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

     /*-
      * SPDX-License-Identifier: BSD-3-Clause
      *
      * Copyright (c) 1989, 1993
      *      The Regents of the University of California.  All rights reserved.
      * (c) UNIX System Laboratories, Inc.
      * All or some portions of this file are derived from material licensed
      * to the University of California by American Telephone and Telegraph
      * Co. or Unix System Laboratories, Inc. and are reproduced herein with
     * the permission of UNIX System Laboratories, Inc.
     *
     * 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_subr.c  8.31 (Berkeley) 5/26/95
     */
    
    /*
     * External virtual filesystem routines
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/12.0/sys/kern/vfs_subr.c 337977 2018-08-17 16:07:06Z markj $");
    
    #include "opt_ddb.h"
    #include "opt_watchdog.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/bio.h>
    #include <sys/buf.h>
    #include <sys/condvar.h>
    #include <sys/conf.h>
    #include <sys/counter.h>
    #include <sys/dirent.h>
    #include <sys/event.h>
    #include <sys/eventhandler.h>
    #include <sys/extattr.h>
    #include <sys/file.h>
    #include <sys/fcntl.h>
    #include <sys/jail.h>
    #include <sys/kdb.h>
    #include <sys/kernel.h>
    #include <sys/kthread.h>
    #include <sys/lockf.h>
    #include <sys/malloc.h>
    #include <sys/mount.h>
    #include <sys/namei.h>
    #include <sys/pctrie.h>
    #include <sys/priv.h>
    #include <sys/reboot.h>
    #include <sys/refcount.h>
    #include <sys/rwlock.h>
    #include <sys/sched.h>
    #include <sys/sleepqueue.h>
    #include <sys/smp.h>
    #include <sys/stat.h>
    #include <sys/sysctl.h>
    #include <sys/syslog.h>
    #include <sys/vmmeter.h>
    #include <sys/vnode.h>
    #include <sys/watchdog.h>
    
    #include <machine/stdarg.h>
    
    #include <security/mac/mac_framework.h>
    
    #include <vm/vm.h>
    #include <vm/vm_object.h>
    #include <vm/vm_extern.h>
    #include <vm/pmap.h>
    #include <vm/vm_map.h>
    #include <vm/vm_page.h>
    #include <vm/vm_kern.h>
    #include <vm/uma.h>
    
    #ifdef DDB
    #include <ddb/ddb.h>
   #endif
   
   static void     delmntque(struct vnode *vp);
   static int      flushbuflist(struct bufv *bufv, int flags, struct bufobj *bo,
                       int slpflag, int slptimeo);
   static void     syncer_shutdown(void *arg, int howto);
   static int      vtryrecycle(struct vnode *vp);
   static void     v_init_counters(struct vnode *);
   static void     v_incr_usecount(struct vnode *);
   static void     v_incr_usecount_locked(struct vnode *);
   static void     v_incr_devcount(struct vnode *);
   static void     v_decr_devcount(struct vnode *);
   static void     vgonel(struct vnode *);
   static void     vfs_knllock(void *arg);
   static void     vfs_knlunlock(void *arg);
   static void     vfs_knl_assert_locked(void *arg);
   static void     vfs_knl_assert_unlocked(void *arg);
   static void     vnlru_return_batches(struct vfsops *mnt_op);
   static void     destroy_vpollinfo(struct vpollinfo *vi);
   
   /*
    * These fences are intended for cases where some synchronization is
    * needed between access of v_iflags and lockless vnode refcount (v_holdcnt
    * and v_usecount) updates.  Access to v_iflags is generally synchronized
    * by the interlock, but we have some internal assertions that check vnode
    * flags without acquiring the lock.  Thus, these fences are INVARIANTS-only
    * for now.
    */
   #ifdef INVARIANTS
   #define VNODE_REFCOUNT_FENCE_ACQ()      atomic_thread_fence_acq()
   #define VNODE_REFCOUNT_FENCE_REL()      atomic_thread_fence_rel()
   #else
   #define VNODE_REFCOUNT_FENCE_ACQ()
   #define VNODE_REFCOUNT_FENCE_REL()
   #endif
   
   /*
    * Number of vnodes in existence.  Increased whenever getnewvnode()
    * allocates a new vnode, decreased in vdropl() for VI_DOOMED vnode.
    */
   static unsigned long    numvnodes;
   
   SYSCTL_ULONG(_vfs, OID_AUTO, numvnodes, CTLFLAG_RD, &numvnodes, 0,
       "Number of vnodes in existence");
   
   static counter_u64_t vnodes_created;
   SYSCTL_COUNTER_U64(_vfs, OID_AUTO, vnodes_created, CTLFLAG_RD, &vnodes_created,
       "Number of vnodes created by getnewvnode");
   
   static u_long mnt_free_list_batch = 128;
   SYSCTL_ULONG(_vfs, OID_AUTO, mnt_free_list_batch, CTLFLAG_RW,
       &mnt_free_list_batch, 0, "Limit of vnodes held on mnt's free list");
   
   /*
    * Conversion tables for conversion from vnode types to inode formats
    * and back.
    */
   enum vtype iftovt_tab[16] = {
           VNON, VFIFO, VCHR, VNON, VDIR, VNON, VBLK, VNON,
           VREG, VNON, VLNK, VNON, VSOCK, VNON, VNON, VBAD,
   };
   int vttoif_tab[10] = {
           0, S_IFREG, S_IFDIR, S_IFBLK, S_IFCHR, S_IFLNK,
           S_IFSOCK, S_IFIFO, S_IFMT, S_IFMT
   };
   
   /*
    * List of vnodes that are ready for recycling.
    */
   static TAILQ_HEAD(freelst, vnode) vnode_free_list;
   
   /*
    * "Free" vnode target.  Free vnodes are rarely completely free, but are
    * just ones that are cheap to recycle.  Usually they are for files which
    * have been stat'd but not read; these usually have inode and namecache
    * data attached to them.  This target is the preferred minimum size of a
    * sub-cache consisting mostly of such files. The system balances the size
    * of this sub-cache with its complement to try to prevent either from
    * thrashing while the other is relatively inactive.  The targets express
    * a preference for the best balance.
    *
    * "Above" this target there are 2 further targets (watermarks) related
    * to recyling of free vnodes.  In the best-operating case, the cache is
    * exactly full, the free list has size between vlowat and vhiwat above the
    * free target, and recycling from it and normal use maintains this state.
    * Sometimes the free list is below vlowat or even empty, but this state
    * is even better for immediate use provided the cache is not full.
    * Otherwise, vnlru_proc() runs to reclaim enough vnodes (usually non-free
    * ones) to reach one of these states.  The watermarks are currently hard-
    * coded as 4% and 9% of the available space higher.  These and the default
    * of 25% for wantfreevnodes are too large if the memory size is large.
    * E.g., 9% of 75% of MAXVNODES is more than 566000 vnodes to reclaim
    * whenever vnlru_proc() becomes active.
    */
   static u_long wantfreevnodes;
   SYSCTL_ULONG(_vfs, OID_AUTO, wantfreevnodes, CTLFLAG_RW,
       &wantfreevnodes, 0, "Target for minimum number of \"free\" vnodes");
   static u_long freevnodes;
   SYSCTL_ULONG(_vfs, OID_AUTO, freevnodes, CTLFLAG_RD,
       &freevnodes, 0, "Number of \"free\" vnodes");
   
   static counter_u64_t recycles_count;
   SYSCTL_COUNTER_U64(_vfs, OID_AUTO, recycles, CTLFLAG_RD, &recycles_count,
       "Number of vnodes recycled to meet vnode cache targets");
   
   /*
    * Various variables used for debugging the new implementation of
    * reassignbuf().
    * XXX these are probably of (very) limited utility now.
    */
   static int reassignbufcalls;
   SYSCTL_INT(_vfs, OID_AUTO, reassignbufcalls, CTLFLAG_RW, &reassignbufcalls, 0,
       "Number of calls to reassignbuf");
   
   static counter_u64_t free_owe_inact;
   SYSCTL_COUNTER_U64(_vfs, OID_AUTO, free_owe_inact, CTLFLAG_RD, &free_owe_inact,
       "Number of times free vnodes kept on active list due to VFS "
       "owing inactivation");
   
   /* To keep more than one thread at a time from running vfs_getnewfsid */
   static struct mtx mntid_mtx;
   
   /*
    * Lock for any access to the following:
    *      vnode_free_list
    *      numvnodes
    *      freevnodes
    */
   static struct mtx vnode_free_list_mtx;
   
   /* Publicly exported FS */
   struct nfs_public nfs_pub;
   
   static uma_zone_t buf_trie_zone;
   
   /* Zone for allocation of new vnodes - used exclusively by getnewvnode() */
   static uma_zone_t vnode_zone;
   static uma_zone_t vnodepoll_zone;
   
   /*
    * The workitem queue.
    *
    * It is useful to delay writes of file data and filesystem metadata
    * for tens of seconds so that quickly created and deleted files need
    * not waste disk bandwidth being created and removed. To realize this,
    * we append vnodes to a "workitem" queue. When running with a soft
    * updates implementation, most pending metadata dependencies should
    * not wait for more than a few seconds. Thus, mounted on block devices
    * are delayed only about a half the time that file data is delayed.
    * Similarly, directory updates are more critical, so are only delayed
    * about a third the time that file data is delayed. Thus, there are
    * SYNCER_MAXDELAY queues that are processed round-robin at a rate of
    * one each second (driven off the filesystem syncer process). The
    * syncer_delayno variable indicates the next queue that is to be processed.
    * Items that need to be processed soon are placed in this queue:
    *
    *      syncer_workitem_pending[syncer_delayno]
    *
    * A delay of fifteen seconds is done by placing the request fifteen
    * entries later in the queue:
    *
    *      syncer_workitem_pending[(syncer_delayno + 15) & syncer_mask]
    *
    */
   static int syncer_delayno;
   static long syncer_mask;
   LIST_HEAD(synclist, bufobj);
   static struct synclist *syncer_workitem_pending;
   /*
    * The sync_mtx protects:
    *      bo->bo_synclist
    *      sync_vnode_count
    *      syncer_delayno
    *      syncer_state
    *      syncer_workitem_pending
    *      syncer_worklist_len
    *      rushjob
    */
   static struct mtx sync_mtx;
   static struct cv sync_wakeup;
   
   #define SYNCER_MAXDELAY         32
   static int syncer_maxdelay = SYNCER_MAXDELAY;   /* maximum delay time */
   static int syncdelay = 30;              /* max time to delay syncing data */
   static int filedelay = 30;              /* time to delay syncing files */
   SYSCTL_INT(_kern, OID_AUTO, filedelay, CTLFLAG_RW, &filedelay, 0,
       "Time to delay syncing files (in seconds)");
   static int dirdelay = 29;               /* time to delay syncing directories */
   SYSCTL_INT(_kern, OID_AUTO, dirdelay, CTLFLAG_RW, &dirdelay, 0,
       "Time to delay syncing directories (in seconds)");
   static int metadelay = 28;              /* time to delay syncing metadata */
   SYSCTL_INT(_kern, OID_AUTO, metadelay, CTLFLAG_RW, &metadelay, 0,
       "Time to delay syncing metadata (in seconds)");
   static int rushjob;             /* number of slots to run ASAP */
   static int stat_rush_requests;  /* number of times I/O speeded up */
   SYSCTL_INT(_debug, OID_AUTO, rush_requests, CTLFLAG_RW, &stat_rush_requests, 0,
       "Number of times I/O speeded up (rush requests)");
   
   /*
    * When shutting down the syncer, run it at four times normal speed.
    */
   #define SYNCER_SHUTDOWN_SPEEDUP         4
   static int sync_vnode_count;
   static int syncer_worklist_len;
   static enum { SYNCER_RUNNING, SYNCER_SHUTTING_DOWN, SYNCER_FINAL_DELAY }
       syncer_state;
   
   /* Target for maximum number of vnodes. */
   int desiredvnodes;
   static int gapvnodes;           /* gap between wanted and desired */
   static int vhiwat;              /* enough extras after expansion */
   static int vlowat;              /* minimal extras before expansion */
   static int vstir;               /* nonzero to stir non-free vnodes */
   static volatile int vsmalltrigger = 8;  /* pref to keep if > this many pages */
   
   static int
   sysctl_update_desiredvnodes(SYSCTL_HANDLER_ARGS)
   {
           int error, old_desiredvnodes;
   
           old_desiredvnodes = desiredvnodes;
           if ((error = sysctl_handle_int(oidp, arg1, arg2, req)) != 0)
                   return (error);
           if (old_desiredvnodes != desiredvnodes) {
                   wantfreevnodes = desiredvnodes / 4;
                   /* XXX locking seems to be incomplete. */
                   vfs_hash_changesize(desiredvnodes);
                   cache_changesize(desiredvnodes);
           }
           return (0);
   }
   
   SYSCTL_PROC(_kern, KERN_MAXVNODES, maxvnodes,
       CTLTYPE_INT | CTLFLAG_MPSAFE | CTLFLAG_RW, &desiredvnodes, 0,
       sysctl_update_desiredvnodes, "I", "Target for maximum number of vnodes");
   SYSCTL_ULONG(_kern, OID_AUTO, minvnodes, CTLFLAG_RW,
       &wantfreevnodes, 0, "Old name for vfs.wantfreevnodes (legacy)");
   static int vnlru_nowhere;
   SYSCTL_INT(_debug, OID_AUTO, vnlru_nowhere, CTLFLAG_RW,
       &vnlru_nowhere, 0, "Number of times the vnlru process ran without success");
   
   /* Shift count for (uintptr_t)vp to initialize vp->v_hash. */
   static int vnsz2log;
   
   /*
    * Support for the bufobj clean & dirty pctrie.
    */
   static void *
   buf_trie_alloc(struct pctrie *ptree)
   {
   
           return uma_zalloc(buf_trie_zone, M_NOWAIT);
   }
   
   static void
   buf_trie_free(struct pctrie *ptree, void *node)
   {
   
           uma_zfree(buf_trie_zone, node);
   }
   PCTRIE_DEFINE(BUF, buf, b_lblkno, buf_trie_alloc, buf_trie_free);
   
   /*
    * Initialize the vnode management data structures.
    *
    * Reevaluate the following cap on the number of vnodes after the physical
    * memory size exceeds 512GB.  In the limit, as the physical memory size
    * grows, the ratio of the memory size in KB to vnodes approaches 64:1.
    */
   #ifndef MAXVNODES_MAX
   #define MAXVNODES_MAX   (512 * 1024 * 1024 / 64)        /* 8M */
   #endif
   
   /*
    * Initialize a vnode as it first enters the zone.
    */
   static int
   vnode_init(void *mem, int size, int flags)
   {
           struct vnode *vp;
   
           vp = mem;
           bzero(vp, size);
           /*
            * Setup locks.
            */
           vp->v_vnlock = &vp->v_lock;
           mtx_init(&vp->v_interlock, "vnode interlock", NULL, MTX_DEF);
           /*
            * By default, don't allow shared locks unless filesystems opt-in.
            */
           lockinit(vp->v_vnlock, PVFS, "vnode", VLKTIMEOUT,
               LK_NOSHARE | LK_IS_VNODE);
           /*
            * Initialize bufobj.
            */
           bufobj_init(&vp->v_bufobj, vp);
           /*
            * Initialize namecache.
            */
           LIST_INIT(&vp->v_cache_src);
           TAILQ_INIT(&vp->v_cache_dst);
           /*
            * Initialize rangelocks.
            */
           rangelock_init(&vp->v_rl);
           return (0);
   }
   
   /*
    * Free a vnode when it is cleared from the zone.
    */
   static void
   vnode_fini(void *mem, int size)
   {
           struct vnode *vp;
           struct bufobj *bo;
   
           vp = mem;
           rangelock_destroy(&vp->v_rl);
           lockdestroy(vp->v_vnlock);
           mtx_destroy(&vp->v_interlock);
           bo = &vp->v_bufobj;
           rw_destroy(BO_LOCKPTR(bo));
   }
   
   /*
    * Provide the size of NFS nclnode and NFS fh for calculation of the
    * vnode memory consumption.  The size is specified directly to
    * eliminate dependency on NFS-private header.
    *
    * Other filesystems may use bigger or smaller (like UFS and ZFS)
    * private inode data, but the NFS-based estimation is ample enough.
    * Still, we care about differences in the size between 64- and 32-bit
    * platforms.
    *
    * Namecache structure size is heuristically
    * sizeof(struct namecache_ts) + CACHE_PATH_CUTOFF + 1.
    */
   #ifdef _LP64
   #define NFS_NCLNODE_SZ  (528 + 64)
   #define NC_SZ           148
   #else
   #define NFS_NCLNODE_SZ  (360 + 32)
   #define NC_SZ           92
   #endif
   
   static void
   vntblinit(void *dummy __unused)
   {
           u_int i;
           int physvnodes, virtvnodes;
   
           /*
            * Desiredvnodes is a function of the physical memory size and the
            * kernel's heap size.  Generally speaking, it scales with the
            * physical memory size.  The ratio of desiredvnodes to the physical
            * memory size is 1:16 until desiredvnodes exceeds 98,304.
            * Thereafter, the
            * marginal ratio of desiredvnodes to the physical memory size is
            * 1:64.  However, desiredvnodes is limited by the kernel's heap
            * size.  The memory required by desiredvnodes vnodes and vm objects
            * must not exceed 1/10th of the kernel's heap size.
            */
           physvnodes = maxproc + pgtok(vm_cnt.v_page_count) / 64 +
               3 * min(98304 * 16, pgtok(vm_cnt.v_page_count)) / 64;
           virtvnodes = vm_kmem_size / (10 * (sizeof(struct vm_object) +
               sizeof(struct vnode) + NC_SZ * ncsizefactor + NFS_NCLNODE_SZ));
           desiredvnodes = min(physvnodes, virtvnodes);
           if (desiredvnodes > MAXVNODES_MAX) {
                   if (bootverbose)
                           printf("Reducing kern.maxvnodes %d -> %d\n",
                               desiredvnodes, MAXVNODES_MAX);
                   desiredvnodes = MAXVNODES_MAX;
           }
           wantfreevnodes = desiredvnodes / 4;
           mtx_init(&mntid_mtx, "mntid", NULL, MTX_DEF);
           TAILQ_INIT(&vnode_free_list);
           mtx_init(&vnode_free_list_mtx, "vnode_free_list", NULL, MTX_DEF);
           vnode_zone = uma_zcreate("VNODE", sizeof (struct vnode), NULL, NULL,
               vnode_init, vnode_fini, UMA_ALIGN_PTR, 0);
           vnodepoll_zone = uma_zcreate("VNODEPOLL", sizeof (struct vpollinfo),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
           /*
            * Preallocate enough nodes to support one-per buf so that
            * we can not fail an insert.  reassignbuf() callers can not
            * tolerate the insertion failure.
            */
           buf_trie_zone = uma_zcreate("BUF TRIE", pctrie_node_size(),
               NULL, NULL, pctrie_zone_init, NULL, UMA_ALIGN_PTR, 
               UMA_ZONE_NOFREE | UMA_ZONE_VM);
           uma_prealloc(buf_trie_zone, nbuf);
   
           vnodes_created = counter_u64_alloc(M_WAITOK);
           recycles_count = counter_u64_alloc(M_WAITOK);
           free_owe_inact = counter_u64_alloc(M_WAITOK);
   
           /*
            * Initialize the filesystem syncer.
            */
           syncer_workitem_pending = hashinit(syncer_maxdelay, M_VNODE,
               &syncer_mask);
           syncer_maxdelay = syncer_mask + 1;
           mtx_init(&sync_mtx, "Syncer mtx", NULL, MTX_DEF);
           cv_init(&sync_wakeup, "syncer");
           for (i = 1; i <= sizeof(struct vnode); i <<= 1)
                   vnsz2log++;
           vnsz2log--;
   }
   SYSINIT(vfs, SI_SUB_VFS, SI_ORDER_FIRST, vntblinit, NULL);
   
   
   /*
    * Mark a mount point as busy. Used to synchronize access and to delay
    * unmounting. Eventually, mountlist_mtx is not released on failure.
    *
    * vfs_busy() is a custom lock, it can block the caller.
    * vfs_busy() only sleeps if the unmount is active on the mount point.
    * For a mountpoint mp, vfs_busy-enforced lock is before lock of any
    * vnode belonging to mp.
    *
    * Lookup uses vfs_busy() to traverse mount points.
    * root fs                      var fs
    * / vnode lock         A       / vnode lock (/var)             D
    * /var vnode lock      B       /log vnode lock(/var/log)       E
    * vfs_busy lock        C       vfs_busy lock                   F
    *
    * Within each file system, the lock order is C->A->B and F->D->E.
    *
    * When traversing across mounts, the system follows that lock order:
    *
    *        C->A->B
    *              |
    *              +->F->D->E
    *
    * The lookup() process for namei("/var") illustrates the process:
    *  VOP_LOOKUP() obtains B while A is held
    *  vfs_busy() obtains a shared lock on F while A and B are held
    *  vput() releases lock on B
    *  vput() releases lock on A
    *  VFS_ROOT() obtains lock on D while shared lock on F is held
    *  vfs_unbusy() releases shared lock on F
    *  vn_lock() obtains lock on deadfs vnode vp_crossmp instead of A.
    *    Attempt to lock A (instead of vp_crossmp) while D is held would
    *    violate the global order, causing deadlocks.
    *
    * dounmount() locks B while F is drained.
    */
   int
   vfs_busy(struct mount *mp, int flags)
   {
   
           MPASS((flags & ~MBF_MASK) == 0);
           CTR3(KTR_VFS, "%s: mp %p with flags %d", __func__, mp, flags);
   
           MNT_ILOCK(mp);
           MNT_REF(mp);
           /*
            * If mount point is currently being unmounted, sleep until the
            * mount point fate is decided.  If thread doing the unmounting fails,
            * it will clear MNTK_UNMOUNT flag before waking us up, indicating
            * that this mount point has survived the unmount attempt and vfs_busy
            * should retry.  Otherwise the unmounter thread will set MNTK_REFEXPIRE
            * flag in addition to MNTK_UNMOUNT, indicating that mount point is
            * about to be really destroyed.  vfs_busy needs to release its
            * reference on the mount point in this case and return with ENOENT,
            * telling the caller that mount mount it tried to busy is no longer
            * valid.
            */
           while (mp->mnt_kern_flag & MNTK_UNMOUNT) {
                   if (flags & MBF_NOWAIT || mp->mnt_kern_flag & MNTK_REFEXPIRE) {
                           MNT_REL(mp);
                           MNT_IUNLOCK(mp);
                           CTR1(KTR_VFS, "%s: failed busying before sleeping",
                               __func__);
                           return (ENOENT);
                   }
                   if (flags & MBF_MNTLSTLOCK)
                           mtx_unlock(&mountlist_mtx);
                   mp->mnt_kern_flag |= MNTK_MWAIT;
                   msleep(mp, MNT_MTX(mp), PVFS | PDROP, "vfs_busy", 0);
                   if (flags & MBF_MNTLSTLOCK)
                           mtx_lock(&mountlist_mtx);
                   MNT_ILOCK(mp);
           }
           if (flags & MBF_MNTLSTLOCK)
                   mtx_unlock(&mountlist_mtx);
           mp->mnt_lockref++;
           MNT_IUNLOCK(mp);
           return (0);
   }
   
   /*
    * Free a busy filesystem.
    */
   void
   vfs_unbusy(struct mount *mp)
   {
   
           CTR2(KTR_VFS, "%s: mp %p", __func__, mp);
           MNT_ILOCK(mp);
           MNT_REL(mp);
           KASSERT(mp->mnt_lockref > 0, ("negative mnt_lockref"));
           mp->mnt_lockref--;
           if (mp->mnt_lockref == 0 && (mp->mnt_kern_flag & MNTK_DRAINING) != 0) {
                   MPASS(mp->mnt_kern_flag & MNTK_UNMOUNT);
                   CTR1(KTR_VFS, "%s: waking up waiters", __func__);
                   mp->mnt_kern_flag &= ~MNTK_DRAINING;
                   wakeup(&mp->mnt_lockref);
           }
           MNT_IUNLOCK(mp);
   }
   
   /*
    * Lookup a mount point by filesystem identifier.
    */
   struct mount *
   vfs_getvfs(fsid_t *fsid)
   {
           struct mount *mp;
   
