FreeBSD/Linux Kernel Cross Reference
sys/kern/vfs_cache.c

     /*-
      * SPDX-License-Identifier: BSD-3-Clause
      *
      * Copyright (c) 1989, 1993, 1995
      *      The Regents of the University of California.  All rights reserved.
      *
      * This code is derived from software contributed to Berkeley by
      * Poul-Henning Kamp of the FreeBSD Project.
      *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)vfs_cache.c 8.5 (Berkeley) 3/22/95
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_ddb.h"
    #include "opt_ktrace.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/capsicum.h>
    #include <sys/counter.h>
    #include <sys/filedesc.h>
    #include <sys/fnv_hash.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/lock.h>
    #include <sys/malloc.h>
    #include <sys/fcntl.h>
    #include <sys/jail.h>
    #include <sys/mount.h>
    #include <sys/namei.h>
    #include <sys/proc.h>
    #include <sys/seqc.h>
    #include <sys/sdt.h>
    #include <sys/smr.h>
    #include <sys/smp.h>
    #include <sys/syscallsubr.h>
    #include <sys/sysctl.h>
    #include <sys/sysproto.h>
    #include <sys/vnode.h>
    #include <ck_queue.h>
    #ifdef KTRACE
    #include <sys/ktrace.h>
    #endif
    #ifdef INVARIANTS
    #include <machine/_inttypes.h>
    #endif
    
    #include <sys/capsicum.h>
    
    #include <security/audit/audit.h>
    #include <security/mac/mac_framework.h>
    
    #ifdef DDB
    #include <ddb/ddb.h>
    #endif
    
    #include <vm/uma.h>
    
    /*
     * High level overview of name caching in the VFS layer.
     *
     * Originally caching was implemented as part of UFS, later extracted to allow
     * use by other filesystems. A decision was made to make it optional and
     * completely detached from the rest of the kernel, which comes with limitations
     * outlined near the end of this comment block.
     *
     * This fundamental choice needs to be revisited. In the meantime, the current
     * state is described below. Significance of all notable routines is explained
     * in comments placed above their implementation. Scattered thoroughout the
     * file are TODO comments indicating shortcomings which can be fixed without
     * reworking everything (most of the fixes will likely be reusable). Various
     * details are omitted from this explanation to not clutter the overview, they
     * have to be checked by reading the code and associated commentary.
    *
    * Keep in mind that it's individual path components which are cached, not full
    * paths. That is, for a fully cached path "foo/bar/baz" there are 3 entries,
    * one for each name.
    *
    * I. Data organization
    *
    * Entries are described by "struct namecache" objects and stored in a hash
    * table. See cache_get_hash for more information.
    *
    * "struct vnode" contains pointers to source entries (names which can be found
    * when traversing through said vnode), destination entries (names of that
    * vnode (see "Limitations" for a breakdown on the subject) and a pointer to
    * the parent vnode.
    *
    * The (directory vnode; name) tuple reliably determines the target entry if
    * it exists.
    *
    * Since there are no small locks at this time (all are 32 bytes in size on
    * LP64), the code works around the problem by introducing lock arrays to
    * protect hash buckets and vnode lists.
    *
    * II. Filesystem integration
    *
    * Filesystems participating in name caching do the following:
    * - set vop_lookup routine to vfs_cache_lookup
    * - set vop_cachedlookup to whatever can perform the lookup if the above fails
    * - if they support lockless lookup (see below), vop_fplookup_vexec and
    *   vop_fplookup_symlink are set along with the MNTK_FPLOOKUP flag on the
    *   mount point
    * - call cache_purge or cache_vop_* routines to eliminate stale entries as
    *   applicable
    * - call cache_enter to add entries depending on the MAKEENTRY flag
    *
    * With the above in mind, there are 2 entry points when doing lookups:
    * - ... -> namei -> cache_fplookup -- this is the default
    * - ... -> VOP_LOOKUP -> vfs_cache_lookup -- normally only called by namei
    *   should the above fail
    *
    * Example code flow how an entry is added:
    * ... -> namei -> cache_fplookup -> cache_fplookup_noentry -> VOP_LOOKUP ->
    * vfs_cache_lookup -> VOP_CACHEDLOOKUP -> ufs_lookup_ino -> cache_enter
    *
    * III. Performance considerations
    *
    * For lockless case forward lookup avoids any writes to shared areas apart
    * from the terminal path component. In other words non-modifying lookups of
    * different files don't suffer any scalability problems in the namecache.
    * Looking up the same file is limited by VFS and goes beyond the scope of this
    * file.
    *
    * At least on amd64 the single-threaded bottleneck for long paths is hashing
    * (see cache_get_hash). There are cases where the code issues acquire fence
    * multiple times, they can be combined on architectures which suffer from it.
    *
    * For locked case each encountered vnode has to be referenced and locked in
    * order to be handed out to the caller (normally that's namei). This
    * introduces significant hit single-threaded and serialization multi-threaded.
    *
    * Reverse lookup (e.g., "getcwd") fully scales provided it is fully cached --
    * avoids any writes to shared areas to any components.
    *
    * Unrelated insertions are partially serialized on updating the global entry
    * counter and possibly serialized on colliding bucket or vnode locks.
    *
    * IV. Observability
    *
    * Note not everything has an explicit dtrace probe nor it should have, thus
    * some of the one-liners below depend on implementation details.
    *
    * Examples:
    *
    * # Check what lookups failed to be handled in a lockless manner. Column 1 is
    * # line number, column 2 is status code (see cache_fpl_status)
    * dtrace -n 'vfs:fplookup:lookup:done { @[arg1, arg2] = count(); }'
    *
    * # Lengths of names added by binary name
    * dtrace -n 'fbt::cache_enter_time:entry { @[execname] = quantize(args[2]->cn_namelen); }'
    *
    * # Same as above but only those which exceed 64 characters
    * dtrace -n 'fbt::cache_enter_time:entry /args[2]->cn_namelen > 64/ { @[execname] = quantize(args[2]->cn_namelen); }'
    *
    * # Who is performing lookups with spurious slashes (e.g., "foo//bar") and what
    * # path is it
    * dtrace -n 'fbt::cache_fplookup_skip_slashes:entry { @[execname, stringof(args[0]->cnp->cn_pnbuf)] = count(); }'
    *
    * V. Limitations and implementation defects
    *
    * - since it is possible there is no entry for an open file, tools like
    *   "procstat" may fail to resolve fd -> vnode -> path to anything
    * - even if a filesystem adds an entry, it may get purged (e.g., due to memory
    *   shortage) in which case the above problem applies
    * - hardlinks are not tracked, thus if a vnode is reachable in more than one
    *   way, resolving a name may return a different path than the one used to
    *   open it (even if said path is still valid)
    * - by default entries are not added for newly created files
    * - adding an entry may need to evict negative entry first, which happens in 2
    *   distinct places (evicting on lookup, adding in a later VOP) making it
    *   impossible to simply reuse it
    * - there is a simple scheme to evict negative entries as the cache is approaching
    *   its capacity, but it is very unclear if doing so is a good idea to begin with
    * - vnodes are subject to being recycled even if target inode is left in memory,
    *   which loses the name cache entries when it perhaps should not. in case of tmpfs
    *   names get duplicated -- kept by filesystem itself and namecache separately
    * - struct namecache has a fixed size and comes in 2 variants, often wasting space.
    *   now hard to replace with malloc due to dependence on SMR.
    * - lack of better integration with the kernel also turns nullfs into a layered
    *   filesystem instead of something which can take advantage of caching
    */
   
   static SYSCTL_NODE(_vfs, OID_AUTO, cache, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
       "Name cache");
   
   SDT_PROVIDER_DECLARE(vfs);
   SDT_PROBE_DEFINE3(vfs, namecache, enter, done, "struct vnode *", "char *",
       "struct vnode *");
   SDT_PROBE_DEFINE3(vfs, namecache, enter, duplicate, "struct vnode *", "char *",
       "struct vnode *");
   SDT_PROBE_DEFINE2(vfs, namecache, enter_negative, done, "struct vnode *",
       "char *");
   SDT_PROBE_DEFINE2(vfs, namecache, fullpath_smr, hit, "struct vnode *",
       "const char *");
   SDT_PROBE_DEFINE4(vfs, namecache, fullpath_smr, miss, "struct vnode *",
       "struct namecache *", "int", "int");
   SDT_PROBE_DEFINE1(vfs, namecache, fullpath, entry, "struct vnode *");
   SDT_PROBE_DEFINE3(vfs, namecache, fullpath, hit, "struct vnode *",
       "char *", "struct vnode *");
   SDT_PROBE_DEFINE1(vfs, namecache, fullpath, miss, "struct vnode *");
   SDT_PROBE_DEFINE3(vfs, namecache, fullpath, return, "int",
       "struct vnode *", "char *");
   SDT_PROBE_DEFINE3(vfs, namecache, lookup, hit, "struct vnode *", "char *",
       "struct vnode *");
   SDT_PROBE_DEFINE2(vfs, namecache, lookup, hit__negative,
       "struct vnode *", "char *");
   SDT_PROBE_DEFINE2(vfs, namecache, lookup, miss, "struct vnode *",
       "char *");
   SDT_PROBE_DEFINE2(vfs, namecache, removecnp, hit, "struct vnode *",
       "struct componentname *");
   SDT_PROBE_DEFINE2(vfs, namecache, removecnp, miss, "struct vnode *",
       "struct componentname *");
   SDT_PROBE_DEFINE3(vfs, namecache, purge, done, "struct vnode *", "size_t", "size_t");
   SDT_PROBE_DEFINE1(vfs, namecache, purge, batch, "int");
   SDT_PROBE_DEFINE1(vfs, namecache, purge_negative, done, "struct vnode *");
   SDT_PROBE_DEFINE1(vfs, namecache, purgevfs, done, "struct mount *");
   SDT_PROBE_DEFINE3(vfs, namecache, zap, done, "struct vnode *", "char *",
       "struct vnode *");
   SDT_PROBE_DEFINE2(vfs, namecache, zap_negative, done, "struct vnode *",
       "char *");
   SDT_PROBE_DEFINE2(vfs, namecache, evict_negative, done, "struct vnode *",
       "char *");
   SDT_PROBE_DEFINE1(vfs, namecache, symlink, alloc__fail, "size_t");
   
   SDT_PROBE_DEFINE3(vfs, fplookup, lookup, done, "struct nameidata", "int", "bool");
   SDT_PROBE_DECLARE(vfs, namei, lookup, entry);
   SDT_PROBE_DECLARE(vfs, namei, lookup, return);
   
   static char __read_frequently cache_fast_lookup_enabled = true;
   
   /*
    * This structure describes the elements in the cache of recent
    * names looked up by namei.
    */
   struct negstate {
           u_char neg_flag;
           u_char neg_hit;
   };
   _Static_assert(sizeof(struct negstate) <= sizeof(struct vnode *),
       "the state must fit in a union with a pointer without growing it");
   
   struct  namecache {
           LIST_ENTRY(namecache) nc_src;   /* source vnode list */
           TAILQ_ENTRY(namecache) nc_dst;  /* destination vnode list */
           CK_SLIST_ENTRY(namecache) nc_hash;/* hash chain */
           struct  vnode *nc_dvp;          /* vnode of parent of name */
           union {
                   struct  vnode *nu_vp;   /* vnode the name refers to */
                   struct  negstate nu_neg;/* negative entry state */
           } n_un;
           u_char  nc_flag;                /* flag bits */
           u_char  nc_nlen;                /* length of name */
           char    nc_name[0];             /* segment name + nul */
   };
   
   /*
    * struct namecache_ts repeats struct namecache layout up to the
    * nc_nlen member.
    * struct namecache_ts is used in place of struct namecache when time(s) need
    * to be stored.  The nc_dotdottime field is used when a cache entry is mapping
    * both a non-dotdot directory name plus dotdot for the directory's
    * parent.
    *
    * See below for alignment requirement.
    */
   struct  namecache_ts {
           struct  timespec nc_time;       /* timespec provided by fs */
           struct  timespec nc_dotdottime; /* dotdot timespec provided by fs */
           int     nc_ticks;               /* ticks value when entry was added */
           int     nc_pad;
           struct namecache nc_nc;
   };
   
   TAILQ_HEAD(cache_freebatch, namecache);
   
   /*
    * At least mips n32 performs 64-bit accesses to timespec as found
    * in namecache_ts and requires them to be aligned. Since others
    * may be in the same spot suffer a little bit and enforce the
    * alignment for everyone. Note this is a nop for 64-bit platforms.
    */
   #define CACHE_ZONE_ALIGNMENT    UMA_ALIGNOF(time_t)
   
   /*
    * TODO: the initial value of CACHE_PATH_CUTOFF was inherited from the
    * 4.4 BSD codebase. Later on struct namecache was tweaked to become
    * smaller and the value was bumped to retain the total size, but it
    * was never re-evaluated for suitability. A simple test counting
    * lengths during package building shows that the value of 45 covers
    * about 86% of all added entries, reaching 99% at 65.
    *
    * Regardless of the above, use of dedicated zones instead of malloc may be
    * inducing additional waste. This may be hard to address as said zones are
    * tied to VFS SMR. Even if retaining them, the current split should be
    * re-evaluated.
    */
   #ifdef __LP64__
   #define CACHE_PATH_CUTOFF       45
   #define CACHE_LARGE_PAD         6
   #else
   #define CACHE_PATH_CUTOFF       41
   #define CACHE_LARGE_PAD         2
   #endif
   
   #define CACHE_ZONE_SMALL_SIZE           (offsetof(struct namecache, nc_name) + CACHE_PATH_CUTOFF + 1)
   #define CACHE_ZONE_SMALL_TS_SIZE        (offsetof(struct namecache_ts, nc_nc) + CACHE_ZONE_SMALL_SIZE)
   #define CACHE_ZONE_LARGE_SIZE           (offsetof(struct namecache, nc_name) + NAME_MAX + 1 + CACHE_LARGE_PAD)
   #define CACHE_ZONE_LARGE_TS_SIZE        (offsetof(struct namecache_ts, nc_nc) + CACHE_ZONE_LARGE_SIZE)
   
   _Static_assert((CACHE_ZONE_SMALL_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");
   _Static_assert((CACHE_ZONE_SMALL_TS_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");
   _Static_assert((CACHE_ZONE_LARGE_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");
   _Static_assert((CACHE_ZONE_LARGE_TS_SIZE % (CACHE_ZONE_ALIGNMENT + 1)) == 0, "bad zone size");
   
   #define nc_vp           n_un.nu_vp
   #define nc_neg          n_un.nu_neg
   
   /*
    * Flags in namecache.nc_flag
    */
   #define NCF_WHITE       0x01
   #define NCF_ISDOTDOT    0x02
   #define NCF_TS          0x04
   #define NCF_DTS         0x08
   #define NCF_DVDROP      0x10
   #define NCF_NEGATIVE    0x20
   #define NCF_INVALID     0x40
   #define NCF_WIP         0x80
   
   /*
    * Flags in negstate.neg_flag
    */
   #define NEG_HOT         0x01
   
   static bool     cache_neg_evict_cond(u_long lnumcache);
   
   /*
    * Mark an entry as invalid.
    *
    * This is called before it starts getting deconstructed.
    */
   static void
   cache_ncp_invalidate(struct namecache *ncp)
   {
   
           KASSERT((ncp->nc_flag & NCF_INVALID) == 0,
               ("%s: entry %p already invalid", __func__, ncp));
           atomic_store_char(&ncp->nc_flag, ncp->nc_flag | NCF_INVALID);
           atomic_thread_fence_rel();
   }
   
   /*
    * Check whether the entry can be safely used.
    *
    * All places which elide locks are supposed to call this after they are
    * done with reading from an entry.
    */
   #define cache_ncp_canuse(ncp)   ({                                      \
           struct namecache *_ncp = (ncp);                                 \
           u_char _nc_flag;                                                \
                                                                           \
           atomic_thread_fence_acq();                                      \
           _nc_flag = atomic_load_char(&_ncp->nc_flag);                    \
           __predict_true((_nc_flag & (NCF_INVALID | NCF_WIP)) == 0);      \
   })
   
   /*
    * Like the above but also checks NCF_WHITE.
    */
   #define cache_fpl_neg_ncp_canuse(ncp)   ({                              \
           struct namecache *_ncp = (ncp);                                 \
           u_char _nc_flag;                                                \
                                                                           \
           atomic_thread_fence_acq();                                      \
           _nc_flag = atomic_load_char(&_ncp->nc_flag);                    \
           __predict_true((_nc_flag & (NCF_INVALID | NCF_WIP | NCF_WHITE)) == 0);  \
   })
   
   VFS_SMR_DECLARE;
   
   static SYSCTL_NODE(_vfs_cache, OID_AUTO, param, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
       "Name cache parameters");
   
   static u_int __read_mostly      ncsize; /* the size as computed on creation or resizing */
   SYSCTL_UINT(_vfs_cache_param, OID_AUTO, size, CTLFLAG_RW, &ncsize, 0,
       "Total namecache capacity");
   
   u_int ncsizefactor = 2;
   SYSCTL_UINT(_vfs_cache_param, OID_AUTO, sizefactor, CTLFLAG_RW, &ncsizefactor, 0,
       "Size factor for namecache");
   
   static u_long __read_mostly     ncnegfactor = 5; /* ratio of negative entries */
   SYSCTL_ULONG(_vfs_cache_param, OID_AUTO, negfactor, CTLFLAG_RW, &ncnegfactor, 0,
       "Ratio of negative namecache entries");
   
   /*
    * Negative entry % of namecache capacity above which automatic eviction is allowed.
    *
    * Check cache_neg_evict_cond for details.
    */
   static u_int ncnegminpct = 3;
   
   static u_int __read_mostly     neg_min; /* the above recomputed against ncsize */
   SYSCTL_UINT(_vfs_cache_param, OID_AUTO, negmin, CTLFLAG_RD, &neg_min, 0,
       "Negative entry count above which automatic eviction is allowed");
   
   /*
    * Structures associated with name caching.
    */
   #define NCHHASH(hash) \
           (&nchashtbl[(hash) & nchash])
   static __read_mostly CK_SLIST_HEAD(nchashhead, namecache) *nchashtbl;/* Hash Table */
   static u_long __read_mostly     nchash;                 /* size of hash table */
   SYSCTL_ULONG(_debug, OID_AUTO, nchash, CTLFLAG_RD, &nchash, 0,
       "Size of namecache hash table");
   static u_long __exclusive_cache_line    numneg; /* number of negative entries allocated */
   static u_long __exclusive_cache_line    numcache;/* number of cache entries allocated */
   
   struct nchstats nchstats;               /* cache effectiveness statistics */
   
   static bool __read_mostly cache_rename_add = true;
   SYSCTL_BOOL(_vfs, OID_AUTO, cache_rename_add, CTLFLAG_RW,
       &cache_rename_add, 0, "");
   
   static u_int __exclusive_cache_line neg_cycle;
   
   #define ncneghash       3
   #define numneglists     (ncneghash + 1)
   
   struct neglist {
           struct mtx              nl_evict_lock;
           struct mtx              nl_lock __aligned(CACHE_LINE_SIZE);
           TAILQ_HEAD(, namecache) nl_list;
           TAILQ_HEAD(, namecache) nl_hotlist;
           u_long                  nl_hotnum;
   } __aligned(CACHE_LINE_SIZE);
   
   static struct neglist neglists[numneglists];
   
   static inline struct neglist *
   NCP2NEGLIST(struct namecache *ncp)
   {
   
           return (&neglists[(((uintptr_t)(ncp) >> 8) & ncneghash)]);
   }
   
   static inline struct negstate *
   NCP2NEGSTATE(struct namecache *ncp)
   {
   
           MPASS(atomic_load_char(&ncp->nc_flag) & NCF_NEGATIVE);
           return (&ncp->nc_neg);
   }
   
   #define numbucketlocks (ncbuckethash + 1)
   static u_int __read_mostly  ncbuckethash;
   static struct mtx_padalign __read_mostly  *bucketlocks;
   #define HASH2BUCKETLOCK(hash) \
           ((struct mtx *)(&bucketlocks[((hash) & ncbuckethash)]))
   
   #define numvnodelocks (ncvnodehash + 1)
   static u_int __read_mostly  ncvnodehash;
   static struct mtx __read_mostly *vnodelocks;
   static inline struct mtx *
   VP2VNODELOCK(struct vnode *vp)
   {
   
           return (&vnodelocks[(((uintptr_t)(vp) >> 8) & ncvnodehash)]);
   }
   
   static void
   cache_out_ts(struct namecache *ncp, struct timespec *tsp, int *ticksp)
   {
           struct namecache_ts *ncp_ts;
   
           KASSERT((ncp->nc_flag & NCF_TS) != 0 ||
               (tsp == NULL && ticksp == NULL),
               ("No NCF_TS"));
   
           if (tsp == NULL)
                   return;
   
           ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
           *tsp = ncp_ts->nc_time;
           *ticksp = ncp_ts->nc_ticks;
   }
   
   #ifdef DEBUG_CACHE
   static int __read_mostly        doingcache = 1; /* 1 => enable the cache */
   SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
       "VFS namecache enabled");
   #endif
   
   /* Export size information to userland */
   SYSCTL_INT(_debug_sizeof, OID_AUTO, namecache, CTLFLAG_RD, SYSCTL_NULL_INT_PTR,
       sizeof(struct namecache), "sizeof(struct namecache)");
   
   /*
    * The new name cache statistics
    */
   static SYSCTL_NODE(_vfs_cache, OID_AUTO, stats, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
       "Name cache statistics");
   
   #define STATNODE_ULONG(name, varname, descr)                                    \
           SYSCTL_ULONG(_vfs_cache_stats, OID_AUTO, name, CTLFLAG_RD, &varname, 0, descr);
   #define STATNODE_COUNTER(name, varname, descr)                                  \
           static COUNTER_U64_DEFINE_EARLY(varname);                               \
           SYSCTL_COUNTER_U64(_vfs_cache_stats, OID_AUTO, name, CTLFLAG_RD, &varname, \
               descr);
   STATNODE_ULONG(neg, numneg, "Number of negative cache entries");
   STATNODE_ULONG(count, numcache, "Number of cache entries");
   STATNODE_COUNTER(heldvnodes, numcachehv, "Number of namecache entries with vnodes held");
   STATNODE_COUNTER(drops, numdrops, "Number of dropped entries due to reaching the limit");
   STATNODE_COUNTER(dothits, dothits, "Number of '.' hits");
   STATNODE_COUNTER(dotdothis, dotdothits, "Number of '..' hits");
   STATNODE_COUNTER(miss, nummiss, "Number of cache misses");
   STATNODE_COUNTER(misszap, nummisszap, "Number of cache misses we do not want to cache");
   STATNODE_COUNTER(posszaps, numposzaps,
       "Number of cache hits (positive) we do not want to cache");
   STATNODE_COUNTER(poshits, numposhits, "Number of cache hits (positive)");
   STATNODE_COUNTER(negzaps, numnegzaps,
       "Number of cache hits (negative) we do not want to cache");
   STATNODE_COUNTER(neghits, numneghits, "Number of cache hits (negative)");
   /* These count for vn_getcwd(), too. */
   STATNODE_COUNTER(fullpathcalls, numfullpathcalls, "Number of fullpath search calls");
   STATNODE_COUNTER(fullpathfail1, numfullpathfail1, "Number of fullpath search errors (ENOTDIR)");
   STATNODE_COUNTER(fullpathfail2, numfullpathfail2,
       "Number of fullpath search errors (VOP_VPTOCNP failures)");
   STATNODE_COUNTER(fullpathfail4, numfullpathfail4, "Number of fullpath search errors (ENOMEM)");
   STATNODE_COUNTER(fullpathfound, numfullpathfound, "Number of successful fullpath calls");
   STATNODE_COUNTER(symlinktoobig, symlinktoobig, "Number of times symlink did not fit the cache");
   
   /*
    * Debug or developer statistics.
    */
   static SYSCTL_NODE(_vfs_cache, OID_AUTO, debug, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
       "Name cache debugging");
   #define DEBUGNODE_ULONG(name, varname, descr)                                   \
           SYSCTL_ULONG(_vfs_cache_debug, OID_AUTO, name, CTLFLAG_RD, &varname, 0, descr);
   #define DEBUGNODE_COUNTER(name, varname, descr)                                 \
           static COUNTER_U64_DEFINE_EARLY(varname);                               \
           SYSCTL_COUNTER_U64(_vfs_cache_debug, OID_AUTO, name, CTLFLAG_RD, &varname, \
               descr);
   DEBUGNODE_COUNTER(zap_bucket_relock_success, zap_bucket_relock_success,
       "Number of successful removals after relocking");
   static long zap_bucket_fail;
   DEBUGNODE_ULONG(zap_bucket_fail, zap_bucket_fail, "");
   static long zap_bucket_fail2;
   DEBUGNODE_ULONG(zap_bucket_fail2, zap_bucket_fail2, "");
   static long cache_lock_vnodes_cel_3_failures;
   DEBUGNODE_ULONG(vnodes_cel_3_failures, cache_lock_vnodes_cel_3_failures,
       "Number of times 3-way vnode locking failed");
   
   static void cache_zap_locked(struct namecache *ncp);
   static int vn_fullpath_any_smr(struct vnode *vp, struct vnode *rdir, char *buf,
       char **retbuf, size_t *buflen, size_t addend);
   static int vn_fullpath_any(struct vnode *vp, struct vnode *rdir, char *buf,
       char **retbuf, size_t *buflen);
   static int vn_fullpath_dir(struct vnode *vp, struct vnode *rdir, char *buf,
       char **retbuf, size_t *len, size_t addend);
   
   static MALLOC_DEFINE(M_VFSCACHE, "vfscache", "VFS name cache entries");
   
   static inline void
   cache_assert_vlp_locked(struct mtx *vlp)
   {
   
           if (vlp != NULL)
                   mtx_assert(vlp, MA_OWNED);
   }
   
   static inline void
   cache_assert_vnode_locked(struct vnode *vp)
   {
           struct mtx *vlp;
   
           vlp = VP2VNODELOCK(vp);
           cache_assert_vlp_locked(vlp);
   }
   
   /*
    * Directory vnodes with entries are held for two reasons:
    * 1. make them less of a target for reclamation in vnlru
    * 2. suffer smaller performance penalty in locked lookup as requeieing is avoided
    *
    * It will be feasible to stop doing it altogether if all filesystems start
    * supporting lockless lookup.
    */
   static void
   cache_hold_vnode(struct vnode *vp)
   {
   
           cache_assert_vnode_locked(vp);
           VNPASS(LIST_EMPTY(&vp->v_cache_src), vp);
           vhold(vp);
           counter_u64_add(numcachehv, 1);
   }
   
   static void
   cache_drop_vnode(struct vnode *vp)
   {
   
           /*
            * Called after all locks are dropped, meaning we can't assert
            * on the state of v_cache_src.
            */
           vdrop(vp);
           counter_u64_add(numcachehv, -1);
   }
   
