
         "Turbo L2ARC warmup");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, norw, INT, ZMOD_RW,
         "No reads during writes");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, meta_percent, UINT, ZMOD_RW,
         "Percent of ARC size allowed for L2ARC-only headers");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_enabled, INT, ZMOD_RW,
         "Rebuild the L2ARC when importing a pool");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_blocks_min_l2size, U64, ZMOD_RW,
         "Min size in bytes to write rebuild log blocks in L2ARC");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, mfuonly, INT, ZMOD_RW,
         "Cache only MFU data from ARC into L2ARC");
 
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, exclude_special, INT, ZMOD_RW,
         "Exclude dbufs on special vdevs from being cached to L2ARC if set.");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, lotsfree_percent, param_set_arc_int,
         param_get_uint, ZMOD_RW, "System free memory I/O throttle in bytes");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, sys_free, param_set_arc_u64,
         spl_param_get_u64, ZMOD_RW, "System free memory target size in bytes");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, dnode_limit, param_set_arc_u64,
         spl_param_get_u64, ZMOD_RW, "Minimum bytes of dnodes in ARC");
 
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, dnode_limit_percent,
     param_set_arc_int, param_get_uint, ZMOD_RW,
         "Percent of ARC meta buffers for dnodes");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, dnode_reduce_percent, UINT, ZMOD_RW,
         "Percentage of excess dnodes to try to unpin");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, eviction_pct, UINT, ZMOD_RW,
         "When full, ARC allocation waits for eviction of this % of alloc size");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, evict_batch_limit, UINT, ZMOD_RW,
         "The number of headers to evict per sublist before moving to the next");
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, prune_task_threads, INT, ZMOD_RW,
         "Number of arc_prune threads");