           CTR2(KTR_VFS, "%s: fsid %p", __func__, fsid);
           mtx_lock(&mountlist_mtx);
           TAILQ_FOREACH(mp, &mountlist, mnt_list) {
                   if (mp->mnt_stat.f_fsid.val[0] == fsid->val[0] &&
                       mp->mnt_stat.f_fsid.val[1] == fsid->val[1]) {
                           vfs_ref(mp);
                           mtx_unlock(&mountlist_mtx);
                           return (mp);
                   }
           }
           mtx_unlock(&mountlist_mtx);
           CTR2(KTR_VFS, "%s: lookup failed for %p id", __func__, fsid);
           return ((struct mount *) 0);
   }
   
   /*
    * Lookup a mount point by filesystem identifier, busying it before
    * returning.
    *
    * To avoid congestion on mountlist_mtx, implement simple direct-mapped
    * cache for popular filesystem identifiers.  The cache is lockess, using
    * the fact that struct mount's are never freed.  In worst case we may
    * get pointer to unmounted or even different filesystem, so we have to
    * check what we got, and go slow way if so.
    */
   struct mount *
   vfs_busyfs(fsid_t *fsid)
   {
   #define FSID_CACHE_SIZE 256
           typedef struct mount * volatile vmp_t;
           static vmp_t cache[FSID_CACHE_SIZE];
           struct mount *mp;
           int error;
           uint32_t hash;
   
           CTR2(KTR_VFS, "%s: fsid %p", __func__, fsid);
           hash = fsid->val[0] ^ fsid->val[1];
           hash = (hash >> 16 ^ hash) & (FSID_CACHE_SIZE - 1);
           mp = cache[hash];
           if (mp == NULL ||
               mp->mnt_stat.f_fsid.val[0] != fsid->val[0] ||
               mp->mnt_stat.f_fsid.val[1] != fsid->val[1])
                   goto slow;
           if (vfs_busy(mp, 0) != 0) {
                   cache[hash] = NULL;
                   goto slow;
           }
           if (mp->mnt_stat.f_fsid.val[0] == fsid->val[0] &&
               mp->mnt_stat.f_fsid.val[1] == fsid->val[1])
                   return (mp);
           else
               vfs_unbusy(mp);
   
   slow:
           mtx_lock(&mountlist_mtx);
           TAILQ_FOREACH(mp, &mountlist, mnt_list) {
                   if (mp->mnt_stat.f_fsid.val[0] == fsid->val[0] &&
                       mp->mnt_stat.f_fsid.val[1] == fsid->val[1]) {
                           error = vfs_busy(mp, MBF_MNTLSTLOCK);
                           if (error) {
                                   cache[hash] = NULL;
                                   mtx_unlock(&mountlist_mtx);
                                   return (NULL);
                           }
                           cache[hash] = mp;
                           return (mp);
                   }
           }
           CTR2(KTR_VFS, "%s: lookup failed for %p id", __func__, fsid);
           mtx_unlock(&mountlist_mtx);
           return ((struct mount *) 0);
   }
   
   /*
    * Check if a user can access privileged mount options.
    */
   int
   vfs_suser(struct mount *mp, struct thread *td)
   {
           int error;
   
           if (jailed(td->td_ucred)) {
                   /*
                    * If the jail of the calling thread lacks permission for
                    * this type of file system, deny immediately.
                    */
                   if (!prison_allow(td->td_ucred, mp->mnt_vfc->vfc_prison_flag))
                           return (EPERM);
   
                   /*
                    * If the file system was mounted outside the jail of the
                    * calling thread, deny immediately.
                    */
                   if (prison_check(td->td_ucred, mp->mnt_cred) != 0)
                           return (EPERM);
           }
   
           /*
            * If file system supports delegated administration, we don't check
            * for the PRIV_VFS_MOUNT_OWNER privilege - it will be better verified
            * by the file system itself.
            * If this is not the user that did original mount, we check for
            * the PRIV_VFS_MOUNT_OWNER privilege.
            */
           if (!(mp->mnt_vfc->vfc_flags & VFCF_DELEGADMIN) &&
               mp->mnt_cred->cr_uid != td->td_ucred->cr_uid) {
                   if ((error = priv_check(td, PRIV_VFS_MOUNT_OWNER)) != 0)
                           return (error);
           }
           return (0);
   }
   
   /*
    * Get a new unique fsid.  Try to make its val[0] unique, since this value
    * will be used to create fake device numbers for stat().  Also try (but
    * not so hard) make its val[0] unique mod 2^16, since some emulators only
    * support 16-bit device numbers.  We end up with unique val[0]'s for the
    * first 2^16 calls and unique val[0]'s mod 2^16 for the first 2^8 calls.
    *
    * Keep in mind that several mounts may be running in parallel.  Starting
    * the search one past where the previous search terminated is both a
    * micro-optimization and a defense against returning the same fsid to
    * different mounts.
    */
   void
   vfs_getnewfsid(struct mount *mp)
   {
           static uint16_t mntid_base;
           struct mount *nmp;
           fsid_t tfsid;
           int mtype;
   
           CTR2(KTR_VFS, "%s: mp %p", __func__, mp);
           mtx_lock(&mntid_mtx);
           mtype = mp->mnt_vfc->vfc_typenum;
           tfsid.val[1] = mtype;
           mtype = (mtype & 0xFF) << 24;
           for (;;) {
                   tfsid.val[0] = makedev(255,
                       mtype | ((mntid_base & 0xFF00) << 8) | (mntid_base & 0xFF));
                   mntid_base++;
                   if ((nmp = vfs_getvfs(&tfsid)) == NULL)
                           break;
                   vfs_rel(nmp);
           }
           mp->mnt_stat.f_fsid.val[0] = tfsid.val[0];
           mp->mnt_stat.f_fsid.val[1] = tfsid.val[1];
           mtx_unlock(&mntid_mtx);
   }
   
   /*
    * Knob to control the precision of file timestamps:
    *
    *   0 = seconds only; nanoseconds zeroed.
    *   1 = seconds and nanoseconds, accurate within 1/HZ.
    *   2 = seconds and nanoseconds, truncated to microseconds.
    * >=3 = seconds and nanoseconds, maximum precision.
    */
   enum { TSP_SEC, TSP_HZ, TSP_USEC, TSP_NSEC };
   
   static int timestamp_precision = TSP_USEC;
   SYSCTL_INT(_vfs, OID_AUTO, timestamp_precision, CTLFLAG_RW,
       &timestamp_precision, 0, "File timestamp precision (0: seconds, "
       "1: sec + ns accurate to 1/HZ, 2: sec + ns truncated to us, "
       "3+: sec + ns (max. precision))");
   
   /*
    * Get a current timestamp.
    */
   void
   vfs_timestamp(struct timespec *tsp)
   {
           struct timeval tv;
   
           switch (timestamp_precision) {
           case TSP_SEC:
                   tsp->tv_sec = time_second;
                   tsp->tv_nsec = 0;
                   break;
           case TSP_HZ:
                   getnanotime(tsp);
                   break;
           case TSP_USEC:
                   microtime(&tv);
                   TIMEVAL_TO_TIMESPEC(&tv, tsp);
                   break;
           case TSP_NSEC:
           default:
                   nanotime(tsp);
                   break;
           }
   }
   
   /*
    * Set vnode attributes to VNOVAL
    */
   void
   vattr_null(struct vattr *vap)
   {
   
           vap->va_type = VNON;
           vap->va_size = VNOVAL;
           vap->va_bytes = VNOVAL;
           vap->va_mode = VNOVAL;
           vap->va_nlink = VNOVAL;
           vap->va_uid = VNOVAL;
           vap->va_gid = VNOVAL;
           vap->va_fsid = VNOVAL;
           vap->va_fileid = VNOVAL;
           vap->va_blocksize = VNOVAL;
           vap->va_rdev = VNOVAL;
           vap->va_atime.tv_sec = VNOVAL;
           vap->va_atime.tv_nsec = VNOVAL;
           vap->va_mtime.tv_sec = VNOVAL;
           vap->va_mtime.tv_nsec = VNOVAL;
           vap->va_ctime.tv_sec = VNOVAL;
           vap->va_ctime.tv_nsec = VNOVAL;
           vap->va_birthtime.tv_sec = VNOVAL;
           vap->va_birthtime.tv_nsec = VNOVAL;
           vap->va_flags = VNOVAL;
           vap->va_gen = VNOVAL;
           vap->va_vaflags = 0;
   }
   
   /*
    * This routine is called when we have too many vnodes.  It attempts
    * to free <count> vnodes and will potentially free vnodes that still
    * have VM backing store (VM backing store is typically the cause
    * of a vnode blowout so we want to do this).  Therefore, this operation
    * is not considered cheap.
    *
    * A number of conditions may prevent a vnode from being reclaimed.
    * the buffer cache may have references on the vnode, a directory
    * vnode may still have references due to the namei cache representing
    * underlying files, or the vnode may be in active use.   It is not
    * desirable to reuse such vnodes.  These conditions may cause the
    * number of vnodes to reach some minimum value regardless of what
    * you set kern.maxvnodes to.  Do not set kern.maxvnodes too low.
    */
   static int
   vlrureclaim(struct mount *mp, int reclaim_nc_src, int trigger)
   {
           struct vnode *vp;
           int count, done, target;
   
           done = 0;
           vn_start_write(NULL, &mp, V_WAIT);
           MNT_ILOCK(mp);
           count = mp->mnt_nvnodelistsize;
           target = count * (int64_t)gapvnodes / imax(desiredvnodes, 1);
           target = target / 10 + 1;
           while (count != 0 && done < target) {
                   vp = TAILQ_FIRST(&mp->mnt_nvnodelist);
                   while (vp != NULL && vp->v_type == VMARKER)
                           vp = TAILQ_NEXT(vp, v_nmntvnodes);
                   if (vp == NULL)
                           break;
                   /*
                    * XXX LRU is completely broken for non-free vnodes.  First
                    * by calling here in mountpoint order, then by moving
                    * unselected vnodes to the end here, and most grossly by
                    * removing the vlruvp() function that was supposed to
                    * maintain the order.  (This function was born broken
                    * since syncer problems prevented it doing anything.)  The
                    * order is closer to LRC (C = Created).
                    *
                    * LRU reclaiming of vnodes seems to have last worked in
                    * FreeBSD-3 where LRU wasn't mentioned under any spelling.
                    * Then there was no hold count, and inactive vnodes were
                    * simply put on the free list in LRU order.  The separate
                    * lists also break LRU.  We prefer to reclaim from the
                    * free list for technical reasons.  This tends to thrash
                    * the free list to keep very unrecently used held vnodes.
                    * The problem is mitigated by keeping the free list large.
                    */
                   TAILQ_REMOVE(&mp->mnt_nvnodelist, vp, v_nmntvnodes);
                   TAILQ_INSERT_TAIL(&mp->mnt_nvnodelist, vp, v_nmntvnodes);
                   --count;
                   if (!VI_TRYLOCK(vp))
                           goto next_iter;
                   /*
                    * If it's been deconstructed already, it's still
                    * referenced, or it exceeds the trigger, skip it.
                    * Also skip free vnodes.  We are trying to make space
                    * to expand the free list, not reduce it.
                    */
                   if (vp->v_usecount ||
                       (!reclaim_nc_src && !LIST_EMPTY(&vp->v_cache_src)) ||
                       ((vp->v_iflag & VI_FREE) != 0) ||
                       (vp->v_iflag & VI_DOOMED) != 0 || (vp->v_object != NULL &&
                       vp->v_object->resident_page_count > trigger)) {
                           VI_UNLOCK(vp);
                           goto next_iter;
                   }
                   MNT_IUNLOCK(mp);
                   vholdl(vp);
                   if (VOP_LOCK(vp, LK_INTERLOCK|LK_EXCLUSIVE|LK_NOWAIT)) {
                           vdrop(vp);
                           goto next_iter_mntunlocked;
                   }
                   VI_LOCK(vp);
                   /*
                    * v_usecount may have been bumped after VOP_LOCK() dropped
                    * the vnode interlock and before it was locked again.
                    *
                    * It is not necessary to recheck VI_DOOMED because it can
                    * only be set by another thread that holds both the vnode
                    * lock and vnode interlock.  If another thread has the
                    * vnode lock before we get to VOP_LOCK() and obtains the
                    * vnode interlock after VOP_LOCK() drops the vnode
                    * interlock, the other thread will be unable to drop the
                    * vnode lock before our VOP_LOCK() call fails.
                    */
                   if (vp->v_usecount ||
                       (!reclaim_nc_src && !LIST_EMPTY(&vp->v_cache_src)) ||
                       (vp->v_iflag & VI_FREE) != 0 ||
                       (vp->v_object != NULL &&
                       vp->v_object->resident_page_count > trigger)) {
                           VOP_UNLOCK(vp, LK_INTERLOCK);
                           vdrop(vp);
                           goto next_iter_mntunlocked;
                   }
                   KASSERT((vp->v_iflag & VI_DOOMED) == 0,
                       ("VI_DOOMED unexpectedly detected in vlrureclaim()"));
                   counter_u64_add(recycles_count, 1);
                   vgonel(vp);
                   VOP_UNLOCK(vp, 0);
                   vdropl(vp);
                   done++;
   next_iter_mntunlocked:
                   if (!should_yield())
                           goto relock_mnt;
                   goto yield;
   next_iter:
                   if (!should_yield())
                           continue;
                   MNT_IUNLOCK(mp);
   yield:
                   kern_yield(PRI_USER);
   relock_mnt:
                   MNT_ILOCK(mp);
           }
           MNT_IUNLOCK(mp);
           vn_finished_write(mp);
           return done;
   }
   
   static int max_vnlru_free = 10000; /* limit on vnode free requests per call */
   SYSCTL_INT(_debug, OID_AUTO, max_vnlru_free, CTLFLAG_RW, &max_vnlru_free,
       0,
       "limit on vnode free requests per call to the vnlru_free routine");
   
   /*
    * Attempt to reduce the free list by the requested amount.
    */
   static void
   vnlru_free_locked(int count, struct vfsops *mnt_op)
   {
           struct vnode *vp;
           struct mount *mp;
           bool tried_batches;
   
           tried_batches = false;
           mtx_assert(&vnode_free_list_mtx, MA_OWNED);
           if (count > max_vnlru_free)
                   count = max_vnlru_free;
           for (; count > 0; count--) {
                   vp = TAILQ_FIRST(&vnode_free_list);
                   /*
                    * The list can be modified while the free_list_mtx
                    * has been dropped and vp could be NULL here.
                    */
                   if (vp == NULL) {
                           if (tried_batches)
                                   break;
                           mtx_unlock(&vnode_free_list_mtx);
                           vnlru_return_batches(mnt_op);
                           tried_batches = true;
                           mtx_lock(&vnode_free_list_mtx);
                          continue;
                  }
  
                  VNASSERT(vp->v_op != NULL, vp,
                      ("vnlru_free: vnode already reclaimed."));
                  KASSERT((vp->v_iflag & VI_FREE) != 0,
                      ("Removing vnode not on freelist"));
                  KASSERT((vp->v_iflag & VI_ACTIVE) == 0,
                      ("Mangling active vnode"));
                  TAILQ_REMOVE(&vnode_free_list, vp, v_actfreelist);
  
                  /*
                   * Don't recycle if our vnode is from different type
                   * of mount point.  Note that mp is type-safe, the
                   * check does not reach unmapped address even if
                   * vnode is reclaimed.
                   * Don't recycle if we can't get the interlock without
                   * blocking.
                   */
                  if ((mnt_op != NULL && (mp = vp->v_mount) != NULL &&
                      mp->mnt_op != mnt_op) || !VI_TRYLOCK(vp)) {
                          TAILQ_INSERT_TAIL(&vnode_free_list, vp, v_actfreelist);
                          continue;
                  }
                  VNASSERT((vp->v_iflag & VI_FREE) != 0 && vp->v_holdcnt == 0,
                      vp, ("vp inconsistent on freelist"));
  
                  /*
                   * The clear of VI_FREE prevents activation of the
                   * vnode.  There is no sense in putting the vnode on
                   * the mount point active list, only to remove it
                   * later during recycling.  Inline the relevant part
                   * of vholdl(), to avoid triggering assertions or
                   * activating.
                   */
                  freevnodes--;
                  vp->v_iflag &= ~VI_FREE;
                  VNODE_REFCOUNT_FENCE_REL();
                  refcount_acquire(&vp->v_holdcnt);
  
                  mtx_unlock(&vnode_free_list_mtx);
                  VI_UNLOCK(vp);
                  vtryrecycle(vp);
                  /*
                   * If the recycled succeeded this vdrop will actually free
                   * the vnode.  If not it will simply place it back on
                   * the free list.
                   */
                  vdrop(vp);
                  mtx_lock(&vnode_free_list_mtx);
          }
  }
  
  void
  vnlru_free(int count, struct vfsops *mnt_op)
  {
  
          mtx_lock(&vnode_free_list_mtx);
          vnlru_free_locked(count, mnt_op);
          mtx_unlock(&vnode_free_list_mtx);
  }
  
  
  /* XXX some names and initialization are bad for limits and watermarks. */
  static int
  vspace(void)
  {
          int space;
  
          gapvnodes = imax(desiredvnodes - wantfreevnodes, 100);
          vhiwat = gapvnodes / 11; /* 9% -- just under the 10% in vlrureclaim() */
          vlowat = vhiwat / 2;
          if (numvnodes > desiredvnodes)
                  return (0);
          space = desiredvnodes - numvnodes;
          if (freevnodes > wantfreevnodes)
                  space += freevnodes - wantfreevnodes;
          return (space);
  }
  
  static void
  vnlru_return_batch_locked(struct mount *mp)
  {
          struct vnode *vp;
  
          mtx_assert(&mp->mnt_listmtx, MA_OWNED);
  
          if (mp->mnt_tmpfreevnodelistsize == 0)
                  return;
  
          TAILQ_FOREACH(vp, &mp->mnt_tmpfreevnodelist, v_actfreelist) {
                  VNASSERT((vp->v_mflag & VMP_TMPMNTFREELIST) != 0, vp,
                      ("vnode without VMP_TMPMNTFREELIST on mnt_tmpfreevnodelist"));
                  vp->v_mflag &= ~VMP_TMPMNTFREELIST;
          }
          mtx_lock(&vnode_free_list_mtx);
          TAILQ_CONCAT(&vnode_free_list, &mp->mnt_tmpfreevnodelist, v_actfreelist);
          freevnodes += mp->mnt_tmpfreevnodelistsize;
          mtx_unlock(&vnode_free_list_mtx);
          mp->mnt_tmpfreevnodelistsize = 0;
  }
  
  static void
  vnlru_return_batch(struct mount *mp)
  {
  
          mtx_lock(&mp->mnt_listmtx);
          vnlru_return_batch_locked(mp);
          mtx_unlock(&mp->mnt_listmtx);
  }
  
  static void
  vnlru_return_batches(struct vfsops *mnt_op)
  {
          struct mount *mp, *nmp;
          bool need_unbusy;
  
          mtx_lock(&mountlist_mtx);
          for (mp = TAILQ_FIRST(&mountlist); mp != NULL; mp = nmp) {
                  need_unbusy = false;
                  if (mnt_op != NULL && mp->mnt_op != mnt_op)
                          goto next;
                  if (mp->mnt_tmpfreevnodelistsize == 0)
                          goto next;
                  if (vfs_busy(mp, MBF_NOWAIT | MBF_MNTLSTLOCK) == 0) {
                          vnlru_return_batch(mp);
                          need_unbusy = true;
                          mtx_lock(&mountlist_mtx);
                  }
  next:
                  nmp = TAILQ_NEXT(mp, mnt_list);
                  if (need_unbusy)
                          vfs_unbusy(mp);
          }
          mtx_unlock(&mountlist_mtx);
  }
  
  /*
   * Attempt to recycle vnodes in a context that is always safe to block.
   * Calling vlrurecycle() from the bowels of filesystem code has some
   * interesting deadlock problems.
   */
  static struct proc *vnlruproc;
  static int vnlruproc_sig;
  
  static void
  vnlru_proc(void)
  {
          struct mount *mp, *nmp;
          unsigned long onumvnodes;
          int done, force, reclaim_nc_src, trigger, usevnodes;
  
          EVENTHANDLER_REGISTER(shutdown_pre_sync, kproc_shutdown, vnlruproc,
              SHUTDOWN_PRI_FIRST);
  
          force = 0;
          for (;;) {
                  kproc_suspend_check(vnlruproc);
                  mtx_lock(&vnode_free_list_mtx);
                  /*
                   * If numvnodes is too large (due to desiredvnodes being
                   * adjusted using its sysctl, or emergency growth), first
                   * try to reduce it by discarding from the free list.
                   */
                  if (numvnodes > desiredvnodes)
                          vnlru_free_locked(numvnodes - desiredvnodes, NULL);
                  /*
                   * Sleep if the vnode cache is in a good state.  This is
                   * when it is not over-full and has space for about a 4%
                   * or 9% expansion (by growing its size or inexcessively
                   * reducing its free list).  Otherwise, try to reclaim
                   * space for a 10% expansion.
                   */
                  if (vstir && force == 0) {
                          force = 1;
                          vstir = 0;
                  }
                  if (vspace() >= vlowat && force == 0) {
                          vnlruproc_sig = 0;
                          wakeup(&vnlruproc_sig);
                          msleep(vnlruproc, &vnode_free_list_mtx,
                              PVFS|PDROP, "vlruwt", hz);
                          continue;
                  }
                  mtx_unlock(&vnode_free_list_mtx);
                  done = 0;
                  onumvnodes = numvnodes;
                  /*
                   * Calculate parameters for recycling.  These are the same
                   * throughout the loop to give some semblance of fairness.
                   * The trigger point is to avoid recycling vnodes with lots
                   * of resident pages.  We aren't trying to free memory; we
                   * are trying to recycle or at least free vnodes.
                   */
                  if (numvnodes <= desiredvnodes)
                          usevnodes = numvnodes - freevnodes;
                  else
                          usevnodes = numvnodes;
                  if (usevnodes <= 0)
                          usevnodes = 1;
                  /*
                   * The trigger value is is chosen to give a conservatively
                   * large value to ensure that it alone doesn't prevent
                   * making progress.  The value can easily be so large that
                   * it is effectively infinite in some congested and
                   * misconfigured cases, and this is necessary.  Normally
                   * it is about 8 to 100 (pages), which is quite large.
                   */
                  trigger = vm_cnt.v_page_count * 2 / usevnodes;
                  if (force < 2)
                          trigger = vsmalltrigger;
                  reclaim_nc_src = force >= 3;
                  mtx_lock(&mountlist_mtx);
                  for (mp = TAILQ_FIRST(&mountlist); mp != NULL; mp = nmp) {
                          if (vfs_busy(mp, MBF_NOWAIT | MBF_MNTLSTLOCK)) {
                                  nmp = TAILQ_NEXT(mp, mnt_list);
                                  continue;
                          }
                          done += vlrureclaim(mp, reclaim_nc_src, trigger);
                          mtx_lock(&mountlist_mtx);
                          nmp = TAILQ_NEXT(mp, mnt_list);
                          vfs_unbusy(mp);
                  }
                  mtx_unlock(&mountlist_mtx);
                  if (onumvnodes > desiredvnodes && numvnodes <= desiredvnodes)
                          uma_reclaim();
                  if (done == 0) {
                          if (force == 0 || force == 1) {
                                  force = 2;
                                  continue;
                          }
                          if (force == 2) {
                                  force = 3;
                                  continue;
                          }
                          force = 0;
                          vnlru_nowhere++;
                          tsleep(vnlruproc, PPAUSE, "vlrup", hz * 3);
                  } else
                          kern_yield(PRI_USER);
                  /*
                   * After becoming active to expand above low water, keep
                   * active until above high water.
                   */
                  force = vspace() < vhiwat;
          }
  }
  
  static struct kproc_desc vnlru_kp = {
          "vnlru",
          vnlru_proc,
          &vnlruproc
  };
  SYSINIT(vnlru, SI_SUB_KTHREAD_UPDATE, SI_ORDER_FIRST, kproc_start,
      &vnlru_kp);
   
  /*
   * Routines having to do with the management of the vnode table.
   */
  
  /*
   * Try to recycle a freed vnode.  We abort if anyone picks up a reference
   * before we actually vgone().  This function must be called with the vnode
   * held to prevent the vnode from being returned to the free list midway
   * through vgone().
   */
  static int
  vtryrecycle(struct vnode *vp)
  {
          struct mount *vnmp;
  