   /*
    * UMA zones.
    */
   static uma_zone_t __read_mostly cache_zone_small;
   static uma_zone_t __read_mostly cache_zone_small_ts;
   static uma_zone_t __read_mostly cache_zone_large;
   static uma_zone_t __read_mostly cache_zone_large_ts;
   
   char *
   cache_symlink_alloc(size_t size, int flags)
   {
   
           if (size < CACHE_ZONE_SMALL_SIZE) {
                   return (uma_zalloc_smr(cache_zone_small, flags));
           }
           if (size < CACHE_ZONE_LARGE_SIZE) {
                   return (uma_zalloc_smr(cache_zone_large, flags));
           }
           counter_u64_add(symlinktoobig, 1);
           SDT_PROBE1(vfs, namecache, symlink, alloc__fail, size);
           return (NULL);
   }
   
   void
   cache_symlink_free(char *string, size_t size)
   {
   
           MPASS(string != NULL);
           KASSERT(size < CACHE_ZONE_LARGE_SIZE,
               ("%s: size %zu too big", __func__, size));
   
           if (size < CACHE_ZONE_SMALL_SIZE) {
                   uma_zfree_smr(cache_zone_small, string);
                   return;
           }
           if (size < CACHE_ZONE_LARGE_SIZE) {
                   uma_zfree_smr(cache_zone_large, string);
                   return;
           }
           __assert_unreachable();
   }
   
   static struct namecache *
   cache_alloc_uma(int len, bool ts)
   {
           struct namecache_ts *ncp_ts;
           struct namecache *ncp;
   
           if (__predict_false(ts)) {
                   if (len <= CACHE_PATH_CUTOFF)
                           ncp_ts = uma_zalloc_smr(cache_zone_small_ts, M_WAITOK);
                   else
                           ncp_ts = uma_zalloc_smr(cache_zone_large_ts, M_WAITOK);
                   ncp = &ncp_ts->nc_nc;
           } else {
                   if (len <= CACHE_PATH_CUTOFF)
                           ncp = uma_zalloc_smr(cache_zone_small, M_WAITOK);
                   else
                           ncp = uma_zalloc_smr(cache_zone_large, M_WAITOK);
           }
           return (ncp);
   }
   
   static void
   cache_free_uma(struct namecache *ncp)
   {
           struct namecache_ts *ncp_ts;
   
           if (__predict_false(ncp->nc_flag & NCF_TS)) {
                   ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
                   if (ncp->nc_nlen <= CACHE_PATH_CUTOFF)
                           uma_zfree_smr(cache_zone_small_ts, ncp_ts);
                   else
                           uma_zfree_smr(cache_zone_large_ts, ncp_ts);
           } else {
                   if (ncp->nc_nlen <= CACHE_PATH_CUTOFF)
                           uma_zfree_smr(cache_zone_small, ncp);
                   else
                           uma_zfree_smr(cache_zone_large, ncp);
           }
   }
   
   static struct namecache *
   cache_alloc(int len, bool ts)
   {
           u_long lnumcache;
   
           /*
            * Avoid blowout in namecache entries.
            *
            * Bugs:
            * 1. filesystems may end up trying to add an already existing entry
            * (for example this can happen after a cache miss during concurrent
            * lookup), in which case we will call cache_neg_evict despite not
            * adding anything.
            * 2. the routine may fail to free anything and no provisions are made
            * to make it try harder (see the inside for failure modes)
            * 3. it only ever looks at negative entries.
            */
           lnumcache = atomic_fetchadd_long(&numcache, 1) + 1;
           if (cache_neg_evict_cond(lnumcache)) {
                   lnumcache = atomic_load_long(&numcache);
           }
           if (__predict_false(lnumcache >= ncsize)) {
                   atomic_subtract_long(&numcache, 1);
                   counter_u64_add(numdrops, 1);
                   return (NULL);
           }
           return (cache_alloc_uma(len, ts));
   }
   
   static void
   cache_free(struct namecache *ncp)
   {
   
           MPASS(ncp != NULL);
           if ((ncp->nc_flag & NCF_DVDROP) != 0) {
                   cache_drop_vnode(ncp->nc_dvp);
           }
           cache_free_uma(ncp);
           atomic_subtract_long(&numcache, 1);
   }
   
   static void
   cache_free_batch(struct cache_freebatch *batch)
   {
           struct namecache *ncp, *nnp;
           int i;
   
           i = 0;
           if (TAILQ_EMPTY(batch))
                   goto out;
           TAILQ_FOREACH_SAFE(ncp, batch, nc_dst, nnp) {
                   if ((ncp->nc_flag & NCF_DVDROP) != 0) {
                           cache_drop_vnode(ncp->nc_dvp);
                   }
                   cache_free_uma(ncp);
                   i++;
           }
           atomic_subtract_long(&numcache, i);
   out:
           SDT_PROBE1(vfs, namecache, purge, batch, i);
   }
   
   /*
    * Hashing.
    *
    * The code was made to use FNV in 2001 and this choice needs to be revisited.
    *
    * Short summary of the difficulty:
    * The longest name which can be inserted is NAME_MAX characters in length (or
    * 255 at the time of writing this comment), while majority of names used in
    * practice are significantly shorter (mostly below 10). More importantly
    * majority of lookups performed find names are even shorter than that.
    *
    * This poses a problem where hashes which do better than FNV past word size
    * (or so) tend to come with additional overhead when finalizing the result,
    * making them noticeably slower for the most commonly used range.
    *
    * Consider a path like: /usr/obj/usr/src/sys/amd64/GENERIC/vnode_if.c
    *
    * When looking it up the most time consuming part by a large margin (at least
    * on amd64) is hashing.  Replacing FNV with something which pessimizes short
    * input would make the slowest part stand out even more.
    */
   
   /*
    * TODO: With the value stored we can do better than computing the hash based
    * on the address.
    */
   static void
   cache_prehash(struct vnode *vp)
   {
   
           vp->v_nchash = fnv_32_buf(&vp, sizeof(vp), FNV1_32_INIT);
   }
   
   static uint32_t
   cache_get_hash(char *name, u_char len, struct vnode *dvp)
   {
   
           return (fnv_32_buf(name, len, dvp->v_nchash));
   }
   
   static uint32_t
   cache_get_hash_iter_start(struct vnode *dvp)
   {
   
           return (dvp->v_nchash);
   }
   
   static uint32_t
   cache_get_hash_iter(char c, uint32_t hash)
   {
   
           return (fnv_32_buf(&c, 1, hash));
   }
   
   static uint32_t
   cache_get_hash_iter_finish(uint32_t hash)
   {
   
           return (hash);
   }
   
   static inline struct nchashhead *
   NCP2BUCKET(struct namecache *ncp)
   {
           uint32_t hash;
   
           hash = cache_get_hash(ncp->nc_name, ncp->nc_nlen, ncp->nc_dvp);
           return (NCHHASH(hash));
   }
   
   static inline struct mtx *
   NCP2BUCKETLOCK(struct namecache *ncp)
   {
           uint32_t hash;
   
           hash = cache_get_hash(ncp->nc_name, ncp->nc_nlen, ncp->nc_dvp);
           return (HASH2BUCKETLOCK(hash));
   }
   
   #ifdef INVARIANTS
   static void
   cache_assert_bucket_locked(struct namecache *ncp)
   {
           struct mtx *blp;
   
           blp = NCP2BUCKETLOCK(ncp);
           mtx_assert(blp, MA_OWNED);
   }
   
   static void
   cache_assert_bucket_unlocked(struct namecache *ncp)
   {
           struct mtx *blp;
   
           blp = NCP2BUCKETLOCK(ncp);
           mtx_assert(blp, MA_NOTOWNED);
   }
   #else
   #define cache_assert_bucket_locked(x) do { } while (0)
   #define cache_assert_bucket_unlocked(x) do { } while (0)
   #endif
   
   #define cache_sort_vnodes(x, y) _cache_sort_vnodes((void **)(x), (void **)(y))
   static void
   _cache_sort_vnodes(void **p1, void **p2)
   {
           void *tmp;
   
           MPASS(*p1 != NULL || *p2 != NULL);
   
           if (*p1 > *p2) {
                   tmp = *p2;
                   *p2 = *p1;
                   *p1 = tmp;
           }
   }
   
   static void
   cache_lock_all_buckets(void)
   {
           u_int i;
   
           for (i = 0; i < numbucketlocks; i++)
                   mtx_lock(&bucketlocks[i]);
   }
   
   static void
   cache_unlock_all_buckets(void)
   {
           u_int i;
   
           for (i = 0; i < numbucketlocks; i++)
                   mtx_unlock(&bucketlocks[i]);
   }
   
   static void
   cache_lock_all_vnodes(void)
   {
           u_int i;
   
           for (i = 0; i < numvnodelocks; i++)
                   mtx_lock(&vnodelocks[i]);
   }
   
   static void
   cache_unlock_all_vnodes(void)
   {
           u_int i;
   
           for (i = 0; i < numvnodelocks; i++)
                   mtx_unlock(&vnodelocks[i]);
   }
   
   static int
   cache_trylock_vnodes(struct mtx *vlp1, struct mtx *vlp2)
   {
   
           cache_sort_vnodes(&vlp1, &vlp2);
   
           if (vlp1 != NULL) {
                   if (!mtx_trylock(vlp1))
                           return (EAGAIN);
           }
           if (!mtx_trylock(vlp2)) {
                   if (vlp1 != NULL)
                           mtx_unlock(vlp1);
                   return (EAGAIN);
           }
   
           return (0);
   }
   
   static void
   cache_lock_vnodes(struct mtx *vlp1, struct mtx *vlp2)
   {
   
           MPASS(vlp1 != NULL || vlp2 != NULL);
           MPASS(vlp1 <= vlp2);
   
           if (vlp1 != NULL)
                   mtx_lock(vlp1);
           if (vlp2 != NULL)
                   mtx_lock(vlp2);
   }
   
   static void
   cache_unlock_vnodes(struct mtx *vlp1, struct mtx *vlp2)
   {
   
           MPASS(vlp1 != NULL || vlp2 != NULL);
   
           if (vlp1 != NULL)
                   mtx_unlock(vlp1);
           if (vlp2 != NULL)
                   mtx_unlock(vlp2);
   }
   
   static int
   sysctl_nchstats(SYSCTL_HANDLER_ARGS)
   {
           struct nchstats snap;
   
           if (req->oldptr == NULL)
                   return (SYSCTL_OUT(req, 0, sizeof(snap)));
   
           snap = nchstats;
           snap.ncs_goodhits = counter_u64_fetch(numposhits);
           snap.ncs_neghits = counter_u64_fetch(numneghits);
           snap.ncs_badhits = counter_u64_fetch(numposzaps) +
               counter_u64_fetch(numnegzaps);
           snap.ncs_miss = counter_u64_fetch(nummisszap) +
               counter_u64_fetch(nummiss);
   
           return (SYSCTL_OUT(req, &snap, sizeof(snap)));
   }
   SYSCTL_PROC(_vfs_cache, OID_AUTO, nchstats, CTLTYPE_OPAQUE | CTLFLAG_RD |
       CTLFLAG_MPSAFE, 0, 0, sysctl_nchstats, "LU",
       "VFS cache effectiveness statistics");
  
  static void
  cache_recalc_neg_min(u_int val)
  {
  
          neg_min = (ncsize * val) / 100;
  }
  
  static int
  sysctl_negminpct(SYSCTL_HANDLER_ARGS)
  {
          u_int val;
          int error;
  
          val = ncnegminpct;
          error = sysctl_handle_int(oidp, &val, 0, req);
          if (error != 0 || req->newptr == NULL)
                  return (error);
  
          if (val == ncnegminpct)
                  return (0);
          if (val < 0 || val > 99)
                  return (EINVAL);
          ncnegminpct = val;
          cache_recalc_neg_min(val);
          return (0);
  }
  
  SYSCTL_PROC(_vfs_cache_param, OID_AUTO, negminpct,
      CTLTYPE_INT | CTLFLAG_MPSAFE | CTLFLAG_RW, NULL, 0, sysctl_negminpct,
      "I", "Negative entry \% of namecache capacity above which automatic eviction is allowed");
  
  #ifdef DEBUG_CACHE
  /*
   * Grab an atomic snapshot of the name cache hash chain lengths
   */
  static SYSCTL_NODE(_debug, OID_AUTO, hashstat,
      CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
      "hash table stats");
  
  static int
  sysctl_debug_hashstat_rawnchash(SYSCTL_HANDLER_ARGS)
  {
          struct nchashhead *ncpp;
          struct namecache *ncp;
          int i, error, n_nchash, *cntbuf;
  
  retry:
          n_nchash = nchash + 1;  /* nchash is max index, not count */
          if (req->oldptr == NULL)
                  return SYSCTL_OUT(req, 0, n_nchash * sizeof(int));
          cntbuf = malloc(n_nchash * sizeof(int), M_TEMP, M_ZERO | M_WAITOK);
          cache_lock_all_buckets();
          if (n_nchash != nchash + 1) {
                  cache_unlock_all_buckets();
                  free(cntbuf, M_TEMP);
                  goto retry;
          }
          /* Scan hash tables counting entries */
          for (ncpp = nchashtbl, i = 0; i < n_nchash; ncpp++, i++)
                  CK_SLIST_FOREACH(ncp, ncpp, nc_hash)
                          cntbuf[i]++;
          cache_unlock_all_buckets();
          for (error = 0, i = 0; i < n_nchash; i++)
                  if ((error = SYSCTL_OUT(req, &cntbuf[i], sizeof(int))) != 0)
                          break;
          free(cntbuf, M_TEMP);
          return (error);
  }
  SYSCTL_PROC(_debug_hashstat, OID_AUTO, rawnchash, CTLTYPE_INT|CTLFLAG_RD|
      CTLFLAG_MPSAFE, 0, 0, sysctl_debug_hashstat_rawnchash, "S,int",
      "nchash chain lengths");
  
  static int
  sysctl_debug_hashstat_nchash(SYSCTL_HANDLER_ARGS)
  {
          int error;
          struct nchashhead *ncpp;
          struct namecache *ncp;
          int n_nchash;
          int count, maxlength, used, pct;
  
          if (!req->oldptr)
                  return SYSCTL_OUT(req, 0, 4 * sizeof(int));
  
          cache_lock_all_buckets();
          n_nchash = nchash + 1;  /* nchash is max index, not count */
          used = 0;
          maxlength = 0;
  
          /* Scan hash tables for applicable entries */
          for (ncpp = nchashtbl; n_nchash > 0; n_nchash--, ncpp++) {
                  count = 0;
                  CK_SLIST_FOREACH(ncp, ncpp, nc_hash) {
                          count++;
                  }
                  if (count)
                          used++;
                  if (maxlength < count)
                          maxlength = count;
          }
          n_nchash = nchash + 1;
          cache_unlock_all_buckets();
          pct = (used * 100) / (n_nchash / 100);
          error = SYSCTL_OUT(req, &n_nchash, sizeof(n_nchash));
          if (error)
                  return (error);
          error = SYSCTL_OUT(req, &used, sizeof(used));
          if (error)
                  return (error);
          error = SYSCTL_OUT(req, &maxlength, sizeof(maxlength));
          if (error)
                  return (error);
          error = SYSCTL_OUT(req, &pct, sizeof(pct));
          if (error)
                  return (error);
          return (0);
  }
  SYSCTL_PROC(_debug_hashstat, OID_AUTO, nchash, CTLTYPE_INT|CTLFLAG_RD|
      CTLFLAG_MPSAFE, 0, 0, sysctl_debug_hashstat_nchash, "I",
      "nchash statistics (number of total/used buckets, maximum chain length, usage percentage)");
  #endif
  
  /*
   * Negative entries management
   *
   * Various workloads create plenty of negative entries and barely use them
   * afterwards. Moreover malicious users can keep performing bogus lookups
   * adding even more entries. For example "make tinderbox" as of writing this
   * comment ends up with 2.6M namecache entries in total, 1.2M of which are
   * negative.
   *
   * As such, a rather aggressive eviction method is needed. The currently
   * employed method is a placeholder.
   *
   * Entries are split over numneglists separate lists, each of which is further
   * split into hot and cold entries. Entries get promoted after getting a hit.
   * Eviction happens on addition of new entry.
   */
  static SYSCTL_NODE(_vfs_cache, OID_AUTO, neg, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
      "Name cache negative entry statistics");
  
  SYSCTL_ULONG(_vfs_cache_neg, OID_AUTO, count, CTLFLAG_RD, &numneg, 0,
      "Number of negative cache entries");
  
  static COUNTER_U64_DEFINE_EARLY(neg_created);
  SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, created, CTLFLAG_RD, &neg_created,
      "Number of created negative entries");
  
  static COUNTER_U64_DEFINE_EARLY(neg_evicted);
  SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evicted, CTLFLAG_RD, &neg_evicted,
      "Number of evicted negative entries");
  
  static COUNTER_U64_DEFINE_EARLY(neg_evict_skipped_empty);
  SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evict_skipped_empty, CTLFLAG_RD,
      &neg_evict_skipped_empty,
      "Number of times evicting failed due to lack of entries");
  
  static COUNTER_U64_DEFINE_EARLY(neg_evict_skipped_missed);
  SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evict_skipped_missed, CTLFLAG_RD,
      &neg_evict_skipped_missed,
      "Number of times evicting failed due to target entry disappearing");
  
  static COUNTER_U64_DEFINE_EARLY(neg_evict_skipped_contended);
  SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, evict_skipped_contended, CTLFLAG_RD,
      &neg_evict_skipped_contended,
      "Number of times evicting failed due to contention");
  
  SYSCTL_COUNTER_U64(_vfs_cache_neg, OID_AUTO, hits, CTLFLAG_RD, &numneghits,
      "Number of cache hits (negative)");
  
  static int
  sysctl_neg_hot(SYSCTL_HANDLER_ARGS)
  {
          int i, out;
  
          out = 0;
          for (i = 0; i < numneglists; i++)
                  out += neglists[i].nl_hotnum;
  
          return (SYSCTL_OUT(req, &out, sizeof(out)));
  }
  SYSCTL_PROC(_vfs_cache_neg, OID_AUTO, hot, CTLTYPE_INT | CTLFLAG_RD |
      CTLFLAG_MPSAFE, 0, 0, sysctl_neg_hot, "I",
      "Number of hot negative entries");
  
  static void
  cache_neg_init(struct namecache *ncp)
  {
          struct negstate *ns;
  
          ncp->nc_flag |= NCF_NEGATIVE;
          ns = NCP2NEGSTATE(ncp);
          ns->neg_flag = 0;
          ns->neg_hit = 0;
          counter_u64_add(neg_created, 1);
  }
  
  #define CACHE_NEG_PROMOTION_THRESH 2
  
  static bool
  cache_neg_hit_prep(struct namecache *ncp)
  {
          struct negstate *ns;
          u_char n;
  
          ns = NCP2NEGSTATE(ncp);
          n = atomic_load_char(&ns->neg_hit);
          for (;;) {
                  if (n >= CACHE_NEG_PROMOTION_THRESH)
                          return (false);
                  if (atomic_fcmpset_8(&ns->neg_hit, &n, n + 1))
                          break;
          }
          return (n + 1 == CACHE_NEG_PROMOTION_THRESH);
  }
  
  /*
   * Nothing to do here but it is provided for completeness as some
   * cache_neg_hit_prep callers may end up returning without even
   * trying to promote.
   */
  #define cache_neg_hit_abort(ncp)        do { } while (0)
  
  static void
  cache_neg_hit_finish(struct namecache *ncp)
  {
  
          SDT_PROBE2(vfs, namecache, lookup, hit__negative, ncp->nc_dvp, ncp->nc_name);
          counter_u64_add(numneghits, 1);
  }
  
  /*
   * Move a negative entry to the hot list.
   */
  static void
  cache_neg_promote_locked(struct namecache *ncp)
  {
          struct neglist *nl;
          struct negstate *ns;
  
          ns = NCP2NEGSTATE(ncp);
          nl = NCP2NEGLIST(ncp);
          mtx_assert(&nl->nl_lock, MA_OWNED);
          if ((ns->neg_flag & NEG_HOT) == 0) {
                  TAILQ_REMOVE(&nl->nl_list, ncp, nc_dst);
                  TAILQ_INSERT_TAIL(&nl->nl_hotlist, ncp, nc_dst);
                  nl->nl_hotnum++;
                  ns->neg_flag |= NEG_HOT;
          }
  }
  
  /*
   * Move a hot negative entry to the cold list.
   */
  static void
  cache_neg_demote_locked(struct namecache *ncp)
  {
          struct neglist *nl;
          struct negstate *ns;
  
          ns = NCP2NEGSTATE(ncp);
          nl = NCP2NEGLIST(ncp);
          mtx_assert(&nl->nl_lock, MA_OWNED);
          MPASS(ns->neg_flag & NEG_HOT);
          TAILQ_REMOVE(&nl->nl_hotlist, ncp, nc_dst);
          TAILQ_INSERT_TAIL(&nl->nl_list, ncp, nc_dst);
          nl->nl_hotnum--;
          ns->neg_flag &= ~NEG_HOT;
          atomic_store_char(&ns->neg_hit, 0);
  }
  
  /*
   * Move a negative entry to the hot list if it matches the lookup.
   *
   * We have to take locks, but they may be contended and in the worst
   * case we may need to go off CPU. We don't want to spin within the
   * smr section and we can't block with it. Exiting the section means
   * the found entry could have been evicted. We are going to look it
   * up again.
   */
  static bool
  cache_neg_promote_cond(struct vnode *dvp, struct componentname *cnp,
      struct namecache *oncp, uint32_t hash)
  {
          struct namecache *ncp;
          struct neglist *nl;
          u_char nc_flag;
  
          nl = NCP2NEGLIST(oncp);
  
          mtx_lock(&nl->nl_lock);
          /*
           * For hash iteration.
           */
          vfs_smr_enter();
  
          /*
           * Avoid all surprises by only succeeding if we got the same entry and
           * bailing completely otherwise.
           * XXX There are no provisions to keep the vnode around, meaning we may
           * end up promoting a negative entry for a *new* vnode and returning
           * ENOENT on its account. This is the error we want to return anyway
           * and promotion is harmless.
           *
           * In particular at this point there can be a new ncp which matches the
           * search but hashes to a different neglist.
           */
          CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                  if (ncp == oncp)
                          break;
          }
  
          /*
           * No match to begin with.
           */
          if (__predict_false(ncp == NULL)) {
                  goto out_abort;
          }
  
          /*
           * The newly found entry may be something different...
           */
          if (!(ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
              !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))) {
                  goto out_abort;
          }
  
          /*
           * ... and not even negative.
           */
          nc_flag = atomic_load_char(&ncp->nc_flag);
          if ((nc_flag & NCF_NEGATIVE) == 0) {
                  goto out_abort;
          }
  
          if (!cache_ncp_canuse(ncp)) {
                  goto out_abort;
          }
  
          cache_neg_promote_locked(ncp);
          cache_neg_hit_finish(ncp);
          vfs_smr_exit();
          mtx_unlock(&nl->nl_lock);
          return (true);
  out_abort:
          vfs_smr_exit();
          mtx_unlock(&nl->nl_lock);
          return (false);
  }
  
  static void
  cache_neg_promote(struct namecache *ncp)
  {
          struct neglist *nl;
  
          nl = NCP2NEGLIST(ncp);
          mtx_lock(&nl->nl_lock);
          cache_neg_promote_locked(ncp);
          mtx_unlock(&nl->nl_lock);
  }
  
  static void
  cache_neg_insert(struct namecache *ncp)
  {
          struct neglist *nl;
  
          MPASS(ncp->nc_flag & NCF_NEGATIVE);
          cache_assert_bucket_locked(ncp);
          nl = NCP2NEGLIST(ncp);
          mtx_lock(&nl->nl_lock);
          TAILQ_INSERT_TAIL(&nl->nl_list, ncp, nc_dst);
          mtx_unlock(&nl->nl_lock);
          atomic_add_long(&numneg, 1);
  }
  
  static void
  cache_neg_remove(struct namecache *ncp)
  {
          struct neglist *nl;
          struct negstate *ns;
  
          cache_assert_bucket_locked(ncp);
          nl = NCP2NEGLIST(ncp);
          ns = NCP2NEGSTATE(ncp);
          mtx_lock(&nl->nl_lock);
          if ((ns->neg_flag & NEG_HOT) != 0) {
                  TAILQ_REMOVE(&nl->nl_hotlist, ncp, nc_dst);
                  nl->nl_hotnum--;
          } else {
                  TAILQ_REMOVE(&nl->nl_list, ncp, nc_dst);
          }
          mtx_unlock(&nl->nl_lock);
          atomic_subtract_long(&numneg, 1);
  }
  
  static struct neglist *
  cache_neg_evict_select_list(void)
  {
          struct neglist *nl;
          u_int c;
  
          c = atomic_fetchadd_int(&neg_cycle, 1) + 1;
          nl = &neglists[c % numneglists];
          if (!mtx_trylock(&nl->nl_evict_lock)) {
                  counter_u64_add(neg_evict_skipped_contended, 1);
                  return (NULL);
          }
          return (nl);
  }
  
  static struct namecache *
  cache_neg_evict_select_entry(struct neglist *nl)
  {
          struct namecache *ncp, *lncp;
          struct negstate *ns, *lns;
          int i;
  
          mtx_assert(&nl->nl_evict_lock, MA_OWNED);
          mtx_assert(&nl->nl_lock, MA_OWNED);
          ncp = TAILQ_FIRST(&nl->nl_list);
          if (ncp == NULL)
                  return (NULL);
          lncp = ncp;
          lns = NCP2NEGSTATE(lncp);
          for (i = 1; i < 4; i++) {
                  ncp = TAILQ_NEXT(ncp, nc_dst);
                  if (ncp == NULL)
                          break;
                  ns = NCP2NEGSTATE(ncp);
                  if (ns->neg_hit < lns->neg_hit) {
                          lncp = ncp;
                          lns = ns;
                  }
          }
          return (lncp);
  }
  
  static bool
  cache_neg_evict(void)
  {
          struct namecache *ncp, *ncp2;
          struct neglist *nl;
          struct vnode *dvp;
          struct mtx *dvlp;
          struct mtx *blp;
          uint32_t hash;
          u_char nlen;
          bool evicted;
  
          nl = cache_neg_evict_select_list();
          if (nl == NULL) {
                  return (false);
          }
  
          mtx_lock(&nl->nl_lock);
          ncp = TAILQ_FIRST(&nl->nl_hotlist);
          if (ncp != NULL) {
                  cache_neg_demote_locked(ncp);
          }
          ncp = cache_neg_evict_select_entry(nl);
          if (ncp == NULL) {
                  counter_u64_add(neg_evict_skipped_empty, 1);
                  mtx_unlock(&nl->nl_lock);
                  mtx_unlock(&nl->nl_evict_lock);
                  return (false);
          }
          nlen = ncp->nc_nlen;
          dvp = ncp->nc_dvp;
          hash = cache_get_hash(ncp->nc_name, nlen, dvp);
          dvlp = VP2VNODELOCK(dvp);
          blp = HASH2BUCKETLOCK(hash);
          mtx_unlock(&nl->nl_lock);
          mtx_unlock(&nl->nl_evict_lock);
          mtx_lock(dvlp);
          mtx_lock(blp);
          /*
           * Note that since all locks were dropped above, the entry may be
           * gone or reallocated to be something else.
           */
          CK_SLIST_FOREACH(ncp2, (NCHHASH(hash)), nc_hash) {
                  if (ncp2 == ncp && ncp2->nc_dvp == dvp &&
                      ncp2->nc_nlen == nlen && (ncp2->nc_flag & NCF_NEGATIVE) != 0)
                          break;
          }
          if (ncp2 == NULL) {
                  counter_u64_add(neg_evict_skipped_missed, 1);
                  ncp = NULL;
                  evicted = false;
          } else {
                  MPASS(dvlp == VP2VNODELOCK(ncp->nc_dvp));
                  MPASS(blp == NCP2BUCKETLOCK(ncp));
                  SDT_PROBE2(vfs, namecache, evict_negative, done, ncp->nc_dvp,
                      ncp->nc_name);
                  cache_zap_locked(ncp);
                  counter_u64_add(neg_evicted, 1);
                  evicted = true;
          }
          mtx_unlock(blp);
          mtx_unlock(dvlp);
          if (ncp != NULL)
                  cache_free(ncp);
          return (evicted);
  }
  