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          VNASSERT(vp->v_holdcnt, vp,
              ("vtryrecycle: Recycling vp %p without a reference.", vp));
          /*
           * This vnode may found and locked via some other list, if so we
           * can't recycle it yet.
           */
          if (VOP_LOCK(vp, LK_EXCLUSIVE | LK_NOWAIT) != 0) {
                  CTR2(KTR_VFS,
                      "%s: impossible to recycle, vp %p lock is already held",
                      __func__, vp);
                  return (EWOULDBLOCK);
          }
          /*
           * Don't recycle if its filesystem is being suspended.
           */
          if (vn_start_write(vp, &vnmp, V_NOWAIT) != 0) {
                  VOP_UNLOCK(vp, 0);
                  CTR2(KTR_VFS,
                      "%s: impossible to recycle, cannot start the write for %p",
                      __func__, vp);
                  return (EBUSY);
          }
          /*
           * If we got this far, we need to acquire the interlock and see if
           * anyone picked up this vnode from another list.  If not, we will
           * mark it with DOOMED via vgonel() so that anyone who does find it
           * will skip over it.
           */
          VI_LOCK(vp);
          if (vp->v_usecount) {
                  VOP_UNLOCK(vp, LK_INTERLOCK);
                  vn_finished_write(vnmp);
                  CTR2(KTR_VFS,
                      "%s: impossible to recycle, %p is already referenced",
                      __func__, vp);
                  return (EBUSY);
          }
          if ((vp->v_iflag & VI_DOOMED) == 0) {
                  counter_u64_add(recycles_count, 1);
                  vgonel(vp);
          }
          VOP_UNLOCK(vp, LK_INTERLOCK);
          vn_finished_write(vnmp);
          return (0);
  }
  
  static void
  vcheckspace(void)
  {
  
          if (vspace() < vlowat && vnlruproc_sig == 0) {
                  vnlruproc_sig = 1;
                  wakeup(vnlruproc);
          }
  }
  
  /*
   * Wait if necessary for space for a new vnode.
   */
  static int
  getnewvnode_wait(int suspended)
  {
  
          mtx_assert(&vnode_free_list_mtx, MA_OWNED);
          if (numvnodes >= desiredvnodes) {
                  if (suspended) {
                          /*
                           * The file system is being suspended.  We cannot
                           * risk a deadlock here, so allow allocation of
                           * another vnode even if this would give too many.
                           */
                          return (0);
                  }
                  if (vnlruproc_sig == 0) {
                          vnlruproc_sig = 1;      /* avoid unnecessary wakeups */
                          wakeup(vnlruproc);
                  }
                  msleep(&vnlruproc_sig, &vnode_free_list_mtx, PVFS,
                      "vlruwk", hz);
          }
          /* Post-adjust like the pre-adjust in getnewvnode(). */
          if (numvnodes + 1 > desiredvnodes && freevnodes > 1)
                  vnlru_free_locked(1, NULL);
          return (numvnodes >= desiredvnodes ? ENFILE : 0);
  }
  
  /*
   * This hack is fragile, and probably not needed any more now that the
   * watermark handling works.
   */
  void
  getnewvnode_reserve(u_int count)
  {
          struct thread *td;
  
          /* Pre-adjust like the pre-adjust in getnewvnode(), with any count. */
          /* XXX no longer so quick, but this part is not racy. */
          mtx_lock(&vnode_free_list_mtx);
          if (numvnodes + count > desiredvnodes && freevnodes > wantfreevnodes)
                  vnlru_free_locked(ulmin(numvnodes + count - desiredvnodes,
                      freevnodes - wantfreevnodes), NULL);
          mtx_unlock(&vnode_free_list_mtx);
  
          td = curthread;
          /* First try to be quick and racy. */
          if (atomic_fetchadd_long(&numvnodes, count) + count <= desiredvnodes) {
                  td->td_vp_reserv += count;
                  vcheckspace();  /* XXX no longer so quick, but more racy */
                  return;
          } else
                  atomic_subtract_long(&numvnodes, count);
  
          mtx_lock(&vnode_free_list_mtx);
          while (count > 0) {
                  if (getnewvnode_wait(0) == 0) {
                          count--;
                          td->td_vp_reserv++;
                          atomic_add_long(&numvnodes, 1);
                  }
          }
          vcheckspace();
          mtx_unlock(&vnode_free_list_mtx);
  }
  
  /*
   * This hack is fragile, especially if desiredvnodes or wantvnodes are
   * misconfgured or changed significantly.  Reducing desiredvnodes below
   * the reserved amount should cause bizarre behaviour like reducing it
   * below the number of active vnodes -- the system will try to reduce
   * numvnodes to match, but should fail, so the subtraction below should
   * not overflow.
   */
  void
  getnewvnode_drop_reserve(void)
  {
          struct thread *td;
  
          td = curthread;
          atomic_subtract_long(&numvnodes, td->td_vp_reserv);
          td->td_vp_reserv = 0;
  }
  
  /*
   * Return the next vnode from the free list.
   */
  int
  getnewvnode(const char *tag, struct mount *mp, struct vop_vector *vops,
      struct vnode **vpp)
  {
          struct vnode *vp;
          struct thread *td;
          struct lock_object *lo;
          static int cyclecount;
          int error __unused;
  
          CTR3(KTR_VFS, "%s: mp %p with tag %s", __func__, mp, tag);
          vp = NULL;
          td = curthread;
          if (td->td_vp_reserv > 0) {
                  td->td_vp_reserv -= 1;
                  goto alloc;
          }
          mtx_lock(&vnode_free_list_mtx);
          if (numvnodes < desiredvnodes)
                  cyclecount = 0;
          else if (cyclecount++ >= freevnodes) {
                  cyclecount = 0;
                  vstir = 1;
          }
          /*
           * Grow the vnode cache if it will not be above its target max
           * after growing.  Otherwise, if the free list is nonempty, try
           * to reclaim 1 item from it before growing the cache (possibly
           * above its target max if the reclamation failed or is delayed).
           * Otherwise, wait for some space.  In all cases, schedule
           * vnlru_proc() if we are getting short of space.  The watermarks
           * should be chosen so that we never wait or even reclaim from
           * the free list to below its target minimum.
           */
          if (numvnodes + 1 <= desiredvnodes)
                  ;
          else if (freevnodes > 0)
                  vnlru_free_locked(1, NULL);
          else {
                  error = getnewvnode_wait(mp != NULL && (mp->mnt_kern_flag &
                      MNTK_SUSPEND));
  #if 0   /* XXX Not all VFS_VGET/ffs_vget callers check returns. */
                  if (error != 0) {
                          mtx_unlock(&vnode_free_list_mtx);
                          return (error);
                  }
  #endif
          }
          vcheckspace();
          atomic_add_long(&numvnodes, 1);
          mtx_unlock(&vnode_free_list_mtx);
  alloc:
          counter_u64_add(vnodes_created, 1);
          vp = (struct vnode *) uma_zalloc(vnode_zone, M_WAITOK);
          /*
           * Locks are given the generic name "vnode" when created.
           * Follow the historic practice of using the filesystem
           * name when they allocated, e.g., "zfs", "ufs", "nfs, etc.
           *
           * Locks live in a witness group keyed on their name. Thus,
           * when a lock is renamed, it must also move from the witness
           * group of its old name to the witness group of its new name.
           *
           * The change only needs to be made when the vnode moves
           * from one filesystem type to another. We ensure that each
           * filesystem use a single static name pointer for its tag so
           * that we can compare pointers rather than doing a strcmp().
           */
          lo = &vp->v_vnlock->lock_object;
          if (lo->lo_name != tag) {
                  lo->lo_name = tag;
                  WITNESS_DESTROY(lo);
                  WITNESS_INIT(lo, tag);
          }
          /*
           * By default, don't allow shared locks unless filesystems opt-in.
           */
          vp->v_vnlock->lock_object.lo_flags |= LK_NOSHARE;
          /*
           * Finalize various vnode identity bits.
           */
          KASSERT(vp->v_object == NULL, ("stale v_object %p", vp));
          KASSERT(vp->v_lockf == NULL, ("stale v_lockf %p", vp));
          KASSERT(vp->v_pollinfo == NULL, ("stale v_pollinfo %p", vp));
          vp->v_type = VNON;
          vp->v_tag = tag;
          vp->v_op = vops;
          v_init_counters(vp);
          vp->v_bufobj.bo_ops = &buf_ops_bio;
  #ifdef DIAGNOSTIC
          if (mp == NULL && vops != &dead_vnodeops)
                  printf("NULL mp in getnewvnode(9), tag %s\n", tag);
  #endif
  #ifdef MAC
          mac_vnode_init(vp);
          if (mp != NULL && (mp->mnt_flag & MNT_MULTILABEL) == 0)
                  mac_vnode_associate_singlelabel(mp, vp);
  #endif
          if (mp != NULL) {
                  vp->v_bufobj.bo_bsize = mp->mnt_stat.f_iosize;
                  if ((mp->mnt_kern_flag & MNTK_NOKNOTE) != 0)
                          vp->v_vflag |= VV_NOKNOTE;
          }
  
          /*
           * For the filesystems which do not use vfs_hash_insert(),
           * still initialize v_hash to have vfs_hash_index() useful.
           * E.g., nullfs uses vfs_hash_index() on the lower vnode for
           * its own hashing.
           */
          vp->v_hash = (uintptr_t)vp >> vnsz2log;
  
          *vpp = vp;
          return (0);
  }
  
  /*
   * Delete from old mount point vnode list, if on one.
   */
  static void
  delmntque(struct vnode *vp)
  {
          struct mount *mp;
          int active;
  
          mp = vp->v_mount;
          if (mp == NULL)
                  return;
          MNT_ILOCK(mp);
          VI_LOCK(vp);
          KASSERT(mp->mnt_activevnodelistsize <= mp->mnt_nvnodelistsize,
              ("Active vnode list size %d > Vnode list size %d",
               mp->mnt_activevnodelistsize, mp->mnt_nvnodelistsize));
          active = vp->v_iflag & VI_ACTIVE;
          vp->v_iflag &= ~VI_ACTIVE;
          if (active) {
                  mtx_lock(&mp->mnt_listmtx);
                  TAILQ_REMOVE(&mp->mnt_activevnodelist, vp, v_actfreelist);
                  mp->mnt_activevnodelistsize--;
                  mtx_unlock(&mp->mnt_listmtx);
          }
          vp->v_mount = NULL;
          VI_UNLOCK(vp);
          VNASSERT(mp->mnt_nvnodelistsize > 0, vp,
                  ("bad mount point vnode list size"));
          TAILQ_REMOVE(&mp->mnt_nvnodelist, vp, v_nmntvnodes);
          mp->mnt_nvnodelistsize--;
          MNT_REL(mp);
          MNT_IUNLOCK(mp);
  }
  
  static void
  insmntque_stddtr(struct vnode *vp, void *dtr_arg)
  {
  
          vp->v_data = NULL;
          vp->v_op = &dead_vnodeops;
          vgone(vp);
          vput(vp);
  }
  
  /*
   * Insert into list of vnodes for the new mount point, if available.
   */
  int
  insmntque1(struct vnode *vp, struct mount *mp,
          void (*dtr)(struct vnode *, void *), void *dtr_arg)
  {
  
          KASSERT(vp->v_mount == NULL,
                  ("insmntque: vnode already on per mount vnode list"));
          VNASSERT(mp != NULL, vp, ("Don't call insmntque(foo, NULL)"));
          ASSERT_VOP_ELOCKED(vp, "insmntque: non-locked vp");
  
          /*
           * We acquire the vnode interlock early to ensure that the
           * vnode cannot be recycled by another process releasing a
           * holdcnt on it before we get it on both the vnode list
           * and the active vnode list. The mount mutex protects only
           * manipulation of the vnode list and the vnode freelist
           * mutex protects only manipulation of the active vnode list.
           * Hence the need to hold the vnode interlock throughout.
           */
          MNT_ILOCK(mp);
          VI_LOCK(vp);
          if (((mp->mnt_kern_flag & MNTK_NOINSMNTQ) != 0 &&
              ((mp->mnt_kern_flag & MNTK_UNMOUNTF) != 0 ||
              mp->mnt_nvnodelistsize == 0)) &&
              (vp->v_vflag & VV_FORCEINSMQ) == 0) {
                  VI_UNLOCK(vp);
                  MNT_IUNLOCK(mp);
                  if (dtr != NULL)
                          dtr(vp, dtr_arg);
                  return (EBUSY);
          }
          vp->v_mount = mp;
          MNT_REF(mp);
          TAILQ_INSERT_TAIL(&mp->mnt_nvnodelist, vp, v_nmntvnodes);
          VNASSERT(mp->mnt_nvnodelistsize >= 0, vp,
                  ("neg mount point vnode list size"));
          mp->mnt_nvnodelistsize++;
          KASSERT((vp->v_iflag & VI_ACTIVE) == 0,
              ("Activating already active vnode"));
          vp->v_iflag |= VI_ACTIVE;
          mtx_lock(&mp->mnt_listmtx);
          TAILQ_INSERT_HEAD(&mp->mnt_activevnodelist, vp, v_actfreelist);
          mp->mnt_activevnodelistsize++;
          mtx_unlock(&mp->mnt_listmtx);
          VI_UNLOCK(vp);
          MNT_IUNLOCK(mp);
          return (0);
  }
  
  int
  insmntque(struct vnode *vp, struct mount *mp)
  {
  
          return (insmntque1(vp, mp, insmntque_stddtr, NULL));
  }
  
  /*
   * Flush out and invalidate all buffers associated with a bufobj
   * Called with the underlying object locked.
   */
  int
  bufobj_invalbuf(struct bufobj *bo, int flags, int slpflag, int slptimeo)
  {
          int error;
  
          BO_LOCK(bo);
          if (flags & V_SAVE) {
                  error = bufobj_wwait(bo, slpflag, slptimeo);
                  if (error) {
                          BO_UNLOCK(bo);
                          return (error);
                  }
                  if (bo->bo_dirty.bv_cnt > 0) {
                          BO_UNLOCK(bo);
                          if ((error = BO_SYNC(bo, MNT_WAIT)) != 0)
                                  return (error);
                          /*
                           * XXX We could save a lock/unlock if this was only
                           * enabled under INVARIANTS
                           */
                          BO_LOCK(bo);
                          if (bo->bo_numoutput > 0 || bo->bo_dirty.bv_cnt > 0)
                                  panic("vinvalbuf: dirty bufs");
                  }
          }
          /*
           * If you alter this loop please notice that interlock is dropped and
           * reacquired in flushbuflist.  Special care is needed to ensure that
           * no race conditions occur from this.
           */
          do {
                  error = flushbuflist(&bo->bo_clean,
                      flags, bo, slpflag, slptimeo);
                  if (error == 0 && !(flags & V_CLEANONLY))
                          error = flushbuflist(&bo->bo_dirty,
                              flags, bo, slpflag, slptimeo);
                  if (error != 0 && error != EAGAIN) {
                          BO_UNLOCK(bo);
                          return (error);
                  }
          } while (error != 0);
  
          /*
           * Wait for I/O to complete.  XXX needs cleaning up.  The vnode can
           * have write I/O in-progress but if there is a VM object then the
           * VM object can also have read-I/O in-progress.
           */
          do {
                  bufobj_wwait(bo, 0, 0);
                  if ((flags & V_VMIO) == 0) {
                          BO_UNLOCK(bo);
                          if (bo->bo_object != NULL) {
                                  VM_OBJECT_WLOCK(bo->bo_object);
                                  vm_object_pip_wait(bo->bo_object, "bovlbx");
                                  VM_OBJECT_WUNLOCK(bo->bo_object);
                          }
                          BO_LOCK(bo);
                  }
          } while (bo->bo_numoutput > 0);
          BO_UNLOCK(bo);
  
          /*
           * Destroy the copy in the VM cache, too.
           */
          if (bo->bo_object != NULL &&
              (flags & (V_ALT | V_NORMAL | V_CLEANONLY | V_VMIO)) == 0) {
                  VM_OBJECT_WLOCK(bo->bo_object);
                  vm_object_page_remove(bo->bo_object, 0, 0, (flags & V_SAVE) ?
                      OBJPR_CLEANONLY : 0);
                  VM_OBJECT_WUNLOCK(bo->bo_object);
          }
  
  #ifdef INVARIANTS
          BO_LOCK(bo);
          if ((flags & (V_ALT | V_NORMAL | V_CLEANONLY | V_VMIO |
              V_ALLOWCLEAN)) == 0 && (bo->bo_dirty.bv_cnt > 0 ||
              bo->bo_clean.bv_cnt > 0))
                  panic("vinvalbuf: flush failed");
          if ((flags & (V_ALT | V_NORMAL | V_CLEANONLY | V_VMIO)) == 0 &&
              bo->bo_dirty.bv_cnt > 0)
                  panic("vinvalbuf: flush dirty failed");
          BO_UNLOCK(bo);
  #endif
          return (0);
  }
  
  /*
   * Flush out and invalidate all buffers associated with a vnode.
   * Called with the underlying object locked.
   */
  int
  vinvalbuf(struct vnode *vp, int flags, int slpflag, int slptimeo)
  {
  
          CTR3(KTR_VFS, "%s: vp %p with flags %d", __func__, vp, flags);
          ASSERT_VOP_LOCKED(vp, "vinvalbuf");
          if (vp->v_object != NULL && vp->v_object->handle != vp)
                  return (0);
          return (bufobj_invalbuf(&vp->v_bufobj, flags, slpflag, slptimeo));
  }
  
  /*
   * Flush out buffers on the specified list.
   *
   */
  static int
  flushbuflist(struct bufv *bufv, int flags, struct bufobj *bo, int slpflag,
      int slptimeo)
  {
          struct buf *bp, *nbp;
          int retval, error;
          daddr_t lblkno;
          b_xflags_t xflags;
  
          ASSERT_BO_WLOCKED(bo);
  
          retval = 0;
          TAILQ_FOREACH_SAFE(bp, &bufv->bv_hd, b_bobufs, nbp) {
                  if (((flags & V_NORMAL) && (bp->b_xflags & BX_ALTDATA)) ||
                      ((flags & V_ALT) && (bp->b_xflags & BX_ALTDATA) == 0)) {
                          continue;
                  }
                  if (nbp != NULL) {
                          lblkno = nbp->b_lblkno;
                          xflags = nbp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN);
                  }
                  retval = EAGAIN;
                  error = BUF_TIMELOCK(bp,
                      LK_EXCLUSIVE | LK_SLEEPFAIL | LK_INTERLOCK, BO_LOCKPTR(bo),
                      "flushbuf", slpflag, slptimeo);
                  if (error) {
                          BO_LOCK(bo);
                          return (error != ENOLCK ? error : EAGAIN);
                  }
                  KASSERT(bp->b_bufobj == bo,
                      ("bp %p wrong b_bufobj %p should be %p",
                      bp, bp->b_bufobj, bo));
                  /*
                   * XXX Since there are no node locks for NFS, I
                   * believe there is a slight chance that a delayed
                   * write will occur while sleeping just above, so
                   * check for it.
                   */
                  if (((bp->b_flags & (B_DELWRI | B_INVAL)) == B_DELWRI) &&
                      (flags & V_SAVE)) {
                          bremfree(bp);
                          bp->b_flags |= B_ASYNC;
                          bwrite(bp);
                          BO_LOCK(bo);
                          return (EAGAIN);        /* XXX: why not loop ? */
                  }
                  bremfree(bp);
                  bp->b_flags |= (B_INVAL | B_RELBUF);
                  bp->b_flags &= ~B_ASYNC;
                  brelse(bp);
                  BO_LOCK(bo);
                  if (nbp == NULL)
                          break;
                  nbp = gbincore(bo, lblkno);
                  if (nbp == NULL || (nbp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN))
                      != xflags)
                          break;                  /* nbp invalid */
          }
          return (retval);
  }
  
  int
  bnoreuselist(struct bufv *bufv, struct bufobj *bo, daddr_t startn, daddr_t endn)
  {
          struct buf *bp;
          int error;
          daddr_t lblkno;
  
          ASSERT_BO_LOCKED(bo);
  
          for (lblkno = startn;;) {
  again:
                  bp = BUF_PCTRIE_LOOKUP_GE(&bufv->bv_root, lblkno);
                  if (bp == NULL || bp->b_lblkno >= endn ||
                      bp->b_lblkno < startn)
                          break;
                  error = BUF_TIMELOCK(bp, LK_EXCLUSIVE | LK_SLEEPFAIL |
                      LK_INTERLOCK, BO_LOCKPTR(bo), "brlsfl", 0, 0);
                  if (error != 0) {
                          BO_RLOCK(bo);
                          if (error == ENOLCK)
                                  goto again;
                          return (error);
                  }
                  KASSERT(bp->b_bufobj == bo,
                      ("bp %p wrong b_bufobj %p should be %p",
                      bp, bp->b_bufobj, bo));
                  lblkno = bp->b_lblkno + 1;
                  if ((bp->b_flags & B_MANAGED) == 0)
                          bremfree(bp);
                  bp->b_flags |= B_RELBUF;
                  /*
                   * In the VMIO case, use the B_NOREUSE flag to hint that the
                   * pages backing each buffer in the range are unlikely to be
                   * reused.  Dirty buffers will have the hint applied once
                   * they've been written.
                   */
                  if (bp->b_vp->v_object != NULL)
                          bp->b_flags |= B_NOREUSE;
                  brelse(bp);
                  BO_RLOCK(bo);
          }
          return (0);
  }
  
  /*
   * Truncate a file's buffer and pages to a specified length.  This
   * is in lieu of the old vinvalbuf mechanism, which performed unneeded
   * sync activity.
   */
  int
  vtruncbuf(struct vnode *vp, struct ucred *cred, off_t length, int blksize)
  {
          struct buf *bp, *nbp;
          int anyfreed;
          int trunclbn;
          struct bufobj *bo;
  
          CTR5(KTR_VFS, "%s: vp %p with cred %p and block %d:%ju", __func__,
              vp, cred, blksize, (uintmax_t)length);
  
          /*
           * Round up to the *next* lbn.
           */
          trunclbn = howmany(length, blksize);
  
          ASSERT_VOP_LOCKED(vp, "vtruncbuf");
  restart:
          bo = &vp->v_bufobj;
          BO_LOCK(bo);
          anyfreed = 1;
          for (;anyfreed;) {
                  anyfreed = 0;
                  TAILQ_FOREACH_SAFE(bp, &bo->bo_clean.bv_hd, b_bobufs, nbp) {
                          if (bp->b_lblkno < trunclbn)
                                  continue;
                          if (BUF_LOCK(bp,
                              LK_EXCLUSIVE | LK_SLEEPFAIL | LK_INTERLOCK,
                              BO_LOCKPTR(bo)) == ENOLCK)
                                  goto restart;
  
                          bremfree(bp);
                          bp->b_flags |= (B_INVAL | B_RELBUF);
                          bp->b_flags &= ~B_ASYNC;
                          brelse(bp);
                          anyfreed = 1;
  
                          BO_LOCK(bo);
                          if (nbp != NULL &&
                              (((nbp->b_xflags & BX_VNCLEAN) == 0) ||
                              (nbp->b_vp != vp) ||
                              (nbp->b_flags & B_DELWRI))) {
                                  BO_UNLOCK(bo);
                                  goto restart;
                          }
                  }
  
                  TAILQ_FOREACH_SAFE(bp, &bo->bo_dirty.bv_hd, b_bobufs, nbp) {
                          if (bp->b_lblkno < trunclbn)
                                  continue;
                          if (BUF_LOCK(bp,
                              LK_EXCLUSIVE | LK_SLEEPFAIL | LK_INTERLOCK,
                              BO_LOCKPTR(bo)) == ENOLCK)
                                  goto restart;
                          bremfree(bp);
                          bp->b_flags |= (B_INVAL | B_RELBUF);
                          bp->b_flags &= ~B_ASYNC;
                          brelse(bp);
                          anyfreed = 1;
  