  /*
   * Maybe evict a negative entry to create more room.
   *
   * The ncnegfactor parameter limits what fraction of the total count
   * can comprise of negative entries. However, if the cache is just
   * warming up this leads to excessive evictions.  As such, ncnegminpct
   * (recomputed to neg_min) dictates whether the above should be
   * applied.
   *
   * Try evicting if the cache is close to full capacity regardless of
   * other considerations.
   */
  static bool
  cache_neg_evict_cond(u_long lnumcache)
  {
          u_long lnumneg;
  
          if (ncsize - 1000 < lnumcache)
                  goto out_evict;
          lnumneg = atomic_load_long(&numneg);
          if (lnumneg < neg_min)
                  return (false);
          if (lnumneg * ncnegfactor < lnumcache)
                  return (false);
  out_evict:
          return (cache_neg_evict());
  }
  
  /*
   * cache_zap_locked():
   *
   *   Removes a namecache entry from cache, whether it contains an actual
   *   pointer to a vnode or if it is just a negative cache entry.
   */
  static void
  cache_zap_locked(struct namecache *ncp)
  {
          struct nchashhead *ncpp;
          struct vnode *dvp, *vp;
  
          dvp = ncp->nc_dvp;
          vp = ncp->nc_vp;
  
          if (!(ncp->nc_flag & NCF_NEGATIVE))
                  cache_assert_vnode_locked(vp);
          cache_assert_vnode_locked(dvp);
          cache_assert_bucket_locked(ncp);
  
          cache_ncp_invalidate(ncp);
  
          ncpp = NCP2BUCKET(ncp);
          CK_SLIST_REMOVE(ncpp, ncp, namecache, nc_hash);
          if (!(ncp->nc_flag & NCF_NEGATIVE)) {
                  SDT_PROBE3(vfs, namecache, zap, done, dvp, ncp->nc_name, vp);
                  TAILQ_REMOVE(&vp->v_cache_dst, ncp, nc_dst);
                  if (ncp == vp->v_cache_dd) {
                          atomic_store_ptr(&vp->v_cache_dd, NULL);
                  }
          } else {
                  SDT_PROBE2(vfs, namecache, zap_negative, done, dvp, ncp->nc_name);
                  cache_neg_remove(ncp);
          }
          if (ncp->nc_flag & NCF_ISDOTDOT) {
                  if (ncp == dvp->v_cache_dd) {
                          atomic_store_ptr(&dvp->v_cache_dd, NULL);
                  }
          } else {
                  LIST_REMOVE(ncp, nc_src);
                  if (LIST_EMPTY(&dvp->v_cache_src)) {
                          ncp->nc_flag |= NCF_DVDROP;
                  }
          }
  }
  
  static void
  cache_zap_negative_locked_vnode_kl(struct namecache *ncp, struct vnode *vp)
  {
          struct mtx *blp;
  
          MPASS(ncp->nc_dvp == vp);
          MPASS(ncp->nc_flag & NCF_NEGATIVE);
          cache_assert_vnode_locked(vp);
  
          blp = NCP2BUCKETLOCK(ncp);
          mtx_lock(blp);
          cache_zap_locked(ncp);
          mtx_unlock(blp);
  }
  
  static bool
  cache_zap_locked_vnode_kl2(struct namecache *ncp, struct vnode *vp,
      struct mtx **vlpp)
  {
          struct mtx *pvlp, *vlp1, *vlp2, *to_unlock;
          struct mtx *blp;
  
          MPASS(vp == ncp->nc_dvp || vp == ncp->nc_vp);
          cache_assert_vnode_locked(vp);
  
          if (ncp->nc_flag & NCF_NEGATIVE) {
                  if (*vlpp != NULL) {
                          mtx_unlock(*vlpp);
                          *vlpp = NULL;
                  }
                  cache_zap_negative_locked_vnode_kl(ncp, vp);
                  return (true);
          }
  
          pvlp = VP2VNODELOCK(vp);
          blp = NCP2BUCKETLOCK(ncp);
          vlp1 = VP2VNODELOCK(ncp->nc_dvp);
          vlp2 = VP2VNODELOCK(ncp->nc_vp);
  
          if (*vlpp == vlp1 || *vlpp == vlp2) {
                  to_unlock = *vlpp;
                  *vlpp = NULL;
          } else {
                  if (*vlpp != NULL) {
                          mtx_unlock(*vlpp);
                          *vlpp = NULL;
                  }
                  cache_sort_vnodes(&vlp1, &vlp2);
                  if (vlp1 == pvlp) {
                          mtx_lock(vlp2);
                          to_unlock = vlp2;
                  } else {
                          if (!mtx_trylock(vlp1))
                                  goto out_relock;
                          to_unlock = vlp1;
                  }
          }
          mtx_lock(blp);
          cache_zap_locked(ncp);
          mtx_unlock(blp);
          if (to_unlock != NULL)
                  mtx_unlock(to_unlock);
          return (true);
  
  out_relock:
          mtx_unlock(vlp2);
          mtx_lock(vlp1);
          mtx_lock(vlp2);
          MPASS(*vlpp == NULL);
          *vlpp = vlp1;
          return (false);
  }
  
  /*
   * If trylocking failed we can get here. We know enough to take all needed locks
   * in the right order and re-lookup the entry.
   */
  static int
  cache_zap_unlocked_bucket(struct namecache *ncp, struct componentname *cnp,
      struct vnode *dvp, struct mtx *dvlp, struct mtx *vlp, uint32_t hash,
      struct mtx *blp)
  {
          struct namecache *rncp;
  
          cache_assert_bucket_unlocked(ncp);
  
          cache_sort_vnodes(&dvlp, &vlp);
          cache_lock_vnodes(dvlp, vlp);
          mtx_lock(blp);
          CK_SLIST_FOREACH(rncp, (NCHHASH(hash)), nc_hash) {
                  if (rncp == ncp && rncp->nc_dvp == dvp &&
                      rncp->nc_nlen == cnp->cn_namelen &&
                      !bcmp(rncp->nc_name, cnp->cn_nameptr, rncp->nc_nlen))
                          break;
          }
          if (rncp != NULL) {
                  cache_zap_locked(rncp);
                  mtx_unlock(blp);
                  cache_unlock_vnodes(dvlp, vlp);
                  counter_u64_add(zap_bucket_relock_success, 1);
                  return (0);
          }
  
          mtx_unlock(blp);
          cache_unlock_vnodes(dvlp, vlp);
          return (EAGAIN);
  }
  
  static int __noinline
  cache_zap_locked_bucket(struct namecache *ncp, struct componentname *cnp,
      uint32_t hash, struct mtx *blp)
  {
          struct mtx *dvlp, *vlp;
          struct vnode *dvp;
  
          cache_assert_bucket_locked(ncp);
  
          dvlp = VP2VNODELOCK(ncp->nc_dvp);
          vlp = NULL;
          if (!(ncp->nc_flag & NCF_NEGATIVE))
                  vlp = VP2VNODELOCK(ncp->nc_vp);
          if (cache_trylock_vnodes(dvlp, vlp) == 0) {
                  cache_zap_locked(ncp);
                  mtx_unlock(blp);
                  cache_unlock_vnodes(dvlp, vlp);
                  return (0);
          }
  
          dvp = ncp->nc_dvp;
          mtx_unlock(blp);
          return (cache_zap_unlocked_bucket(ncp, cnp, dvp, dvlp, vlp, hash, blp));
  }
  
  static __noinline int
  cache_remove_cnp(struct vnode *dvp, struct componentname *cnp)
  {
          struct namecache *ncp;
          struct mtx *blp;
          struct mtx *dvlp, *dvlp2;
          uint32_t hash;
          int error;
  
          if (cnp->cn_namelen == 2 &&
              cnp->cn_nameptr[0] == '.' && cnp->cn_nameptr[1] == '.') {
                  dvlp = VP2VNODELOCK(dvp);
                  dvlp2 = NULL;
                  mtx_lock(dvlp);
  retry_dotdot:
                  ncp = dvp->v_cache_dd;
                  if (ncp == NULL) {
                          mtx_unlock(dvlp);
                          if (dvlp2 != NULL)
                                  mtx_unlock(dvlp2);
                          SDT_PROBE2(vfs, namecache, removecnp, miss, dvp, cnp);
                          return (0);
                  }
                  if ((ncp->nc_flag & NCF_ISDOTDOT) != 0) {
                          if (!cache_zap_locked_vnode_kl2(ncp, dvp, &dvlp2))
                                  goto retry_dotdot;
                          MPASS(dvp->v_cache_dd == NULL);
                          mtx_unlock(dvlp);
                          if (dvlp2 != NULL)
                                  mtx_unlock(dvlp2);
                          cache_free(ncp);
                  } else {
                          atomic_store_ptr(&dvp->v_cache_dd, NULL);
                          mtx_unlock(dvlp);
                          if (dvlp2 != NULL)
                                  mtx_unlock(dvlp2);
                  }
                  SDT_PROBE2(vfs, namecache, removecnp, hit, dvp, cnp);
                  return (1);
          }
  
          hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);
          blp = HASH2BUCKETLOCK(hash);
  retry:
          if (CK_SLIST_EMPTY(NCHHASH(hash)))
                  goto out_no_entry;
  
          mtx_lock(blp);
  
          CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                  if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                      !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                          break;
          }
  
          if (ncp == NULL) {
                  mtx_unlock(blp);
                  goto out_no_entry;
          }
  
          error = cache_zap_locked_bucket(ncp, cnp, hash, blp);
          if (__predict_false(error != 0)) {
                  zap_bucket_fail++;
                  goto retry;
          }
          counter_u64_add(numposzaps, 1);
          SDT_PROBE2(vfs, namecache, removecnp, hit, dvp, cnp);
          cache_free(ncp);
          return (1);
  out_no_entry:
          counter_u64_add(nummisszap, 1);
          SDT_PROBE2(vfs, namecache, removecnp, miss, dvp, cnp);
          return (0);
  }
  
  static int __noinline
  cache_lookup_dot(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
      struct timespec *tsp, int *ticksp)
  {
          int ltype;
  
          *vpp = dvp;
          counter_u64_add(dothits, 1);
          SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ".", *vpp);
          if (tsp != NULL)
                  timespecclear(tsp);
          if (ticksp != NULL)
                  *ticksp = ticks;
          vrefact(*vpp);
          /*
           * When we lookup "." we still can be asked to lock it
           * differently...
           */
          ltype = cnp->cn_lkflags & LK_TYPE_MASK;
          if (ltype != VOP_ISLOCKED(*vpp)) {
                  if (ltype == LK_EXCLUSIVE) {
                          vn_lock(*vpp, LK_UPGRADE | LK_RETRY);
                          if (VN_IS_DOOMED((*vpp))) {
                                  /* forced unmount */
                                  vrele(*vpp);
                                  *vpp = NULL;
                                  return (ENOENT);
                          }
                  } else
                          vn_lock(*vpp, LK_DOWNGRADE | LK_RETRY);
          }
          return (-1);
  }
  
  static int __noinline
  cache_lookup_dotdot(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
      struct timespec *tsp, int *ticksp)
  {
          struct namecache_ts *ncp_ts;
          struct namecache *ncp;
          struct mtx *dvlp;
          enum vgetstate vs;
          int error, ltype;
          bool whiteout;
  
          MPASS((cnp->cn_flags & ISDOTDOT) != 0);
  
          if ((cnp->cn_flags & MAKEENTRY) == 0) {
                  cache_remove_cnp(dvp, cnp);
                  return (0);
          }
  
          counter_u64_add(dotdothits, 1);
  retry:
          dvlp = VP2VNODELOCK(dvp);
          mtx_lock(dvlp);
          ncp = dvp->v_cache_dd;
          if (ncp == NULL) {
                  SDT_PROBE2(vfs, namecache, lookup, miss, dvp, "..");
                  mtx_unlock(dvlp);
                  return (0);
          }
          if ((ncp->nc_flag & NCF_ISDOTDOT) != 0) {
                  if (ncp->nc_flag & NCF_NEGATIVE)
                          *vpp = NULL;
                  else
                          *vpp = ncp->nc_vp;
          } else
                  *vpp = ncp->nc_dvp;
          if (*vpp == NULL)
                  goto negative_success;
          SDT_PROBE3(vfs, namecache, lookup, hit, dvp, "..", *vpp);
          cache_out_ts(ncp, tsp, ticksp);
          if ((ncp->nc_flag & (NCF_ISDOTDOT | NCF_DTS)) ==
              NCF_DTS && tsp != NULL) {
                  ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
                  *tsp = ncp_ts->nc_dotdottime;
          }
  
          MPASS(dvp != *vpp);
          ltype = VOP_ISLOCKED(dvp);
          VOP_UNLOCK(dvp);
          vs = vget_prep(*vpp);
          mtx_unlock(dvlp);
          error = vget_finish(*vpp, cnp->cn_lkflags, vs);
          vn_lock(dvp, ltype | LK_RETRY);
          if (VN_IS_DOOMED(dvp)) {
                  if (error == 0)
                          vput(*vpp);
                  *vpp = NULL;
                  return (ENOENT);
          }
          if (error) {
                  *vpp = NULL;
                  goto retry;
          }
          return (-1);
  negative_success:
          if (__predict_false(cnp->cn_nameiop == CREATE)) {
                  if (cnp->cn_flags & ISLASTCN) {
                          counter_u64_add(numnegzaps, 1);
                          cache_zap_negative_locked_vnode_kl(ncp, dvp);
                          mtx_unlock(dvlp);
                          cache_free(ncp);
                          return (0);
                  }
          }
  
          whiteout = (ncp->nc_flag & NCF_WHITE);
          cache_out_ts(ncp, tsp, ticksp);
          if (cache_neg_hit_prep(ncp))
                  cache_neg_promote(ncp);
          else
                  cache_neg_hit_finish(ncp);
          mtx_unlock(dvlp);
          if (whiteout)
                  cnp->cn_flags |= ISWHITEOUT;
          return (ENOENT);
  }
  
  /**
   * Lookup a name in the name cache
   *
   * # Arguments
   *
   * - dvp:       Parent directory in which to search.
   * - vpp:       Return argument.  Will contain desired vnode on cache hit.
   * - cnp:       Parameters of the name search.  The most interesting bits of
   *              the cn_flags field have the following meanings:
   *      - MAKEENTRY:    If clear, free an entry from the cache rather than look
   *                      it up.
   *      - ISDOTDOT:     Must be set if and only if cn_nameptr == ".."
   * - tsp:       Return storage for cache timestamp.  On a successful (positive
   *              or negative) lookup, tsp will be filled with any timespec that
   *              was stored when this cache entry was created.  However, it will
   *              be clear for "." entries.
   * - ticks:     Return storage for alternate cache timestamp.  On a successful
   *              (positive or negative) lookup, it will contain the ticks value
   *              that was current when the cache entry was created, unless cnp
   *              was ".".
   *
   * Either both tsp and ticks have to be provided or neither of them.
   *
   * # Returns
   *
   * - -1:        A positive cache hit.  vpp will contain the desired vnode.
   * - ENOENT:    A negative cache hit, or dvp was recycled out from under us due
   *              to a forced unmount.  vpp will not be modified.  If the entry
   *              is a whiteout, then the ISWHITEOUT flag will be set in
   *              cnp->cn_flags.
   * - 0:         A cache miss.  vpp will not be modified.
   *
   * # Locking
   *
   * On a cache hit, vpp will be returned locked and ref'd.  If we're looking up
   * .., dvp is unlocked.  If we're looking up . an extra ref is taken, but the
   * lock is not recursively acquired.
   */
  static int __noinline
  cache_lookup_fallback(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
      struct timespec *tsp, int *ticksp)
  {
          struct namecache *ncp;
          struct mtx *blp;
          uint32_t hash;
          enum vgetstate vs;
          int error;
          bool whiteout;
  
          MPASS((cnp->cn_flags & ISDOTDOT) == 0);
          MPASS((cnp->cn_flags & (MAKEENTRY | NC_KEEPPOSENTRY)) != 0);
  
  retry:
          hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);
          blp = HASH2BUCKETLOCK(hash);
          mtx_lock(blp);
  
          CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                  if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                      !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                          break;
          }
  
          if (__predict_false(ncp == NULL)) {
                  mtx_unlock(blp);
                  SDT_PROBE2(vfs, namecache, lookup, miss, dvp, cnp->cn_nameptr);
                  counter_u64_add(nummiss, 1);
                  return (0);
          }
  
          if (ncp->nc_flag & NCF_NEGATIVE)
                  goto negative_success;
  
          counter_u64_add(numposhits, 1);
          *vpp = ncp->nc_vp;
          SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ncp->nc_name, *vpp);
          cache_out_ts(ncp, tsp, ticksp);
          MPASS(dvp != *vpp);
          vs = vget_prep(*vpp);
          mtx_unlock(blp);
          error = vget_finish(*vpp, cnp->cn_lkflags, vs);
          if (error) {
                  *vpp = NULL;
                  goto retry;
          }
          return (-1);
  negative_success:
          /*
           * We don't get here with regular lookup apart from corner cases.
           */
          if (__predict_true(cnp->cn_nameiop == CREATE)) {
                  if (cnp->cn_flags & ISLASTCN) {
                          counter_u64_add(numnegzaps, 1);
                          error = cache_zap_locked_bucket(ncp, cnp, hash, blp);
                          if (__predict_false(error != 0)) {
                                  zap_bucket_fail2++;
                                  goto retry;
                          }
                          cache_free(ncp);
                          return (0);
                  }
          }
  
          whiteout = (ncp->nc_flag & NCF_WHITE);
          cache_out_ts(ncp, tsp, ticksp);
          if (cache_neg_hit_prep(ncp))
                  cache_neg_promote(ncp);
          else
                  cache_neg_hit_finish(ncp);
          mtx_unlock(blp);
          if (whiteout)
                  cnp->cn_flags |= ISWHITEOUT;
          return (ENOENT);
  }
  
  int
  cache_lookup(struct vnode *dvp, struct vnode **vpp, struct componentname *cnp,
      struct timespec *tsp, int *ticksp)
  {
          struct namecache *ncp;
          uint32_t hash;
          enum vgetstate vs;
          int error;
          bool whiteout, neg_promote;
          u_short nc_flag;
  
          MPASS((tsp == NULL && ticksp == NULL) || (tsp != NULL && ticksp != NULL));
  
  #ifdef DEBUG_CACHE
          if (__predict_false(!doingcache)) {
                  cnp->cn_flags &= ~MAKEENTRY;
                  return (0);
          }
  #endif
  
          if (__predict_false(cnp->cn_nameptr[0] == '.')) {
                  if (cnp->cn_namelen == 1)
                          return (cache_lookup_dot(dvp, vpp, cnp, tsp, ticksp));
                  if (cnp->cn_namelen == 2 && cnp->cn_nameptr[1] == '.')
                          return (cache_lookup_dotdot(dvp, vpp, cnp, tsp, ticksp));
          }
  
          MPASS((cnp->cn_flags & ISDOTDOT) == 0);
  
          if ((cnp->cn_flags & (MAKEENTRY | NC_KEEPPOSENTRY)) == 0) {
                  cache_remove_cnp(dvp, cnp);
                  return (0);
          }
  
          hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);
          vfs_smr_enter();
  
          CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                  if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                      !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                          break;
          }
  
          if (__predict_false(ncp == NULL)) {
                  vfs_smr_exit();
                  SDT_PROBE2(vfs, namecache, lookup, miss, dvp, cnp->cn_nameptr);
                  counter_u64_add(nummiss, 1);
                  return (0);
          }
  
          nc_flag = atomic_load_char(&ncp->nc_flag);
          if (nc_flag & NCF_NEGATIVE)
                  goto negative_success;
  
          counter_u64_add(numposhits, 1);
          *vpp = ncp->nc_vp;
          SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ncp->nc_name, *vpp);
          cache_out_ts(ncp, tsp, ticksp);
          MPASS(dvp != *vpp);
          if (!cache_ncp_canuse(ncp)) {
                  vfs_smr_exit();
                  *vpp = NULL;
                  goto out_fallback;
          }
          vs = vget_prep_smr(*vpp);
          vfs_smr_exit();
          if (__predict_false(vs == VGET_NONE)) {
                  *vpp = NULL;
                  goto out_fallback;
          }
          error = vget_finish(*vpp, cnp->cn_lkflags, vs);
          if (error) {
                  *vpp = NULL;
                  goto out_fallback;
          }
          return (-1);
  negative_success:
          if (cnp->cn_nameiop == CREATE) {
                  if (cnp->cn_flags & ISLASTCN) {
                          vfs_smr_exit();
                          goto out_fallback;
                  }
          }
  
          cache_out_ts(ncp, tsp, ticksp);
          whiteout = (atomic_load_char(&ncp->nc_flag) & NCF_WHITE);
          neg_promote = cache_neg_hit_prep(ncp);
          if (!cache_ncp_canuse(ncp)) {
                  cache_neg_hit_abort(ncp);
                  vfs_smr_exit();
                  goto out_fallback;
          }
          if (neg_promote) {
                  vfs_smr_exit();
                  if (!cache_neg_promote_cond(dvp, cnp, ncp, hash))
                          goto out_fallback;
          } else {
                  cache_neg_hit_finish(ncp);
                  vfs_smr_exit();
          }
          if (whiteout)
                  cnp->cn_flags |= ISWHITEOUT;
          return (ENOENT);
  out_fallback:
          return (cache_lookup_fallback(dvp, vpp, cnp, tsp, ticksp));
  }
  
  struct celockstate {
          struct mtx *vlp[3];
          struct mtx *blp[2];
  };
  CTASSERT((nitems(((struct celockstate *)0)->vlp) == 3));
  CTASSERT((nitems(((struct celockstate *)0)->blp) == 2));
  
  static inline void
  cache_celockstate_init(struct celockstate *cel)
  {
  
          bzero(cel, sizeof(*cel));
  }
  
  static void
  cache_lock_vnodes_cel(struct celockstate *cel, struct vnode *vp,
      struct vnode *dvp)
  {
          struct mtx *vlp1, *vlp2;
  
          MPASS(cel->vlp[0] == NULL);
          MPASS(cel->vlp[1] == NULL);
          MPASS(cel->vlp[2] == NULL);
  
          MPASS(vp != NULL || dvp != NULL);
  
          vlp1 = VP2VNODELOCK(vp);
          vlp2 = VP2VNODELOCK(dvp);
          cache_sort_vnodes(&vlp1, &vlp2);
  
          if (vlp1 != NULL) {
                  mtx_lock(vlp1);
                  cel->vlp[0] = vlp1;
          }
          mtx_lock(vlp2);
          cel->vlp[1] = vlp2;
  }
  
  static void
  cache_unlock_vnodes_cel(struct celockstate *cel)
  {
  
          MPASS(cel->vlp[0] != NULL || cel->vlp[1] != NULL);
  
          if (cel->vlp[0] != NULL)
                  mtx_unlock(cel->vlp[0]);
          if (cel->vlp[1] != NULL)
                  mtx_unlock(cel->vlp[1]);
          if (cel->vlp[2] != NULL)
                  mtx_unlock(cel->vlp[2]);
  }
  
  static bool
  cache_lock_vnodes_cel_3(struct celockstate *cel, struct vnode *vp)
  {
          struct mtx *vlp;
          bool ret;
  
          cache_assert_vlp_locked(cel->vlp[0]);
          cache_assert_vlp_locked(cel->vlp[1]);
          MPASS(cel->vlp[2] == NULL);
  
          MPASS(vp != NULL);
          vlp = VP2VNODELOCK(vp);
  
          ret = true;
          if (vlp >= cel->vlp[1]) {
                  mtx_lock(vlp);
          } else {
                  if (mtx_trylock(vlp))
                          goto out;
                  cache_lock_vnodes_cel_3_failures++;
                  cache_unlock_vnodes_cel(cel);
                  if (vlp < cel->vlp[0]) {
                          mtx_lock(vlp);
                          mtx_lock(cel->vlp[0]);
                          mtx_lock(cel->vlp[1]);
                  } else {
                          if (cel->vlp[0] != NULL)
                                  mtx_lock(cel->vlp[0]);
                          mtx_lock(vlp);
                          mtx_lock(cel->vlp[1]);
                  }
                  ret = false;
          }
  out:
          cel->vlp[2] = vlp;
          return (ret);
  }
  
  static void
  cache_lock_buckets_cel(struct celockstate *cel, struct mtx *blp1,
      struct mtx *blp2)
  {
  
          MPASS(cel->blp[0] == NULL);
          MPASS(cel->blp[1] == NULL);
  
          cache_sort_vnodes(&blp1, &blp2);
  
          if (blp1 != NULL) {
                  mtx_lock(blp1);
                  cel->blp[0] = blp1;
          }
          mtx_lock(blp2);
          cel->blp[1] = blp2;
  }
  
  static void
  cache_unlock_buckets_cel(struct celockstate *cel)
  {
  
          if (cel->blp[0] != NULL)
                  mtx_unlock(cel->blp[0]);
          mtx_unlock(cel->blp[1]);
  }
  