                          BO_LOCK(bo);
                          if (nbp != NULL &&
                              (((nbp->b_xflags & BX_VNDIRTY) == 0) ||
                              (nbp->b_vp != vp) ||
                              (nbp->b_flags & B_DELWRI) == 0)) {
                                  BO_UNLOCK(bo);
                                  goto restart;
                          }
                  }
          }
  
          if (length > 0) {
  restartsync:
                  TAILQ_FOREACH_SAFE(bp, &bo->bo_dirty.bv_hd, b_bobufs, nbp) {
                          if (bp->b_lblkno > 0)
                                  continue;
                          /*
                           * Since we hold the vnode lock this should only
                           * fail if we're racing with the buf daemon.
                           */
                          if (BUF_LOCK(bp,
                              LK_EXCLUSIVE | LK_SLEEPFAIL | LK_INTERLOCK,
                              BO_LOCKPTR(bo)) == ENOLCK) {
                                  goto restart;
                          }
                          VNASSERT((bp->b_flags & B_DELWRI), vp,
                              ("buf(%p) on dirty queue without DELWRI", bp));
  
                          bremfree(bp);
                          bawrite(bp);
                          BO_LOCK(bo);
                          goto restartsync;
                  }
          }
  
          bufobj_wwait(bo, 0, 0);
          BO_UNLOCK(bo);
          vnode_pager_setsize(vp, length);
  
          return (0);
  }
  
  static void
  buf_vlist_remove(struct buf *bp)
  {
          struct bufv *bv;
  
          KASSERT(bp->b_bufobj != NULL, ("No b_bufobj %p", bp));
          ASSERT_BO_WLOCKED(bp->b_bufobj);
          KASSERT((bp->b_xflags & (BX_VNDIRTY|BX_VNCLEAN)) !=
              (BX_VNDIRTY|BX_VNCLEAN),
              ("buf_vlist_remove: Buf %p is on two lists", bp));
          if (bp->b_xflags & BX_VNDIRTY)
                  bv = &bp->b_bufobj->bo_dirty;
          else
                  bv = &bp->b_bufobj->bo_clean;
          BUF_PCTRIE_REMOVE(&bv->bv_root, bp->b_lblkno);
          TAILQ_REMOVE(&bv->bv_hd, bp, b_bobufs);
          bv->bv_cnt--;
          bp->b_xflags &= ~(BX_VNDIRTY | BX_VNCLEAN);
  }
  
  /*
   * Add the buffer to the sorted clean or dirty block list.
   *
   * NOTE: xflags is passed as a constant, optimizing this inline function!
   */
  static void
  buf_vlist_add(struct buf *bp, struct bufobj *bo, b_xflags_t xflags)
  {
          struct bufv *bv;
          struct buf *n;
          int error;
  
          ASSERT_BO_WLOCKED(bo);
          KASSERT((xflags & BX_VNDIRTY) == 0 || (bo->bo_flag & BO_DEAD) == 0,
              ("dead bo %p", bo));
          KASSERT((bp->b_xflags & (BX_VNDIRTY|BX_VNCLEAN)) == 0,
              ("buf_vlist_add: Buf %p has existing xflags %d", bp, bp->b_xflags));
          bp->b_xflags |= xflags;
          if (xflags & BX_VNDIRTY)
                  bv = &bo->bo_dirty;
          else
                  bv = &bo->bo_clean;
  
          /*
           * Keep the list ordered.  Optimize empty list insertion.  Assume
           * we tend to grow at the tail so lookup_le should usually be cheaper
           * than _ge. 
           */
          if (bv->bv_cnt == 0 ||
              bp->b_lblkno > TAILQ_LAST(&bv->bv_hd, buflists)->b_lblkno)
                  TAILQ_INSERT_TAIL(&bv->bv_hd, bp, b_bobufs);
          else if ((n = BUF_PCTRIE_LOOKUP_LE(&bv->bv_root, bp->b_lblkno)) == NULL)
                  TAILQ_INSERT_HEAD(&bv->bv_hd, bp, b_bobufs);
          else
                  TAILQ_INSERT_AFTER(&bv->bv_hd, n, bp, b_bobufs);
          error = BUF_PCTRIE_INSERT(&bv->bv_root, bp);
          if (error)
                  panic("buf_vlist_add:  Preallocated nodes insufficient.");
          bv->bv_cnt++;
  }
  
  /*
   * Look up a buffer using the buffer tries.
   */
  struct buf *
  gbincore(struct bufobj *bo, daddr_t lblkno)
  {
          struct buf *bp;
  
          ASSERT_BO_LOCKED(bo);
          bp = BUF_PCTRIE_LOOKUP(&bo->bo_clean.bv_root, lblkno);
          if (bp != NULL)
                  return (bp);
          return BUF_PCTRIE_LOOKUP(&bo->bo_dirty.bv_root, lblkno);
  }
  
  /*
   * Associate a buffer with a vnode.
   */
  void
  bgetvp(struct vnode *vp, struct buf *bp)
  {
          struct bufobj *bo;
  
          bo = &vp->v_bufobj;
          ASSERT_BO_WLOCKED(bo);
          VNASSERT(bp->b_vp == NULL, bp->b_vp, ("bgetvp: not free"));
  
          CTR3(KTR_BUF, "bgetvp(%p) vp %p flags %X", bp, vp, bp->b_flags);
          VNASSERT((bp->b_xflags & (BX_VNDIRTY|BX_VNCLEAN)) == 0, vp,
              ("bgetvp: bp already attached! %p", bp));
  
          vhold(vp);
          bp->b_vp = vp;
          bp->b_bufobj = bo;
          /*
           * Insert onto list for new vnode.
           */
          buf_vlist_add(bp, bo, BX_VNCLEAN);
  }
  
  /*
   * Disassociate a buffer from a vnode.
   */
  void
  brelvp(struct buf *bp)
  {
          struct bufobj *bo;
          struct vnode *vp;
  
          CTR3(KTR_BUF, "brelvp(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
          KASSERT(bp->b_vp != NULL, ("brelvp: NULL"));
  
          /*
           * Delete from old vnode list, if on one.
           */
          vp = bp->b_vp;          /* XXX */
          bo = bp->b_bufobj;
          BO_LOCK(bo);
          if (bp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN))
                  buf_vlist_remove(bp);
          else
                  panic("brelvp: Buffer %p not on queue.", bp);
          if ((bo->bo_flag & BO_ONWORKLST) && bo->bo_dirty.bv_cnt == 0) {
                  bo->bo_flag &= ~BO_ONWORKLST;
                  mtx_lock(&sync_mtx);
                  LIST_REMOVE(bo, bo_synclist);
                  syncer_worklist_len--;
                  mtx_unlock(&sync_mtx);
          }
          bp->b_vp = NULL;
          bp->b_bufobj = NULL;
          BO_UNLOCK(bo);
          vdrop(vp);
  }
  
  /*
   * Add an item to the syncer work queue.
   */
  static void
  vn_syncer_add_to_worklist(struct bufobj *bo, int delay)
  {
          int slot;
  
          ASSERT_BO_WLOCKED(bo);
  
          mtx_lock(&sync_mtx);
          if (bo->bo_flag & BO_ONWORKLST)
                  LIST_REMOVE(bo, bo_synclist);
          else {
                  bo->bo_flag |= BO_ONWORKLST;
                  syncer_worklist_len++;
          }
  
          if (delay > syncer_maxdelay - 2)
                  delay = syncer_maxdelay - 2;
          slot = (syncer_delayno + delay) & syncer_mask;
  
          LIST_INSERT_HEAD(&syncer_workitem_pending[slot], bo, bo_synclist);
          mtx_unlock(&sync_mtx);
  }
  
  static int
  sysctl_vfs_worklist_len(SYSCTL_HANDLER_ARGS)
  {
          int error, len;
  
          mtx_lock(&sync_mtx);
          len = syncer_worklist_len - sync_vnode_count;
          mtx_unlock(&sync_mtx);
          error = SYSCTL_OUT(req, &len, sizeof(len));
          return (error);
  }
  
  SYSCTL_PROC(_vfs, OID_AUTO, worklist_len, CTLTYPE_INT | CTLFLAG_RD, NULL, 0,
      sysctl_vfs_worklist_len, "I", "Syncer thread worklist length");
  
  static struct proc *updateproc;
  static void sched_sync(void);
  static struct kproc_desc up_kp = {
          "syncer",
          sched_sync,
          &updateproc
  };
  SYSINIT(syncer, SI_SUB_KTHREAD_UPDATE, SI_ORDER_FIRST, kproc_start, &up_kp);
  
  static int
  sync_vnode(struct synclist *slp, struct bufobj **bo, struct thread *td)
  {
          struct vnode *vp;
          struct mount *mp;
  
          *bo = LIST_FIRST(slp);
          if (*bo == NULL)
                  return (0);
          vp = bo2vnode(*bo);
          if (VOP_ISLOCKED(vp) != 0 || VI_TRYLOCK(vp) == 0)
                  return (1);
          /*
           * We use vhold in case the vnode does not
           * successfully sync.  vhold prevents the vnode from
           * going away when we unlock the sync_mtx so that
           * we can acquire the vnode interlock.
           */
          vholdl(vp);
          mtx_unlock(&sync_mtx);
          VI_UNLOCK(vp);
          if (vn_start_write(vp, &mp, V_NOWAIT) != 0) {
                  vdrop(vp);
                  mtx_lock(&sync_mtx);
                  return (*bo == LIST_FIRST(slp));
          }
          vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
          (void) VOP_FSYNC(vp, MNT_LAZY, td);
          VOP_UNLOCK(vp, 0);
          vn_finished_write(mp);
          BO_LOCK(*bo);
          if (((*bo)->bo_flag & BO_ONWORKLST) != 0) {
                  /*
                   * Put us back on the worklist.  The worklist
                   * routine will remove us from our current
                   * position and then add us back in at a later
                   * position.
                   */
                  vn_syncer_add_to_worklist(*bo, syncdelay);
          }
          BO_UNLOCK(*bo);
          vdrop(vp);
          mtx_lock(&sync_mtx);
          return (0);
  }
  
  static int first_printf = 1;
  
  /*
   * System filesystem synchronizer daemon.
   */
  static void
  sched_sync(void)
  {
          struct synclist *next, *slp;
          struct bufobj *bo;
          long starttime;
          struct thread *td = curthread;
          int last_work_seen;
          int net_worklist_len;
          int syncer_final_iter;
          int error;
  
          last_work_seen = 0;
          syncer_final_iter = 0;
          syncer_state = SYNCER_RUNNING;
          starttime = time_uptime;
          td->td_pflags |= TDP_NORUNNINGBUF;
  
          EVENTHANDLER_REGISTER(shutdown_pre_sync, syncer_shutdown, td->td_proc,
              SHUTDOWN_PRI_LAST);
  
          mtx_lock(&sync_mtx);
          for (;;) {
                  if (syncer_state == SYNCER_FINAL_DELAY &&
                      syncer_final_iter == 0) {
                          mtx_unlock(&sync_mtx);
                          kproc_suspend_check(td->td_proc);
                          mtx_lock(&sync_mtx);
                  }
                  net_worklist_len = syncer_worklist_len - sync_vnode_count;
                  if (syncer_state != SYNCER_RUNNING &&
                      starttime != time_uptime) {
                          if (first_printf) {
                                  printf("\nSyncing disks, vnodes remaining... ");
                                  first_printf = 0;
                          }
                          printf("%d ", net_worklist_len);
                  }
                  starttime = time_uptime;
  
                  /*
                   * Push files whose dirty time has expired.  Be careful
                   * of interrupt race on slp queue.
                   *
                   * Skip over empty worklist slots when shutting down.
                   */
                  do {
                          slp = &syncer_workitem_pending[syncer_delayno];
                          syncer_delayno += 1;
                          if (syncer_delayno == syncer_maxdelay)
                                  syncer_delayno = 0;
                          next = &syncer_workitem_pending[syncer_delayno];
                          /*
                           * If the worklist has wrapped since the
                           * it was emptied of all but syncer vnodes,
                           * switch to the FINAL_DELAY state and run
                           * for one more second.
                           */
                          if (syncer_state == SYNCER_SHUTTING_DOWN &&
                              net_worklist_len == 0 &&
                              last_work_seen == syncer_delayno) {
                                  syncer_state = SYNCER_FINAL_DELAY;
                                  syncer_final_iter = SYNCER_SHUTDOWN_SPEEDUP;
                          }
                  } while (syncer_state != SYNCER_RUNNING && LIST_EMPTY(slp) &&
                      syncer_worklist_len > 0);
  
                  /*
                   * Keep track of the last time there was anything
                   * on the worklist other than syncer vnodes.
                   * Return to the SHUTTING_DOWN state if any
                   * new work appears.
                   */
                  if (net_worklist_len > 0 || syncer_state == SYNCER_RUNNING)
                          last_work_seen = syncer_delayno;
                  if (net_worklist_len > 0 && syncer_state == SYNCER_FINAL_DELAY)
                          syncer_state = SYNCER_SHUTTING_DOWN;
                  while (!LIST_EMPTY(slp)) {
                          error = sync_vnode(slp, &bo, td);
                          if (error == 1) {
                                  LIST_REMOVE(bo, bo_synclist);
                                  LIST_INSERT_HEAD(next, bo, bo_synclist);
                                  continue;
                          }
  
                          if (first_printf == 0) {
                                  /*
                                   * Drop the sync mutex, because some watchdog
                                   * drivers need to sleep while patting
                                   */
                                  mtx_unlock(&sync_mtx);
                                  wdog_kern_pat(WD_LASTVAL);
                                  mtx_lock(&sync_mtx);
                          }
  
                  }
                  if (syncer_state == SYNCER_FINAL_DELAY && syncer_final_iter > 0)
                          syncer_final_iter--;
                  /*
                   * The variable rushjob allows the kernel to speed up the
                   * processing of the filesystem syncer process. A rushjob
                   * value of N tells the filesystem syncer to process the next
                   * N seconds worth of work on its queue ASAP. Currently rushjob
                   * is used by the soft update code to speed up the filesystem
                   * syncer process when the incore state is getting so far
                   * ahead of the disk that the kernel memory pool is being
                   * threatened with exhaustion.
                   */
                  if (rushjob > 0) {
                          rushjob -= 1;
                          continue;
                  }
                  /*
                   * Just sleep for a short period of time between
                   * iterations when shutting down to allow some I/O
                   * to happen.
                   *
                   * If it has taken us less than a second to process the
                   * current work, then wait. Otherwise start right over
                   * again. We can still lose time if any single round
                   * takes more than two seconds, but it does not really
                   * matter as we are just trying to generally pace the
                   * filesystem activity.
                   */
                  if (syncer_state != SYNCER_RUNNING ||
                      time_uptime == starttime) {
                          thread_lock(td);
                          sched_prio(td, PPAUSE);
                          thread_unlock(td);
                  }
                  if (syncer_state != SYNCER_RUNNING)
                          cv_timedwait(&sync_wakeup, &sync_mtx,
                              hz / SYNCER_SHUTDOWN_SPEEDUP);
                  else if (time_uptime == starttime)
                          cv_timedwait(&sync_wakeup, &sync_mtx, hz);
          }
  }
  
  /*
   * Request the syncer daemon to speed up its work.
   * We never push it to speed up more than half of its
   * normal turn time, otherwise it could take over the cpu.
   */
  int
  speedup_syncer(void)
  {
          int ret = 0;
  
          mtx_lock(&sync_mtx);
          if (rushjob < syncdelay / 2) {
                  rushjob += 1;
                  stat_rush_requests += 1;
                  ret = 1;
          }
          mtx_unlock(&sync_mtx);
          cv_broadcast(&sync_wakeup);
          return (ret);
  }
  
  /*
   * Tell the syncer to speed up its work and run though its work
   * list several times, then tell it to shut down.
   */
  static void
  syncer_shutdown(void *arg, int howto)
  {
  
          if (howto & RB_NOSYNC)
                  return;
          mtx_lock(&sync_mtx);
          syncer_state = SYNCER_SHUTTING_DOWN;
          rushjob = 0;
          mtx_unlock(&sync_mtx);
          cv_broadcast(&sync_wakeup);
          kproc_shutdown(arg, howto);
  }
  
  void
  syncer_suspend(void)
  {
  
          syncer_shutdown(updateproc, 0);
  }
  
  void
  syncer_resume(void)
  {
  
          mtx_lock(&sync_mtx);
          first_printf = 1;
          syncer_state = SYNCER_RUNNING;
          mtx_unlock(&sync_mtx);
          cv_broadcast(&sync_wakeup);
          kproc_resume(updateproc);
  }
  
  /*
   * Reassign a buffer from one vnode to another.
   * Used to assign file specific control information
   * (indirect blocks) to the vnode to which they belong.
   */
  void
  reassignbuf(struct buf *bp)
  {
          struct vnode *vp;
          struct bufobj *bo;
          int delay;
  #ifdef INVARIANTS
          struct bufv *bv;
  #endif
  
          vp = bp->b_vp;
          bo = bp->b_bufobj;
          ++reassignbufcalls;
  
          CTR3(KTR_BUF, "reassignbuf(%p) vp %p flags %X",
              bp, bp->b_vp, bp->b_flags);
          /*
           * B_PAGING flagged buffers cannot be reassigned because their vp
           * is not fully linked in.
           */
          if (bp->b_flags & B_PAGING)
                  panic("cannot reassign paging buffer");
  
          /*
           * Delete from old vnode list, if on one.
           */
          BO_LOCK(bo);
          if (bp->b_xflags & (BX_VNDIRTY | BX_VNCLEAN))
                  buf_vlist_remove(bp);
          else
                  panic("reassignbuf: Buffer %p not on queue.", bp);
          /*
           * If dirty, put on list of dirty buffers; otherwise insert onto list
           * of clean buffers.
           */
          if (bp->b_flags & B_DELWRI) {
                  if ((bo->bo_flag & BO_ONWORKLST) == 0) {
                          switch (vp->v_type) {
                          case VDIR:
                                  delay = dirdelay;
                                  break;
                          case VCHR:
                                  delay = metadelay;
                                  break;
                          default:
                                  delay = filedelay;
                          }
                          vn_syncer_add_to_worklist(bo, delay);
                  }
                  buf_vlist_add(bp, bo, BX_VNDIRTY);
          } else {
                  buf_vlist_add(bp, bo, BX_VNCLEAN);
  
                  if ((bo->bo_flag & BO_ONWORKLST) && bo->bo_dirty.bv_cnt == 0) {
                          mtx_lock(&sync_mtx);
                          LIST_REMOVE(bo, bo_synclist);
                          syncer_worklist_len--;
                          mtx_unlock(&sync_mtx);
                          bo->bo_flag &= ~BO_ONWORKLST;
                  }
          }
  #ifdef INVARIANTS
          bv = &bo->bo_clean;
          bp = TAILQ_FIRST(&bv->bv_hd);
          KASSERT(bp == NULL || bp->b_bufobj == bo,
              ("bp %p wrong b_bufobj %p should be %p", bp, bp->b_bufobj, bo));
          bp = TAILQ_LAST(&bv->bv_hd, buflists);
          KASSERT(bp == NULL || bp->b_bufobj == bo,
              ("bp %p wrong b_bufobj %p should be %p", bp, bp->b_bufobj, bo));
          bv = &bo->bo_dirty;
          bp = TAILQ_FIRST(&bv->bv_hd);
          KASSERT(bp == NULL || bp->b_bufobj == bo,
              ("bp %p wrong b_bufobj %p should be %p", bp, bp->b_bufobj, bo));
          bp = TAILQ_LAST(&bv->bv_hd, buflists);
          KASSERT(bp == NULL || bp->b_bufobj == bo,
              ("bp %p wrong b_bufobj %p should be %p", bp, bp->b_bufobj, bo));
  #endif
          BO_UNLOCK(bo);
  }
  
  static void
  v_init_counters(struct vnode *vp)
  {
  
          VNASSERT(vp->v_type == VNON && vp->v_data == NULL && vp->v_iflag == 0,
              vp, ("%s called for an initialized vnode", __FUNCTION__));
          ASSERT_VI_UNLOCKED(vp, __FUNCTION__);
  
          refcount_init(&vp->v_holdcnt, 1);
          refcount_init(&vp->v_usecount, 1);
  }
  
  static void
  v_incr_usecount_locked(struct vnode *vp)
  {
  
          ASSERT_VI_LOCKED(vp, __func__);
          if ((vp->v_iflag & VI_OWEINACT) != 0) {
                  VNASSERT(vp->v_usecount == 0, vp,
                      ("vnode with usecount and VI_OWEINACT set"));
                  vp->v_iflag &= ~VI_OWEINACT;
          }
          refcount_acquire(&vp->v_usecount);
          v_incr_devcount(vp);
  }
  
  /*
   * Increment the use count on the vnode, taking care to reference
   * the driver's usecount if this is a chardev.
   */
  static void
  v_incr_usecount(struct vnode *vp)
  {
  
          ASSERT_VI_UNLOCKED(vp, __func__);
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
  
          if (vp->v_type != VCHR &&
              refcount_acquire_if_not_zero(&vp->v_usecount)) {
                  VNODE_REFCOUNT_FENCE_ACQ();
                  VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
                      ("vnode with usecount and VI_OWEINACT set"));
          } else {
                  VI_LOCK(vp);
                  v_incr_usecount_locked(vp);
                  VI_UNLOCK(vp);
          }
  }
  
  /*
   * Increment si_usecount of the associated device, if any.
   */
  static void
  v_incr_devcount(struct vnode *vp)
  {
  
          ASSERT_VI_LOCKED(vp, __FUNCTION__);
          if (vp->v_type == VCHR && vp->v_rdev != NULL) {
                  dev_lock();
                  vp->v_rdev->si_usecount++;
                  dev_unlock();
          }
  }
  
  /*
   * Decrement si_usecount of the associated device, if any.
   */
  static void
  v_decr_devcount(struct vnode *vp)
  {
  
          ASSERT_VI_LOCKED(vp, __FUNCTION__);
          if (vp->v_type == VCHR && vp->v_rdev != NULL) {
                  dev_lock();
                  vp->v_rdev->si_usecount--;
                  dev_unlock();
          }
  }
  
  /*
   * Grab a particular vnode from the free list, increment its
   * reference count and lock it.  VI_DOOMED is set if the vnode
   * is being destroyed.  Only callers who specify LK_RETRY will
   * see doomed vnodes.  If inactive processing was delayed in
   * vput try to do it here.
   *
   * Notes on lockless counter manipulation:
   * _vhold, vputx and other routines make various decisions based
   * on either holdcnt or usecount being 0. As long as either counter
   * is not transitioning 0->1 nor 1->0, the manipulation can be done
   * with atomic operations. Otherwise the interlock is taken covering
   * both the atomic and additional actions.
   */
  int
  vget(struct vnode *vp, int flags, struct thread *td)
  {
          int error, oweinact;
  
          VNASSERT((flags & LK_TYPE_MASK) != 0, vp,
              ("vget: invalid lock operation"));
  
          if ((flags & LK_INTERLOCK) != 0)
                  ASSERT_VI_LOCKED(vp, __func__);
          else
                  ASSERT_VI_UNLOCKED(vp, __func__);
          if ((flags & LK_VNHELD) != 0)
                  VNASSERT((vp->v_holdcnt > 0), vp,
                      ("vget: LK_VNHELD passed but vnode not held"));
  
          CTR3(KTR_VFS, "%s: vp %p with flags %d", __func__, vp, flags);
  
          if ((flags & LK_VNHELD) == 0)
                  _vhold(vp, (flags & LK_INTERLOCK) != 0);
  
          if ((error = vn_lock(vp, flags)) != 0) {
                  vdrop(vp);
                  CTR2(KTR_VFS, "%s: impossible to lock vnode %p", __func__,
                      vp);
                  return (error);
          }
          if (vp->v_iflag & VI_DOOMED && (flags & LK_RETRY) == 0)
                  panic("vget: vn_lock failed to return ENOENT\n");
          /*
           * We don't guarantee that any particular close will
           * trigger inactive processing so just make a best effort
           * here at preventing a reference to a removed file.  If
           * we don't succeed no harm is done.
           *
           * Upgrade our holdcnt to a usecount.
           */
          if (vp->v_type == VCHR ||
              !refcount_acquire_if_not_zero(&vp->v_usecount)) {
                  VI_LOCK(vp);
                  if ((vp->v_iflag & VI_OWEINACT) == 0) {
                          oweinact = 0;
                  } else {
                          oweinact = 1;
                          vp->v_iflag &= ~VI_OWEINACT;
                          VNODE_REFCOUNT_FENCE_REL();
                  }
                  refcount_acquire(&vp->v_usecount);
                  v_incr_devcount(vp);
                  if (oweinact && VOP_ISLOCKED(vp) == LK_EXCLUSIVE &&
                      (flags & LK_NOWAIT) == 0)
                          vinactive(vp, td);
                  VI_UNLOCK(vp);
          }
          return (0);
  }
  