  /*
   * Lock part of the cache affected by the insertion.
   *
   * This means vnodelocks for dvp, vp and the relevant bucketlock.
   * However, insertion can result in removal of an old entry. In this
   * case we have an additional vnode and bucketlock pair to lock.
   *
   * That is, in the worst case we have to lock 3 vnodes and 2 bucketlocks, while
   * preserving the locking order (smaller address first).
   */
  static void
  cache_enter_lock(struct celockstate *cel, struct vnode *dvp, struct vnode *vp,
      uint32_t hash)
  {
          struct namecache *ncp;
          struct mtx *blps[2];
          u_char nc_flag;
  
          blps[0] = HASH2BUCKETLOCK(hash);
          for (;;) {
                  blps[1] = NULL;
                  cache_lock_vnodes_cel(cel, dvp, vp);
                  if (vp == NULL || vp->v_type != VDIR)
                          break;
                  ncp = atomic_load_consume_ptr(&vp->v_cache_dd);
                  if (ncp == NULL)
                          break;
                  nc_flag = atomic_load_char(&ncp->nc_flag);
                  if ((nc_flag & NCF_ISDOTDOT) == 0)
                          break;
                  MPASS(ncp->nc_dvp == vp);
                  blps[1] = NCP2BUCKETLOCK(ncp);
                  if ((nc_flag & NCF_NEGATIVE) != 0)
                          break;
                  if (cache_lock_vnodes_cel_3(cel, ncp->nc_vp))
                          break;
                  /*
                   * All vnodes got re-locked. Re-validate the state and if
                   * nothing changed we are done. Otherwise restart.
                   */
                  if (ncp == vp->v_cache_dd &&
                      (ncp->nc_flag & NCF_ISDOTDOT) != 0 &&
                      blps[1] == NCP2BUCKETLOCK(ncp) &&
                      VP2VNODELOCK(ncp->nc_vp) == cel->vlp[2])
                          break;
                  cache_unlock_vnodes_cel(cel);
                  cel->vlp[0] = NULL;
                  cel->vlp[1] = NULL;
                  cel->vlp[2] = NULL;
          }
          cache_lock_buckets_cel(cel, blps[0], blps[1]);
  }
  
  static void
  cache_enter_lock_dd(struct celockstate *cel, struct vnode *dvp, struct vnode *vp,
      uint32_t hash)
  {
          struct namecache *ncp;
          struct mtx *blps[2];
          u_char nc_flag;
  
          blps[0] = HASH2BUCKETLOCK(hash);
          for (;;) {
                  blps[1] = NULL;
                  cache_lock_vnodes_cel(cel, dvp, vp);
                  ncp = atomic_load_consume_ptr(&dvp->v_cache_dd);
                  if (ncp == NULL)
                          break;
                  nc_flag = atomic_load_char(&ncp->nc_flag);
                  if ((nc_flag & NCF_ISDOTDOT) == 0)
                          break;
                  MPASS(ncp->nc_dvp == dvp);
                  blps[1] = NCP2BUCKETLOCK(ncp);
                  if ((nc_flag & NCF_NEGATIVE) != 0)
                          break;
                  if (cache_lock_vnodes_cel_3(cel, ncp->nc_vp))
                          break;
                  if (ncp == dvp->v_cache_dd &&
                      (ncp->nc_flag & NCF_ISDOTDOT) != 0 &&
                      blps[1] == NCP2BUCKETLOCK(ncp) &&
                      VP2VNODELOCK(ncp->nc_vp) == cel->vlp[2])
                          break;
                  cache_unlock_vnodes_cel(cel);
                  cel->vlp[0] = NULL;
                  cel->vlp[1] = NULL;
                  cel->vlp[2] = NULL;
          }
          cache_lock_buckets_cel(cel, blps[0], blps[1]);
  }
  
  static void
  cache_enter_unlock(struct celockstate *cel)
  {
  
          cache_unlock_buckets_cel(cel);
          cache_unlock_vnodes_cel(cel);
  }
  
  static void __noinline
  cache_enter_dotdot_prep(struct vnode *dvp, struct vnode *vp,
      struct componentname *cnp)
  {
          struct celockstate cel;
          struct namecache *ncp;
          uint32_t hash;
          int len;
  
          if (atomic_load_ptr(&dvp->v_cache_dd) == NULL)
                  return;
          len = cnp->cn_namelen;
          cache_celockstate_init(&cel);
          hash = cache_get_hash(cnp->cn_nameptr, len, dvp);
          cache_enter_lock_dd(&cel, dvp, vp, hash);
          ncp = dvp->v_cache_dd;
          if (ncp != NULL && (ncp->nc_flag & NCF_ISDOTDOT)) {
                  KASSERT(ncp->nc_dvp == dvp, ("wrong isdotdot parent"));
                  cache_zap_locked(ncp);
          } else {
                  ncp = NULL;
          }
          atomic_store_ptr(&dvp->v_cache_dd, NULL);
          cache_enter_unlock(&cel);
          if (ncp != NULL)
                  cache_free(ncp);
  }
  
  /*
   * Add an entry to the cache.
   */
  void
  cache_enter_time(struct vnode *dvp, struct vnode *vp, struct componentname *cnp,
      struct timespec *tsp, struct timespec *dtsp)
  {
          struct celockstate cel;
          struct namecache *ncp, *n2, *ndd;
          struct namecache_ts *ncp_ts;
          struct nchashhead *ncpp;
          uint32_t hash;
          int flag;
          int len;
  
          KASSERT(cnp->cn_namelen <= NAME_MAX,
              ("%s: passed len %ld exceeds NAME_MAX (%d)", __func__, cnp->cn_namelen,
              NAME_MAX));
          VNPASS(!VN_IS_DOOMED(dvp), dvp);
          VNPASS(dvp->v_type != VNON, dvp);
          if (vp != NULL) {
                  VNPASS(!VN_IS_DOOMED(vp), vp);
                  VNPASS(vp->v_type != VNON, vp);
          }
          if (cnp->cn_namelen == 1 && cnp->cn_nameptr[0] == '.') {
                  KASSERT(dvp == vp,
                      ("%s: different vnodes for dot entry (%p; %p)\n", __func__,
                      dvp, vp));
          } else {
                  KASSERT(dvp != vp,
                      ("%s: same vnode for non-dot entry [%s] (%p)\n", __func__,
                      cnp->cn_nameptr, dvp));
          }
  
  #ifdef DEBUG_CACHE
          if (__predict_false(!doingcache))
                  return;
  #endif
  
          flag = 0;
          if (__predict_false(cnp->cn_nameptr[0] == '.')) {
                  if (cnp->cn_namelen == 1)
                          return;
                  if (cnp->cn_namelen == 2 && cnp->cn_nameptr[1] == '.') {
                          cache_enter_dotdot_prep(dvp, vp, cnp);
                          flag = NCF_ISDOTDOT;
                  }
          }
  
          ncp = cache_alloc(cnp->cn_namelen, tsp != NULL);
          if (ncp == NULL)
                  return;
  
          cache_celockstate_init(&cel);
          ndd = NULL;
          ncp_ts = NULL;
  
          /*
           * Calculate the hash key and setup as much of the new
           * namecache entry as possible before acquiring the lock.
           */
          ncp->nc_flag = flag | NCF_WIP;
          ncp->nc_vp = vp;
          if (vp == NULL)
                  cache_neg_init(ncp);
          ncp->nc_dvp = dvp;
          if (tsp != NULL) {
                  ncp_ts = __containerof(ncp, struct namecache_ts, nc_nc);
                  ncp_ts->nc_time = *tsp;
                  ncp_ts->nc_ticks = ticks;
                  ncp_ts->nc_nc.nc_flag |= NCF_TS;
                  if (dtsp != NULL) {
                          ncp_ts->nc_dotdottime = *dtsp;
                          ncp_ts->nc_nc.nc_flag |= NCF_DTS;
                  }
          }
          len = ncp->nc_nlen = cnp->cn_namelen;
          hash = cache_get_hash(cnp->cn_nameptr, len, dvp);
          memcpy(ncp->nc_name, cnp->cn_nameptr, len);
          ncp->nc_name[len] = '\0';
          cache_enter_lock(&cel, dvp, vp, hash);
  
          /*
           * See if this vnode or negative entry is already in the cache
           * with this name.  This can happen with concurrent lookups of
           * the same path name.
           */
          ncpp = NCHHASH(hash);
          CK_SLIST_FOREACH(n2, ncpp, nc_hash) {
                  if (n2->nc_dvp == dvp &&
                      n2->nc_nlen == cnp->cn_namelen &&
                      !bcmp(n2->nc_name, cnp->cn_nameptr, n2->nc_nlen)) {
                          MPASS(cache_ncp_canuse(n2));
                          if ((n2->nc_flag & NCF_NEGATIVE) != 0)
                                  KASSERT(vp == NULL,
                                      ("%s: found entry pointing to a different vnode (%p != %p) ; name [%s]",
                                      __func__, NULL, vp, cnp->cn_nameptr));
                          else
                                  KASSERT(n2->nc_vp == vp,
                                      ("%s: found entry pointing to a different vnode (%p != %p) ; name [%s]",
                                      __func__, n2->nc_vp, vp, cnp->cn_nameptr));
                          /*
                           * Entries are supposed to be immutable unless in the
                           * process of getting destroyed. Accommodating for
                           * changing timestamps is possible but not worth it.
                           * This should be harmless in terms of correctness, in
                           * the worst case resulting in an earlier expiration.
                           * Alternatively, the found entry can be replaced
                           * altogether.
                           */
                          MPASS((n2->nc_flag & (NCF_TS | NCF_DTS)) == (ncp->nc_flag & (NCF_TS | NCF_DTS)));
  #if 0
                          if (tsp != NULL) {
                                  KASSERT((n2->nc_flag & NCF_TS) != 0,
                                      ("no NCF_TS"));
                                  n2_ts = __containerof(n2, struct namecache_ts, nc_nc);
                                  n2_ts->nc_time = ncp_ts->nc_time;
                                  n2_ts->nc_ticks = ncp_ts->nc_ticks;
                                  if (dtsp != NULL) {
                                          n2_ts->nc_dotdottime = ncp_ts->nc_dotdottime;
                                          n2_ts->nc_nc.nc_flag |= NCF_DTS;
                                  }
                          }
  #endif
                          SDT_PROBE3(vfs, namecache, enter, duplicate, dvp, ncp->nc_name,
                              vp);
                          goto out_unlock_free;
                  }
          }
  
          if (flag == NCF_ISDOTDOT) {
                  /*
                   * See if we are trying to add .. entry, but some other lookup
                   * has populated v_cache_dd pointer already.
                   */
                  if (dvp->v_cache_dd != NULL)
                          goto out_unlock_free;
                  KASSERT(vp == NULL || vp->v_type == VDIR,
                      ("wrong vnode type %p", vp));
                  atomic_thread_fence_rel();
                  atomic_store_ptr(&dvp->v_cache_dd, ncp);
          }
  
          if (vp != NULL) {
                  if (flag != NCF_ISDOTDOT) {
                          /*
                           * For this case, the cache entry maps both the
                           * directory name in it and the name ".." for the
                           * directory's parent.
                           */
                          if ((ndd = vp->v_cache_dd) != NULL) {
                                  if ((ndd->nc_flag & NCF_ISDOTDOT) != 0)
                                          cache_zap_locked(ndd);
                                  else
                                          ndd = NULL;
                          }
                          atomic_thread_fence_rel();
                          atomic_store_ptr(&vp->v_cache_dd, ncp);
                  } else if (vp->v_type != VDIR) {
                          if (vp->v_cache_dd != NULL) {
                                  atomic_store_ptr(&vp->v_cache_dd, NULL);
                          }
                  }
          }
  
          if (flag != NCF_ISDOTDOT) {
                  if (LIST_EMPTY(&dvp->v_cache_src)) {
                          cache_hold_vnode(dvp);
                  }
                  LIST_INSERT_HEAD(&dvp->v_cache_src, ncp, nc_src);
          }
  
          /*
           * If the entry is "negative", we place it into the
           * "negative" cache queue, otherwise, we place it into the
           * destination vnode's cache entries queue.
           */
          if (vp != NULL) {
                  TAILQ_INSERT_HEAD(&vp->v_cache_dst, ncp, nc_dst);
                  SDT_PROBE3(vfs, namecache, enter, done, dvp, ncp->nc_name,
                      vp);
          } else {
                  if (cnp->cn_flags & ISWHITEOUT)
                          atomic_store_char(&ncp->nc_flag, ncp->nc_flag | NCF_WHITE);
                  cache_neg_insert(ncp);
                  SDT_PROBE2(vfs, namecache, enter_negative, done, dvp,
                      ncp->nc_name);
          }
  
          /*
           * Insert the new namecache entry into the appropriate chain
           * within the cache entries table.
           */
          CK_SLIST_INSERT_HEAD(ncpp, ncp, nc_hash);
  
          atomic_thread_fence_rel();
          /*
           * Mark the entry as fully constructed.
           * It is immutable past this point until its removal.
           */
          atomic_store_char(&ncp->nc_flag, ncp->nc_flag & ~NCF_WIP);
  
          cache_enter_unlock(&cel);
          if (ndd != NULL)
                  cache_free(ndd);
          return;
  out_unlock_free:
          cache_enter_unlock(&cel);
          cache_free(ncp);
          return;
  }
  
  /*
   * A variant of the above accepting flags.
   *
   * - VFS_CACHE_DROPOLD -- if a conflicting entry is found, drop it.
   *
   * TODO: this routine is a hack. It blindly removes the old entry, even if it
   * happens to match and it is doing it in an inefficient manner. It was added
   * to accommodate NFS which runs into a case where the target for a given name
   * may change from under it. Note this does nothing to solve the following
   * race: 2 callers of cache_enter_time_flags pass a different target vnode for
   * the same [dvp, cnp]. It may be argued that code doing this is broken.
   */
  void
  cache_enter_time_flags(struct vnode *dvp, struct vnode *vp, struct componentname *cnp,
      struct timespec *tsp, struct timespec *dtsp, int flags)
  {
  
          MPASS((flags & ~(VFS_CACHE_DROPOLD)) == 0);
  
          if (flags & VFS_CACHE_DROPOLD)
                  cache_remove_cnp(dvp, cnp);
          cache_enter_time(dvp, vp, cnp, tsp, dtsp);
  }
  
  static u_int
  cache_roundup_2(u_int val)
  {
          u_int res;
  
          for (res = 1; res <= val; res <<= 1)
                  continue;
  
          return (res);
  }
  
  static struct nchashhead *
  nchinittbl(u_long elements, u_long *hashmask)
  {
          struct nchashhead *hashtbl;
          u_long hashsize, i;
  
          hashsize = cache_roundup_2(elements) / 2;
  
          hashtbl = malloc((u_long)hashsize * sizeof(*hashtbl), M_VFSCACHE, M_WAITOK);
          for (i = 0; i < hashsize; i++)
                  CK_SLIST_INIT(&hashtbl[i]);
          *hashmask = hashsize - 1;
          return (hashtbl);
  }
  
  static void
  ncfreetbl(struct nchashhead *hashtbl)
  {
  
          free(hashtbl, M_VFSCACHE);
  }
  
  /*
   * Name cache initialization, from vfs_init() when we are booting
   */
  static void
  nchinit(void *dummy __unused)
  {
          u_int i;
  
          cache_zone_small = uma_zcreate("S VFS Cache", CACHE_ZONE_SMALL_SIZE,
              NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);
          cache_zone_small_ts = uma_zcreate("STS VFS Cache", CACHE_ZONE_SMALL_TS_SIZE,
              NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);
          cache_zone_large = uma_zcreate("L VFS Cache", CACHE_ZONE_LARGE_SIZE,
              NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);
          cache_zone_large_ts = uma_zcreate("LTS VFS Cache", CACHE_ZONE_LARGE_TS_SIZE,
              NULL, NULL, NULL, NULL, CACHE_ZONE_ALIGNMENT, UMA_ZONE_ZINIT);
  
          VFS_SMR_ZONE_SET(cache_zone_small);
          VFS_SMR_ZONE_SET(cache_zone_small_ts);
          VFS_SMR_ZONE_SET(cache_zone_large);
          VFS_SMR_ZONE_SET(cache_zone_large_ts);
  
          ncsize = desiredvnodes * ncsizefactor;
          cache_recalc_neg_min(ncnegminpct);
          nchashtbl = nchinittbl(desiredvnodes * 2, &nchash);
          ncbuckethash = cache_roundup_2(mp_ncpus * mp_ncpus) - 1;
          if (ncbuckethash < 7) /* arbitrarily chosen to avoid having one lock */
                  ncbuckethash = 7;
          if (ncbuckethash > nchash)
                  ncbuckethash = nchash;
          bucketlocks = malloc(sizeof(*bucketlocks) * numbucketlocks, M_VFSCACHE,
              M_WAITOK | M_ZERO);
          for (i = 0; i < numbucketlocks; i++)
                  mtx_init(&bucketlocks[i], "ncbuc", NULL, MTX_DUPOK | MTX_RECURSE);
          ncvnodehash = ncbuckethash;
          vnodelocks = malloc(sizeof(*vnodelocks) * numvnodelocks, M_VFSCACHE,
              M_WAITOK | M_ZERO);
          for (i = 0; i < numvnodelocks; i++)
                  mtx_init(&vnodelocks[i], "ncvn", NULL, MTX_DUPOK | MTX_RECURSE);
  
          for (i = 0; i < numneglists; i++) {
                  mtx_init(&neglists[i].nl_evict_lock, "ncnege", NULL, MTX_DEF);
                  mtx_init(&neglists[i].nl_lock, "ncnegl", NULL, MTX_DEF);
                  TAILQ_INIT(&neglists[i].nl_list);
                  TAILQ_INIT(&neglists[i].nl_hotlist);
          }
  }
  SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_SECOND, nchinit, NULL);
  
  void
  cache_vnode_init(struct vnode *vp)
  {
  
          LIST_INIT(&vp->v_cache_src);
          TAILQ_INIT(&vp->v_cache_dst);
          vp->v_cache_dd = NULL;
          cache_prehash(vp);
  }
  
  /*
   * Induce transient cache misses for lockless operation in cache_lookup() by
   * using a temporary hash table.
   *
   * This will force a fs lookup.
   *
   * Synchronisation is done in 2 steps, calling vfs_smr_synchronize each time
   * to observe all CPUs not performing the lookup.
   */
  static void
  cache_changesize_set_temp(struct nchashhead *temptbl, u_long temphash)
  {
  
          MPASS(temphash < nchash);
          /*
           * Change the size. The new size is smaller and can safely be used
           * against the existing table. All lookups which now hash wrong will
           * result in a cache miss, which all callers are supposed to know how
           * to handle.
           */
          atomic_store_long(&nchash, temphash);
          atomic_thread_fence_rel();
          vfs_smr_synchronize();
          /*
           * At this point everyone sees the updated hash value, but they still
           * see the old table.
           */
          atomic_store_ptr(&nchashtbl, temptbl);
          atomic_thread_fence_rel();
          vfs_smr_synchronize();
          /*
           * At this point everyone sees the updated table pointer and size pair.
           */
  }
  
  /*
   * Set the new hash table.
   *
   * Similarly to cache_changesize_set_temp(), this has to synchronize against
   * lockless operation in cache_lookup().
   */
  static void
  cache_changesize_set_new(struct nchashhead *new_tbl, u_long new_hash)
  {
  
          MPASS(nchash < new_hash);
          /*
           * Change the pointer first. This wont result in out of bounds access
           * since the temporary table is guaranteed to be smaller.
           */
          atomic_store_ptr(&nchashtbl, new_tbl);
          atomic_thread_fence_rel();
          vfs_smr_synchronize();
          /*
           * At this point everyone sees the updated pointer value, but they
           * still see the old size.
           */
          atomic_store_long(&nchash, new_hash);
          atomic_thread_fence_rel();
          vfs_smr_synchronize();
          /*
           * At this point everyone sees the updated table pointer and size pair.
           */
  }
  
  void
  cache_changesize(u_long newmaxvnodes)
  {
          struct nchashhead *new_nchashtbl, *old_nchashtbl, *temptbl;
          u_long new_nchash, old_nchash, temphash;
          struct namecache *ncp;
          uint32_t hash;
          u_long newncsize;
          int i;
  
          newncsize = newmaxvnodes * ncsizefactor;
          newmaxvnodes = cache_roundup_2(newmaxvnodes * 2);
          if (newmaxvnodes < numbucketlocks)
                  newmaxvnodes = numbucketlocks;
  
          new_nchashtbl = nchinittbl(newmaxvnodes, &new_nchash);
          /* If same hash table size, nothing to do */
          if (nchash == new_nchash) {
                  ncfreetbl(new_nchashtbl);
                  return;
          }
  
          temptbl = nchinittbl(1, &temphash);
  
          /*
           * Move everything from the old hash table to the new table.
           * None of the namecache entries in the table can be removed
           * because to do so, they have to be removed from the hash table.
           */
          cache_lock_all_vnodes();
          cache_lock_all_buckets();
          old_nchashtbl = nchashtbl;
          old_nchash = nchash;
          cache_changesize_set_temp(temptbl, temphash);
          for (i = 0; i <= old_nchash; i++) {
                  while ((ncp = CK_SLIST_FIRST(&old_nchashtbl[i])) != NULL) {
                          hash = cache_get_hash(ncp->nc_name, ncp->nc_nlen,
                              ncp->nc_dvp);
                          CK_SLIST_REMOVE(&old_nchashtbl[i], ncp, namecache, nc_hash);
                          CK_SLIST_INSERT_HEAD(&new_nchashtbl[hash & new_nchash], ncp, nc_hash);
                  }
          }
          ncsize = newncsize;
          cache_recalc_neg_min(ncnegminpct);
          cache_changesize_set_new(new_nchashtbl, new_nchash);
          cache_unlock_all_buckets();
          cache_unlock_all_vnodes();
          ncfreetbl(old_nchashtbl);
          ncfreetbl(temptbl);
  }
  
  /*
   * Remove all entries from and to a particular vnode.
   */
  static void
  cache_purge_impl(struct vnode *vp)
  {
          struct cache_freebatch batch;
          struct namecache *ncp;
          struct mtx *vlp, *vlp2;
  
          TAILQ_INIT(&batch);
          vlp = VP2VNODELOCK(vp);
          vlp2 = NULL;
          mtx_lock(vlp);
  retry:
          while (!LIST_EMPTY(&vp->v_cache_src)) {
                  ncp = LIST_FIRST(&vp->v_cache_src);
                  if (!cache_zap_locked_vnode_kl2(ncp, vp, &vlp2))
                          goto retry;
                  TAILQ_INSERT_TAIL(&batch, ncp, nc_dst);
          }
          while (!TAILQ_EMPTY(&vp->v_cache_dst)) {
                  ncp = TAILQ_FIRST(&vp->v_cache_dst);
                  if (!cache_zap_locked_vnode_kl2(ncp, vp, &vlp2))
                          goto retry;
                  TAILQ_INSERT_TAIL(&batch, ncp, nc_dst);
          }
          ncp = vp->v_cache_dd;
          if (ncp != NULL) {
                  KASSERT(ncp->nc_flag & NCF_ISDOTDOT,
                     ("lost dotdot link"));
                  if (!cache_zap_locked_vnode_kl2(ncp, vp, &vlp2))
                          goto retry;
                  TAILQ_INSERT_TAIL(&batch, ncp, nc_dst);
          }
          KASSERT(vp->v_cache_dd == NULL, ("incomplete purge"));
          mtx_unlock(vlp);
          if (vlp2 != NULL)
                  mtx_unlock(vlp2);
          cache_free_batch(&batch);
  }
  
  /*
   * Opportunistic check to see if there is anything to do.
   */
  static bool
  cache_has_entries(struct vnode *vp)
  {
  
          if (LIST_EMPTY(&vp->v_cache_src) && TAILQ_EMPTY(&vp->v_cache_dst) &&
              atomic_load_ptr(&vp->v_cache_dd) == NULL)
                  return (false);
          return (true);
  }
  
  void
  cache_purge(struct vnode *vp)
  {
  
          SDT_PROBE1(vfs, namecache, purge, done, vp);
          if (!cache_has_entries(vp))
                  return;
          cache_purge_impl(vp);
  }
  
  /*
   * Only to be used by vgone.
   */
  void
  cache_purge_vgone(struct vnode *vp)
  {
          struct mtx *vlp;
  
          VNPASS(VN_IS_DOOMED(vp), vp);
          if (cache_has_entries(vp)) {
                  cache_purge_impl(vp);
                  return;
          }
  
          /*
           * Serialize against a potential thread doing cache_purge.
           */
          vlp = VP2VNODELOCK(vp);
          mtx_wait_unlocked(vlp);
          if (cache_has_entries(vp)) {
                  cache_purge_impl(vp);
                  return;
          }
          return;
  }
  
  /*
   * Remove all negative entries for a particular directory vnode.
   */
  void
  cache_purge_negative(struct vnode *vp)
  {
          struct cache_freebatch batch;
          struct namecache *ncp, *nnp;
          struct mtx *vlp;
  
          SDT_PROBE1(vfs, namecache, purge_negative, done, vp);
          if (LIST_EMPTY(&vp->v_cache_src))
                  return;
          TAILQ_INIT(&batch);
          vlp = VP2VNODELOCK(vp);
          mtx_lock(vlp);
          LIST_FOREACH_SAFE(ncp, &vp->v_cache_src, nc_src, nnp) {
                  if (!(ncp->nc_flag & NCF_NEGATIVE))
                          continue;
                  cache_zap_negative_locked_vnode_kl(ncp, vp);
                  TAILQ_INSERT_TAIL(&batch, ncp, nc_dst);
          }
          mtx_unlock(vlp);
          cache_free_batch(&batch);
  }
  
  /*
   * Entry points for modifying VOP operations.
   */
  void
  cache_vop_rename(struct vnode *fdvp, struct vnode *fvp, struct vnode *tdvp,
      struct vnode *tvp, struct componentname *fcnp, struct componentname *tcnp)
  {
  
          ASSERT_VOP_IN_SEQC(fdvp);
          ASSERT_VOP_IN_SEQC(fvp);
          ASSERT_VOP_IN_SEQC(tdvp);
          if (tvp != NULL)
                  ASSERT_VOP_IN_SEQC(tvp);
  
          cache_purge(fvp);
          if (tvp != NULL) {
                  cache_purge(tvp);
                  KASSERT(!cache_remove_cnp(tdvp, tcnp),
                      ("%s: lingering negative entry", __func__));
          } else {
                  cache_remove_cnp(tdvp, tcnp);
          }
  