  /*
   * Increase the reference (use) and hold count of a vnode.
   * This will also remove the vnode from the free list if it is presently free.
   */
  void
  vref(struct vnode *vp)
  {
  
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          _vhold(vp, false);
          v_incr_usecount(vp);
  }
  
  void
  vrefl(struct vnode *vp)
  {
  
          ASSERT_VI_LOCKED(vp, __func__);
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          _vhold(vp, true);
          v_incr_usecount_locked(vp);
  }
  
  void
  vrefact(struct vnode *vp)
  {
  
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          if (__predict_false(vp->v_type == VCHR)) {
                  VNASSERT(vp->v_holdcnt > 0 && vp->v_usecount > 0, vp,
                      ("%s: wrong ref counts", __func__));
                  vref(vp);
                  return;
          }
  #ifdef INVARIANTS
          int old = atomic_fetchadd_int(&vp->v_holdcnt, 1);
          VNASSERT(old > 0, vp, ("%s: wrong hold count", __func__));
          old = atomic_fetchadd_int(&vp->v_usecount, 1);
          VNASSERT(old > 0, vp, ("%s: wrong use count", __func__));
  #else
          refcount_acquire(&vp->v_holdcnt);
          refcount_acquire(&vp->v_usecount);
  #endif
  }
  
  /*
   * Return reference count of a vnode.
   *
   * The results of this call are only guaranteed when some mechanism is used to
   * stop other processes from gaining references to the vnode.  This may be the
   * case if the caller holds the only reference.  This is also useful when stale
   * data is acceptable as race conditions may be accounted for by some other
   * means.
   */
  int
  vrefcnt(struct vnode *vp)
  {
  
          return (vp->v_usecount);
  }
  
  #define VPUTX_VRELE     1
  #define VPUTX_VPUT      2
  #define VPUTX_VUNREF    3
  
  /*
   * Decrement the use and hold counts for a vnode.
   *
   * See an explanation near vget() as to why atomic operation is safe.
   */
  static void
  vputx(struct vnode *vp, int func)
  {
          int error;
  
          KASSERT(vp != NULL, ("vputx: null vp"));
          if (func == VPUTX_VUNREF)
                  ASSERT_VOP_LOCKED(vp, "vunref");
          else if (func == VPUTX_VPUT)
                  ASSERT_VOP_LOCKED(vp, "vput");
          else
                  KASSERT(func == VPUTX_VRELE, ("vputx: wrong func"));
          ASSERT_VI_UNLOCKED(vp, __func__);
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
  
          if (vp->v_type != VCHR &&
              refcount_release_if_not_last(&vp->v_usecount)) {
                  if (func == VPUTX_VPUT)
                          VOP_UNLOCK(vp, 0);
                  vdrop(vp);
                  return;
          }
  
          VI_LOCK(vp);
  
          /*
           * We want to hold the vnode until the inactive finishes to
           * prevent vgone() races.  We drop the use count here and the
           * hold count below when we're done.
           */
          if (!refcount_release(&vp->v_usecount) ||
              (vp->v_iflag & VI_DOINGINACT)) {
                  if (func == VPUTX_VPUT)
                          VOP_UNLOCK(vp, 0);
                  v_decr_devcount(vp);
                  vdropl(vp);
                  return;
          }
  
          v_decr_devcount(vp);
  
          error = 0;
  
          if (vp->v_usecount != 0) {
                  vn_printf(vp, "vputx: usecount not zero for vnode ");
                  panic("vputx: usecount not zero");
          }
  
          CTR2(KTR_VFS, "%s: return vnode %p to the freelist", __func__, vp);
  
          /*
           * We must call VOP_INACTIVE with the node locked. Mark
           * as VI_DOINGINACT to avoid recursion.
           */
          vp->v_iflag |= VI_OWEINACT;
          switch (func) {
          case VPUTX_VRELE:
                  error = vn_lock(vp, LK_EXCLUSIVE | LK_INTERLOCK);
                  VI_LOCK(vp);
                  break;
          case VPUTX_VPUT:
                  if (VOP_ISLOCKED(vp) != LK_EXCLUSIVE) {
                          error = VOP_LOCK(vp, LK_UPGRADE | LK_INTERLOCK |
                              LK_NOWAIT);
                          VI_LOCK(vp);
                  }
                  break;
          case VPUTX_VUNREF:
                  if (VOP_ISLOCKED(vp) != LK_EXCLUSIVE) {
                          error = VOP_LOCK(vp, LK_TRYUPGRADE | LK_INTERLOCK);
                          VI_LOCK(vp);
                  }
                  break;
          }
          VNASSERT(vp->v_usecount == 0 || (vp->v_iflag & VI_OWEINACT) == 0, vp,
              ("vnode with usecount and VI_OWEINACT set"));
          if (error == 0) {
                  if (vp->v_iflag & VI_OWEINACT)
                          vinactive(vp, curthread);
                  if (func != VPUTX_VUNREF)
                          VOP_UNLOCK(vp, 0);
          }
          vdropl(vp);
  }
  
  /*
   * Vnode put/release.
   * If count drops to zero, call inactive routine and return to freelist.
   */
  void
  vrele(struct vnode *vp)
  {
  
          vputx(vp, VPUTX_VRELE);
  }
  
  /*
   * Release an already locked vnode.  This give the same effects as
   * unlock+vrele(), but takes less time and avoids releasing and
   * re-aquiring the lock (as vrele() acquires the lock internally.)
   */
  void
  vput(struct vnode *vp)
  {
  
          vputx(vp, VPUTX_VPUT);
  }
  
  /*
   * Release an exclusively locked vnode. Do not unlock the vnode lock.
   */
  void
  vunref(struct vnode *vp)
  {
  
          vputx(vp, VPUTX_VUNREF);
  }
  
  /*
   * Increase the hold count and activate if this is the first reference.
   */
  void
  _vhold(struct vnode *vp, bool locked)
  {
          struct mount *mp;
  
          if (locked)
                  ASSERT_VI_LOCKED(vp, __func__);
          else
                  ASSERT_VI_UNLOCKED(vp, __func__);
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          if (!locked) {
                  if (refcount_acquire_if_not_zero(&vp->v_holdcnt)) {
                          VNODE_REFCOUNT_FENCE_ACQ();
                          VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
                              ("_vhold: vnode with holdcnt is free"));
                          return;
                  }
                  VI_LOCK(vp);
          }
          if ((vp->v_iflag & VI_FREE) == 0) {
                  refcount_acquire(&vp->v_holdcnt);
                  if (!locked)
                          VI_UNLOCK(vp);
                  return;
          }
          VNASSERT(vp->v_holdcnt == 0, vp,
              ("%s: wrong hold count", __func__));
          VNASSERT(vp->v_op != NULL, vp,
              ("%s: vnode already reclaimed.", __func__));
          /*
           * Remove a vnode from the free list, mark it as in use,
           * and put it on the active list.
           */
          VNASSERT(vp->v_mount != NULL, vp,
              ("_vhold: vnode not on per mount vnode list"));
          mp = vp->v_mount;
          mtx_lock(&mp->mnt_listmtx);
          if ((vp->v_mflag & VMP_TMPMNTFREELIST) != 0) {
                  TAILQ_REMOVE(&mp->mnt_tmpfreevnodelist, vp, v_actfreelist);
                  mp->mnt_tmpfreevnodelistsize--;
                  vp->v_mflag &= ~VMP_TMPMNTFREELIST;
          } else {
                  mtx_lock(&vnode_free_list_mtx);
                  TAILQ_REMOVE(&vnode_free_list, vp, v_actfreelist);
                  freevnodes--;
                  mtx_unlock(&vnode_free_list_mtx);
          }
          KASSERT((vp->v_iflag & VI_ACTIVE) == 0,
              ("Activating already active vnode"));
          vp->v_iflag &= ~VI_FREE;
          vp->v_iflag |= VI_ACTIVE;
          TAILQ_INSERT_HEAD(&mp->mnt_activevnodelist, vp, v_actfreelist);
          mp->mnt_activevnodelistsize++;
          mtx_unlock(&mp->mnt_listmtx);
          refcount_acquire(&vp->v_holdcnt);
          if (!locked)
                  VI_UNLOCK(vp);
  }
  
  /*
   * Drop the hold count of the vnode.  If this is the last reference to
   * the vnode we place it on the free list unless it has been vgone'd
   * (marked VI_DOOMED) in which case we will free it.
   *
   * Because the vnode vm object keeps a hold reference on the vnode if
   * there is at least one resident non-cached page, the vnode cannot
   * leave the active list without the page cleanup done.
   */
  void
  _vdrop(struct vnode *vp, bool locked)
  {
          struct bufobj *bo;
          struct mount *mp;
          int active;
  
          if (locked)
                  ASSERT_VI_LOCKED(vp, __func__);
          else
                  ASSERT_VI_UNLOCKED(vp, __func__);
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          if ((int)vp->v_holdcnt <= 0)
                  panic("vdrop: holdcnt %d", vp->v_holdcnt);
          if (!locked) {
                  if (refcount_release_if_not_last(&vp->v_holdcnt))
                          return;
                  VI_LOCK(vp);
          }
          if (refcount_release(&vp->v_holdcnt) == 0) {
                  VI_UNLOCK(vp);
                  return;
          }
          if ((vp->v_iflag & VI_DOOMED) == 0) {
                  /*
                   * Mark a vnode as free: remove it from its active list
                   * and put it up for recycling on the freelist.
                   */
                  VNASSERT(vp->v_op != NULL, vp,
                      ("vdropl: vnode already reclaimed."));
                  VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
                      ("vnode already free"));
                  VNASSERT(vp->v_holdcnt == 0, vp,
                      ("vdropl: freeing when we shouldn't"));
                  active = vp->v_iflag & VI_ACTIVE;
                  if ((vp->v_iflag & VI_OWEINACT) == 0) {
                          vp->v_iflag &= ~VI_ACTIVE;
                          mp = vp->v_mount;
                          if (mp != NULL) {
                                  mtx_lock(&mp->mnt_listmtx);
                                  if (active) {
                                          TAILQ_REMOVE(&mp->mnt_activevnodelist,
                                              vp, v_actfreelist);
                                          mp->mnt_activevnodelistsize--;
                                  }
                                  TAILQ_INSERT_TAIL(&mp->mnt_tmpfreevnodelist,
                                      vp, v_actfreelist);
                                  mp->mnt_tmpfreevnodelistsize++;
                                  vp->v_iflag |= VI_FREE;
                                  vp->v_mflag |= VMP_TMPMNTFREELIST;
                                  VI_UNLOCK(vp);
                                  if (mp->mnt_tmpfreevnodelistsize >=
                                      mnt_free_list_batch)
                                          vnlru_return_batch_locked(mp);
                                  mtx_unlock(&mp->mnt_listmtx);
                          } else {
                                  VNASSERT(active == 0, vp,
                                      ("vdropl: active vnode not on per mount "
                                      "vnode list"));
                                  mtx_lock(&vnode_free_list_mtx);
                                  TAILQ_INSERT_TAIL(&vnode_free_list, vp,
                                      v_actfreelist);
                                  freevnodes++;
                                  vp->v_iflag |= VI_FREE;
                                  VI_UNLOCK(vp);
                                  mtx_unlock(&vnode_free_list_mtx);
                          }
                  } else {
                          VI_UNLOCK(vp);
                          counter_u64_add(free_owe_inact, 1);
                  }
                  return;
          }
          /*
           * The vnode has been marked for destruction, so free it.
           *
           * The vnode will be returned to the zone where it will
           * normally remain until it is needed for another vnode. We
           * need to cleanup (or verify that the cleanup has already
           * been done) any residual data left from its current use
           * so as not to contaminate the freshly allocated vnode.
           */
          CTR2(KTR_VFS, "%s: destroying the vnode %p", __func__, vp);
          atomic_subtract_long(&numvnodes, 1);
          bo = &vp->v_bufobj;
          VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
              ("cleaned vnode still on the free list."));
          VNASSERT(vp->v_data == NULL, vp, ("cleaned vnode isn't"));
          VNASSERT(vp->v_holdcnt == 0, vp, ("Non-zero hold count"));
          VNASSERT(vp->v_usecount == 0, vp, ("Non-zero use count"));
          VNASSERT(vp->v_writecount == 0, vp, ("Non-zero write count"));
          VNASSERT(bo->bo_numoutput == 0, vp, ("Clean vnode has pending I/O's"));
          VNASSERT(bo->bo_clean.bv_cnt == 0, vp, ("cleanbufcnt not 0"));
          VNASSERT(pctrie_is_empty(&bo->bo_clean.bv_root), vp,
              ("clean blk trie not empty"));
          VNASSERT(bo->bo_dirty.bv_cnt == 0, vp, ("dirtybufcnt not 0"));
          VNASSERT(pctrie_is_empty(&bo->bo_dirty.bv_root), vp,
              ("dirty blk trie not empty"));
          VNASSERT(TAILQ_EMPTY(&vp->v_cache_dst), vp, ("vp has namecache dst"));
          VNASSERT(LIST_EMPTY(&vp->v_cache_src), vp, ("vp has namecache src"));
          VNASSERT(vp->v_cache_dd == NULL, vp, ("vp has namecache for .."));
          VNASSERT(TAILQ_EMPTY(&vp->v_rl.rl_waiters), vp,
              ("Dangling rangelock waiters"));
          VI_UNLOCK(vp);
  #ifdef MAC
          mac_vnode_destroy(vp);
  #endif
          if (vp->v_pollinfo != NULL) {
                  destroy_vpollinfo(vp->v_pollinfo);
                  vp->v_pollinfo = NULL;
          }
  #ifdef INVARIANTS
          /* XXX Elsewhere we detect an already freed vnode via NULL v_op. */
          vp->v_op = NULL;
  #endif
          vp->v_mountedhere = NULL;
          vp->v_unpcb = NULL;
          vp->v_rdev = NULL;
          vp->v_fifoinfo = NULL;
          vp->v_lasta = vp->v_clen = vp->v_cstart = vp->v_lastw = 0;
          vp->v_iflag = 0;
          vp->v_vflag = 0;
          bo->bo_flag = 0;
          uma_zfree(vnode_zone, vp);
  }
  
  /*
   * Call VOP_INACTIVE on the vnode and manage the DOINGINACT and OWEINACT
   * flags.  DOINGINACT prevents us from recursing in calls to vinactive.
   * OWEINACT tracks whether a vnode missed a call to inactive due to a
   * failed lock upgrade.
   */
  void
  vinactive(struct vnode *vp, struct thread *td)
  {
          struct vm_object *obj;
  
          ASSERT_VOP_ELOCKED(vp, "vinactive");
          ASSERT_VI_LOCKED(vp, "vinactive");
          VNASSERT((vp->v_iflag & VI_DOINGINACT) == 0, vp,
              ("vinactive: recursed on VI_DOINGINACT"));
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          vp->v_iflag |= VI_DOINGINACT;
          vp->v_iflag &= ~VI_OWEINACT;
          VI_UNLOCK(vp);
          /*
           * Before moving off the active list, we must be sure that any
           * modified pages are converted into the vnode's dirty
           * buffers, since these will no longer be checked once the
           * vnode is on the inactive list.
           *
           * The write-out of the dirty pages is asynchronous.  At the
           * point that VOP_INACTIVE() is called, there could still be
           * pending I/O and dirty pages in the object.
           */
          if ((obj = vp->v_object) != NULL && (vp->v_vflag & VV_NOSYNC) == 0 &&
              (obj->flags & OBJ_MIGHTBEDIRTY) != 0) {
                  VM_OBJECT_WLOCK(obj);
                  vm_object_page_clean(obj, 0, 0, 0);
                  VM_OBJECT_WUNLOCK(obj);
          }
          VOP_INACTIVE(vp, td);
          VI_LOCK(vp);
          VNASSERT(vp->v_iflag & VI_DOINGINACT, vp,
              ("vinactive: lost VI_DOINGINACT"));
          vp->v_iflag &= ~VI_DOINGINACT;
  }
  
  /*
   * Remove any vnodes in the vnode table belonging to mount point mp.
   *
   * If FORCECLOSE is not specified, there should not be any active ones,
   * return error if any are found (nb: this is a user error, not a
   * system error). If FORCECLOSE is specified, detach any active vnodes
   * that are found.
   *
   * If WRITECLOSE is set, only flush out regular file vnodes open for
   * writing.
   *
   * SKIPSYSTEM causes any vnodes marked VV_SYSTEM to be skipped.
   *
   * `rootrefs' specifies the base reference count for the root vnode
   * of this filesystem. The root vnode is considered busy if its
   * v_usecount exceeds this value. On a successful return, vflush(, td)
   * will call vrele() on the root vnode exactly rootrefs times.
   * If the SKIPSYSTEM or WRITECLOSE flags are specified, rootrefs must
   * be zero.
   */
  #ifdef DIAGNOSTIC
  static int busyprt = 0;         /* print out busy vnodes */
  SYSCTL_INT(_debug, OID_AUTO, busyprt, CTLFLAG_RW, &busyprt, 0, "Print out busy vnodes");
  #endif
  
  int
  vflush(struct mount *mp, int rootrefs, int flags, struct thread *td)
  {
          struct vnode *vp, *mvp, *rootvp = NULL;
          struct vattr vattr;
          int busy = 0, error;
  
          CTR4(KTR_VFS, "%s: mp %p with rootrefs %d and flags %d", __func__, mp,
              rootrefs, flags);
          if (rootrefs > 0) {
                  KASSERT((flags & (SKIPSYSTEM | WRITECLOSE)) == 0,
                      ("vflush: bad args"));
                  /*
                   * Get the filesystem root vnode. We can vput() it
                   * immediately, since with rootrefs > 0, it won't go away.
                   */
                  if ((error = VFS_ROOT(mp, LK_EXCLUSIVE, &rootvp)) != 0) {
                          CTR2(KTR_VFS, "%s: vfs_root lookup failed with %d",
                              __func__, error);
                          return (error);
                  }
                  vput(rootvp);
          }
  loop:
          MNT_VNODE_FOREACH_ALL(vp, mp, mvp) {
                  vholdl(vp);
                  error = vn_lock(vp, LK_INTERLOCK | LK_EXCLUSIVE);
                  if (error) {
                          vdrop(vp);
                          MNT_VNODE_FOREACH_ALL_ABORT(mp, mvp);
                          goto loop;
                  }
                  /*
                   * Skip over a vnodes marked VV_SYSTEM.
                   */
                  if ((flags & SKIPSYSTEM) && (vp->v_vflag & VV_SYSTEM)) {
                          VOP_UNLOCK(vp, 0);
                          vdrop(vp);
                          continue;
                  }
                  /*
                   * If WRITECLOSE is set, flush out unlinked but still open
                   * files (even if open only for reading) and regular file
                   * vnodes open for writing.
                   */
                  if (flags & WRITECLOSE) {
                          if (vp->v_object != NULL) {
                                  VM_OBJECT_WLOCK(vp->v_object);
                                  vm_object_page_clean(vp->v_object, 0, 0, 0);
                                  VM_OBJECT_WUNLOCK(vp->v_object);
                          }
                          error = VOP_FSYNC(vp, MNT_WAIT, td);
                          if (error != 0) {
                                  VOP_UNLOCK(vp, 0);
                                  vdrop(vp);
                                  MNT_VNODE_FOREACH_ALL_ABORT(mp, mvp);
                                  return (error);
                          }
                          error = VOP_GETATTR(vp, &vattr, td->td_ucred);
                          VI_LOCK(vp);
  
                          if ((vp->v_type == VNON ||
                              (error == 0 && vattr.va_nlink > 0)) &&
                              (vp->v_writecount == 0 || vp->v_type != VREG)) {
                                  VOP_UNLOCK(vp, 0);
                                  vdropl(vp);
                                  continue;
                          }
                  } else
                          VI_LOCK(vp);
                  /*
                   * With v_usecount == 0, all we need to do is clear out the
                   * vnode data structures and we are done.
                   *
                   * If FORCECLOSE is set, forcibly close the vnode.
                   */
                  if (vp->v_usecount == 0 || (flags & FORCECLOSE)) {
                          vgonel(vp);
                  } else {
                          busy++;
  #ifdef DIAGNOSTIC
                          if (busyprt)
                                  vn_printf(vp, "vflush: busy vnode ");
  #endif
                  }
                  VOP_UNLOCK(vp, 0);
                  vdropl(vp);
          }
          if (rootrefs > 0 && (flags & FORCECLOSE) == 0) {
                  /*
                   * If just the root vnode is busy, and if its refcount
                   * is equal to `rootrefs', then go ahead and kill it.
                   */
                  VI_LOCK(rootvp);
                  KASSERT(busy > 0, ("vflush: not busy"));
                  VNASSERT(rootvp->v_usecount >= rootrefs, rootvp,
                      ("vflush: usecount %d < rootrefs %d",
                       rootvp->v_usecount, rootrefs));
                  if (busy == 1 && rootvp->v_usecount == rootrefs) {
                          VOP_LOCK(rootvp, LK_EXCLUSIVE|LK_INTERLOCK);
                          vgone(rootvp);
                          VOP_UNLOCK(rootvp, 0);
                          busy = 0;
                  } else
                          VI_UNLOCK(rootvp);
          }
          if (busy) {
                  CTR2(KTR_VFS, "%s: failing as %d vnodes are busy", __func__,
                      busy);
                  return (EBUSY);
          }
          for (; rootrefs > 0; rootrefs--)
                  vrele(rootvp);
          return (0);
  }
  
  /*
   * Recycle an unused vnode to the front of the free list.
   */
  int
  vrecycle(struct vnode *vp)
  {
          int recycled;
  
          VI_LOCK(vp);
          recycled = vrecyclel(vp);
          VI_UNLOCK(vp);
          return (recycled);
  }
  
  /*
   * vrecycle, with the vp interlock held.
   */
  int
  vrecyclel(struct vnode *vp)
  {
          int recycled;
  
          ASSERT_VOP_ELOCKED(vp, __func__);
          ASSERT_VI_LOCKED(vp, __func__);
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          recycled = 0;
          if (vp->v_usecount == 0) {
                  recycled = 1;
                  vgonel(vp);
          }
          return (recycled);
  }
  
  /*
   * Eliminate all activity associated with a vnode
   * in preparation for reuse.
   */
  void
  vgone(struct vnode *vp)
  {
          VI_LOCK(vp);
          vgonel(vp);
          VI_UNLOCK(vp);
  }
  
  static void
  notify_lowervp_vfs_dummy(struct mount *mp __unused,
      struct vnode *lowervp __unused)
  {
  }
  
  /*
   * Notify upper mounts about reclaimed or unlinked vnode.
   */
  void
  vfs_notify_upper(struct vnode *vp, int event)
  {
          static struct vfsops vgonel_vfsops = {
                  .vfs_reclaim_lowervp = notify_lowervp_vfs_dummy,
                  .vfs_unlink_lowervp = notify_lowervp_vfs_dummy,
          };
          struct mount *mp, *ump, *mmp;
  
          mp = vp->v_mount;
          if (mp == NULL)
                  return;
  
          MNT_ILOCK(mp);
          if (TAILQ_EMPTY(&mp->mnt_uppers))
                  goto unlock;
          MNT_IUNLOCK(mp);
          mmp = malloc(sizeof(struct mount), M_TEMP, M_WAITOK | M_ZERO);
          mmp->mnt_op = &vgonel_vfsops;
          mmp->mnt_kern_flag |= MNTK_MARKER;
          MNT_ILOCK(mp);
          mp->mnt_kern_flag |= MNTK_VGONE_UPPER;
          for (ump = TAILQ_FIRST(&mp->mnt_uppers); ump != NULL;) {
                  if ((ump->mnt_kern_flag & MNTK_MARKER) != 0) {
                          ump = TAILQ_NEXT(ump, mnt_upper_link);
                          continue;
                  }
                  TAILQ_INSERT_AFTER(&mp->mnt_uppers, ump, mmp, mnt_upper_link);
                  MNT_IUNLOCK(mp);
                  switch (event) {
                  case VFS_NOTIFY_UPPER_RECLAIM:
                          VFS_RECLAIM_LOWERVP(ump, vp);
                          break;
                  case VFS_NOTIFY_UPPER_UNLINK:
                          VFS_UNLINK_LOWERVP(ump, vp);
                          break;
                  default:
                          KASSERT(0, ("invalid event %d", event));
                          break;
                  }
                  MNT_ILOCK(mp);
                  ump = TAILQ_NEXT(mmp, mnt_upper_link);
                  TAILQ_REMOVE(&mp->mnt_uppers, mmp, mnt_upper_link);
          }
          free(mmp, M_TEMP);
          mp->mnt_kern_flag &= ~MNTK_VGONE_UPPER;
          if ((mp->mnt_kern_flag & MNTK_VGONE_WAITER) != 0) {
                  mp->mnt_kern_flag &= ~MNTK_VGONE_WAITER;
                  wakeup(&mp->mnt_uppers);
          }
  unlock:
          MNT_IUNLOCK(mp);
  }
  
  /*
   * vgone, with the vp interlock held.
   */
  static void
  vgonel(struct vnode *vp)
  {
          struct thread *td;
          int oweinact;
          int active;
          struct mount *mp;
  
          ASSERT_VOP_ELOCKED(vp, "vgonel");
          ASSERT_VI_LOCKED(vp, "vgonel");
          VNASSERT(vp->v_holdcnt, vp,
              ("vgonel: vp %p has no reference.", vp));
          CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
          td = curthread;
  
          /*
           * Don't vgonel if we're already doomed.
           */
          if (vp->v_iflag & VI_DOOMED)
                  return;
          vp->v_iflag |= VI_DOOMED;
  
          /*
           * Check to see if the vnode is in use.  If so, we have to call
           * VOP_CLOSE() and VOP_INACTIVE().
           */
          active = vp->v_usecount;
          oweinact = (vp->v_iflag & VI_OWEINACT);
          VI_UNLOCK(vp);
          vfs_notify_upper(vp, VFS_NOTIFY_UPPER_RECLAIM);
  
          /*
           * If purging an active vnode, it must be closed and
           * deactivated before being reclaimed.
           */
          if (active)
                  VOP_CLOSE(vp, FNONBLOCK, NOCRED, td);
          if (oweinact || active) {
                  VI_LOCK(vp);
                  if ((vp->v_iflag & VI_DOINGINACT) == 0)
                          vinactive(vp, td);
                  VI_UNLOCK(vp);
          }
          if (vp->v_type == VSOCK)
                  vfs_unp_reclaim(vp);
  
          /*
           * Clean out any buffers associated with the vnode.
           * If the flush fails, just toss the buffers.
           */
          mp = NULL;
          if (!TAILQ_EMPTY(&vp->v_bufobj.bo_dirty.bv_hd))
                  (void) vn_start_secondary_write(vp, &mp, V_WAIT);
          if (vinvalbuf(vp, V_SAVE, 0, 0) != 0) {
                  while (vinvalbuf(vp, 0, 0, 0) != 0)
                          ;
          }
  
          BO_LOCK(&vp->v_bufobj);
          KASSERT(TAILQ_EMPTY(&vp->v_bufobj.bo_dirty.bv_hd) &&
              vp->v_bufobj.bo_dirty.bv_cnt == 0 &&
              TAILQ_EMPTY(&vp->v_bufobj.bo_clean.bv_hd) &&
              vp->v_bufobj.bo_clean.bv_cnt == 0,
              ("vp %p bufobj not invalidated", vp));
  
          /*
           * For VMIO bufobj, BO_DEAD is set in vm_object_terminate()
           * after the object's page queue is flushed.
           */
          if (vp->v_bufobj.bo_object == NULL)
                  vp->v_bufobj.bo_flag |= BO_DEAD;
          BO_UNLOCK(&vp->v_bufobj);
  
          /*
           * Reclaim the vnode.
           */
          if (VOP_RECLAIM(vp, td))
                  panic("vgone: cannot reclaim");
          if (mp != NULL)
                  vn_finished_secondary_write(mp);
          VNASSERT(vp->v_object == NULL, vp,
              ("vop_reclaim left v_object vp=%p, tag=%s", vp, vp->v_tag));
          /*
           * Clear the advisory locks and wake up waiting threads.
           */
          (void)VOP_ADVLOCKPURGE(vp);
          vp->v_lockf = NULL;
          /*
           * Delete from old mount point vnode list.
           */
          delmntque(vp);
          cache_purge(vp);
          /*
           * Done with purge, reset to the standard lock and invalidate
           * the vnode.
           */
          VI_LOCK(vp);
          vp->v_vnlock = &vp->v_lock;
          vp->v_op = &dead_vnodeops;
          vp->v_tag = "none";
          vp->v_type = VBAD;
  }
  
  /*
   * Calculate the total number of references to a special device.
   */
  int
  vcount(struct vnode *vp)
  {
          int count;
  
          dev_lock();
          count = vp->v_rdev->si_usecount;
          dev_unlock();
          return (count);
  }
  
  /*
   * Same as above, but using the struct cdev *as argument
   */
  int
  count_dev(struct cdev *dev)
  {
          int count;
  
          dev_lock();
          count = dev->si_usecount;
          dev_unlock();
          return(count);
  }
  
  /*
   * Print out a description of a vnode.
   */
  static char *typename[] =
  {"VNON", "VREG", "VDIR", "VBLK", "VCHR", "VLNK", "VSOCK", "VFIFO", "VBAD",
   "VMARKER"};
  
  void
  vn_printf(struct vnode *vp, const char *fmt, ...)
  {
          va_list ap;
          char buf[256], buf2[16];
          u_long flags;
  
          va_start(ap, fmt);
          vprintf(fmt, ap);
          va_end(ap);
          printf("%p: ", (void *)vp);
          printf("tag %s, type %s\n", vp->v_tag, typename[vp->v_type]);
          printf("    usecount %d, writecount %d, refcount %d",
              vp->v_usecount, vp->v_writecount, vp->v_holdcnt);
          switch (vp->v_type) {
          case VDIR:
                  printf(" mountedhere %p\n", vp->v_mountedhere);
                  break;
          case VCHR:
                  printf(" rdev %p\n", vp->v_rdev);
                  break;
          case VSOCK:
                  printf(" socket %p\n", vp->v_unpcb);
                  break;
          case VFIFO:
                  printf(" fifoinfo %p\n", vp->v_fifoinfo);
                  break;
          default:
                  printf("\n");
                  break;
          }
          buf[0] = '\0';
          buf[1] = '\0';
          if (vp->v_vflag & VV_ROOT)
                  strlcat(buf, "|VV_ROOT", sizeof(buf));
          if (vp->v_vflag & VV_ISTTY)
                  strlcat(buf, "|VV_ISTTY", sizeof(buf));
          if (vp->v_vflag & VV_NOSYNC)
                  strlcat(buf, "|VV_NOSYNC", sizeof(buf));
          if (vp->v_vflag & VV_ETERNALDEV)
                  strlcat(buf, "|VV_ETERNALDEV", sizeof(buf));
          if (vp->v_vflag & VV_CACHEDLABEL)
                  strlcat(buf, "|VV_CACHEDLABEL", sizeof(buf));
          if (vp->v_vflag & VV_TEXT)
                  strlcat(buf, "|VV_TEXT", sizeof(buf));
          if (vp->v_vflag & VV_COPYONWRITE)
                  strlcat(buf, "|VV_COPYONWRITE", sizeof(buf));
          if (vp->v_vflag & VV_SYSTEM)
                  strlcat(buf, "|VV_SYSTEM", sizeof(buf));
          if (vp->v_vflag & VV_PROCDEP)
                  strlcat(buf, "|VV_PROCDEP", sizeof(buf));
          if (vp->v_vflag & VV_NOKNOTE)
                  strlcat(buf, "|VV_NOKNOTE", sizeof(buf));
          if (vp->v_vflag & VV_DELETED)
                  strlcat(buf, "|VV_DELETED", sizeof(buf));
          if (vp->v_vflag & VV_MD)
                  strlcat(buf, "|VV_MD", sizeof(buf));
          if (vp->v_vflag & VV_FORCEINSMQ)
                  strlcat(buf, "|VV_FORCEINSMQ", sizeof(buf));
          flags = vp->v_vflag & ~(VV_ROOT | VV_ISTTY | VV_NOSYNC | VV_ETERNALDEV |
              VV_CACHEDLABEL | VV_TEXT | VV_COPYONWRITE | VV_SYSTEM | VV_PROCDEP |
              VV_NOKNOTE | VV_DELETED | VV_MD | VV_FORCEINSMQ);
          if (flags != 0) {
                  snprintf(buf2, sizeof(buf2), "|VV(0x%lx)", flags);
                  strlcat(buf, buf2, sizeof(buf));
          }
          if (vp->v_iflag & VI_MOUNT)
                  strlcat(buf, "|VI_MOUNT", sizeof(buf));
          if (vp->v_iflag & VI_DOOMED)
                  strlcat(buf, "|VI_DOOMED", sizeof(buf));
          if (vp->v_iflag & VI_FREE)
                  strlcat(buf, "|VI_FREE", sizeof(buf));
          if (vp->v_iflag & VI_ACTIVE)
                  strlcat(buf, "|VI_ACTIVE", sizeof(buf));
          if (vp->v_iflag & VI_DOINGINACT)
                  strlcat(buf, "|VI_DOINGINACT", sizeof(buf));
          if (vp->v_iflag & VI_OWEINACT)
                  strlcat(buf, "|VI_OWEINACT", sizeof(buf));
          flags = vp->v_iflag & ~(VI_MOUNT | VI_DOOMED | VI_FREE |
              VI_ACTIVE | VI_DOINGINACT | VI_OWEINACT);
          if (flags != 0) {
                  snprintf(buf2, sizeof(buf2), "|VI(0x%lx)", flags);
                  strlcat(buf, buf2, sizeof(buf));
          }
          printf("    flags (%s)\n", buf + 1);
          if (mtx_owned(VI_MTX(vp)))
                  printf(" VI_LOCKed");
          if (vp->v_object != NULL)
                  printf("    v_object %p ref %d pages %d "
                      "cleanbuf %d dirtybuf %d\n",
                      vp->v_object, vp->v_object->ref_count,
                      vp->v_object->resident_page_count,
                      vp->v_bufobj.bo_clean.bv_cnt,
                      vp->v_bufobj.bo_dirty.bv_cnt);
          printf("    ");
          lockmgr_printinfo(vp->v_vnlock);
          if (vp->v_data != NULL)
                  VOP_PRINT(vp);
  }
  
  #ifdef DDB
  /*
   * List all of the locked vnodes in the system.
   * Called when debugging the kernel.
   */
  DB_SHOW_COMMAND(lockedvnods, lockedvnodes)
  {
          struct mount *mp;
          struct vnode *vp;
  
          /*
           * Note: because this is DDB, we can't obey the locking semantics
           * for these structures, which means we could catch an inconsistent
           * state and dereference a nasty pointer.  Not much to be done
           * about that.
           */
          db_printf("Locked vnodes\n");
          TAILQ_FOREACH(mp, &mountlist, mnt_list) {
                  TAILQ_FOREACH(vp, &mp->mnt_nvnodelist, v_nmntvnodes) {
                          if (vp->v_type != VMARKER && VOP_ISLOCKED(vp))
                                  vn_printf(vp, "vnode ");
                  }
          }
  }
  
  /*
   * Show details about the given vnode.
   */
  DB_SHOW_COMMAND(vnode, db_show_vnode)
  {
          struct vnode *vp;
  
          if (!have_addr)
                  return;
          vp = (struct vnode *)addr;
          vn_printf(vp, "vnode ");
  }
  
  /*
   * Show details about the given mount point.
   */
  DB_SHOW_COMMAND(mount, db_show_mount)
  {
          struct mount *mp;
          struct vfsopt *opt;
          struct statfs *sp;
          struct vnode *vp;
          char buf[512];
          uint64_t mflags;
          u_int flags;
  
          if (!have_addr) {
                  /* No address given, print short info about all mount points. */
                  TAILQ_FOREACH(mp, &mountlist, mnt_list) {
                          db_printf("%p %s on %s (%s)\n", mp,
                              mp->mnt_stat.f_mntfromname,
                              mp->mnt_stat.f_mntonname,
                              mp->mnt_stat.f_fstypename);
                          if (db_pager_quit)
                                  break;
                  }
                  db_printf("\nMore info: show mount <addr>\n");
                  return;
          }
  
          mp = (struct mount *)addr;
          db_printf("%p %s on %s (%s)\n", mp, mp->mnt_stat.f_mntfromname,
              mp->mnt_stat.f_mntonname, mp->mnt_stat.f_fstypename);
  
          buf[0] = '\0';
          mflags = mp->mnt_flag;
  #define MNT_FLAG(flag)  do {                                            \
          if (mflags & (flag)) {                                          \
                  if (buf[0] != '\0')                                     \
                          strlcat(buf, ", ", sizeof(buf));                \
                  strlcat(buf, (#flag) + 4, sizeof(buf));                 \
                  mflags &= ~(flag);                                      \
          }                                                               \
  } while (0)
          MNT_FLAG(MNT_RDONLY);
          MNT_FLAG(MNT_SYNCHRONOUS);
          MNT_FLAG(MNT_NOEXEC);
          MNT_FLAG(MNT_NOSUID);
          MNT_FLAG(MNT_NFS4ACLS);
          MNT_FLAG(MNT_UNION);
          MNT_FLAG(MNT_ASYNC);
          MNT_FLAG(MNT_SUIDDIR);
          MNT_FLAG(MNT_SOFTDEP);
          MNT_FLAG(MNT_NOSYMFOLLOW);
          MNT_FLAG(MNT_GJOURNAL);
          MNT_FLAG(MNT_MULTILABEL);
          MNT_FLAG(MNT_ACLS);
          MNT_FLAG(MNT_NOATIME);
          MNT_FLAG(MNT_NOCLUSTERR);
          MNT_FLAG(MNT_NOCLUSTERW);
          MNT_FLAG(MNT_SUJ);
          MNT_FLAG(MNT_EXRDONLY);
          MNT_FLAG(MNT_EXPORTED);
          MNT_FLAG(MNT_DEFEXPORTED);
          MNT_FLAG(MNT_EXPORTANON);
          MNT_FLAG(MNT_EXKERB);
          MNT_FLAG(MNT_EXPUBLIC);
          MNT_FLAG(MNT_LOCAL);
          MNT_FLAG(MNT_QUOTA);
          MNT_FLAG(MNT_ROOTFS);
          MNT_FLAG(MNT_USER);
          MNT_FLAG(MNT_IGNORE);
          MNT_FLAG(MNT_UPDATE);
          MNT_FLAG(MNT_DELEXPORT);
          MNT_FLAG(MNT_RELOAD);
          MNT_FLAG(MNT_FORCE);
          MNT_FLAG(MNT_SNAPSHOT);
          MNT_FLAG(MNT_BYFSID);
  #undef MNT_FLAG
          if (mflags != 0) {
                  if (buf[0] != '\0')
                          strlcat(buf, ", ", sizeof(buf));
                  snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf),
                      "0x%016jx", mflags);
          }
          db_printf("    mnt_flag = %s\n", buf);
  
          buf[0] = '\0';
          flags = mp->mnt_kern_flag;
  #define MNT_KERN_FLAG(flag)     do {                                    \
          if (flags & (flag)) {                                           \
                  if (buf[0] != '\0')                                     \
                          strlcat(buf, ", ", sizeof(buf));                \
                  strlcat(buf, (#flag) + 5, sizeof(buf));                 \
                  flags &= ~(flag);                                       \
          }                                                               \
  } while (0)
          MNT_KERN_FLAG(MNTK_UNMOUNTF);
          MNT_KERN_FLAG(MNTK_ASYNC);
          MNT_KERN_FLAG(MNTK_SOFTDEP);
          MNT_KERN_FLAG(MNTK_NOINSMNTQ);
          MNT_KERN_FLAG(MNTK_DRAINING);
          MNT_KERN_FLAG(MNTK_REFEXPIRE);
          MNT_KERN_FLAG(MNTK_EXTENDED_SHARED);
          MNT_KERN_FLAG(MNTK_SHARED_WRITES);
          MNT_KERN_FLAG(MNTK_NO_IOPF);
          MNT_KERN_FLAG(MNTK_VGONE_UPPER);
          MNT_KERN_FLAG(MNTK_VGONE_WAITER);
          MNT_KERN_FLAG(MNTK_LOOKUP_EXCL_DOTDOT);
          MNT_KERN_FLAG(MNTK_MARKER);
          MNT_KERN_FLAG(MNTK_USES_BCACHE);
          MNT_KERN_FLAG(MNTK_NOASYNC);
          MNT_KERN_FLAG(MNTK_UNMOUNT);
          MNT_KERN_FLAG(MNTK_MWAIT);
          MNT_KERN_FLAG(MNTK_SUSPEND);
          MNT_KERN_FLAG(MNTK_SUSPEND2);
          MNT_KERN_FLAG(MNTK_SUSPENDED);
          MNT_KERN_FLAG(MNTK_LOOKUP_SHARED);
          MNT_KERN_FLAG(MNTK_NOKNOTE);
  #undef MNT_KERN_FLAG
          if (flags != 0) {
                  if (buf[0] != '\0')
                          strlcat(buf, ", ", sizeof(buf));
                  snprintf(buf + strlen(buf), sizeof(buf) - strlen(buf),
                      "0x%08x", flags);
          }
          db_printf("    mnt_kern_flag = %s\n", buf);
  
          db_printf("    mnt_opt = ");
          opt = TAILQ_FIRST(mp->mnt_opt);
          if (opt != NULL) {
                  db_printf("%s", opt->name);
                  opt = TAILQ_NEXT(opt, link);
                  while (opt != NULL) {
                          db_printf(", %s", opt->name);
                          opt = TAILQ_NEXT(opt, link);
                  }
          }
          db_printf("\n");
  
          sp = &mp->mnt_stat;
          db_printf("    mnt_stat = { version=%u type=%u flags=0x%016jx "
              "bsize=%ju iosize=%ju blocks=%ju bfree=%ju bavail=%jd files=%ju "
              "ffree=%jd syncwrites=%ju asyncwrites=%ju syncreads=%ju "
              "asyncreads=%ju namemax=%u owner=%u fsid=[%d, %d] }\n",
              (u_int)sp->f_version, (u_int)sp->f_type, (uintmax_t)sp->f_flags,
              (uintmax_t)sp->f_bsize, (uintmax_t)sp->f_iosize,
              (uintmax_t)sp->f_blocks, (uintmax_t)sp->f_bfree,
              (intmax_t)sp->f_bavail, (uintmax_t)sp->f_files,
              (intmax_t)sp->f_ffree, (uintmax_t)sp->f_syncwrites,
              (uintmax_t)sp->f_asyncwrites, (uintmax_t)sp->f_syncreads,
              (uintmax_t)sp->f_asyncreads, (u_int)sp->f_namemax,
              (u_int)sp->f_owner, (int)sp->f_fsid.val[0], (int)sp->f_fsid.val[1]);
  
          db_printf("    mnt_cred = { uid=%u ruid=%u",
              (u_int)mp->mnt_cred->cr_uid, (u_int)mp->mnt_cred->cr_ruid);
          if (jailed(mp->mnt_cred))
                  db_printf(", jail=%d", mp->mnt_cred->cr_prison->pr_id);
          db_printf(" }\n");
          db_printf("    mnt_ref = %d\n", mp->mnt_ref);
          db_printf("    mnt_gen = %d\n", mp->mnt_gen);
          db_printf("    mnt_nvnodelistsize = %d\n", mp->mnt_nvnodelistsize);
          db_printf("    mnt_activevnodelistsize = %d\n",
              mp->mnt_activevnodelistsize);
          db_printf("    mnt_writeopcount = %d\n", mp->mnt_writeopcount);
          db_printf("    mnt_maxsymlinklen = %d\n", mp->mnt_maxsymlinklen);
          db_printf("    mnt_iosize_max = %d\n", mp->mnt_iosize_max);
          db_printf("    mnt_hashseed = %u\n", mp->mnt_hashseed);
          db_printf("    mnt_lockref = %d\n", mp->mnt_lockref);
          db_printf("    mnt_secondary_writes = %d\n", mp->mnt_secondary_writes);
          db_printf("    mnt_secondary_accwrites = %d\n",
              mp->mnt_secondary_accwrites);
          db_printf("    mnt_gjprovider = %s\n",
              mp->mnt_gjprovider != NULL ? mp->mnt_gjprovider : "NULL");
  
          db_printf("\n\nList of active vnodes\n");
          TAILQ_FOREACH(vp, &mp->mnt_activevnodelist, v_actfreelist) {
                  if (vp->v_type != VMARKER) {
                          vn_printf(vp, "vnode ");
                          if (db_pager_quit)
                                  break;
                  }
          }
          db_printf("\n\nList of inactive vnodes\n");
          TAILQ_FOREACH(vp, &mp->mnt_nvnodelist, v_nmntvnodes) {
                  if (vp->v_type != VMARKER && (vp->v_iflag & VI_ACTIVE) == 0) {
                          vn_printf(vp, "vnode ");
                          if (db_pager_quit)
                                  break;
                  }
          }
  }
  #endif  /* DDB */
  
  /*
   * Fill in a struct xvfsconf based on a struct vfsconf.
   */
  static int
  vfsconf2x(struct sysctl_req *req, struct vfsconf *vfsp)
  {
          struct xvfsconf xvfsp;
  
          bzero(&xvfsp, sizeof(xvfsp));
          strcpy(xvfsp.vfc_name, vfsp->vfc_name);
          xvfsp.vfc_typenum = vfsp->vfc_typenum;
          xvfsp.vfc_refcount = vfsp->vfc_refcount;
          xvfsp.vfc_flags = vfsp->vfc_flags;
          /*
           * These are unused in userland, we keep them
           * to not break binary compatibility.
           */
          xvfsp.vfc_vfsops = NULL;
          xvfsp.vfc_next = NULL;
          return (SYSCTL_OUT(req, &xvfsp, sizeof(xvfsp)));
  }
  
  #ifdef COMPAT_FREEBSD32
  struct xvfsconf32 {
          uint32_t        vfc_vfsops;
          char            vfc_name[MFSNAMELEN];
          int32_t         vfc_typenum;
          int32_t         vfc_refcount;
          int32_t         vfc_flags;
          uint32_t        vfc_next;
  };
  
  static int
  vfsconf2x32(struct sysctl_req *req, struct vfsconf *vfsp)
  {
          struct xvfsconf32 xvfsp;
  
          bzero(&xvfsp, sizeof(xvfsp));
          strcpy(xvfsp.vfc_name, vfsp->vfc_name);
          xvfsp.vfc_typenum = vfsp->vfc_typenum;
          xvfsp.vfc_refcount = vfsp->vfc_refcount;
          xvfsp.vfc_flags = vfsp->vfc_flags;
          return (SYSCTL_OUT(req, &xvfsp, sizeof(xvfsp)));
  }
  #endif
  
  /*
   * Top level filesystem related information gathering.
   */
  static int
  sysctl_vfs_conflist(SYSCTL_HANDLER_ARGS)
  {
          struct vfsconf *vfsp;
          int error;
  
          error = 0;
          vfsconf_slock();
          TAILQ_FOREACH(vfsp, &vfsconf, vfc_list) {
  #ifdef COMPAT_FREEBSD32
                  if (req->flags & SCTL_MASK32)
                          error = vfsconf2x32(req, vfsp);
                  else
  #endif
                          error = vfsconf2x(req, vfsp);
                  if (error)
                          break;
          }
          vfsconf_sunlock();
          return (error);
  }
  
  SYSCTL_PROC(_vfs, OID_AUTO, conflist, CTLTYPE_OPAQUE | CTLFLAG_RD |
      CTLFLAG_MPSAFE, NULL, 0, sysctl_vfs_conflist,
      "S,xvfsconf", "List of all configured filesystems");
  
  #ifndef BURN_BRIDGES
  static int      sysctl_ovfs_conf(SYSCTL_HANDLER_ARGS);
  
  static int
  vfs_sysctl(SYSCTL_HANDLER_ARGS)
  {
          int *name = (int *)arg1 - 1;    /* XXX */
          u_int namelen = arg2 + 1;       /* XXX */
          struct vfsconf *vfsp;
  
          log(LOG_WARNING, "userland calling deprecated sysctl, "
              "please rebuild world\n");
  