          /*
           * TODO
           *
           * Historically renaming was always purging all revelang entries,
           * but that's quite wasteful. In particular turns out that in many cases
           * the target file is immediately accessed after rename, inducing a cache
           * miss.
           *
           * Recode this to reduce relocking and reuse the existing entry (if any)
           * instead of just removing it above and allocating a new one here.
           */
          if (cache_rename_add) {
                  cache_enter(tdvp, fvp, tcnp);
          }
  }
  
  void
  cache_vop_rmdir(struct vnode *dvp, struct vnode *vp)
  {
  
          ASSERT_VOP_IN_SEQC(dvp);
          ASSERT_VOP_IN_SEQC(vp);
          cache_purge(vp);
  }
  
  #ifdef INVARIANTS
  /*
   * Validate that if an entry exists it matches.
   */
  void
  cache_validate(struct vnode *dvp, struct vnode *vp, struct componentname *cnp)
  {
          struct namecache *ncp;
          struct mtx *blp;
          uint32_t hash;
  
          hash = cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp);
          if (CK_SLIST_EMPTY(NCHHASH(hash)))
                  return;
          blp = HASH2BUCKETLOCK(hash);
          mtx_lock(blp);
          CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                  if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                      !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen)) {
                          if (ncp->nc_vp != vp)
                                  panic("%s: mismatch (%p != %p); ncp %p [%s] dvp %p\n",
                                      __func__, vp, ncp->nc_vp, ncp, ncp->nc_name, ncp->nc_dvp);
                  }
          }
          mtx_unlock(blp);
  }
  #endif
  
  /*
   * Flush all entries referencing a particular filesystem.
   */
  void
  cache_purgevfs(struct mount *mp)
  {
          struct vnode *vp, *mvp;
          size_t visited, purged;
  
          visited = purged = 0;
          /*
           * Somewhat wasteful iteration over all vnodes. Would be better to
           * support filtering and avoid the interlock to begin with.
           */
          MNT_VNODE_FOREACH_ALL(vp, mp, mvp) {
                  visited++;
                  if (!cache_has_entries(vp)) {
                          VI_UNLOCK(vp);
                          continue;
                  }
                  vholdl(vp);
                  VI_UNLOCK(vp);
                  cache_purge(vp);
                  purged++;
                  vdrop(vp);
          }
  
          SDT_PROBE3(vfs, namecache, purgevfs, done, mp, visited, purged);
  }
  
  /*
   * Perform canonical checks and cache lookup and pass on to filesystem
   * through the vop_cachedlookup only if needed.
   */
  
  int
  vfs_cache_lookup(struct vop_lookup_args *ap)
  {
          struct vnode *dvp;
          int error;
          struct vnode **vpp = ap->a_vpp;
          struct componentname *cnp = ap->a_cnp;
          int flags = cnp->cn_flags;
  
          *vpp = NULL;
          dvp = ap->a_dvp;
  
          if (dvp->v_type != VDIR)
                  return (ENOTDIR);
  
          if ((flags & ISLASTCN) && (dvp->v_mount->mnt_flag & MNT_RDONLY) &&
              (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME))
                  return (EROFS);
  
          error = vn_dir_check_exec(dvp, cnp);
          if (error != 0)
                  return (error);
  
          error = cache_lookup(dvp, vpp, cnp, NULL, NULL);
          if (error == 0)
                  return (VOP_CACHEDLOOKUP(dvp, vpp, cnp));
          if (error == -1)
                  return (0);
          return (error);
  }
  
  /* Implementation of the getcwd syscall. */
  int
  sys___getcwd(struct thread *td, struct __getcwd_args *uap)
  {
          char *buf, *retbuf;
          size_t buflen;
          int error;
  
          buflen = uap->buflen;
          if (__predict_false(buflen < 2))
                  return (EINVAL);
          if (buflen > MAXPATHLEN)
                  buflen = MAXPATHLEN;
  
          buf = uma_zalloc(namei_zone, M_WAITOK);
          error = vn_getcwd(buf, &retbuf, &buflen);
          if (error == 0)
                  error = copyout(retbuf, uap->buf, buflen);
          uma_zfree(namei_zone, buf);
          return (error);
  }
  
  int
  vn_getcwd(char *buf, char **retbuf, size_t *buflen)
  {
          struct pwd *pwd;
          int error;
  
          vfs_smr_enter();
          pwd = pwd_get_smr();
          error = vn_fullpath_any_smr(pwd->pwd_cdir, pwd->pwd_rdir, buf, retbuf,
              buflen, 0);
          VFS_SMR_ASSERT_NOT_ENTERED();
          if (error < 0) {
                  pwd = pwd_hold(curthread);
                  error = vn_fullpath_any(pwd->pwd_cdir, pwd->pwd_rdir, buf,
                      retbuf, buflen);
                  pwd_drop(pwd);
          }
  
  #ifdef KTRACE
          if (KTRPOINT(curthread, KTR_NAMEI) && error == 0)
                  ktrnamei(*retbuf);
  #endif
          return (error);
  }
  
  static int
  kern___realpathat(struct thread *td, int fd, const char *path, char *buf,
      size_t size, int flags, enum uio_seg pathseg)
  {
          struct nameidata nd;
          char *retbuf, *freebuf;
          int error;
  
          if (flags != 0)
                  return (EINVAL);
          NDINIT_ATRIGHTS(&nd, LOOKUP, FOLLOW | SAVENAME | WANTPARENT | AUDITVNODE1,
              pathseg, path, fd, &cap_fstat_rights, td);
          if ((error = namei(&nd)) != 0)
                  return (error);
  
          if (nd.ni_vp->v_type == VREG && nd.ni_dvp->v_type != VDIR &&
              (nd.ni_vp->v_vflag & VV_ROOT) != 0) {
                  /*
                   * This happens if vp is a file mount. The call to
                   * vn_fullpath_hardlink can panic if path resolution can't be
                   * handled without the directory.
                   *
                   * To resolve this, we find the vnode which was mounted on -
                   * this should have a unique global path since we disallow
                   * mounting on linked files.
                   */
                  struct vnode *covered_vp;
                  error = vn_lock(nd.ni_vp, LK_SHARED);
                  if (error != 0)
                          goto out;
                  covered_vp = nd.ni_vp->v_mount->mnt_vnodecovered;
                  vref(covered_vp);
                  VOP_UNLOCK(nd.ni_vp);
                  error = vn_fullpath(covered_vp, &retbuf, &freebuf);
                  vrele(covered_vp);
          } else {
                  error = vn_fullpath_hardlink(nd.ni_vp, nd.ni_dvp, nd.ni_cnd.cn_nameptr,
                      nd.ni_cnd.cn_namelen, &retbuf, &freebuf, &size);
          }
          if (error == 0) {
                  error = copyout(retbuf, buf, size);
                  free(freebuf, M_TEMP);
          }
  out:
          NDFREE(&nd, 0);
          return (error);
  }
  
  int
  sys___realpathat(struct thread *td, struct __realpathat_args *uap)
  {
  
          return (kern___realpathat(td, uap->fd, uap->path, uap->buf, uap->size,
              uap->flags, UIO_USERSPACE));
  }
  
  /*
   * Retrieve the full filesystem path that correspond to a vnode from the name
   * cache (if available)
   */
  int
  vn_fullpath(struct vnode *vp, char **retbuf, char **freebuf)
  {
          struct pwd *pwd;
          char *buf;
          size_t buflen;
          int error;
  
          if (__predict_false(vp == NULL))
                  return (EINVAL);
  
          buflen = MAXPATHLEN;
          buf = malloc(buflen, M_TEMP, M_WAITOK);
          vfs_smr_enter();
          pwd = pwd_get_smr();
          error = vn_fullpath_any_smr(vp, pwd->pwd_rdir, buf, retbuf, &buflen, 0);
          VFS_SMR_ASSERT_NOT_ENTERED();
          if (error < 0) {
                  pwd = pwd_hold(curthread);
                  error = vn_fullpath_any(vp, pwd->pwd_rdir, buf, retbuf, &buflen);
                  pwd_drop(pwd);
          }
          if (error == 0)
                  *freebuf = buf;
          else
                  free(buf, M_TEMP);
          return (error);
  }
  
  /*
   * This function is similar to vn_fullpath, but it attempts to lookup the
   * pathname relative to the global root mount point.  This is required for the
   * auditing sub-system, as audited pathnames must be absolute, relative to the
   * global root mount point.
   */
  int
  vn_fullpath_global(struct vnode *vp, char **retbuf, char **freebuf)
  {
          char *buf;
          size_t buflen;
          int error;
  
          if (__predict_false(vp == NULL))
                  return (EINVAL);
          buflen = MAXPATHLEN;
          buf = malloc(buflen, M_TEMP, M_WAITOK);
          vfs_smr_enter();
          error = vn_fullpath_any_smr(vp, rootvnode, buf, retbuf, &buflen, 0);
          VFS_SMR_ASSERT_NOT_ENTERED();
          if (error < 0) {
                  error = vn_fullpath_any(vp, rootvnode, buf, retbuf, &buflen);
          }
          if (error == 0)
                  *freebuf = buf;
          else
                  free(buf, M_TEMP);
          return (error);
  }
  
  static struct namecache *
  vn_dd_from_dst(struct vnode *vp)
  {
          struct namecache *ncp;
  
          cache_assert_vnode_locked(vp);
          TAILQ_FOREACH(ncp, &vp->v_cache_dst, nc_dst) {
                  if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
                          return (ncp);
          }
          return (NULL);
  }
  
  int
  vn_vptocnp(struct vnode **vp, char *buf, size_t *buflen)
  {
          struct vnode *dvp;
          struct namecache *ncp;
          struct mtx *vlp;
          int error;
  
          vlp = VP2VNODELOCK(*vp);
          mtx_lock(vlp);
          ncp = (*vp)->v_cache_dd;
          if (ncp != NULL && (ncp->nc_flag & NCF_ISDOTDOT) == 0) {
                  KASSERT(ncp == vn_dd_from_dst(*vp),
                      ("%s: mismatch for dd entry (%p != %p)", __func__,
                      ncp, vn_dd_from_dst(*vp)));
          } else {
                  ncp = vn_dd_from_dst(*vp);
          }
          if (ncp != NULL) {
                  if (*buflen < ncp->nc_nlen) {
                          mtx_unlock(vlp);
                          vrele(*vp);
                          counter_u64_add(numfullpathfail4, 1);
                          error = ENOMEM;
                          SDT_PROBE3(vfs, namecache, fullpath, return, error,
                              vp, NULL);
                          return (error);
                  }
                  *buflen -= ncp->nc_nlen;
                  memcpy(buf + *buflen, ncp->nc_name, ncp->nc_nlen);
                  SDT_PROBE3(vfs, namecache, fullpath, hit, ncp->nc_dvp,
                      ncp->nc_name, vp);
                  dvp = *vp;
                  *vp = ncp->nc_dvp;
                  vref(*vp);
                  mtx_unlock(vlp);
                  vrele(dvp);
                  return (0);
          }
          SDT_PROBE1(vfs, namecache, fullpath, miss, vp);
  
          mtx_unlock(vlp);
          vn_lock(*vp, LK_SHARED | LK_RETRY);
          error = VOP_VPTOCNP(*vp, &dvp, buf, buflen);
          vput(*vp);
          if (error) {
                  counter_u64_add(numfullpathfail2, 1);
                  SDT_PROBE3(vfs, namecache, fullpath, return,  error, vp, NULL);
                  return (error);
          }
  
          *vp = dvp;
          if (VN_IS_DOOMED(dvp)) {
                  /* forced unmount */
                  vrele(dvp);
                  error = ENOENT;
                  SDT_PROBE3(vfs, namecache, fullpath, return, error, vp, NULL);
                  return (error);
          }
          /*
           * *vp has its use count incremented still.
           */
  
          return (0);
  }
  
  /*
   * Resolve a directory to a pathname.
   *
   * The name of the directory can always be found in the namecache or fetched
   * from the filesystem. There is also guaranteed to be only one parent, meaning
   * we can just follow vnodes up until we find the root.
   *
   * The vnode must be referenced.
   */
  static int
  vn_fullpath_dir(struct vnode *vp, struct vnode *rdir, char *buf, char **retbuf,
      size_t *len, size_t addend)
  {
  #ifdef KDTRACE_HOOKS
          struct vnode *startvp = vp;
  #endif
          struct vnode *vp1;
          size_t buflen;
          int error;
          bool slash_prefixed;
  
          VNPASS(vp->v_type == VDIR || VN_IS_DOOMED(vp), vp);
          VNPASS(vp->v_usecount > 0, vp);
  
          buflen = *len;
  
          slash_prefixed = true;
          if (addend == 0) {
                  MPASS(*len >= 2);
                  buflen--;
                  buf[buflen] = '\0';
                  slash_prefixed = false;
          }
  
          error = 0;
  
          SDT_PROBE1(vfs, namecache, fullpath, entry, vp);
          counter_u64_add(numfullpathcalls, 1);
          while (vp != rdir && vp != rootvnode) {
                  /*
                   * The vp vnode must be already fully constructed,
                   * since it is either found in namecache or obtained
                   * from VOP_VPTOCNP().  We may test for VV_ROOT safely
                   * without obtaining the vnode lock.
                   */
                  if ((vp->v_vflag & VV_ROOT) != 0) {
                          vn_lock(vp, LK_RETRY | LK_SHARED);
  
                          /*
                           * With the vnode locked, check for races with
                           * unmount, forced or not.  Note that we
                           * already verified that vp is not equal to
                           * the root vnode, which means that
                           * mnt_vnodecovered can be NULL only for the
                           * case of unmount.
                           */
                          if (VN_IS_DOOMED(vp) ||
                              (vp1 = vp->v_mount->mnt_vnodecovered) == NULL ||
                              vp1->v_mountedhere != vp->v_mount) {
                                  vput(vp);
                                  error = ENOENT;
                                  SDT_PROBE3(vfs, namecache, fullpath, return,
                                      error, vp, NULL);
                                  break;
                          }
  
                          vref(vp1);
                          vput(vp);
                          vp = vp1;
                          continue;
                  }
                  if (vp->v_type != VDIR) {
                          vrele(vp);
                          counter_u64_add(numfullpathfail1, 1);
                          error = ENOTDIR;
                          SDT_PROBE3(vfs, namecache, fullpath, return,
                              error, vp, NULL);
                          break;
                  }
                  error = vn_vptocnp(&vp, buf, &buflen);
                  if (error)
                          break;
                  if (buflen == 0) {
                          vrele(vp);
                          error = ENOMEM;
                          SDT_PROBE3(vfs, namecache, fullpath, return, error,
                              startvp, NULL);
                          break;
                  }
                  buf[--buflen] = '/';
                  slash_prefixed = true;
          }
          if (error)
                  return (error);
          if (!slash_prefixed) {
                  if (buflen == 0) {
                          vrele(vp);
                          counter_u64_add(numfullpathfail4, 1);
                          SDT_PROBE3(vfs, namecache, fullpath, return, ENOMEM,
                              startvp, NULL);
                          return (ENOMEM);
                  }
                  buf[--buflen] = '/';
          }
          counter_u64_add(numfullpathfound, 1);
          vrele(vp);
  
          *retbuf = buf + buflen;
          SDT_PROBE3(vfs, namecache, fullpath, return, 0, startvp, *retbuf);
          *len -= buflen;
          *len += addend;
          return (0);
  }
  
  /*
   * Resolve an arbitrary vnode to a pathname.
   *
   * Note 2 caveats:
   * - hardlinks are not tracked, thus if the vnode is not a directory this can
   *   resolve to a different path than the one used to find it
   * - namecache is not mandatory, meaning names are not guaranteed to be added
   *   (in which case resolving fails)
   */
  static void __inline
  cache_rev_failed_impl(int *reason, int line)
  {
  
          *reason = line;
  }
  #define cache_rev_failed(var)   cache_rev_failed_impl((var), __LINE__)
  
  static int
  vn_fullpath_any_smr(struct vnode *vp, struct vnode *rdir, char *buf,
      char **retbuf, size_t *buflen, size_t addend)
  {
  #ifdef KDTRACE_HOOKS
          struct vnode *startvp = vp;
  #endif
          struct vnode *tvp;
          struct mount *mp;
          struct namecache *ncp;
          size_t orig_buflen;
          int reason;
          int error;
  #ifdef KDTRACE_HOOKS
          int i;
  #endif
          seqc_t vp_seqc, tvp_seqc;
          u_char nc_flag;
  
          VFS_SMR_ASSERT_ENTERED();
  
          if (!atomic_load_char(&cache_fast_lookup_enabled)) {
                  vfs_smr_exit();
                  return (-1);
          }
  
          orig_buflen = *buflen;
  
          if (addend == 0) {
                  MPASS(*buflen >= 2);
                  *buflen -= 1;
                  buf[*buflen] = '\0';
          }
  
          if (vp == rdir || vp == rootvnode) {
                  if (addend == 0) {
                          *buflen -= 1;
                          buf[*buflen] = '/';
                  }
                  goto out_ok;
          }
  
  #ifdef KDTRACE_HOOKS
          i = 0;
  #endif
          error = -1;
          ncp = NULL; /* for sdt probe down below */
          vp_seqc = vn_seqc_read_any(vp);
          if (seqc_in_modify(vp_seqc)) {
                  cache_rev_failed(&reason);
                  goto out_abort;
          }
  
          for (;;) {
  #ifdef KDTRACE_HOOKS
                  i++;
  #endif
                  if ((vp->v_vflag & VV_ROOT) != 0) {
                          mp = atomic_load_ptr(&vp->v_mount);
                          if (mp == NULL) {
                                  cache_rev_failed(&reason);
                                  goto out_abort;
                          }
                          tvp = atomic_load_ptr(&mp->mnt_vnodecovered);
                          tvp_seqc = vn_seqc_read_any(tvp);
                          if (seqc_in_modify(tvp_seqc)) {
                                  cache_rev_failed(&reason);
                                  goto out_abort;
                          }
                          if (!vn_seqc_consistent(vp, vp_seqc)) {
                                  cache_rev_failed(&reason);
                                  goto out_abort;
                          }
                          vp = tvp;
                          vp_seqc = tvp_seqc;
                          continue;
                  }
                  ncp = atomic_load_consume_ptr(&vp->v_cache_dd);
                  if (ncp == NULL) {
                          cache_rev_failed(&reason);
                          goto out_abort;
                  }
                  nc_flag = atomic_load_char(&ncp->nc_flag);
                  if ((nc_flag & NCF_ISDOTDOT) != 0) {
                          cache_rev_failed(&reason);
                          goto out_abort;
                  }
                  if (ncp->nc_nlen >= *buflen) {
                          cache_rev_failed(&reason);
                          error = ENOMEM;
                          goto out_abort;
                  }
                  *buflen -= ncp->nc_nlen;
                  memcpy(buf + *buflen, ncp->nc_name, ncp->nc_nlen);
                  *buflen -= 1;
                  buf[*buflen] = '/';
                  tvp = ncp->nc_dvp;
                  tvp_seqc = vn_seqc_read_any(tvp);
                  if (seqc_in_modify(tvp_seqc)) {
                          cache_rev_failed(&reason);
                          goto out_abort;
                  }
                  if (!vn_seqc_consistent(vp, vp_seqc)) {
                          cache_rev_failed(&reason);
                          goto out_abort;
                  }
                  /*
                   * Acquire fence provided by vn_seqc_read_any above.
                   */
                  if (__predict_false(atomic_load_ptr(&vp->v_cache_dd) != ncp)) {
                          cache_rev_failed(&reason);
                          goto out_abort;
                  }
                  if (!cache_ncp_canuse(ncp)) {
                          cache_rev_failed(&reason);
                          goto out_abort;
                  }
                  vp = tvp;
                  vp_seqc = tvp_seqc;
                  if (vp == rdir || vp == rootvnode)
                          break;
          }
  out_ok:
          vfs_smr_exit();
          *retbuf = buf + *buflen;
          *buflen = orig_buflen - *buflen + addend;
          SDT_PROBE2(vfs, namecache, fullpath_smr, hit, startvp, *retbuf);
          return (0);
  
  out_abort:
          *buflen = orig_buflen;
          SDT_PROBE4(vfs, namecache, fullpath_smr, miss, startvp, ncp, reason, i);
          vfs_smr_exit();
          return (error);
  }
  
  static int
  vn_fullpath_any(struct vnode *vp, struct vnode *rdir, char *buf, char **retbuf,
      size_t *buflen)
  {
          size_t orig_buflen, addend;
          int error;
  
          if (*buflen < 2)
                  return (EINVAL);
  
          orig_buflen = *buflen;
  
          vref(vp);
          addend = 0;
          if (vp->v_type != VDIR) {
                  *buflen -= 1;
                  buf[*buflen] = '\0';
                  error = vn_vptocnp(&vp, buf, buflen);
                  if (error)
                          return (error);
                  if (*buflen == 0) {
                          vrele(vp);
                          return (ENOMEM);
                  }
                  *buflen -= 1;
                  buf[*buflen] = '/';
                  addend = orig_buflen - *buflen;
          }
  
          return (vn_fullpath_dir(vp, rdir, buf, retbuf, buflen, addend));
  }
  
  /*
   * Resolve an arbitrary vnode to a pathname (taking care of hardlinks).
   *
   * Since the namecache does not track hardlinks, the caller is
   * expected to first look up the target vnode with SAVENAME |
   * WANTPARENT flags passed to namei to get dvp and vp.
   *
   * Then we have 2 cases:
   * - if the found vnode is a directory, the path can be constructed just by
   *   following names up the chain
   * - otherwise we populate the buffer with the saved name and start resolving
   *   from the parent
   */
  int
  vn_fullpath_hardlink(struct vnode *vp, struct vnode *dvp,
      const char *hrdl_name, size_t hrdl_name_length,
      char **retbuf, char **freebuf, size_t *buflen)
  {
          char *buf, *tmpbuf;
          struct pwd *pwd;
          size_t addend;
          int error;
          enum vtype type;
  
          if (*buflen < 2)
                  return (EINVAL);
          if (*buflen > MAXPATHLEN)
                  *buflen = MAXPATHLEN;
  
          buf = malloc(*buflen, M_TEMP, M_WAITOK);
  
          addend = 0;
  
          /*
           * Check for VBAD to work around the vp_crossmp bug in lookup().
           *
           * For example consider tmpfs on /tmp and realpath /tmp. ni_vp will be
           * set to mount point's root vnode while ni_dvp will be vp_crossmp.
           * If the type is VDIR (like in this very case) we can skip looking
           * at ni_dvp in the first place. However, since vnodes get passed here
           * unlocked the target may transition to doomed state (type == VBAD)
           * before we get to evaluate the condition. If this happens, we will
           * populate part of the buffer and descend to vn_fullpath_dir with
           * vp == vp_crossmp. Prevent the problem by checking for VBAD.
           *
           * This should be atomic_load(&vp->v_type) but it is illegal to take
           * an address of a bit field, even if said field is sized to char.
           * Work around the problem by reading the value into a full-sized enum
           * and then re-reading it with atomic_load which will still prevent
           * the compiler from re-reading down the road.
           */
          type = vp->v_type;
          type = atomic_load_int(&type);
          if (type == VBAD) {
                  error = ENOENT;
                  goto out_bad;
          }
          if (type != VDIR) {
                  addend = hrdl_name_length + 2;
                  if (*buflen < addend) {
                          error = ENOMEM;
                          goto out_bad;
                  }
                  *buflen -= addend;
                  tmpbuf = buf + *buflen;
                  tmpbuf[0] = '/';
                  memcpy(&tmpbuf[1], hrdl_name, hrdl_name_length);
                  tmpbuf[addend - 1] = '\0';
                  vp = dvp;
          }
  
          vfs_smr_enter();
          pwd = pwd_get_smr();
          error = vn_fullpath_any_smr(vp, pwd->pwd_rdir, buf, retbuf, buflen,
              addend);
          VFS_SMR_ASSERT_NOT_ENTERED();
          if (error < 0) {
                  pwd = pwd_hold(curthread);
                  vref(vp);
                  error = vn_fullpath_dir(vp, pwd->pwd_rdir, buf, retbuf, buflen,
                      addend);
                  pwd_drop(pwd);
          }
          if (error != 0)
                  goto out_bad;
  
          *freebuf = buf;
  
          return (0);
  out_bad:
          free(buf, M_TEMP);
          return (error);
  }
  
  struct vnode *
  vn_dir_dd_ino(struct vnode *vp)
  {
          struct namecache *ncp;
          struct vnode *ddvp;
          struct mtx *vlp;
          enum vgetstate vs;
  
          ASSERT_VOP_LOCKED(vp, "vn_dir_dd_ino");
          vlp = VP2VNODELOCK(vp);
          mtx_lock(vlp);
          TAILQ_FOREACH(ncp, &(vp->v_cache_dst), nc_dst) {
                  if ((ncp->nc_flag & NCF_ISDOTDOT) != 0)
                          continue;
                  ddvp = ncp->nc_dvp;
                  vs = vget_prep(ddvp);
                  mtx_unlock(vlp);
                  if (vget_finish(ddvp, LK_SHARED | LK_NOWAIT, vs))
                          return (NULL);
                  return (ddvp);
          }
          mtx_unlock(vlp);
          return (NULL);
  }
  
  int
  vn_commname(struct vnode *vp, char *buf, u_int buflen)
  {
          struct namecache *ncp;
          struct mtx *vlp;
          int l;
  
          vlp = VP2VNODELOCK(vp);
          mtx_lock(vlp);
          TAILQ_FOREACH(ncp, &vp->v_cache_dst, nc_dst)
                  if ((ncp->nc_flag & NCF_ISDOTDOT) == 0)
                          break;
          if (ncp == NULL) {
                  mtx_unlock(vlp);
                  return (ENOENT);
          }
          l = min(ncp->nc_nlen, buflen - 1);
          memcpy(buf, ncp->nc_name, l);
          mtx_unlock(vlp);
          buf[l] = '\0';
          return (0);
  }
  
  /*
   * This function updates path string to vnode's full global path
   * and checks the size of the new path string against the pathlen argument.
   *
   * Requires a locked, referenced vnode.
   * Vnode is re-locked on success or ENODEV, otherwise unlocked.
   *
   * If vp is a directory, the call to vn_fullpath_global() always succeeds
   * because it falls back to the ".." lookup if the namecache lookup fails.
   */
  int
  vn_path_to_global_path(struct thread *td, struct vnode *vp, char *path,
      u_int pathlen)
  {
          struct nameidata nd;
          struct vnode *vp1;
          char *rpath, *fbuf;
          int error;
  
          ASSERT_VOP_ELOCKED(vp, __func__);
  
          /* Construct global filesystem path from vp. */
          VOP_UNLOCK(vp);
          error = vn_fullpath_global(vp, &rpath, &fbuf);
  
          if (error != 0) {
                  vrele(vp);
                  return (error);
          }
  
          if (strlen(rpath) >= pathlen) {
                  vrele(vp);
                  error = ENAMETOOLONG;
                  goto out;
          }
  
          /*
           * Re-lookup the vnode by path to detect a possible rename.
           * As a side effect, the vnode is relocked.
           * If vnode was renamed, return ENOENT.
           */
          NDINIT(&nd, LOOKUP, FOLLOW | LOCKLEAF | AUDITVNODE1,
              UIO_SYSSPACE, path, td);
          error = namei(&nd);
          if (error != 0) {
                  vrele(vp);
                  goto out;
          }
          NDFREE(&nd, NDF_ONLY_PNBUF);
          vp1 = nd.ni_vp;
          vrele(vp);
          if (vp1 == vp)
                  strcpy(path, rpath);
          else {
                  vput(vp1);
                  error = ENOENT;
          }
  
  out:
          free(fbuf, M_TEMP);
          return (error);
  }
  
  /*
   * This is similar to vn_path_to_global_path but allows for regular
   * files which may not be present in the cache.
   *
   * Requires a locked, referenced vnode.
   * Vnode is re-locked on success or ENODEV, otherwise unlocked.
   */
  int
  vn_path_to_global_path_hardlink(struct thread *td, struct vnode *vp,
      struct vnode *dvp, char *path, u_int pathlen, const char *leaf_name,
      size_t leaf_length)
  {
          struct nameidata nd;
          struct vnode *vp1;
          char *rpath, *fbuf;
          size_t len;
          int error;
  
          ASSERT_VOP_ELOCKED(vp, __func__);
  
          /*
           * Construct global filesystem path from dvp, vp and leaf
           * name.
           */
          VOP_UNLOCK(vp);
          error = vn_fullpath_hardlink(vp, dvp, leaf_name, leaf_length,
              &rpath, &fbuf, &len);
  
          if (error != 0) {
                  vrele(vp);
                  goto out;
          }
  
          if (strlen(rpath) >= pathlen) {
                  vrele(vp);
                  error = ENAMETOOLONG;
                  goto out;
          }
  
          /*
           * Re-lookup the vnode by path to detect a possible rename.
           * As a side effect, the vnode is relocked.
           * If vnode was renamed, return ENOENT.
           */
          NDINIT(&nd, LOOKUP, FOLLOW | LOCKLEAF | AUDITVNODE1, UIO_SYSSPACE, path, td);
          error = namei(&nd);
          if (error != 0) {
                  vrele(vp);
                  goto out;
          }
          NDFREE_PNBUF(&nd);
          vp1 = nd.ni_vp;
          vrele(vp);
          if (vp1 == vp)
                  strcpy(path, rpath);
          else {
                  vput(vp1);
                  error = ENOENT;
          }
  
  out:
          free(fbuf, M_TEMP);
          return (error);
  }
  
  #ifdef DDB
  static void
  db_print_vpath(struct vnode *vp)
  {
  
          while (vp != NULL) {
                  db_printf("%p: ", vp);
                  if (vp == rootvnode) {
                          db_printf("/");
                          vp = NULL;
                  } else {
                          if (vp->v_vflag & VV_ROOT) {
                                  db_printf("<mount point>");
                                  vp = vp->v_mount->mnt_vnodecovered;
                          } else {
                                  struct namecache *ncp;
                                  char *ncn;
                                  int i;
  
                                  ncp = TAILQ_FIRST(&vp->v_cache_dst);
                                  if (ncp != NULL) {
                                          ncn = ncp->nc_name;
                                          for (i = 0; i < ncp->nc_nlen; i++)
                                                  db_printf("%c", *ncn++);
                                          vp = ncp->nc_dvp;
                                  } else {
                                          vp = NULL;
                                  }
                          }
                  }
                  db_printf("\n");
          }
  
          return;
  }
  
  DB_SHOW_COMMAND(vpath, db_show_vpath)
  {
          struct vnode *vp;
  
          if (!have_addr) {
                  db_printf("usage: show vpath <struct vnode *>\n");
                  return;
          }
  
          vp = (struct vnode *)addr;
          db_print_vpath(vp);
  }
  
  #endif
  
  static int cache_fast_lookup = 1;
  
  #define CACHE_FPL_FAILED        -2020
  
  void
  cache_fast_lookup_enabled_recalc(void)
  {
          int lookup_flag;
          int mac_on;
  
  #ifdef MAC
          mac_on = mac_vnode_check_lookup_enabled();
          mac_on |= mac_vnode_check_readlink_enabled();
  #else
          mac_on = 0;
  #endif
  
          lookup_flag = atomic_load_int(&cache_fast_lookup);
          if (lookup_flag && !mac_on) {
                  atomic_store_char(&cache_fast_lookup_enabled, true);
          } else {
                  atomic_store_char(&cache_fast_lookup_enabled, false);
          }
  }
  
  static int
  syscal_vfs_cache_fast_lookup(SYSCTL_HANDLER_ARGS)
  {
          int error, old;
  
          old = atomic_load_int(&cache_fast_lookup);
          error = sysctl_handle_int(oidp, arg1, arg2, req);
          if (error == 0 && req->newptr && old != atomic_load_int(&cache_fast_lookup))
                  cache_fast_lookup_enabled_recalc();
          return (error);
  }
  SYSCTL_PROC(_vfs, OID_AUTO, cache_fast_lookup, CTLTYPE_INT|CTLFLAG_RW|CTLFLAG_MPSAFE,
      &cache_fast_lookup, 0, syscal_vfs_cache_fast_lookup, "IU", "");
  
  /*
   * Components of nameidata (or objects it can point to) which may
   * need restoring in case fast path lookup fails.
   */
  struct nameidata_outer {
          size_t ni_pathlen;
          int cn_flags;
  };
  
  struct nameidata_saved {
  #ifdef INVARIANTS
          char *cn_nameptr;
          size_t ni_pathlen;
  #endif
  };
  
  #ifdef INVARIANTS
  struct cache_fpl_debug {
          size_t ni_pathlen;
  };
  #endif
  
  struct cache_fpl {
          struct nameidata *ndp;
          struct componentname *cnp;
          char *nulchar;
          struct vnode *dvp;
          struct vnode *tvp;
          seqc_t dvp_seqc;
          seqc_t tvp_seqc;
          uint32_t hash;
          struct nameidata_saved snd;
          struct nameidata_outer snd_outer;
          int line;
          enum cache_fpl_status status:8;
          bool in_smr;
          bool fsearch;
          bool savename;
          struct pwd **pwd;
  #ifdef INVARIANTS
          struct cache_fpl_debug debug;
  #endif
  };
  
  static bool cache_fplookup_mp_supported(struct mount *mp);
  static bool cache_fplookup_is_mp(struct cache_fpl *fpl);
  static int cache_fplookup_cross_mount(struct cache_fpl *fpl);
  static int cache_fplookup_partial_setup(struct cache_fpl *fpl);
  static int cache_fplookup_skip_slashes(struct cache_fpl *fpl);
  static int cache_fplookup_trailingslash(struct cache_fpl *fpl);
  static void cache_fpl_pathlen_dec(struct cache_fpl *fpl);
  static void cache_fpl_pathlen_inc(struct cache_fpl *fpl);
  static void cache_fpl_pathlen_add(struct cache_fpl *fpl, size_t n);
  static void cache_fpl_pathlen_sub(struct cache_fpl *fpl, size_t n);
  
  static void
  cache_fpl_cleanup_cnp(struct componentname *cnp)
  {
  
          uma_zfree(namei_zone, cnp->cn_pnbuf);
  #ifdef DIAGNOSTIC
          cnp->cn_pnbuf = NULL;
          cnp->cn_nameptr = NULL;
  #endif
  }
  
  static struct vnode *
  cache_fpl_handle_root(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
  
          MPASS(*(cnp->cn_nameptr) == '/');
          cnp->cn_nameptr++;
          cache_fpl_pathlen_dec(fpl);
  
          if (__predict_false(*(cnp->cn_nameptr) == '/')) {
                  do {
                          cnp->cn_nameptr++;
                          cache_fpl_pathlen_dec(fpl);
                  } while (*(cnp->cn_nameptr) == '/');
          }
  
          return (ndp->ni_rootdir);
  }
  
  static void
  cache_fpl_checkpoint_outer(struct cache_fpl *fpl)
  {
  
          fpl->snd_outer.ni_pathlen = fpl->ndp->ni_pathlen;
          fpl->snd_outer.cn_flags = fpl->ndp->ni_cnd.cn_flags;
  }
  
  static void
  cache_fpl_checkpoint(struct cache_fpl *fpl)
  {
  
  #ifdef INVARIANTS
          fpl->snd.cn_nameptr = fpl->ndp->ni_cnd.cn_nameptr;
          fpl->snd.ni_pathlen = fpl->debug.ni_pathlen;
  #endif
  }
  
  static void
  cache_fpl_restore_partial(struct cache_fpl *fpl)
  {
  
          fpl->ndp->ni_cnd.cn_flags = fpl->snd_outer.cn_flags;
  #ifdef INVARIANTS
          fpl->debug.ni_pathlen = fpl->snd.ni_pathlen;
  #endif
  }
  
  static void
  cache_fpl_restore_abort(struct cache_fpl *fpl)
  {
  
          cache_fpl_restore_partial(fpl);
          /*
           * It is 0 on entry by API contract.
           */
          fpl->ndp->ni_resflags = 0;
          fpl->ndp->ni_cnd.cn_nameptr = fpl->ndp->ni_cnd.cn_pnbuf;
          fpl->ndp->ni_pathlen = fpl->snd_outer.ni_pathlen;
  }
  
  #ifdef INVARIANTS
  #define cache_fpl_smr_assert_entered(fpl) ({                    \
          struct cache_fpl *_fpl = (fpl);                         \
          MPASS(_fpl->in_smr == true);                            \
          VFS_SMR_ASSERT_ENTERED();                               \
  })
  #define cache_fpl_smr_assert_not_entered(fpl) ({                \
          struct cache_fpl *_fpl = (fpl);                         \
          MPASS(_fpl->in_smr == false);                           \
          VFS_SMR_ASSERT_NOT_ENTERED();                           \
  })
  static void
  cache_fpl_assert_status(struct cache_fpl *fpl)
  {
  
          switch (fpl->status) {
          case CACHE_FPL_STATUS_UNSET:
                  __assert_unreachable();
                  break;
          case CACHE_FPL_STATUS_DESTROYED:
          case CACHE_FPL_STATUS_ABORTED:
          case CACHE_FPL_STATUS_PARTIAL:
          case CACHE_FPL_STATUS_HANDLED:
                  break;
          }
  }
  #else
  #define cache_fpl_smr_assert_entered(fpl) do { } while (0)
  #define cache_fpl_smr_assert_not_entered(fpl) do { } while (0)
  #define cache_fpl_assert_status(fpl) do { } while (0)
  #endif
  
  #define cache_fpl_smr_enter_initial(fpl) ({                     \
          struct cache_fpl *_fpl = (fpl);                         \
          vfs_smr_enter();                                        \
          _fpl->in_smr = true;                                    \
  })
  
  #define cache_fpl_smr_enter(fpl) ({                             \
          struct cache_fpl *_fpl = (fpl);                         \
          MPASS(_fpl->in_smr == false);                           \
          vfs_smr_enter();                                        \
          _fpl->in_smr = true;                                    \
  })
  
  #define cache_fpl_smr_exit(fpl) ({                              \
          struct cache_fpl *_fpl = (fpl);                         \
          MPASS(_fpl->in_smr == true);                            \
          vfs_smr_exit();                                         \
          _fpl->in_smr = false;                                   \
  })
  
  static int
  cache_fpl_aborted_early_impl(struct cache_fpl *fpl, int line)
  {
  
          if (fpl->status != CACHE_FPL_STATUS_UNSET) {
                  KASSERT(fpl->status == CACHE_FPL_STATUS_PARTIAL,
                      ("%s: converting to abort from %d at %d, set at %d\n",
                      __func__, fpl->status, line, fpl->line));
          }
          cache_fpl_smr_assert_not_entered(fpl);
          fpl->status = CACHE_FPL_STATUS_ABORTED;
          fpl->line = line;
          return (CACHE_FPL_FAILED);
  }
  
  #define cache_fpl_aborted_early(x)      cache_fpl_aborted_early_impl((x), __LINE__)
  
  static int __noinline
  cache_fpl_aborted_impl(struct cache_fpl *fpl, int line)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
  
          if (fpl->status != CACHE_FPL_STATUS_UNSET) {
                  KASSERT(fpl->status == CACHE_FPL_STATUS_PARTIAL,
                      ("%s: converting to abort from %d at %d, set at %d\n",
                      __func__, fpl->status, line, fpl->line));
          }
          fpl->status = CACHE_FPL_STATUS_ABORTED;
          fpl->line = line;
          if (fpl->in_smr)
                  cache_fpl_smr_exit(fpl);
          cache_fpl_restore_abort(fpl);
          /*
           * Resolving symlinks overwrites data passed by the caller.
           * Let namei know.
           */
          if (ndp->ni_loopcnt > 0) {
                  fpl->status = CACHE_FPL_STATUS_DESTROYED;
                  cache_fpl_cleanup_cnp(cnp);
          }
          return (CACHE_FPL_FAILED);
  }
  
  #define cache_fpl_aborted(x)    cache_fpl_aborted_impl((x), __LINE__)
  
  static int __noinline
  cache_fpl_partial_impl(struct cache_fpl *fpl, int line)
  {
  
          KASSERT(fpl->status == CACHE_FPL_STATUS_UNSET,
              ("%s: setting to partial at %d, but already set to %d at %d\n",
              __func__, line, fpl->status, fpl->line));
          cache_fpl_smr_assert_entered(fpl);
          fpl->status = CACHE_FPL_STATUS_PARTIAL;
          fpl->line = line;
          return (cache_fplookup_partial_setup(fpl));
  }
  
  #define cache_fpl_partial(x)    cache_fpl_partial_impl((x), __LINE__)
  
  static int
  cache_fpl_handled_impl(struct cache_fpl *fpl, int line)
  {
  
          KASSERT(fpl->status == CACHE_FPL_STATUS_UNSET,
              ("%s: setting to handled at %d, but already set to %d at %d\n",
              __func__, line, fpl->status, fpl->line));
          cache_fpl_smr_assert_not_entered(fpl);
          fpl->status = CACHE_FPL_STATUS_HANDLED;
          fpl->line = line;
          return (0);
  }
  
  #define cache_fpl_handled(x)    cache_fpl_handled_impl((x), __LINE__)
  
  static int
  cache_fpl_handled_error_impl(struct cache_fpl *fpl, int error, int line)
  {
  
          KASSERT(fpl->status == CACHE_FPL_STATUS_UNSET,
              ("%s: setting to handled at %d, but already set to %d at %d\n",
              __func__, line, fpl->status, fpl->line));
          MPASS(error != 0);
          MPASS(error != CACHE_FPL_FAILED);
          cache_fpl_smr_assert_not_entered(fpl);
          fpl->status = CACHE_FPL_STATUS_HANDLED;
          fpl->line = line;
          fpl->dvp = NULL;
          fpl->tvp = NULL;
          fpl->savename = false;
          return (error);
  }
  
  #define cache_fpl_handled_error(x, e)   cache_fpl_handled_error_impl((x), (e), __LINE__)
  
  static bool
  cache_fpl_terminated(struct cache_fpl *fpl)
  {
  
          return (fpl->status != CACHE_FPL_STATUS_UNSET);
  }
  
  #define CACHE_FPL_SUPPORTED_CN_FLAGS \
          (NC_NOMAKEENTRY | NC_KEEPPOSENTRY | LOCKLEAF | LOCKPARENT | WANTPARENT | \
           FAILIFEXISTS | FOLLOW | EMPTYPATH | LOCKSHARED | SAVENAME | SAVESTART | \
           WILLBEDIR | ISOPEN | NOMACCHECK | AUDITVNODE1 | AUDITVNODE2 | NOCAPCHECK | \
           WANTIOCTLCAPS)
  
  #define CACHE_FPL_INTERNAL_CN_FLAGS \
          (ISDOTDOT | MAKEENTRY | ISLASTCN)
  
  _Static_assert((CACHE_FPL_SUPPORTED_CN_FLAGS & CACHE_FPL_INTERNAL_CN_FLAGS) == 0,
      "supported and internal flags overlap");
  
  static bool
  cache_fpl_islastcn(struct nameidata *ndp)
  {
  
          return (*ndp->ni_next == 0);
  }
  
  static bool
  cache_fpl_istrailingslash(struct cache_fpl *fpl)
  {
  
          MPASS(fpl->nulchar > fpl->cnp->cn_pnbuf);
          return (*(fpl->nulchar - 1) == '/');
  }
  
  static bool
  cache_fpl_isdotdot(struct componentname *cnp)
  {
  
          if (cnp->cn_namelen == 2 &&
              cnp->cn_nameptr[1] == '.' && cnp->cn_nameptr[0] == '.')
                  return (true);
          return (false);
  }
  
  static bool
  cache_can_fplookup(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          struct thread *td;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          td = cnp->cn_thread;
  
          if (!atomic_load_char(&cache_fast_lookup_enabled)) {
                  cache_fpl_aborted_early(fpl);
                  return (false);
          }
          if ((cnp->cn_flags & ~CACHE_FPL_SUPPORTED_CN_FLAGS) != 0) {
                  cache_fpl_aborted_early(fpl);
                  return (false);
          }
          if (IN_CAPABILITY_MODE(td)) {
                  cache_fpl_aborted_early(fpl);
                  return (false);
          }
          if (AUDITING_TD(td)) {
                  cache_fpl_aborted_early(fpl);
                  return (false);
          }
          if (ndp->ni_startdir != NULL) {
                  cache_fpl_aborted_early(fpl);
                  return (false);
          }
          return (true);
  }
  
  static int __noinline
  cache_fplookup_dirfd(struct cache_fpl *fpl, struct vnode **vpp)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          int error;
          bool fsearch;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
  
          error = fgetvp_lookup_smr(ndp->ni_dirfd, ndp, vpp, &fsearch);
          if (__predict_false(error != 0)) {
                  return (cache_fpl_aborted(fpl));
          }
          fpl->fsearch = fsearch;
          if ((*vpp)->v_type != VDIR) {
                  if (!((cnp->cn_flags & EMPTYPATH) != 0 && cnp->cn_pnbuf[0] == '\0')) {
                          cache_fpl_smr_exit(fpl);
                          return (cache_fpl_handled_error(fpl, ENOTDIR));
                  }
          }
          return (0);
  }
  
  static int __noinline
  cache_fplookup_negative_promote(struct cache_fpl *fpl, struct namecache *oncp,
      uint32_t hash)
  {
          struct componentname *cnp;
          struct vnode *dvp;
  
          cnp = fpl->cnp;
          dvp = fpl->dvp;
  
          cache_fpl_smr_exit(fpl);
          if (cache_neg_promote_cond(dvp, cnp, oncp, hash))
                  return (cache_fpl_handled_error(fpl, ENOENT));
          else
                  return (cache_fpl_aborted(fpl));
  }
  
  /*
   * The target vnode is not supported, prepare for the slow path to take over.
   */
  static int __noinline
  cache_fplookup_partial_setup(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          enum vgetstate dvs;
          struct vnode *dvp;
          struct pwd *pwd;
          seqc_t dvp_seqc;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          pwd = *(fpl->pwd);
          dvp = fpl->dvp;
          dvp_seqc = fpl->dvp_seqc;
  
          if (!pwd_hold_smr(pwd)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          /*
           * Note that seqc is checked before the vnode is locked, so by
           * the time regular lookup gets to it it may have moved.
           *
           * Ultimately this does not affect correctness, any lookup errors
           * are userspace racing with itself. It is guaranteed that any
           * path which ultimately gets found could also have been found
           * by regular lookup going all the way in absence of concurrent
           * modifications.
           */
          dvs = vget_prep_smr(dvp);
          cache_fpl_smr_exit(fpl);
          if (__predict_false(dvs == VGET_NONE)) {
                  pwd_drop(pwd);
                  return (cache_fpl_aborted(fpl));
          }
  
          vget_finish_ref(dvp, dvs);
          if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                  vrele(dvp);
                  pwd_drop(pwd);
                  return (cache_fpl_aborted(fpl));
          }
  
          cache_fpl_restore_partial(fpl);
  #ifdef INVARIANTS
          if (cnp->cn_nameptr != fpl->snd.cn_nameptr) {
                  panic("%s: cn_nameptr mismatch (%p != %p) full [%s]\n", __func__,
                      cnp->cn_nameptr, fpl->snd.cn_nameptr, cnp->cn_pnbuf);
          }
  #endif
  
          ndp->ni_startdir = dvp;
          cnp->cn_flags |= MAKEENTRY;
          if (cache_fpl_islastcn(ndp))
                  cnp->cn_flags |= ISLASTCN;
          if (cache_fpl_isdotdot(cnp))
                  cnp->cn_flags |= ISDOTDOT;
  
          /*
           * Skip potential extra slashes parsing did not take care of.
           * cache_fplookup_skip_slashes explains the mechanism.
           */
          if (__predict_false(*(cnp->cn_nameptr) == '/')) {
                  do {
                          cnp->cn_nameptr++;
                          cache_fpl_pathlen_dec(fpl);
                  } while (*(cnp->cn_nameptr) == '/');
          }
  
          ndp->ni_pathlen = fpl->nulchar - cnp->cn_nameptr + 1;
  #ifdef INVARIANTS
          if (ndp->ni_pathlen != fpl->debug.ni_pathlen) {
                  panic("%s: mismatch (%zu != %zu) nulchar %p nameptr %p [%s] ; full string [%s]\n",
                      __func__, ndp->ni_pathlen, fpl->debug.ni_pathlen, fpl->nulchar,
                      cnp->cn_nameptr, cnp->cn_nameptr, cnp->cn_pnbuf);
          }
  #endif
          return (0);
  }
  
  static int
  cache_fplookup_final_child(struct cache_fpl *fpl, enum vgetstate tvs)
  {
          struct componentname *cnp;
          struct vnode *tvp;
          seqc_t tvp_seqc;
          int error, lkflags;
  
          cnp = fpl->cnp;
          tvp = fpl->tvp;
          tvp_seqc = fpl->tvp_seqc;
  
          if ((cnp->cn_flags & LOCKLEAF) != 0) {
                  lkflags = LK_SHARED;
                  if ((cnp->cn_flags & LOCKSHARED) == 0)
                          lkflags = LK_EXCLUSIVE;
                  error = vget_finish(tvp, lkflags, tvs);
                  if (__predict_false(error != 0)) {
                          return (cache_fpl_aborted(fpl));
                  }
          } else {
                  vget_finish_ref(tvp, tvs);
          }
  
          if (!vn_seqc_consistent(tvp, tvp_seqc)) {
                  if ((cnp->cn_flags & LOCKLEAF) != 0)
                          vput(tvp);
                  else
                          vrele(tvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          return (cache_fpl_handled(fpl));
  }
  
  /*
   * They want to possibly modify the state of the namecache.
   */
  static int __noinline
  cache_fplookup_final_modifying(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          enum vgetstate dvs;
          struct vnode *dvp, *tvp;
          struct mount *mp;
          seqc_t dvp_seqc;
          int error;
          bool docache;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          dvp = fpl->dvp;
          dvp_seqc = fpl->dvp_seqc;
  
          MPASS(*(cnp->cn_nameptr) != '/');
          MPASS(cache_fpl_islastcn(ndp));
          if ((cnp->cn_flags & LOCKPARENT) == 0)
                  MPASS((cnp->cn_flags & WANTPARENT) != 0);
          MPASS((cnp->cn_flags & TRAILINGSLASH) == 0);
          MPASS(cnp->cn_nameiop == CREATE || cnp->cn_nameiop == DELETE ||
              cnp->cn_nameiop == RENAME);
          MPASS((cnp->cn_flags & MAKEENTRY) == 0);
          MPASS((cnp->cn_flags & ISDOTDOT) == 0);
  
          docache = (cnp->cn_flags & NOCACHE) ^ NOCACHE;
          if (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)
                  docache = false;
  
          /*
           * Regular lookup nulifies the slash, which we don't do here.
           * Don't take chances with filesystem routines seeing it for
           * the last entry.
           */
          if (cache_fpl_istrailingslash(fpl)) {
                  return (cache_fpl_partial(fpl));
          }
  
          mp = atomic_load_ptr(&dvp->v_mount);
          if (__predict_false(mp == NULL)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          if (__predict_false(mp->mnt_flag & MNT_RDONLY)) {
                  cache_fpl_smr_exit(fpl);
                  /*
                   * Original code keeps not checking for CREATE which
                   * might be a bug. For now let the old lookup decide.
                   */
                  if (cnp->cn_nameiop == CREATE) {
                          return (cache_fpl_aborted(fpl));
                  }
                  return (cache_fpl_handled_error(fpl, EROFS));
          }
  
          if (fpl->tvp != NULL && (cnp->cn_flags & FAILIFEXISTS) != 0) {
                  cache_fpl_smr_exit(fpl);
                  return (cache_fpl_handled_error(fpl, EEXIST));
          }
  
          /*
           * Secure access to dvp; check cache_fplookup_partial_setup for
           * reasoning.
           *
           * XXX At least UFS requires its lookup routine to be called for
           * the last path component, which leads to some level of complication
           * and inefficiency:
           * - the target routine always locks the target vnode, but our caller
           *   may not need it locked
           * - some of the VOP machinery asserts that the parent is locked, which
           *   once more may be not required
           *
           * TODO: add a flag for filesystems which don't need this.
           */
          dvs = vget_prep_smr(dvp);
          cache_fpl_smr_exit(fpl);
          if (__predict_false(dvs == VGET_NONE)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          vget_finish_ref(dvp, dvs);
          if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                  vrele(dvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          error = vn_lock(dvp, LK_EXCLUSIVE);
          if (__predict_false(error != 0)) {
                  vrele(dvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          tvp = NULL;
          cnp->cn_flags |= ISLASTCN;
          if (docache)
                  cnp->cn_flags |= MAKEENTRY;
          if (cache_fpl_isdotdot(cnp))
                  cnp->cn_flags |= ISDOTDOT;
          cnp->cn_lkflags = LK_EXCLUSIVE;
          error = VOP_LOOKUP(dvp, &tvp, cnp);
          switch (error) {
          case EJUSTRETURN:
          case 0:
                  break;
          case ENOTDIR:
          case ENOENT:
                  vput(dvp);
                  return (cache_fpl_handled_error(fpl, error));
          default:
                  vput(dvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          fpl->tvp = tvp;
          fpl->savename = (cnp->cn_flags & SAVENAME) != 0;
  
          if (tvp == NULL) {
                  if ((cnp->cn_flags & SAVESTART) != 0) {
                          ndp->ni_startdir = dvp;
                          vrefact(ndp->ni_startdir);
                          cnp->cn_flags |= SAVENAME;
                          fpl->savename = true;
                  }
                  MPASS(error == EJUSTRETURN);
                  if ((cnp->cn_flags & LOCKPARENT) == 0) {
                          VOP_UNLOCK(dvp);
                  }
                  return (cache_fpl_handled(fpl));
          }
  