  #if 1 || defined(COMPAT_PRELITE2)
          /* Resolve ambiguity between VFS_VFSCONF and VFS_GENERIC. */
          if (namelen == 1)
                  return (sysctl_ovfs_conf(oidp, arg1, arg2, req));
  #endif
  
          switch (name[1]) {
          case VFS_MAXTYPENUM:
                  if (namelen != 2)
                          return (ENOTDIR);
                  return (SYSCTL_OUT(req, &maxvfsconf, sizeof(int)));
          case VFS_CONF:
                  if (namelen != 3)
                          return (ENOTDIR);       /* overloaded */
                  vfsconf_slock();
                  TAILQ_FOREACH(vfsp, &vfsconf, vfc_list) {
                          if (vfsp->vfc_typenum == name[2])
                                  break;
                  }
                  vfsconf_sunlock();
                  if (vfsp == NULL)
                          return (EOPNOTSUPP);
  #ifdef COMPAT_FREEBSD32
                  if (req->flags & SCTL_MASK32)
                          return (vfsconf2x32(req, vfsp));
                  else
  #endif
                          return (vfsconf2x(req, vfsp));
          }
          return (EOPNOTSUPP);
  }
  
  static SYSCTL_NODE(_vfs, VFS_GENERIC, generic, CTLFLAG_RD | CTLFLAG_SKIP |
      CTLFLAG_MPSAFE, vfs_sysctl,
      "Generic filesystem");
  
  #if 1 || defined(COMPAT_PRELITE2)
  
  static int
  sysctl_ovfs_conf(SYSCTL_HANDLER_ARGS)
  {
          int error;
          struct vfsconf *vfsp;
          struct ovfsconf ovfs;
  
          vfsconf_slock();
          TAILQ_FOREACH(vfsp, &vfsconf, vfc_list) {
                  bzero(&ovfs, sizeof(ovfs));
                  ovfs.vfc_vfsops = vfsp->vfc_vfsops;     /* XXX used as flag */
                  strcpy(ovfs.vfc_name, vfsp->vfc_name);
                  ovfs.vfc_index = vfsp->vfc_typenum;
                  ovfs.vfc_refcount = vfsp->vfc_refcount;
                  ovfs.vfc_flags = vfsp->vfc_flags;
                  error = SYSCTL_OUT(req, &ovfs, sizeof ovfs);
                  if (error != 0) {
                          vfsconf_sunlock();
                          return (error);
                  }
          }
          vfsconf_sunlock();
          return (0);
  }
  
  #endif /* 1 || COMPAT_PRELITE2 */
  #endif /* !BURN_BRIDGES */
  
  #define KINFO_VNODESLOP         10
  #ifdef notyet
  /*
   * Dump vnode list (via sysctl).
   */
  /* ARGSUSED */
  static int
  sysctl_vnode(SYSCTL_HANDLER_ARGS)
  {
          struct xvnode *xvn;
          struct mount *mp;
          struct vnode *vp;
          int error, len, n;
  
          /*
           * Stale numvnodes access is not fatal here.
           */
          req->lock = 0;
          len = (numvnodes + KINFO_VNODESLOP) * sizeof *xvn;
          if (!req->oldptr)
                  /* Make an estimate */
                  return (SYSCTL_OUT(req, 0, len));
  
          error = sysctl_wire_old_buffer(req, 0);
          if (error != 0)
                  return (error);
          xvn = malloc(len, M_TEMP, M_ZERO | M_WAITOK);
          n = 0;
          mtx_lock(&mountlist_mtx);
          TAILQ_FOREACH(mp, &mountlist, mnt_list) {
                  if (vfs_busy(mp, MBF_NOWAIT | MBF_MNTLSTLOCK))
                          continue;
                  MNT_ILOCK(mp);
                  TAILQ_FOREACH(vp, &mp->mnt_nvnodelist, v_nmntvnodes) {
                          if (n == len)
                                  break;
                          vref(vp);
                          xvn[n].xv_size = sizeof *xvn;
                          xvn[n].xv_vnode = vp;
                          xvn[n].xv_id = 0;       /* XXX compat */
  #define XV_COPY(field) xvn[n].xv_##field = vp->v_##field
                          XV_COPY(usecount);
                          XV_COPY(writecount);
                          XV_COPY(holdcnt);
                          XV_COPY(mount);
                          XV_COPY(numoutput);
                          XV_COPY(type);
  #undef XV_COPY
                          xvn[n].xv_flag = vp->v_vflag;
  
                          switch (vp->v_type) {
                          case VREG:
                          case VDIR:
                          case VLNK:
                                  break;
                          case VBLK:
                          case VCHR:
                                  if (vp->v_rdev == NULL) {
                                          vrele(vp);
                                          continue;
                                  }
                                  xvn[n].xv_dev = dev2udev(vp->v_rdev);
                                  break;
                          case VSOCK:
                                  xvn[n].xv_socket = vp->v_socket;
                                  break;
                          case VFIFO:
                                  xvn[n].xv_fifo = vp->v_fifoinfo;
                                  break;
                          case VNON:
                          case VBAD:
                          default:
                                  /* shouldn't happen? */
                                  vrele(vp);
                                  continue;
                          }
                          vrele(vp);
                          ++n;
                  }
                  MNT_IUNLOCK(mp);
                  mtx_lock(&mountlist_mtx);
                  vfs_unbusy(mp);
                  if (n == len)
                          break;
          }
          mtx_unlock(&mountlist_mtx);
  
          error = SYSCTL_OUT(req, xvn, n * sizeof *xvn);
          free(xvn, M_TEMP);
          return (error);
  }
  
  SYSCTL_PROC(_kern, KERN_VNODE, vnode, CTLTYPE_OPAQUE | CTLFLAG_RD |
      CTLFLAG_MPSAFE, 0, 0, sysctl_vnode, "S,xvnode",
      "");
  #endif
  
  static void
  unmount_or_warn(struct mount *mp)
  {
          int error;
  
          error = dounmount(mp, MNT_FORCE, curthread);
          if (error != 0) {
                  printf("unmount of %s failed (", mp->mnt_stat.f_mntonname);
                  if (error == EBUSY)
                          printf("BUSY)\n");
                  else
                          printf("%d)\n", error);
          }
  }
  
  /*
   * Unmount all filesystems. The list is traversed in reverse order
   * of mounting to avoid dependencies.
   */
  void
  vfs_unmountall(void)
  {
          struct mount *mp, *tmp;
  
          CTR1(KTR_VFS, "%s: unmounting all filesystems", __func__);
  
          /*
           * Since this only runs when rebooting, it is not interlocked.
           */
          TAILQ_FOREACH_REVERSE_SAFE(mp, &mountlist, mntlist, mnt_list, tmp) {
                  vfs_ref(mp);
  
                  /*
                   * Forcibly unmounting "/dev" before "/" would prevent clean
                   * unmount of the latter.
                   */
                  if (mp == rootdevmp)
                          continue;
  
                  unmount_or_warn(mp);
          }
  
          if (rootdevmp != NULL)
                  unmount_or_warn(rootdevmp);
  }
  
  /*
   * perform msync on all vnodes under a mount point
   * the mount point must be locked.
   */
  void
  vfs_msync(struct mount *mp, int flags)
  {
          struct vnode *vp, *mvp;
          struct vm_object *obj;
  
          CTR2(KTR_VFS, "%s: mp %p", __func__, mp);
  
          vnlru_return_batch(mp);
  
          MNT_VNODE_FOREACH_ACTIVE(vp, mp, mvp) {
                  obj = vp->v_object;
                  if (obj != NULL && (obj->flags & OBJ_MIGHTBEDIRTY) != 0 &&
                      (flags == MNT_WAIT || VOP_ISLOCKED(vp) == 0)) {
                          if (!vget(vp,
                              LK_EXCLUSIVE | LK_RETRY | LK_INTERLOCK,
                              curthread)) {
                                  if (vp->v_vflag & VV_NOSYNC) {  /* unlinked */
                                          vput(vp);
                                          continue;
                                  }
  
                                  obj = vp->v_object;
                                  if (obj != NULL) {
                                          VM_OBJECT_WLOCK(obj);
                                          vm_object_page_clean(obj, 0, 0,
                                              flags == MNT_WAIT ?
                                              OBJPC_SYNC : OBJPC_NOSYNC);
                                          VM_OBJECT_WUNLOCK(obj);
                                  }
                                  vput(vp);
                          }
                  } else
                          VI_UNLOCK(vp);
          }
  }
  
  static void
  destroy_vpollinfo_free(struct vpollinfo *vi)
  {
  
          knlist_destroy(&vi->vpi_selinfo.si_note);
          mtx_destroy(&vi->vpi_lock);
          uma_zfree(vnodepoll_zone, vi);
  }
  
  static void
  destroy_vpollinfo(struct vpollinfo *vi)
  {
  
          knlist_clear(&vi->vpi_selinfo.si_note, 1);
          seldrain(&vi->vpi_selinfo);
          destroy_vpollinfo_free(vi);
  }
  
  /*
   * Initialize per-vnode helper structure to hold poll-related state.
   */
  void
  v_addpollinfo(struct vnode *vp)
  {
          struct vpollinfo *vi;
  
          if (vp->v_pollinfo != NULL)
                  return;
          vi = uma_zalloc(vnodepoll_zone, M_WAITOK | M_ZERO);
          mtx_init(&vi->vpi_lock, "vnode pollinfo", NULL, MTX_DEF);
          knlist_init(&vi->vpi_selinfo.si_note, vp, vfs_knllock,
              vfs_knlunlock, vfs_knl_assert_locked, vfs_knl_assert_unlocked);
          VI_LOCK(vp);
          if (vp->v_pollinfo != NULL) {
                  VI_UNLOCK(vp);
                  destroy_vpollinfo_free(vi);
                  return;
          }
          vp->v_pollinfo = vi;
          VI_UNLOCK(vp);
  }
  
  /*
   * Record a process's interest in events which might happen to
   * a vnode.  Because poll uses the historic select-style interface
   * internally, this routine serves as both the ``check for any
   * pending events'' and the ``record my interest in future events''
   * functions.  (These are done together, while the lock is held,
   * to avoid race conditions.)
   */
  int
  vn_pollrecord(struct vnode *vp, struct thread *td, int events)
  {
  
          v_addpollinfo(vp);
          mtx_lock(&vp->v_pollinfo->vpi_lock);
          if (vp->v_pollinfo->vpi_revents & events) {
                  /*
                   * This leaves events we are not interested
                   * in available for the other process which
                   * which presumably had requested them
                   * (otherwise they would never have been
                   * recorded).
                   */
                  events &= vp->v_pollinfo->vpi_revents;
                  vp->v_pollinfo->vpi_revents &= ~events;
  
                  mtx_unlock(&vp->v_pollinfo->vpi_lock);
                  return (events);
          }
          vp->v_pollinfo->vpi_events |= events;
          selrecord(td, &vp->v_pollinfo->vpi_selinfo);
          mtx_unlock(&vp->v_pollinfo->vpi_lock);
          return (0);
  }
  
  /*
   * Routine to create and manage a filesystem syncer vnode.
   */
  #define sync_close ((int (*)(struct  vop_close_args *))nullop)
  static int      sync_fsync(struct  vop_fsync_args *);
  static int      sync_inactive(struct  vop_inactive_args *);
  static int      sync_reclaim(struct  vop_reclaim_args *);
  
  static struct vop_vector sync_vnodeops = {
          .vop_bypass =   VOP_EOPNOTSUPP,
          .vop_close =    sync_close,             /* close */
          .vop_fsync =    sync_fsync,             /* fsync */
          .vop_inactive = sync_inactive,  /* inactive */
          .vop_reclaim =  sync_reclaim,   /* reclaim */
          .vop_lock1 =    vop_stdlock,    /* lock */
          .vop_unlock =   vop_stdunlock,  /* unlock */
          .vop_islocked = vop_stdislocked,        /* islocked */
  };
  
  /*
   * Create a new filesystem syncer vnode for the specified mount point.
   */
  void
  vfs_allocate_syncvnode(struct mount *mp)
  {
          struct vnode *vp;
          struct bufobj *bo;
          static long start, incr, next;
          int error;
  
          /* Allocate a new vnode */
          error = getnewvnode("syncer", mp, &sync_vnodeops, &vp);
          if (error != 0)
                  panic("vfs_allocate_syncvnode: getnewvnode() failed");
          vp->v_type = VNON;
          vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
          vp->v_vflag |= VV_FORCEINSMQ;
          error = insmntque(vp, mp);
          if (error != 0)
                  panic("vfs_allocate_syncvnode: insmntque() failed");
          vp->v_vflag &= ~VV_FORCEINSMQ;
          VOP_UNLOCK(vp, 0);
          /*
           * Place the vnode onto the syncer worklist. We attempt to
           * scatter them about on the list so that they will go off
           * at evenly distributed times even if all the filesystems
           * are mounted at once.
           */
          next += incr;
          if (next == 0 || next > syncer_maxdelay) {
                  start /= 2;
                  incr /= 2;
                  if (start == 0) {
                          start = syncer_maxdelay / 2;
                          incr = syncer_maxdelay;
                  }
                  next = start;
          }
          bo = &vp->v_bufobj;
          BO_LOCK(bo);
          vn_syncer_add_to_worklist(bo, syncdelay > 0 ? next % syncdelay : 0);
          /* XXX - vn_syncer_add_to_worklist() also grabs and drops sync_mtx. */
          mtx_lock(&sync_mtx);
          sync_vnode_count++;
          if (mp->mnt_syncer == NULL) {
                  mp->mnt_syncer = vp;
                  vp = NULL;
          }
          mtx_unlock(&sync_mtx);
          BO_UNLOCK(bo);
          if (vp != NULL) {
                  vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
                  vgone(vp);
                  vput(vp);
          }
  }
  
  void
  vfs_deallocate_syncvnode(struct mount *mp)
  {
          struct vnode *vp;
  
          mtx_lock(&sync_mtx);
          vp = mp->mnt_syncer;
          if (vp != NULL)
                  mp->mnt_syncer = NULL;
          mtx_unlock(&sync_mtx);
          if (vp != NULL)
                  vrele(vp);
  }
  
  /*
   * Do a lazy sync of the filesystem.
   */
  static int
  sync_fsync(struct vop_fsync_args *ap)
  {
          struct vnode *syncvp = ap->a_vp;
          struct mount *mp = syncvp->v_mount;
          int error, save;
          struct bufobj *bo;
  
          /*
           * We only need to do something if this is a lazy evaluation.
           */
          if (ap->a_waitfor != MNT_LAZY)
                  return (0);
  
          /*
           * Move ourselves to the back of the sync list.
           */
          bo = &syncvp->v_bufobj;
          BO_LOCK(bo);
          vn_syncer_add_to_worklist(bo, syncdelay);
          BO_UNLOCK(bo);
  
          /*
           * Walk the list of vnodes pushing all that are dirty and
           * not already on the sync list.
           */
          if (vfs_busy(mp, MBF_NOWAIT) != 0)
                  return (0);
          if (vn_start_write(NULL, &mp, V_NOWAIT) != 0) {
                  vfs_unbusy(mp);
                  return (0);
          }
          save = curthread_pflags_set(TDP_SYNCIO);
          vfs_msync(mp, MNT_NOWAIT);
          error = VFS_SYNC(mp, MNT_LAZY);
          curthread_pflags_restore(save);
          vn_finished_write(mp);
          vfs_unbusy(mp);
          return (error);
  }
  
  /*
   * The syncer vnode is no referenced.
   */
  static int
  sync_inactive(struct vop_inactive_args *ap)
  {
  
          vgone(ap->a_vp);
          return (0);
  }
  
  /*
   * The syncer vnode is no longer needed and is being decommissioned.
   *
   * Modifications to the worklist must be protected by sync_mtx.
   */
  static int
  sync_reclaim(struct vop_reclaim_args *ap)
  {
          struct vnode *vp = ap->a_vp;
          struct bufobj *bo;
  
          bo = &vp->v_bufobj;
          BO_LOCK(bo);
          mtx_lock(&sync_mtx);
          if (vp->v_mount->mnt_syncer == vp)
                  vp->v_mount->mnt_syncer = NULL;
          if (bo->bo_flag & BO_ONWORKLST) {
                  LIST_REMOVE(bo, bo_synclist);
                  syncer_worklist_len--;
                  sync_vnode_count--;
                  bo->bo_flag &= ~BO_ONWORKLST;
          }
          mtx_unlock(&sync_mtx);
          BO_UNLOCK(bo);
  
          return (0);
  }
  
  /*
   * Check if vnode represents a disk device
   */
  int
  vn_isdisk(struct vnode *vp, int *errp)
  {
          int error;
  
          if (vp->v_type != VCHR) {
                  error = ENOTBLK;
                  goto out;
          }
          error = 0;
          dev_lock();
          if (vp->v_rdev == NULL)
                  error = ENXIO;
          else if (vp->v_rdev->si_devsw == NULL)
                  error = ENXIO;
          else if (!(vp->v_rdev->si_devsw->d_flags & D_DISK))
                  error = ENOTBLK;
          dev_unlock();
  out:
          if (errp != NULL)
                  *errp = error;
          return (error == 0);
  }
  
  /*
   * Common filesystem object access control check routine.  Accepts a
   * vnode's type, "mode", uid and gid, requested access mode, credentials,
   * and optional call-by-reference privused argument allowing vaccess()
   * to indicate to the caller whether privilege was used to satisfy the
   * request (obsoleted).  Returns 0 on success, or an errno on failure.
   */
  int
  vaccess(enum vtype type, mode_t file_mode, uid_t file_uid, gid_t file_gid,
      accmode_t accmode, struct ucred *cred, int *privused)
  {
          accmode_t dac_granted;
          accmode_t priv_granted;
  
          KASSERT((accmode & ~(VEXEC | VWRITE | VREAD | VADMIN | VAPPEND)) == 0,
              ("invalid bit in accmode"));
          KASSERT((accmode & VAPPEND) == 0 || (accmode & VWRITE),
              ("VAPPEND without VWRITE"));
  
          /*
           * Look for a normal, non-privileged way to access the file/directory
           * as requested.  If it exists, go with that.
           */
  
          if (privused != NULL)
                  *privused = 0;
  
          dac_granted = 0;
  
          /* Check the owner. */
          if (cred->cr_uid == file_uid) {
                  dac_granted |= VADMIN;
                  if (file_mode & S_IXUSR)
                          dac_granted |= VEXEC;
                  if (file_mode & S_IRUSR)
                          dac_granted |= VREAD;
                  if (file_mode & S_IWUSR)
                          dac_granted |= (VWRITE | VAPPEND);
  
                  if ((accmode & dac_granted) == accmode)
                          return (0);
  
                  goto privcheck;
          }
  
          /* Otherwise, check the groups (first match) */
          if (groupmember(file_gid, cred)) {
                  if (file_mode & S_IXGRP)
                          dac_granted |= VEXEC;
                  if (file_mode & S_IRGRP)
                          dac_granted |= VREAD;
                  if (file_mode & S_IWGRP)
                          dac_granted |= (VWRITE | VAPPEND);
  
                  if ((accmode & dac_granted) == accmode)
                          return (0);
  
                  goto privcheck;
          }
  
          /* Otherwise, check everyone else. */
          if (file_mode & S_IXOTH)
                  dac_granted |= VEXEC;
          if (file_mode & S_IROTH)
                  dac_granted |= VREAD;
          if (file_mode & S_IWOTH)
                  dac_granted |= (VWRITE | VAPPEND);
          if ((accmode & dac_granted) == accmode)
                  return (0);
  
  privcheck:
          /*
           * Build a privilege mask to determine if the set of privileges
           * satisfies the requirements when combined with the granted mask
           * from above.  For each privilege, if the privilege is required,
           * bitwise or the request type onto the priv_granted mask.
           */
          priv_granted = 0;
  
          if (type == VDIR) {
                  /*
                   * For directories, use PRIV_VFS_LOOKUP to satisfy VEXEC
                   * requests, instead of PRIV_VFS_EXEC.
                   */
                  if ((accmode & VEXEC) && ((dac_granted & VEXEC) == 0) &&
                      !priv_check_cred(cred, PRIV_VFS_LOOKUP, 0))
                          priv_granted |= VEXEC;
          } else {
                  /*
                   * Ensure that at least one execute bit is on. Otherwise,
                   * a privileged user will always succeed, and we don't want
                   * this to happen unless the file really is executable.
                   */
                  if ((accmode & VEXEC) && ((dac_granted & VEXEC) == 0) &&
                      (file_mode & (S_IXUSR | S_IXGRP | S_IXOTH)) != 0 &&
                      !priv_check_cred(cred, PRIV_VFS_EXEC, 0))
                          priv_granted |= VEXEC;
          }
  
          if ((accmode & VREAD) && ((dac_granted & VREAD) == 0) &&
              !priv_check_cred(cred, PRIV_VFS_READ, 0))
                  priv_granted |= VREAD;
  
          if ((accmode & VWRITE) && ((dac_granted & VWRITE) == 0) &&
              !priv_check_cred(cred, PRIV_VFS_WRITE, 0))
                  priv_granted |= (VWRITE | VAPPEND);
  
          if ((accmode & VADMIN) && ((dac_granted & VADMIN) == 0) &&
              !priv_check_cred(cred, PRIV_VFS_ADMIN, 0))
                  priv_granted |= VADMIN;
  
          if ((accmode & (priv_granted | dac_granted)) == accmode) {
                  /* XXX audit: privilege used */
                  if (privused != NULL)
                          *privused = 1;
                  return (0);
          }
  
          return ((accmode & VADMIN) ? EPERM : EACCES);
  }
  
  /*
   * Credential check based on process requesting service, and per-attribute
   * permissions.
   */
  int
  extattr_check_cred(struct vnode *vp, int attrnamespace, struct ucred *cred,
      struct thread *td, accmode_t accmode)
  {
  
          /*
           * Kernel-invoked always succeeds.
           */
          if (cred == NOCRED)
                  return (0);
  
          /*
           * Do not allow privileged processes in jail to directly manipulate
           * system attributes.
           */
          switch (attrnamespace) {
          case EXTATTR_NAMESPACE_SYSTEM:
                  /* Potentially should be: return (EPERM); */
                  return (priv_check_cred(cred, PRIV_VFS_EXTATTR_SYSTEM, 0));
          case EXTATTR_NAMESPACE_USER:
                  return (VOP_ACCESS(vp, accmode, cred, td));
          default:
                  return (EPERM);
          }
  }
  
  #ifdef DEBUG_VFS_LOCKS
  /*
   * This only exists to suppress warnings from unlocked specfs accesses.  It is
   * no longer ok to have an unlocked VFS.
   */
  #define IGNORE_LOCK(vp) (panicstr != NULL || (vp) == NULL ||            \
          (vp)->v_type == VCHR || (vp)->v_type == VBAD)
  
  int vfs_badlock_ddb = 1;        /* Drop into debugger on violation. */
  SYSCTL_INT(_debug, OID_AUTO, vfs_badlock_ddb, CTLFLAG_RW, &vfs_badlock_ddb, 0,
      "Drop into debugger on lock violation");
  
  int vfs_badlock_mutex = 1;      /* Check for interlock across VOPs. */
  SYSCTL_INT(_debug, OID_AUTO, vfs_badlock_mutex, CTLFLAG_RW, &vfs_badlock_mutex,
      0, "Check for interlock across VOPs");
  
  int vfs_badlock_print = 1;      /* Print lock violations. */
  SYSCTL_INT(_debug, OID_AUTO, vfs_badlock_print, CTLFLAG_RW, &vfs_badlock_print,
      0, "Print lock violations");
  
  int vfs_badlock_vnode = 1;      /* Print vnode details on lock violations. */
  SYSCTL_INT(_debug, OID_AUTO, vfs_badlock_vnode, CTLFLAG_RW, &vfs_badlock_vnode,
      0, "Print vnode details on lock violations");
  
  #ifdef KDB
  int vfs_badlock_backtrace = 1;  /* Print backtrace at lock violations. */
  SYSCTL_INT(_debug, OID_AUTO, vfs_badlock_backtrace, CTLFLAG_RW,
      &vfs_badlock_backtrace, 0, "Print backtrace at lock violations");
  #endif
  
  static void
  vfs_badlock(const char *msg, const char *str, struct vnode *vp)
  {
  