          /*
           * There are very hairy corner cases concerning various flag combinations
           * and locking state. In particular here we only hold one lock instead of
           * two.
           *
           * Skip the complexity as it is of no significance for normal workloads.
           */
          if (__predict_false(tvp == dvp)) {
                  vput(dvp);
                  vrele(tvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          /*
           * If they want the symlink itself we are fine, but if they want to
           * follow it regular lookup has to be engaged.
           */
          if (tvp->v_type == VLNK) {
                  if ((cnp->cn_flags & FOLLOW) != 0) {
                          vput(dvp);
                          vput(tvp);
                          return (cache_fpl_aborted(fpl));
                  }
          }
  
          /*
           * Since we expect this to be the terminal vnode it should almost never
           * be a mount point.
           */
          if (__predict_false(cache_fplookup_is_mp(fpl))) {
                  vput(dvp);
                  vput(tvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          if ((cnp->cn_flags & FAILIFEXISTS) != 0) {
                  vput(dvp);
                  vput(tvp);
                  return (cache_fpl_handled_error(fpl, EEXIST));
          }
  
          if ((cnp->cn_flags & LOCKLEAF) == 0) {
                  VOP_UNLOCK(tvp);
          }
  
          if ((cnp->cn_flags & LOCKPARENT) == 0) {
                  VOP_UNLOCK(dvp);
          }
  
          if ((cnp->cn_flags & SAVESTART) != 0) {
                  ndp->ni_startdir = dvp;
                  vrefact(ndp->ni_startdir);
                  cnp->cn_flags |= SAVENAME;
                  fpl->savename = true;
          }
  
          return (cache_fpl_handled(fpl));
  }
  
  static int __noinline
  cache_fplookup_modifying(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
  
          ndp = fpl->ndp;
  
          if (!cache_fpl_islastcn(ndp)) {
                  return (cache_fpl_partial(fpl));
          }
          return (cache_fplookup_final_modifying(fpl));
  }
  
  static int __noinline
  cache_fplookup_final_withparent(struct cache_fpl *fpl)
  {
          struct componentname *cnp;
          enum vgetstate dvs, tvs;
          struct vnode *dvp, *tvp;
          seqc_t dvp_seqc;
          int error;
  
          cnp = fpl->cnp;
          dvp = fpl->dvp;
          dvp_seqc = fpl->dvp_seqc;
          tvp = fpl->tvp;
  
          MPASS((cnp->cn_flags & (LOCKPARENT|WANTPARENT)) != 0);
  
          /*
           * This is less efficient than it can be for simplicity.
           */
          dvs = vget_prep_smr(dvp);
          if (__predict_false(dvs == VGET_NONE)) {
                  return (cache_fpl_aborted(fpl));
          }
          tvs = vget_prep_smr(tvp);
          if (__predict_false(tvs == VGET_NONE)) {
                  cache_fpl_smr_exit(fpl);
                  vget_abort(dvp, dvs);
                  return (cache_fpl_aborted(fpl));
          }
  
          cache_fpl_smr_exit(fpl);
  
          if ((cnp->cn_flags & LOCKPARENT) != 0) {
                  error = vget_finish(dvp, LK_EXCLUSIVE, dvs);
                  if (__predict_false(error != 0)) {
                          vget_abort(tvp, tvs);
                          return (cache_fpl_aborted(fpl));
                  }
          } else {
                  vget_finish_ref(dvp, dvs);
          }
  
          if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                  vget_abort(tvp, tvs);
                  if ((cnp->cn_flags & LOCKPARENT) != 0)
                          vput(dvp);
                  else
                          vrele(dvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          error = cache_fplookup_final_child(fpl, tvs);
          if (__predict_false(error != 0)) {
                  MPASS(fpl->status == CACHE_FPL_STATUS_ABORTED ||
                      fpl->status == CACHE_FPL_STATUS_DESTROYED);
                  if ((cnp->cn_flags & LOCKPARENT) != 0)
                          vput(dvp);
                  else
                          vrele(dvp);
                  return (error);
          }
  
          MPASS(fpl->status == CACHE_FPL_STATUS_HANDLED);
          return (0);
  }
  
  static int
  cache_fplookup_final(struct cache_fpl *fpl)
  {
          struct componentname *cnp;
          enum vgetstate tvs;
          struct vnode *dvp, *tvp;
          seqc_t dvp_seqc;
  
          cnp = fpl->cnp;
          dvp = fpl->dvp;
          dvp_seqc = fpl->dvp_seqc;
          tvp = fpl->tvp;
  
          MPASS(*(cnp->cn_nameptr) != '/');
  
          if (cnp->cn_nameiop != LOOKUP) {
                  return (cache_fplookup_final_modifying(fpl));
          }
  
          if ((cnp->cn_flags & (LOCKPARENT|WANTPARENT)) != 0)
                  return (cache_fplookup_final_withparent(fpl));
  
          tvs = vget_prep_smr(tvp);
          if (__predict_false(tvs == VGET_NONE)) {
                  return (cache_fpl_partial(fpl));
          }
  
          if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                  cache_fpl_smr_exit(fpl);
                  vget_abort(tvp, tvs);
                  return (cache_fpl_aborted(fpl));
          }
  
          cache_fpl_smr_exit(fpl);
          return (cache_fplookup_final_child(fpl, tvs));
  }
  
  /*
   * Comment from locked lookup:
   * Check for degenerate name (e.g. / or "") which is a way of talking about a
   * directory, e.g. like "/." or ".".
   */
  static int __noinline
  cache_fplookup_degenerate(struct cache_fpl *fpl)
  {
          struct componentname *cnp;
          struct vnode *dvp;
          enum vgetstate dvs;
          int error, lkflags;
  #ifdef INVARIANTS
          char *cp;
  #endif
  
          fpl->tvp = fpl->dvp;
          fpl->tvp_seqc = fpl->dvp_seqc;
  
          cnp = fpl->cnp;
          dvp = fpl->dvp;
  
  #ifdef INVARIANTS
          for (cp = cnp->cn_pnbuf; *cp != '\0'; cp++) {
                  KASSERT(*cp == '/',
                      ("%s: encountered non-slash; string [%s]\n", __func__,
                      cnp->cn_pnbuf));
          }
  #endif
  
          if (__predict_false(cnp->cn_nameiop != LOOKUP)) {
                  cache_fpl_smr_exit(fpl);
                  return (cache_fpl_handled_error(fpl, EISDIR));
          }
  
          MPASS((cnp->cn_flags & SAVESTART) == 0);
  
          if ((cnp->cn_flags & (LOCKPARENT|WANTPARENT)) != 0) {
                  return (cache_fplookup_final_withparent(fpl));
          }
  
          dvs = vget_prep_smr(dvp);
          cache_fpl_smr_exit(fpl);
          if (__predict_false(dvs == VGET_NONE)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          if ((cnp->cn_flags & LOCKLEAF) != 0) {
                  lkflags = LK_SHARED;
                  if ((cnp->cn_flags & LOCKSHARED) == 0)
                          lkflags = LK_EXCLUSIVE;
                  error = vget_finish(dvp, lkflags, dvs);
                  if (__predict_false(error != 0)) {
                          return (cache_fpl_aborted(fpl));
                  }
          } else {
                  vget_finish_ref(dvp, dvs);
          }
          return (cache_fpl_handled(fpl));
  }
  
  static int __noinline
  cache_fplookup_emptypath(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          enum vgetstate tvs;
          struct vnode *tvp;
          int error, lkflags;
  
          fpl->tvp = fpl->dvp;
          fpl->tvp_seqc = fpl->dvp_seqc;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          tvp = fpl->tvp;
  
          MPASS(*cnp->cn_pnbuf == '\0');
  
          if (__predict_false((cnp->cn_flags & EMPTYPATH) == 0)) {
                  cache_fpl_smr_exit(fpl);
                  return (cache_fpl_handled_error(fpl, ENOENT));
          }
  
          MPASS((cnp->cn_flags & (LOCKPARENT | WANTPARENT)) == 0);
  
          tvs = vget_prep_smr(tvp);
          cache_fpl_smr_exit(fpl);
          if (__predict_false(tvs == VGET_NONE)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          if ((cnp->cn_flags & LOCKLEAF) != 0) {
                  lkflags = LK_SHARED;
                  if ((cnp->cn_flags & LOCKSHARED) == 0)
                          lkflags = LK_EXCLUSIVE;
                  error = vget_finish(tvp, lkflags, tvs);
                  if (__predict_false(error != 0)) {
                          return (cache_fpl_aborted(fpl));
                  }
          } else {
                  vget_finish_ref(tvp, tvs);
          }
  
          ndp->ni_resflags |= NIRES_EMPTYPATH;
          return (cache_fpl_handled(fpl));
  }
  
  static int __noinline
  cache_fplookup_noentry(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          enum vgetstate dvs;
          struct vnode *dvp, *tvp;
          seqc_t dvp_seqc;
          int error;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          dvp = fpl->dvp;
          dvp_seqc = fpl->dvp_seqc;
  
          MPASS((cnp->cn_flags & MAKEENTRY) == 0);
          MPASS((cnp->cn_flags & ISDOTDOT) == 0);
          if (cnp->cn_nameiop == LOOKUP)
                  MPASS((cnp->cn_flags & NOCACHE) == 0);
          MPASS(!cache_fpl_isdotdot(cnp));
  
          /*
           * Hack: delayed name len checking.
           */
          if (__predict_false(cnp->cn_namelen > NAME_MAX)) {
                  cache_fpl_smr_exit(fpl);
                  return (cache_fpl_handled_error(fpl, ENAMETOOLONG));
          }
  
          if (cnp->cn_nameptr[0] == '/') {
                  return (cache_fplookup_skip_slashes(fpl));
          }
  
          if (cnp->cn_pnbuf[0] == '\0') {
                  return (cache_fplookup_emptypath(fpl));
          }
  
          if (cnp->cn_nameptr[0] == '\0') {
                  if (fpl->tvp == NULL) {
                          return (cache_fplookup_degenerate(fpl));
                  }
                  return (cache_fplookup_trailingslash(fpl));
          }
  
          if (cnp->cn_nameiop != LOOKUP) {
                  fpl->tvp = NULL;
                  return (cache_fplookup_modifying(fpl));
          }
  
          MPASS((cnp->cn_flags & SAVESTART) == 0);
  
          /*
           * Only try to fill in the component if it is the last one,
           * otherwise not only there may be several to handle but the
           * walk may be complicated.
           */
          if (!cache_fpl_islastcn(ndp)) {
                  return (cache_fpl_partial(fpl));
          }
  
          /*
           * Regular lookup nulifies the slash, which we don't do here.
           * Don't take chances with filesystem routines seeing it for
           * the last entry.
           */
          if (cache_fpl_istrailingslash(fpl)) {
                  return (cache_fpl_partial(fpl));
          }
  
          /*
           * Secure access to dvp; check cache_fplookup_partial_setup for
           * reasoning.
           */
          dvs = vget_prep_smr(dvp);
          cache_fpl_smr_exit(fpl);
          if (__predict_false(dvs == VGET_NONE)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          vget_finish_ref(dvp, dvs);
          if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                  vrele(dvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          error = vn_lock(dvp, LK_SHARED);
          if (__predict_false(error != 0)) {
                  vrele(dvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          tvp = NULL;
          /*
           * TODO: provide variants which don't require locking either vnode.
           */
          cnp->cn_flags |= ISLASTCN | MAKEENTRY;
          cnp->cn_lkflags = LK_SHARED;
          if ((cnp->cn_flags & LOCKSHARED) == 0) {
                  cnp->cn_lkflags = LK_EXCLUSIVE;
          }
          error = VOP_LOOKUP(dvp, &tvp, cnp);
          switch (error) {
          case EJUSTRETURN:
          case 0:
                  break;
          case ENOTDIR:
          case ENOENT:
                  vput(dvp);
                  return (cache_fpl_handled_error(fpl, error));
          default:
                  vput(dvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          fpl->tvp = tvp;
          if (!fpl->savename) {
                  MPASS((cnp->cn_flags & SAVENAME) == 0);
          }
  
          if (tvp == NULL) {
                  MPASS(error == EJUSTRETURN);
                  if ((cnp->cn_flags & (WANTPARENT | LOCKPARENT)) == 0) {
                          vput(dvp);
                  } else if ((cnp->cn_flags & LOCKPARENT) == 0) {
                          VOP_UNLOCK(dvp);
                  }
                  return (cache_fpl_handled(fpl));
          }
  
          if (tvp->v_type == VLNK) {
                  if ((cnp->cn_flags & FOLLOW) != 0) {
                          vput(dvp);
                          vput(tvp);
                          return (cache_fpl_aborted(fpl));
                  }
          }
  
          if (__predict_false(cache_fplookup_is_mp(fpl))) {
                  vput(dvp);
                  vput(tvp);
                  return (cache_fpl_aborted(fpl));
          }
  
          if ((cnp->cn_flags & LOCKLEAF) == 0) {
                  VOP_UNLOCK(tvp);
          }
  
          if ((cnp->cn_flags & (WANTPARENT | LOCKPARENT)) == 0) {
                  vput(dvp);
          } else if ((cnp->cn_flags & LOCKPARENT) == 0) {
                  VOP_UNLOCK(dvp);
          }
          return (cache_fpl_handled(fpl));
  }
  
  static int __noinline
  cache_fplookup_dot(struct cache_fpl *fpl)
  {
          int error;
  
          MPASS(!seqc_in_modify(fpl->dvp_seqc));
          /*
           * Just re-assign the value. seqc will be checked later for the first
           * non-dot path component in line and/or before deciding to return the
           * vnode.
           */
          fpl->tvp = fpl->dvp;
          fpl->tvp_seqc = fpl->dvp_seqc;
  
          counter_u64_add(dothits, 1);
          SDT_PROBE3(vfs, namecache, lookup, hit, fpl->dvp, ".", fpl->dvp);
  
          error = 0;
          if (cache_fplookup_is_mp(fpl)) {
                  error = cache_fplookup_cross_mount(fpl);
          }
          return (error);
  }
  
  static int __noinline
  cache_fplookup_dotdot(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          struct namecache *ncp;
          struct vnode *dvp;
          struct prison *pr;
          u_char nc_flag;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          dvp = fpl->dvp;
  
          MPASS(cache_fpl_isdotdot(cnp));
  
          /*
           * XXX this is racy the same way regular lookup is
           */
          for (pr = cnp->cn_cred->cr_prison; pr != NULL;
              pr = pr->pr_parent)
                  if (dvp == pr->pr_root)
                          break;
  
          if (dvp == ndp->ni_rootdir ||
              dvp == ndp->ni_topdir ||
              dvp == rootvnode ||
              pr != NULL) {
                  fpl->tvp = dvp;
                  fpl->tvp_seqc = vn_seqc_read_any(dvp);
                  if (seqc_in_modify(fpl->tvp_seqc)) {
                          return (cache_fpl_aborted(fpl));
                  }
                  return (0);
          }
  
          if ((dvp->v_vflag & VV_ROOT) != 0) {
                  /*
                   * TODO
                   * The opposite of climb mount is needed here.
                   */
                  return (cache_fpl_partial(fpl));
          }
  
          ncp = atomic_load_consume_ptr(&dvp->v_cache_dd);
          if (ncp == NULL) {
                  return (cache_fpl_aborted(fpl));
          }
  
          nc_flag = atomic_load_char(&ncp->nc_flag);
          if ((nc_flag & NCF_ISDOTDOT) != 0) {
                  if ((nc_flag & NCF_NEGATIVE) != 0)
                          return (cache_fpl_aborted(fpl));
                  fpl->tvp = ncp->nc_vp;
          } else {
                  fpl->tvp = ncp->nc_dvp;
          }
  
          fpl->tvp_seqc = vn_seqc_read_any(fpl->tvp);
          if (seqc_in_modify(fpl->tvp_seqc)) {
                  return (cache_fpl_partial(fpl));
          }
  
          /*
           * Acquire fence provided by vn_seqc_read_any above.
           */
          if (__predict_false(atomic_load_ptr(&dvp->v_cache_dd) != ncp)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          if (!cache_ncp_canuse(ncp)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          counter_u64_add(dotdothits, 1);
          return (0);
  }
  
  static int __noinline
  cache_fplookup_neg(struct cache_fpl *fpl, struct namecache *ncp, uint32_t hash)
  {
          u_char nc_flag __diagused;
          bool neg_promote;
  
  #ifdef INVARIANTS
          nc_flag = atomic_load_char(&ncp->nc_flag);
          MPASS((nc_flag & NCF_NEGATIVE) != 0);
  #endif
          /*
           * If they want to create an entry we need to replace this one.
           */
          if (__predict_false(fpl->cnp->cn_nameiop != LOOKUP)) {
                  fpl->tvp = NULL;
                  return (cache_fplookup_modifying(fpl));
          }
          neg_promote = cache_neg_hit_prep(ncp);
          if (!cache_fpl_neg_ncp_canuse(ncp)) {
                  cache_neg_hit_abort(ncp);
                  return (cache_fpl_partial(fpl));
          }
          if (neg_promote) {
                  return (cache_fplookup_negative_promote(fpl, ncp, hash));
          }
          cache_neg_hit_finish(ncp);
          cache_fpl_smr_exit(fpl);
          return (cache_fpl_handled_error(fpl, ENOENT));
  }
  
  /*
   * Resolve a symlink. Called by filesystem-specific routines.
   *
   * Code flow is:
   * ... -> cache_fplookup_symlink -> VOP_FPLOOKUP_SYMLINK -> cache_symlink_resolve
   */
  int
  cache_symlink_resolve(struct cache_fpl *fpl, const char *string, size_t len)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          size_t adjust;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
  
          if (__predict_false(len == 0)) {
                  return (ENOENT);
          }
  
          if (__predict_false(len > MAXPATHLEN - 2)) {
                  if (cache_fpl_istrailingslash(fpl)) {
                          return (EAGAIN);
                  }
          }
  
          ndp->ni_pathlen = fpl->nulchar - cnp->cn_nameptr - cnp->cn_namelen + 1;
  #ifdef INVARIANTS
          if (ndp->ni_pathlen != fpl->debug.ni_pathlen) {
                  panic("%s: mismatch (%zu != %zu) nulchar %p nameptr %p [%s] ; full string [%s]\n",
                      __func__, ndp->ni_pathlen, fpl->debug.ni_pathlen, fpl->nulchar,
                      cnp->cn_nameptr, cnp->cn_nameptr, cnp->cn_pnbuf);
          }
  #endif
  
          if (__predict_false(len + ndp->ni_pathlen > MAXPATHLEN)) {
                  return (ENAMETOOLONG);
          }
  
          if (__predict_false(ndp->ni_loopcnt++ >= MAXSYMLINKS)) {
                  return (ELOOP);
          }
  
          adjust = len;
          if (ndp->ni_pathlen > 1) {
                  bcopy(ndp->ni_next, cnp->cn_pnbuf + len, ndp->ni_pathlen);
          } else {
                  if (cache_fpl_istrailingslash(fpl)) {
                          adjust = len + 1;
                          cnp->cn_pnbuf[len] = '/';
                          cnp->cn_pnbuf[len + 1] = '\0';
                  } else {
                          cnp->cn_pnbuf[len] = '\0';
                  }
          }
          bcopy(string, cnp->cn_pnbuf, len);
  
          ndp->ni_pathlen += adjust;
          cache_fpl_pathlen_add(fpl, adjust);
          cnp->cn_nameptr = cnp->cn_pnbuf;
          fpl->nulchar = &cnp->cn_nameptr[ndp->ni_pathlen - 1];
          fpl->tvp = NULL;
          return (0);
  }
  
  static int __noinline
  cache_fplookup_symlink(struct cache_fpl *fpl)
  {
          struct mount *mp;
          struct nameidata *ndp;
          struct componentname *cnp;
          struct vnode *dvp, *tvp;
          int error;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          dvp = fpl->dvp;
          tvp = fpl->tvp;
  
          if (cache_fpl_islastcn(ndp)) {
                  if ((cnp->cn_flags & FOLLOW) == 0) {
                          return (cache_fplookup_final(fpl));
                  }
          }
  
          mp = atomic_load_ptr(&dvp->v_mount);
          if (__predict_false(mp == NULL)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          /*
           * Note this check races against setting the flag just like regular
           * lookup.
           */
          if (__predict_false((mp->mnt_flag & MNT_NOSYMFOLLOW) != 0)) {
                  cache_fpl_smr_exit(fpl);
                  return (cache_fpl_handled_error(fpl, EACCES));
          }
  
          error = VOP_FPLOOKUP_SYMLINK(tvp, fpl);
          if (__predict_false(error != 0)) {
                  switch (error) {
                  case EAGAIN:
                          return (cache_fpl_partial(fpl));
                  case ENOENT:
                  case ENAMETOOLONG:
                  case ELOOP:
                          cache_fpl_smr_exit(fpl);
                          return (cache_fpl_handled_error(fpl, error));
                  default:
                          return (cache_fpl_aborted(fpl));
                  }
          }
  
          if (*(cnp->cn_nameptr) == '/') {
                  fpl->dvp = cache_fpl_handle_root(fpl);
                  fpl->dvp_seqc = vn_seqc_read_any(fpl->dvp);
                  if (seqc_in_modify(fpl->dvp_seqc)) {
                          return (cache_fpl_aborted(fpl));
                  }
                  /*
                   * The main loop assumes that ->dvp points to a vnode belonging
                   * to a filesystem which can do lockless lookup, but the absolute
                   * symlink can be wandering off to one which does not.
                   */
                  mp = atomic_load_ptr(&fpl->dvp->v_mount);
                  if (__predict_false(mp == NULL)) {
                          return (cache_fpl_aborted(fpl));
                  }
                  if (!cache_fplookup_mp_supported(mp)) {
                          cache_fpl_checkpoint(fpl);
                          return (cache_fpl_partial(fpl));
                  }
          }
          return (0);
  }
  
  static int
  cache_fplookup_next(struct cache_fpl *fpl)
  {
          struct componentname *cnp;
          struct namecache *ncp;
          struct vnode *dvp, *tvp;
          u_char nc_flag;
          uint32_t hash;
          int error;
  
          cnp = fpl->cnp;
          dvp = fpl->dvp;
          hash = fpl->hash;
  
          if (__predict_false(cnp->cn_nameptr[0] == '.')) {
                  if (cnp->cn_namelen == 1) {
                          return (cache_fplookup_dot(fpl));
                  }
                  if (cnp->cn_namelen == 2 && cnp->cn_nameptr[1] == '.') {
                          return (cache_fplookup_dotdot(fpl));
                  }
          }
  
          MPASS(!cache_fpl_isdotdot(cnp));
  
          CK_SLIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                  if (ncp->nc_dvp == dvp && ncp->nc_nlen == cnp->cn_namelen &&
                      !bcmp(ncp->nc_name, cnp->cn_nameptr, ncp->nc_nlen))
                          break;
          }
  
          if (__predict_false(ncp == NULL)) {
                  return (cache_fplookup_noentry(fpl));
          }
  
          tvp = atomic_load_ptr(&ncp->nc_vp);
          nc_flag = atomic_load_char(&ncp->nc_flag);
          if ((nc_flag & NCF_NEGATIVE) != 0) {
                  return (cache_fplookup_neg(fpl, ncp, hash));
          }
  
          if (!cache_ncp_canuse(ncp)) {
                  return (cache_fpl_partial(fpl));
          }
  
          fpl->tvp = tvp;
          fpl->tvp_seqc = vn_seqc_read_any(tvp);
          if (seqc_in_modify(fpl->tvp_seqc)) {
                  return (cache_fpl_partial(fpl));
          }
  
          counter_u64_add(numposhits, 1);
          SDT_PROBE3(vfs, namecache, lookup, hit, dvp, ncp->nc_name, tvp);
  
          error = 0;
          if (cache_fplookup_is_mp(fpl)) {
                  error = cache_fplookup_cross_mount(fpl);
          }
          return (error);
  }
  
  static bool
  cache_fplookup_mp_supported(struct mount *mp)
  {
  
          MPASS(mp != NULL);
          if ((mp->mnt_kern_flag & MNTK_FPLOOKUP) == 0)
                  return (false);
          return (true);
  }
  
  /*
   * Walk up the mount stack (if any).
   *
   * Correctness is provided in the following ways:
   * - all vnodes are protected from freeing with SMR
   * - struct mount objects are type stable making them always safe to access
   * - stability of the particular mount is provided by busying it
   * - relationship between the vnode which is mounted on and the mount is
   *   verified with the vnode sequence counter after busying
   * - association between root vnode of the mount and the mount is protected
   *   by busy
   *
   * From that point on we can read the sequence counter of the root vnode
   * and get the next mount on the stack (if any) using the same protection.
   *
   * By the end of successful walk we are guaranteed the reached state was
   * indeed present at least at some point which matches the regular lookup.
   */
  static int __noinline
  cache_fplookup_climb_mount(struct cache_fpl *fpl)
  {
          struct mount *mp, *prev_mp;
          struct mount_pcpu *mpcpu, *prev_mpcpu;
          struct vnode *vp;
          seqc_t vp_seqc;
  
          vp = fpl->tvp;
          vp_seqc = fpl->tvp_seqc;
  
          VNPASS(vp->v_type == VDIR || vp->v_type == VREG || vp->v_type == VBAD, vp);
          mp = atomic_load_ptr(&vp->v_mountedhere);
          if (__predict_false(mp == NULL)) {
                  return (0);
          }
  
          prev_mp = NULL;
          for (;;) {
                  if (!vfs_op_thread_enter_crit(mp, mpcpu)) {
                          if (prev_mp != NULL)
                                  vfs_op_thread_exit_crit(prev_mp, prev_mpcpu);
                          return (cache_fpl_partial(fpl));
                  }
                  if (prev_mp != NULL)
                          vfs_op_thread_exit_crit(prev_mp, prev_mpcpu);
                  if (!vn_seqc_consistent(vp, vp_seqc)) {
                          vfs_op_thread_exit_crit(mp, mpcpu);
                          return (cache_fpl_partial(fpl));
                  }
                  if (!cache_fplookup_mp_supported(mp)) {
                          vfs_op_thread_exit_crit(mp, mpcpu);
                          return (cache_fpl_partial(fpl));
                  }
                  vp = atomic_load_ptr(&mp->mnt_rootvnode);
                  if (vp == NULL) {
                          vfs_op_thread_exit_crit(mp, mpcpu);
                          return (cache_fpl_partial(fpl));
                  }
                  vp_seqc = vn_seqc_read_any(vp);
                  if (seqc_in_modify(vp_seqc)) {
                          vfs_op_thread_exit_crit(mp, mpcpu);
                          return (cache_fpl_partial(fpl));
                  }
                  prev_mp = mp;
                  prev_mpcpu = mpcpu;
                  mp = atomic_load_ptr(&vp->v_mountedhere);
                  if (mp == NULL)
                          break;
          }
  
          vfs_op_thread_exit_crit(prev_mp, prev_mpcpu);
          fpl->tvp = vp;
          fpl->tvp_seqc = vp_seqc;
          return (0);
  }
  
  static int __noinline
  cache_fplookup_cross_mount(struct cache_fpl *fpl)
  {
          struct mount *mp;
          struct mount_pcpu *mpcpu;
          struct vnode *vp;
          seqc_t vp_seqc;
  
          vp = fpl->tvp;
          vp_seqc = fpl->tvp_seqc;
  
          VNPASS(vp->v_type == VDIR || vp->v_type == VREG || vp->v_type == VBAD, vp);
          mp = atomic_load_ptr(&vp->v_mountedhere);
          if (__predict_false(mp == NULL)) {
                  return (0);
          }
  
          if (!vfs_op_thread_enter_crit(mp, mpcpu)) {
                  return (cache_fpl_partial(fpl));
          }
          if (!vn_seqc_consistent(vp, vp_seqc)) {
                  vfs_op_thread_exit_crit(mp, mpcpu);
                  return (cache_fpl_partial(fpl));
          }
          if (!cache_fplookup_mp_supported(mp)) {
                  vfs_op_thread_exit_crit(mp, mpcpu);
                  return (cache_fpl_partial(fpl));
          }
          vp = atomic_load_ptr(&mp->mnt_rootvnode);
          if (__predict_false(vp == NULL)) {
                  vfs_op_thread_exit_crit(mp, mpcpu);
                  return (cache_fpl_partial(fpl));
          }
          vp_seqc = vn_seqc_read_any(vp);
          vfs_op_thread_exit_crit(mp, mpcpu);
          if (seqc_in_modify(vp_seqc)) {
                  return (cache_fpl_partial(fpl));
          }
          mp = atomic_load_ptr(&vp->v_mountedhere);
          if (__predict_false(mp != NULL)) {
                  /*
                   * There are possibly more mount points on top.
                   * Normally this does not happen so for simplicity just start
                   * over.
                   */
                  return (cache_fplookup_climb_mount(fpl));
          }
  
          fpl->tvp = vp;
          fpl->tvp_seqc = vp_seqc;
          return (0);
  }
  
  /*
   * Check if a vnode is mounted on.
   */
  static bool
  cache_fplookup_is_mp(struct cache_fpl *fpl)
  {
          struct vnode *vp;
  
          vp = fpl->tvp;
          return ((vn_irflag_read(vp) & VIRF_MOUNTPOINT) != 0);
  }
  
  /*
   * Parse the path.
   *
   * The code was originally copy-pasted from regular lookup and despite
   * clean ups leaves performance on the table. Any modifications here
   * must take into account that in case off fallback the resulting
   * nameidata state has to be compatible with the original.
   */
  
  /*
   * Debug ni_pathlen tracking.
   */
  #ifdef INVARIANTS
  static void
  cache_fpl_pathlen_add(struct cache_fpl *fpl, size_t n)
  {
  
          fpl->debug.ni_pathlen += n;
          KASSERT(fpl->debug.ni_pathlen <= PATH_MAX,
              ("%s: pathlen overflow to %zd\n", __func__, fpl->debug.ni_pathlen));
  }
  
  static void
  cache_fpl_pathlen_sub(struct cache_fpl *fpl, size_t n)
  {
  
          fpl->debug.ni_pathlen -= n;
          KASSERT(fpl->debug.ni_pathlen <= PATH_MAX,
              ("%s: pathlen underflow to %zd\n", __func__, fpl->debug.ni_pathlen));
  }
  
  static void
  cache_fpl_pathlen_inc(struct cache_fpl *fpl)
  {
  
          cache_fpl_pathlen_add(fpl, 1);
  }
  
  static void
  cache_fpl_pathlen_dec(struct cache_fpl *fpl)
  {
  
          cache_fpl_pathlen_sub(fpl, 1);
  }
  #else
  static void
  cache_fpl_pathlen_add(struct cache_fpl *fpl, size_t n)
  {
  }
  
  static void
  cache_fpl_pathlen_sub(struct cache_fpl *fpl, size_t n)
  {
  }
  
  static void
  cache_fpl_pathlen_inc(struct cache_fpl *fpl)
  {
  }
  
  static void
  cache_fpl_pathlen_dec(struct cache_fpl *fpl)
  {
  }
  #endif
  
  static void
  cache_fplookup_parse(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          struct vnode *dvp;
          char *cp;
          uint32_t hash;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          dvp = fpl->dvp;
  
          /*
           * Find the end of this path component, it is either / or nul.
           *
           * Store / as a temporary sentinel so that we only have one character
           * to test for. Pathnames tend to be short so this should not be
           * resulting in cache misses.
           *
           * TODO: fix this to be word-sized.
           */
          MPASS(&cnp->cn_nameptr[fpl->debug.ni_pathlen - 1] >= cnp->cn_pnbuf);
          KASSERT(&cnp->cn_nameptr[fpl->debug.ni_pathlen - 1] == fpl->nulchar,
              ("%s: mismatch between pathlen (%zu) and nulchar (%p != %p), string [%s]\n",
              __func__, fpl->debug.ni_pathlen, &cnp->cn_nameptr[fpl->debug.ni_pathlen - 1],
              fpl->nulchar, cnp->cn_pnbuf));
          KASSERT(*fpl->nulchar == '\0',
              ("%s: expected nul at %p; string [%s]\n", __func__, fpl->nulchar,
              cnp->cn_pnbuf));
          hash = cache_get_hash_iter_start(dvp);
          *fpl->nulchar = '/';
          for (cp = cnp->cn_nameptr; *cp != '/'; cp++) {
                  KASSERT(*cp != '\0',
                      ("%s: encountered unexpected nul; string [%s]\n", __func__,
                      cnp->cn_nameptr));
                  hash = cache_get_hash_iter(*cp, hash);
                  continue;
          }
          *fpl->nulchar = '\0';
          fpl->hash = cache_get_hash_iter_finish(hash);
  
          cnp->cn_namelen = cp - cnp->cn_nameptr;
          cache_fpl_pathlen_sub(fpl, cnp->cn_namelen);
  
  #ifdef INVARIANTS
          /*
           * cache_get_hash only accepts lengths up to NAME_MAX. This is fine since
           * we are going to fail this lookup with ENAMETOOLONG (see below).
           */
          if (cnp->cn_namelen <= NAME_MAX) {
                  if (fpl->hash != cache_get_hash(cnp->cn_nameptr, cnp->cn_namelen, dvp)) {
                          panic("%s: mismatched hash for [%s] len %ld", __func__,
                              cnp->cn_nameptr, cnp->cn_namelen);
                  }
          }
  #endif
  
          /*
           * Hack: we have to check if the found path component's length exceeds
           * NAME_MAX. However, the condition is very rarely true and check can
           * be elided in the common case -- if an entry was found in the cache,
           * then it could not have been too long to begin with.
           */
          ndp->ni_next = cp;
  }
  
  static void
  cache_fplookup_parse_advance(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
  
          cnp->cn_nameptr = ndp->ni_next;
          KASSERT(*(cnp->cn_nameptr) == '/',
              ("%s: should have seen slash at %p ; buf %p [%s]\n", __func__,
              cnp->cn_nameptr, cnp->cn_pnbuf, cnp->cn_pnbuf));
          cnp->cn_nameptr++;
          cache_fpl_pathlen_dec(fpl);
  }
  
  /*
   * Skip spurious slashes in a pathname (e.g., "foo///bar") and retry.
   *
   * Lockless lookup tries to elide checking for spurious slashes and should they
   * be present is guaranteed to fail to find an entry. In this case the caller
   * must check if the name starts with a slash and call this routine.  It is
   * going to fast forward across the spurious slashes and set the state up for
   * retry.
   */
  static int __noinline
  cache_fplookup_skip_slashes(struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
  
          MPASS(*(cnp->cn_nameptr) == '/');
          do {
                  cnp->cn_nameptr++;
                  cache_fpl_pathlen_dec(fpl);
          } while (*(cnp->cn_nameptr) == '/');
  
          /*
           * Go back to one slash so that cache_fplookup_parse_advance has
           * something to skip.
           */
          cnp->cn_nameptr--;
          cache_fpl_pathlen_inc(fpl);
  
          /*
           * cache_fplookup_parse_advance starts from ndp->ni_next
           */
          ndp->ni_next = cnp->cn_nameptr;
  
          /*
           * See cache_fplookup_dot.
           */
          fpl->tvp = fpl->dvp;
          fpl->tvp_seqc = fpl->dvp_seqc;
  
          return (0);
  }
  
  /*
   * Handle trailing slashes (e.g., "foo/").
   *
   * If a trailing slash is found the terminal vnode must be a directory.
   * Regular lookup shortens the path by nulifying the first trailing slash and
   * sets the TRAILINGSLASH flag to denote this took place. There are several
   * checks on it performed later.
   *
   * Similarly to spurious slashes, lockless lookup handles this in a speculative
   * manner relying on an invariant that a non-directory vnode will get a miss.
   * In this case cn_nameptr[0] == '\0' and cn_namelen == 0.
   *
   * Thus for a path like "foo/bar/" the code unwinds the state back to "bar/"
   * and denotes this is the last path component, which avoids looping back.
   *
   * Only plain lookups are supported for now to restrict corner cases to handle.
   */
  static int __noinline
  cache_fplookup_trailingslash(struct cache_fpl *fpl)
  {
  #ifdef INVARIANTS
          size_t ni_pathlen;
  #endif
          struct nameidata *ndp;
          struct componentname *cnp;
          struct namecache *ncp;
          struct vnode *tvp;
          char *cn_nameptr_orig, *cn_nameptr_slash;
          seqc_t tvp_seqc;
          u_char nc_flag;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
          tvp = fpl->tvp;
          tvp_seqc = fpl->tvp_seqc;
  
          MPASS(fpl->dvp == fpl->tvp);
          KASSERT(cache_fpl_istrailingslash(fpl),
              ("%s: expected trailing slash at %p; string [%s]\n", __func__, fpl->nulchar - 1,
              cnp->cn_pnbuf));
          KASSERT(cnp->cn_nameptr[0] == '\0',
              ("%s: expected nul char at %p; string [%s]\n", __func__, &cnp->cn_nameptr[0],
              cnp->cn_pnbuf));
          KASSERT(cnp->cn_namelen == 0,
              ("%s: namelen 0 but got %ld; string [%s]\n", __func__, cnp->cn_namelen,
              cnp->cn_pnbuf));
          MPASS(cnp->cn_nameptr > cnp->cn_pnbuf);
  
          if (cnp->cn_nameiop != LOOKUP) {
                  return (cache_fpl_aborted(fpl));
          }
  
          if (__predict_false(tvp->v_type != VDIR)) {
                  if (!vn_seqc_consistent(tvp, tvp_seqc)) {
                          return (cache_fpl_aborted(fpl));
                  }
                  cache_fpl_smr_exit(fpl);
                  return (cache_fpl_handled_error(fpl, ENOTDIR));
          }
  
          /*
           * Denote the last component.
           */
          ndp->ni_next = &cnp->cn_nameptr[0];
          MPASS(cache_fpl_islastcn(ndp));
  
          /*
           * Unwind trailing slashes.
           */
          cn_nameptr_orig = cnp->cn_nameptr;
          while (cnp->cn_nameptr >= cnp->cn_pnbuf) {
                  cnp->cn_nameptr--;
                  if (cnp->cn_nameptr[0] != '/') {
                          break;
                  }
          }
  
          /*
           * Unwind to the beginning of the path component.
           *
           * Note the path may or may not have started with a slash.
           */
          cn_nameptr_slash = cnp->cn_nameptr;
          while (cnp->cn_nameptr > cnp->cn_pnbuf) {
                  cnp->cn_nameptr--;
                  if (cnp->cn_nameptr[0] == '/') {
                          break;
                  }
          }
          if (cnp->cn_nameptr[0] == '/') {
                  cnp->cn_nameptr++;
          }
  
          cnp->cn_namelen = cn_nameptr_slash - cnp->cn_nameptr + 1;
          cache_fpl_pathlen_add(fpl, cn_nameptr_orig - cnp->cn_nameptr);
          cache_fpl_checkpoint(fpl);
  
  #ifdef INVARIANTS
          ni_pathlen = fpl->nulchar - cnp->cn_nameptr + 1;
          if (ni_pathlen != fpl->debug.ni_pathlen) {
                  panic("%s: mismatch (%zu != %zu) nulchar %p nameptr %p [%s] ; full string [%s]\n",
                      __func__, ni_pathlen, fpl->debug.ni_pathlen, fpl->nulchar,
                      cnp->cn_nameptr, cnp->cn_nameptr, cnp->cn_pnbuf);
          }
  #endif
  
          /*
           * If this was a "./" lookup the parent directory is already correct.
           */
          if (cnp->cn_nameptr[0] == '.' && cnp->cn_namelen == 1) {
                  return (0);
          }
  
          /*
           * Otherwise we need to look it up.
           */
          tvp = fpl->tvp;
          ncp = atomic_load_consume_ptr(&tvp->v_cache_dd);
          if (__predict_false(ncp == NULL)) {
                  return (cache_fpl_aborted(fpl));
          }
          nc_flag = atomic_load_char(&ncp->nc_flag);
          if ((nc_flag & NCF_ISDOTDOT) != 0) {
                  return (cache_fpl_aborted(fpl));
          }
          fpl->dvp = ncp->nc_dvp;
          fpl->dvp_seqc = vn_seqc_read_any(fpl->dvp);
          if (seqc_in_modify(fpl->dvp_seqc)) {
                  return (cache_fpl_aborted(fpl));
          }
          return (0);
  }
  
  /*
   * See the API contract for VOP_FPLOOKUP_VEXEC.
   */
  static int __noinline
  cache_fplookup_failed_vexec(struct cache_fpl *fpl, int error)
  {
          struct componentname *cnp;
          struct vnode *dvp;
          seqc_t dvp_seqc;
  
          cnp = fpl->cnp;
          dvp = fpl->dvp;
          dvp_seqc = fpl->dvp_seqc;
  
          /*
           * Hack: delayed empty path checking.
           */
          if (cnp->cn_pnbuf[0] == '\0') {
                  return (cache_fplookup_emptypath(fpl));
          }
  
          /*
           * TODO: Due to ignoring trailing slashes lookup will perform a
           * permission check on the last dir when it should not be doing it.  It
           * may fail, but said failure should be ignored. It is possible to fix
           * it up fully without resorting to regular lookup, but for now just
           * abort.
           */
          if (cache_fpl_istrailingslash(fpl)) {
                  return (cache_fpl_aborted(fpl));
          }
  
          /*
           * Hack: delayed degenerate path checking.
           */
          if (cnp->cn_nameptr[0] == '\0' && fpl->tvp == NULL) {
                  return (cache_fplookup_degenerate(fpl));
          }
  
          /*
           * Hack: delayed name len checking.
           */
          if (__predict_false(cnp->cn_namelen > NAME_MAX)) {
                  cache_fpl_smr_exit(fpl);
                  return (cache_fpl_handled_error(fpl, ENAMETOOLONG));
          }
  
          /*
           * Hack: they may be looking up foo/bar, where foo is not a directory.
           * In such a case we need to return ENOTDIR, but we may happen to get
           * here with a different error.
           */
          if (dvp->v_type != VDIR) {
                  error = ENOTDIR;
          }
  
          /*
           * Hack: handle O_SEARCH.
           *
           * Open Group Base Specifications Issue 7, 2018 edition states:
           * <quote>
           * If the access mode of the open file description associated with the
           * file descriptor is not O_SEARCH, the function shall check whether
           * directory searches are permitted using the current permissions of
           * the directory underlying the file descriptor. If the access mode is
           * O_SEARCH, the function shall not perform the check.
           * </quote>
           *
           * Regular lookup tests for the NOEXECCHECK flag for every path
           * component to decide whether to do the permission check. However,
           * since most lookups never have the flag (and when they do it is only
           * present for the first path component), lockless lookup only acts on
           * it if there is a permission problem. Here the flag is represented
           * with a boolean so that we don't have to clear it on the way out.
           *
           * For simplicity this always aborts.
           * TODO: check if this is the first lookup and ignore the permission
           * problem. Note the flag has to survive fallback (if it happens to be
           * performed).
           */
          if (fpl->fsearch) {
                  return (cache_fpl_aborted(fpl));
          }
  
          switch (error) {
          case EAGAIN:
                  if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                          error = cache_fpl_aborted(fpl);
                  } else {
                          cache_fpl_partial(fpl);
                  }
                  break;
          default:
                  if (!vn_seqc_consistent(dvp, dvp_seqc)) {
                          error = cache_fpl_aborted(fpl);
                  } else {
                          cache_fpl_smr_exit(fpl);
                          cache_fpl_handled_error(fpl, error);
                  }
                  break;
          }
          return (error);
  }
  
  static int
  cache_fplookup_impl(struct vnode *dvp, struct cache_fpl *fpl)
  {
          struct nameidata *ndp;
          struct componentname *cnp;
          struct mount *mp;
          int error;
  
          ndp = fpl->ndp;
          cnp = fpl->cnp;
  
          cache_fpl_checkpoint(fpl);
  
          /*
           * The vnode at hand is almost always stable, skip checking for it.
           * Worst case this postpones the check towards the end of the iteration
           * of the main loop.
           */
          fpl->dvp = dvp;
          fpl->dvp_seqc = vn_seqc_read_notmodify(fpl->dvp);
  
          mp = atomic_load_ptr(&dvp->v_mount);
          if (__predict_false(mp == NULL || !cache_fplookup_mp_supported(mp))) {
                  return (cache_fpl_aborted(fpl));
          }
  
          MPASS(fpl->tvp == NULL);
  
          for (;;) {
                  cache_fplookup_parse(fpl);
  
                  error = VOP_FPLOOKUP_VEXEC(fpl->dvp, cnp->cn_cred);
                  if (__predict_false(error != 0)) {
                          error = cache_fplookup_failed_vexec(fpl, error);
                          break;
                  }
  
                  error = cache_fplookup_next(fpl);
                  if (__predict_false(cache_fpl_terminated(fpl))) {
                          break;
                  }
  
                  VNPASS(!seqc_in_modify(fpl->tvp_seqc), fpl->tvp);
  
                  if (fpl->tvp->v_type == VLNK) {
                          error = cache_fplookup_symlink(fpl);
                          if (cache_fpl_terminated(fpl)) {
                                  break;
                          }
                  } else {
                          if (cache_fpl_islastcn(ndp)) {
                                  error = cache_fplookup_final(fpl);
                                  break;
                          }
  
                          if (!vn_seqc_consistent(fpl->dvp, fpl->dvp_seqc)) {
                                  error = cache_fpl_aborted(fpl);
                                  break;
                          }
  
                          fpl->dvp = fpl->tvp;
                          fpl->dvp_seqc = fpl->tvp_seqc;
                          cache_fplookup_parse_advance(fpl);
                  }
  
                  cache_fpl_checkpoint(fpl);
          }
  
          return (error);
  }
  
  /*
   * Fast path lookup protected with SMR and sequence counters.
   *
   * Note: all VOP_FPLOOKUP_VEXEC routines have a comment referencing this one.
   *
   * Filesystems can opt in by setting the MNTK_FPLOOKUP flag and meeting criteria
   * outlined below.
   *
   * Traditional vnode lookup conceptually looks like this:
   *
   * vn_lock(current);
   * for (;;) {
   *      next = find();
   *      vn_lock(next);
   *      vn_unlock(current);
   *      current = next;
   *      if (last)
   *          break;
   * }
   * return (current);
   *
   * Each jump to the next vnode is safe memory-wise and atomic with respect to
   * any modifications thanks to holding respective locks.
   *
   * The same guarantee can be provided with a combination of safe memory
   * reclamation and sequence counters instead. If all operations which affect
   * the relationship between the current vnode and the one we are looking for
   * also modify the counter, we can verify whether all the conditions held as
   * we made the jump. This includes things like permissions, mount points etc.
   * Counter modification is provided by enclosing relevant places in
   * vn_seqc_write_begin()/end() calls.
   *
   * Thus this translates to:
   *
   * vfs_smr_enter();
   * dvp_seqc = seqc_read_any(dvp);
   * if (seqc_in_modify(dvp_seqc)) // someone is altering the vnode
   *     abort();
   * for (;;) {
   *      tvp = find();
   *      tvp_seqc = seqc_read_any(tvp);
   *      if (seqc_in_modify(tvp_seqc)) // someone is altering the target vnode
   *          abort();
   *      if (!seqc_consistent(dvp, dvp_seqc) // someone is altering the vnode
   *          abort();
   *      dvp = tvp; // we know nothing of importance has changed
   *      dvp_seqc = tvp_seqc; // store the counter for the tvp iteration
   *      if (last)
   *          break;
   * }
   * vget(); // secure the vnode
   * if (!seqc_consistent(tvp, tvp_seqc) // final check
   *          abort();
   * // at this point we know nothing has changed for any parent<->child pair
   * // as they were crossed during the lookup, meaning we matched the guarantee
   * // of the locked variant
   * return (tvp);
   *
   * The API contract for VOP_FPLOOKUP_VEXEC routines is as follows:
   * - they are called while within vfs_smr protection which they must never exit
   * - EAGAIN can be returned to denote checking could not be performed, it is
   *   always valid to return it
   * - if the sequence counter has not changed the result must be valid
   * - if the sequence counter has changed both false positives and false negatives
   *   are permitted (since the result will be rejected later)
   * - for simple cases of unix permission checks vaccess_vexec_smr can be used
   *
   * Caveats to watch out for:
   * - vnodes are passed unlocked and unreferenced with nothing stopping
   *   VOP_RECLAIM, in turn meaning that ->v_data can become NULL. It is advised
   *   to use atomic_load_ptr to fetch it.
   * - the aforementioned object can also get freed, meaning absent other means it
   *   should be protected with vfs_smr
   * - either safely checking permissions as they are modified or guaranteeing
   *   their stability is left to the routine
   */
  int
  cache_fplookup(struct nameidata *ndp, enum cache_fpl_status *status,
      struct pwd **pwdp)
  {
          struct cache_fpl fpl;
          struct pwd *pwd;
          struct vnode *dvp;
          struct componentname *cnp;
          int error;
  
          fpl.status = CACHE_FPL_STATUS_UNSET;
          fpl.in_smr = false;
          fpl.ndp = ndp;
          fpl.cnp = cnp = &ndp->ni_cnd;
          MPASS(ndp->ni_lcf == 0);
          MPASS(curthread == cnp->cn_thread);
          KASSERT ((cnp->cn_flags & CACHE_FPL_INTERNAL_CN_FLAGS) == 0,
              ("%s: internal flags found in cn_flags %" PRIx64, __func__,
              cnp->cn_flags));
          if ((cnp->cn_flags & SAVESTART) != 0) {
                  MPASS(cnp->cn_nameiop != LOOKUP);
          }
          MPASS(cnp->cn_nameptr == cnp->cn_pnbuf);
  
          if (__predict_false(!cache_can_fplookup(&fpl))) {
                  *status = fpl.status;
                  SDT_PROBE3(vfs, fplookup, lookup, done, ndp, fpl.line, fpl.status);
                  return (EOPNOTSUPP);
          }
  
          cache_fpl_checkpoint_outer(&fpl);
  
          cache_fpl_smr_enter_initial(&fpl);
  #ifdef INVARIANTS
          fpl.debug.ni_pathlen = ndp->ni_pathlen;
  #endif
          fpl.nulchar = &cnp->cn_nameptr[ndp->ni_pathlen - 1];
          fpl.fsearch = false;
          fpl.savename = (cnp->cn_flags & SAVENAME) != 0;
          fpl.tvp = NULL; /* for degenerate path handling */
          fpl.pwd = pwdp;
          pwd = pwd_get_smr();
          *(fpl.pwd) = pwd;
          ndp->ni_rootdir = pwd->pwd_rdir;
          ndp->ni_topdir = pwd->pwd_jdir;
  
          if (cnp->cn_pnbuf[0] == '/') {
                  dvp = cache_fpl_handle_root(&fpl);
                  MPASS(ndp->ni_resflags == 0);
                  ndp->ni_resflags = NIRES_ABS;
          } else {
                  if (ndp->ni_dirfd == AT_FDCWD) {
                          dvp = pwd->pwd_cdir;
                  } else {
                          error = cache_fplookup_dirfd(&fpl, &dvp);
                          if (__predict_false(error != 0)) {
                                  goto out;
                          }
                  }
          }
  
          SDT_PROBE4(vfs, namei, lookup, entry, dvp, cnp->cn_pnbuf, cnp->cn_flags, true);
          error = cache_fplookup_impl(dvp, &fpl);
  out:
          cache_fpl_smr_assert_not_entered(&fpl);
          cache_fpl_assert_status(&fpl);
          *status = fpl.status;
          if (SDT_PROBES_ENABLED()) {
                  SDT_PROBE3(vfs, fplookup, lookup, done, ndp, fpl.line, fpl.status);
                  if (fpl.status == CACHE_FPL_STATUS_HANDLED)
                          SDT_PROBE4(vfs, namei, lookup, return, error, ndp->ni_vp, true,
                              ndp);
          }
  
          if (__predict_true(fpl.status == CACHE_FPL_STATUS_HANDLED)) {
                  MPASS(error != CACHE_FPL_FAILED);
                  if (error != 0) {
                          MPASS(fpl.dvp == NULL);
                          MPASS(fpl.tvp == NULL);
                          MPASS(fpl.savename == false);
                  }
                  ndp->ni_dvp = fpl.dvp;
                  ndp->ni_vp = fpl.tvp;
                  if (fpl.savename) {
                          cnp->cn_flags |= HASBUF;
                  } else {
                          cache_fpl_cleanup_cnp(cnp);
                  }
          }
          return (error);
  }