  #ifdef KDB
          if (vfs_badlock_backtrace)
                  kdb_backtrace();
  #endif
          if (vfs_badlock_vnode)
                  vn_printf(vp, "vnode ");
          if (vfs_badlock_print)
                  printf("%s: %p %s\n", str, (void *)vp, msg);
          if (vfs_badlock_ddb)
                  kdb_enter(KDB_WHY_VFSLOCK, "lock violation");
  }
  
  void
  assert_vi_locked(struct vnode *vp, const char *str)
  {
  
          if (vfs_badlock_mutex && !mtx_owned(VI_MTX(vp)))
                  vfs_badlock("interlock is not locked but should be", str, vp);
  }
  
  void
  assert_vi_unlocked(struct vnode *vp, const char *str)
  {
  
          if (vfs_badlock_mutex && mtx_owned(VI_MTX(vp)))
                  vfs_badlock("interlock is locked but should not be", str, vp);
  }
  
  void
  assert_vop_locked(struct vnode *vp, const char *str)
  {
          int locked;
  
          if (!IGNORE_LOCK(vp)) {
                  locked = VOP_ISLOCKED(vp);
                  if (locked == 0 || locked == LK_EXCLOTHER)
                          vfs_badlock("is not locked but should be", str, vp);
          }
  }
  
  void
  assert_vop_unlocked(struct vnode *vp, const char *str)
  {
  
          if (!IGNORE_LOCK(vp) && VOP_ISLOCKED(vp) == LK_EXCLUSIVE)
                  vfs_badlock("is locked but should not be", str, vp);
  }
  
  void
  assert_vop_elocked(struct vnode *vp, const char *str)
  {
  
          if (!IGNORE_LOCK(vp) && VOP_ISLOCKED(vp) != LK_EXCLUSIVE)
                  vfs_badlock("is not exclusive locked but should be", str, vp);
  }
  #endif /* DEBUG_VFS_LOCKS */
  
  void
  vop_rename_fail(struct vop_rename_args *ap)
  {
  
          if (ap->a_tvp != NULL)
                  vput(ap->a_tvp);
          if (ap->a_tdvp == ap->a_tvp)
                  vrele(ap->a_tdvp);
          else
                  vput(ap->a_tdvp);
          vrele(ap->a_fdvp);
          vrele(ap->a_fvp);
  }
  
  void
  vop_rename_pre(void *ap)
  {
          struct vop_rename_args *a = ap;
  
  #ifdef DEBUG_VFS_LOCKS
          if (a->a_tvp)
                  ASSERT_VI_UNLOCKED(a->a_tvp, "VOP_RENAME");
          ASSERT_VI_UNLOCKED(a->a_tdvp, "VOP_RENAME");
          ASSERT_VI_UNLOCKED(a->a_fvp, "VOP_RENAME");
          ASSERT_VI_UNLOCKED(a->a_fdvp, "VOP_RENAME");
  
          /* Check the source (from). */
          if (a->a_tdvp->v_vnlock != a->a_fdvp->v_vnlock &&
              (a->a_tvp == NULL || a->a_tvp->v_vnlock != a->a_fdvp->v_vnlock))
                  ASSERT_VOP_UNLOCKED(a->a_fdvp, "vop_rename: fdvp locked");
          if (a->a_tvp == NULL || a->a_tvp->v_vnlock != a->a_fvp->v_vnlock)
                  ASSERT_VOP_UNLOCKED(a->a_fvp, "vop_rename: fvp locked");
  
          /* Check the target. */
          if (a->a_tvp)
                  ASSERT_VOP_LOCKED(a->a_tvp, "vop_rename: tvp not locked");
          ASSERT_VOP_LOCKED(a->a_tdvp, "vop_rename: tdvp not locked");
  #endif
          if (a->a_tdvp != a->a_fdvp)
                  vhold(a->a_fdvp);
          if (a->a_tvp != a->a_fvp)
                  vhold(a->a_fvp);
          vhold(a->a_tdvp);
          if (a->a_tvp)
                  vhold(a->a_tvp);
  }
  
  #ifdef DEBUG_VFS_LOCKS
  void
  vop_strategy_pre(void *ap)
  {
          struct vop_strategy_args *a;
          struct buf *bp;
  
          a = ap;
          bp = a->a_bp;
  
          /*
           * Cluster ops lock their component buffers but not the IO container.
           */
          if ((bp->b_flags & B_CLUSTER) != 0)
                  return;
  
          if (panicstr == NULL && !BUF_ISLOCKED(bp)) {
                  if (vfs_badlock_print)
                          printf(
                              "VOP_STRATEGY: bp is not locked but should be\n");
                  if (vfs_badlock_ddb)
                          kdb_enter(KDB_WHY_VFSLOCK, "lock violation");
          }
  }
  
  void
  vop_lock_pre(void *ap)
  {
          struct vop_lock1_args *a = ap;
  
          if ((a->a_flags & LK_INTERLOCK) == 0)
                  ASSERT_VI_UNLOCKED(a->a_vp, "VOP_LOCK");
          else
                  ASSERT_VI_LOCKED(a->a_vp, "VOP_LOCK");
  }
  
  void
  vop_lock_post(void *ap, int rc)
  {
          struct vop_lock1_args *a = ap;
  
          ASSERT_VI_UNLOCKED(a->a_vp, "VOP_LOCK");
          if (rc == 0 && (a->a_flags & LK_EXCLOTHER) == 0)
                  ASSERT_VOP_LOCKED(a->a_vp, "VOP_LOCK");
  }
  
  void
  vop_unlock_pre(void *ap)
  {
          struct vop_unlock_args *a = ap;
  
          if (a->a_flags & LK_INTERLOCK)
                  ASSERT_VI_LOCKED(a->a_vp, "VOP_UNLOCK");
          ASSERT_VOP_LOCKED(a->a_vp, "VOP_UNLOCK");
  }
  
  void
  vop_unlock_post(void *ap, int rc)
  {
          struct vop_unlock_args *a = ap;
  
          if (a->a_flags & LK_INTERLOCK)
                  ASSERT_VI_UNLOCKED(a->a_vp, "VOP_UNLOCK");
  }
  #endif
  
  void
  vop_create_post(void *ap, int rc)
  {
          struct vop_create_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_dvp, NOTE_WRITE);
  }
  
  void
  vop_deleteextattr_post(void *ap, int rc)
  {
          struct vop_deleteextattr_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_ATTRIB);
  }
  
  void
  vop_link_post(void *ap, int rc)
  {
          struct vop_link_args *a = ap;
  
          if (!rc) {
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_LINK);
                  VFS_KNOTE_LOCKED(a->a_tdvp, NOTE_WRITE);
          }
  }
  
  void
  vop_mkdir_post(void *ap, int rc)
  {
          struct vop_mkdir_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_dvp, NOTE_WRITE | NOTE_LINK);
  }
  
  void
  vop_mknod_post(void *ap, int rc)
  {
          struct vop_mknod_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_dvp, NOTE_WRITE);
  }
  
  void
  vop_reclaim_post(void *ap, int rc)
  {
          struct vop_reclaim_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_REVOKE);
  }
  
  void
  vop_remove_post(void *ap, int rc)
  {
          struct vop_remove_args *a = ap;
  
          if (!rc) {
                  VFS_KNOTE_LOCKED(a->a_dvp, NOTE_WRITE);
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_DELETE);
          }
  }
  
  void
  vop_rename_post(void *ap, int rc)
  {
          struct vop_rename_args *a = ap;
          long hint;
  
          if (!rc) {
                  hint = NOTE_WRITE;
                  if (a->a_fdvp == a->a_tdvp) {
                          if (a->a_tvp != NULL && a->a_tvp->v_type == VDIR)
                                  hint |= NOTE_LINK;
                          VFS_KNOTE_UNLOCKED(a->a_fdvp, hint);
                          VFS_KNOTE_UNLOCKED(a->a_tdvp, hint);
                  } else {
                          hint |= NOTE_EXTEND;
                          if (a->a_fvp->v_type == VDIR)
                                  hint |= NOTE_LINK;
                          VFS_KNOTE_UNLOCKED(a->a_fdvp, hint);
  
                          if (a->a_fvp->v_type == VDIR && a->a_tvp != NULL &&
                              a->a_tvp->v_type == VDIR)
                                  hint &= ~NOTE_LINK;
                          VFS_KNOTE_UNLOCKED(a->a_tdvp, hint);
                  }
  
                  VFS_KNOTE_UNLOCKED(a->a_fvp, NOTE_RENAME);
                  if (a->a_tvp)
                          VFS_KNOTE_UNLOCKED(a->a_tvp, NOTE_DELETE);
          }
          if (a->a_tdvp != a->a_fdvp)
                  vdrop(a->a_fdvp);
          if (a->a_tvp != a->a_fvp)
                  vdrop(a->a_fvp);
          vdrop(a->a_tdvp);
          if (a->a_tvp)
                  vdrop(a->a_tvp);
  }
  
  void
  vop_rmdir_post(void *ap, int rc)
  {
          struct vop_rmdir_args *a = ap;
  
          if (!rc) {
                  VFS_KNOTE_LOCKED(a->a_dvp, NOTE_WRITE | NOTE_LINK);
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_DELETE);
          }
  }
  
  void
  vop_setattr_post(void *ap, int rc)
  {
          struct vop_setattr_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_ATTRIB);
  }
  
  void
  vop_setextattr_post(void *ap, int rc)
  {
          struct vop_setextattr_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_ATTRIB);
  }
  
  void
  vop_symlink_post(void *ap, int rc)
  {
          struct vop_symlink_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_dvp, NOTE_WRITE);
  }
  
  void
  vop_open_post(void *ap, int rc)
  {
          struct vop_open_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_OPEN);
  }
  
  void
  vop_close_post(void *ap, int rc)
  {
          struct vop_close_args *a = ap;
  
          if (!rc && (a->a_cred != NOCRED || /* filter out revokes */
              (a->a_vp->v_iflag & VI_DOOMED) == 0)) {
                  VFS_KNOTE_LOCKED(a->a_vp, (a->a_fflag & FWRITE) != 0 ?
                      NOTE_CLOSE_WRITE : NOTE_CLOSE);
          }
  }
  
  void
  vop_read_post(void *ap, int rc)
  {
          struct vop_read_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_READ);
  }
  
  void
  vop_readdir_post(void *ap, int rc)
  {
          struct vop_readdir_args *a = ap;
  
          if (!rc)
                  VFS_KNOTE_LOCKED(a->a_vp, NOTE_READ);
  }
  
  static struct knlist fs_knlist;
  
  static void
  vfs_event_init(void *arg)
  {
          knlist_init_mtx(&fs_knlist, NULL);
  }
  /* XXX - correct order? */
  SYSINIT(vfs_knlist, SI_SUB_VFS, SI_ORDER_ANY, vfs_event_init, NULL);
  
  void
  vfs_event_signal(fsid_t *fsid, uint32_t event, intptr_t data __unused)
  {
  
          KNOTE_UNLOCKED(&fs_knlist, event);
  }
  
  static int      filt_fsattach(struct knote *kn);
  static void     filt_fsdetach(struct knote *kn);
  static int      filt_fsevent(struct knote *kn, long hint);
  
  struct filterops fs_filtops = {
          .f_isfd = 0,
          .f_attach = filt_fsattach,
          .f_detach = filt_fsdetach,
          .f_event = filt_fsevent
  };
  
  static int
  filt_fsattach(struct knote *kn)
  {
  
          kn->kn_flags |= EV_CLEAR;
          knlist_add(&fs_knlist, kn, 0);
          return (0);
  }
  
  static void
  filt_fsdetach(struct knote *kn)
  {
  
          knlist_remove(&fs_knlist, kn, 0);
  }
  
  static int
  filt_fsevent(struct knote *kn, long hint)
  {
  
          kn->kn_fflags |= hint;
          return (kn->kn_fflags != 0);
  }
  
  static int
  sysctl_vfs_ctl(SYSCTL_HANDLER_ARGS)
  {
          struct vfsidctl vc;
          int error;
          struct mount *mp;
  
          error = SYSCTL_IN(req, &vc, sizeof(vc));
          if (error)
                  return (error);
          if (vc.vc_vers != VFS_CTL_VERS1)
                  return (EINVAL);
          mp = vfs_getvfs(&vc.vc_fsid);
          if (mp == NULL)
                  return (ENOENT);
          /* ensure that a specific sysctl goes to the right filesystem. */
          if (strcmp(vc.vc_fstypename, "*") != 0 &&
              strcmp(vc.vc_fstypename, mp->mnt_vfc->vfc_name) != 0) {
                  vfs_rel(mp);
                  return (EINVAL);
          }
          VCTLTOREQ(&vc, req);
          error = VFS_SYSCTL(mp, vc.vc_op, req);
          vfs_rel(mp);
          return (error);
  }
  
  SYSCTL_PROC(_vfs, OID_AUTO, ctl, CTLTYPE_OPAQUE | CTLFLAG_WR,
      NULL, 0, sysctl_vfs_ctl, "",
      "Sysctl by fsid");
  
  /*
   * Function to initialize a va_filerev field sensibly.
   * XXX: Wouldn't a random number make a lot more sense ??
   */
  u_quad_t
  init_va_filerev(void)
  {
          struct bintime bt;
  
          getbinuptime(&bt);
          return (((u_quad_t)bt.sec << 32LL) | (bt.frac >> 32LL));
  }
  
  static int      filt_vfsread(struct knote *kn, long hint);
  static int      filt_vfswrite(struct knote *kn, long hint);
  static int      filt_vfsvnode(struct knote *kn, long hint);
  static void     filt_vfsdetach(struct knote *kn);
  static struct filterops vfsread_filtops = {
          .f_isfd = 1,
          .f_detach = filt_vfsdetach,
          .f_event = filt_vfsread
  };
  static struct filterops vfswrite_filtops = {
          .f_isfd = 1,
          .f_detach = filt_vfsdetach,
          .f_event = filt_vfswrite
  };
  static struct filterops vfsvnode_filtops = {
          .f_isfd = 1,
          .f_detach = filt_vfsdetach,
          .f_event = filt_vfsvnode
  };
  
  static void
  vfs_knllock(void *arg)
  {
          struct vnode *vp = arg;
  
          vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
  }
  
  static void
  vfs_knlunlock(void *arg)
  {
          struct vnode *vp = arg;
  
          VOP_UNLOCK(vp, 0);
  }
  
  static void
  vfs_knl_assert_locked(void *arg)
  {
  #ifdef DEBUG_VFS_LOCKS
          struct vnode *vp = arg;
  
          ASSERT_VOP_LOCKED(vp, "vfs_knl_assert_locked");
  #endif
  }
  
  static void
  vfs_knl_assert_unlocked(void *arg)
  {
  #ifdef DEBUG_VFS_LOCKS
          struct vnode *vp = arg;
  
          ASSERT_VOP_UNLOCKED(vp, "vfs_knl_assert_unlocked");
  #endif
  }
  
  int
  vfs_kqfilter(struct vop_kqfilter_args *ap)
  {
          struct vnode *vp = ap->a_vp;
          struct knote *kn = ap->a_kn;
          struct knlist *knl;
  
          switch (kn->kn_filter) {
          case EVFILT_READ:
                  kn->kn_fop = &vfsread_filtops;
                  break;
          case EVFILT_WRITE:
                  kn->kn_fop = &vfswrite_filtops;
                  break;
          case EVFILT_VNODE:
                  kn->kn_fop = &vfsvnode_filtops;
                  break;
          default:
                  return (EINVAL);
          }
  
          kn->kn_hook = (caddr_t)vp;
  
          v_addpollinfo(vp);
          if (vp->v_pollinfo == NULL)
                  return (ENOMEM);
          knl = &vp->v_pollinfo->vpi_selinfo.si_note;
          vhold(vp);
          knlist_add(knl, kn, 0);
  
          return (0);
  }
  
  /*
   * Detach knote from vnode
   */
  static void
  filt_vfsdetach(struct knote *kn)
  {
          struct vnode *vp = (struct vnode *)kn->kn_hook;
  
          KASSERT(vp->v_pollinfo != NULL, ("Missing v_pollinfo"));
          knlist_remove(&vp->v_pollinfo->vpi_selinfo.si_note, kn, 0);
          vdrop(vp);
  }
  
  /*ARGSUSED*/
  static int
  filt_vfsread(struct knote *kn, long hint)
  {
          struct vnode *vp = (struct vnode *)kn->kn_hook;
          struct vattr va;
          int res;
  
          /*
           * filesystem is gone, so set the EOF flag and schedule
           * the knote for deletion.
           */
          if (hint == NOTE_REVOKE || (hint == 0 && vp->v_type == VBAD)) {
                  VI_LOCK(vp);
                  kn->kn_flags |= (EV_EOF | EV_ONESHOT);
                  VI_UNLOCK(vp);
                  return (1);
          }
  
          if (VOP_GETATTR(vp, &va, curthread->td_ucred))
                  return (0);
  
          VI_LOCK(vp);
          kn->kn_data = va.va_size - kn->kn_fp->f_offset;
          res = (kn->kn_sfflags & NOTE_FILE_POLL) != 0 || kn->kn_data != 0;
          VI_UNLOCK(vp);
          return (res);
  }
  
  /*ARGSUSED*/
  static int
  filt_vfswrite(struct knote *kn, long hint)
  {
          struct vnode *vp = (struct vnode *)kn->kn_hook;
  
          VI_LOCK(vp);
  
          /*
           * filesystem is gone, so set the EOF flag and schedule
           * the knote for deletion.
           */
          if (hint == NOTE_REVOKE || (hint == 0 && vp->v_type == VBAD))
                  kn->kn_flags |= (EV_EOF | EV_ONESHOT);
  
          kn->kn_data = 0;
          VI_UNLOCK(vp);
          return (1);
  }
  
  static int
  filt_vfsvnode(struct knote *kn, long hint)
  {
          struct vnode *vp = (struct vnode *)kn->kn_hook;
          int res;
  
          VI_LOCK(vp);
          if (kn->kn_sfflags & hint)
                  kn->kn_fflags |= hint;
          if (hint == NOTE_REVOKE || (hint == 0 && vp->v_type == VBAD)) {
                  kn->kn_flags |= EV_EOF;
                  VI_UNLOCK(vp);
                  return (1);
          }
          res = (kn->kn_fflags != 0);
          VI_UNLOCK(vp);
          return (res);
  }
  
  int
  vfs_read_dirent(struct vop_readdir_args *ap, struct dirent *dp, off_t off)
  {
          int error;
  
          if (dp->d_reclen > ap->a_uio->uio_resid)
                  return (ENAMETOOLONG);
          error = uiomove(dp, dp->d_reclen, ap->a_uio);
          if (error) {
                  if (ap->a_ncookies != NULL) {
                          if (ap->a_cookies != NULL)
                                  free(ap->a_cookies, M_TEMP);
                          ap->a_cookies = NULL;
                          *ap->a_ncookies = 0;
                  }
                  return (error);
          }
          if (ap->a_ncookies == NULL)
                  return (0);
  
          KASSERT(ap->a_cookies,
              ("NULL ap->a_cookies value with non-NULL ap->a_ncookies!"));
  
          *ap->a_cookies = realloc(*ap->a_cookies,
              (*ap->a_ncookies + 1) * sizeof(u_long), M_TEMP, M_WAITOK | M_ZERO);
          (*ap->a_cookies)[*ap->a_ncookies] = off;
          *ap->a_ncookies += 1;
          return (0);
  }
  
  /*
   * Mark for update the access time of the file if the filesystem
   * supports VOP_MARKATIME.  This functionality is used by execve and
   * mmap, so we want to avoid the I/O implied by directly setting
   * va_atime for the sake of efficiency.
   */
  void
  vfs_mark_atime(struct vnode *vp, struct ucred *cred)
  {
          struct mount *mp;
  
          mp = vp->v_mount;
          ASSERT_VOP_LOCKED(vp, "vfs_mark_atime");
          if (mp != NULL && (mp->mnt_flag & (MNT_NOATIME | MNT_RDONLY)) == 0)
                  (void)VOP_MARKATIME(vp);
  }
  
  /*
   * The purpose of this routine is to remove granularity from accmode_t,
   * reducing it into standard unix access bits - VEXEC, VREAD, VWRITE,
   * VADMIN and VAPPEND.
   *
   * If it returns 0, the caller is supposed to continue with the usual
   * access checks using 'accmode' as modified by this routine.  If it
   * returns nonzero value, the caller is supposed to return that value
   * as errno.
   *
   * Note that after this routine runs, accmode may be zero.
   */
  int
  vfs_unixify_accmode(accmode_t *accmode)
  {
          /*
           * There is no way to specify explicit "deny" rule using
           * file mode or POSIX.1e ACLs.
           */
          if (*accmode & VEXPLICIT_DENY) {
                  *accmode = 0;
                  return (0);
          }
  
          /*
           * None of these can be translated into usual access bits.
           * Also, the common case for NFSv4 ACLs is to not contain
           * either of these bits. Caller should check for VWRITE
           * on the containing directory instead.
           */
          if (*accmode & (VDELETE_CHILD | VDELETE))
                  return (EPERM);
  
          if (*accmode & VADMIN_PERMS) {
                  *accmode &= ~VADMIN_PERMS;
                  *accmode |= VADMIN;
          }
  
          /*
           * There is no way to deny VREAD_ATTRIBUTES, VREAD_ACL
           * or VSYNCHRONIZE using file mode or POSIX.1e ACL.
           */
          *accmode &= ~(VSTAT_PERMS | VSYNCHRONIZE);
  
          return (0);
  }
  
  /*
   * These are helper functions for filesystems to traverse all
   * their vnodes.  See MNT_VNODE_FOREACH_ALL() in sys/mount.h.
   *
   * This interface replaces MNT_VNODE_FOREACH.
   */
  
  MALLOC_DEFINE(M_VNODE_MARKER, "vnodemarker", "vnode marker");
  
  struct vnode *
  __mnt_vnode_next_all(struct vnode **mvp, struct mount *mp)
  {
          struct vnode *vp;
  
          if (should_yield())
                  kern_yield(PRI_USER);
          MNT_ILOCK(mp);
          KASSERT((*mvp)->v_mount == mp, ("marker vnode mount list mismatch"));
          for (vp = TAILQ_NEXT(*mvp, v_nmntvnodes); vp != NULL;
              vp = TAILQ_NEXT(vp, v_nmntvnodes)) {
                  /* Allow a racy peek at VI_DOOMED to save a lock acquisition. */
                  if (vp->v_type == VMARKER || (vp->v_iflag & VI_DOOMED) != 0)
                          continue;
                  VI_LOCK(vp);
                  if ((vp->v_iflag & VI_DOOMED) != 0) {
                          VI_UNLOCK(vp);
                          continue;
                  }
                  break;
          }
          if (vp == NULL) {
                  __mnt_vnode_markerfree_all(mvp, mp);
                  /* MNT_IUNLOCK(mp); -- done in above function */
                  mtx_assert(MNT_MTX(mp), MA_NOTOWNED);
                  return (NULL);
          }
          TAILQ_REMOVE(&mp->mnt_nvnodelist, *mvp, v_nmntvnodes);
          TAILQ_INSERT_AFTER(&mp->mnt_nvnodelist, vp, *mvp, v_nmntvnodes);
          MNT_IUNLOCK(mp);
          return (vp);
  }
  
  struct vnode *
  __mnt_vnode_first_all(struct vnode **mvp, struct mount *mp)
  {
          struct vnode *vp;
  
          *mvp = malloc(sizeof(struct vnode), M_VNODE_MARKER, M_WAITOK | M_ZERO);
          MNT_ILOCK(mp);
          MNT_REF(mp);
          (*mvp)->v_mount = mp;
          (*mvp)->v_type = VMARKER;
  
          TAILQ_FOREACH(vp, &mp->mnt_nvnodelist, v_nmntvnodes) {
                  /* Allow a racy peek at VI_DOOMED to save a lock acquisition. */
                  if (vp->v_type == VMARKER || (vp->v_iflag & VI_DOOMED) != 0)
                          continue;
                  VI_LOCK(vp);
                  if ((vp->v_iflag & VI_DOOMED) != 0) {
                          VI_UNLOCK(vp);
                          continue;
                  }
                  break;
          }
          if (vp == NULL) {
                  MNT_REL(mp);
                  MNT_IUNLOCK(mp);
                  free(*mvp, M_VNODE_MARKER);
                  *mvp = NULL;
                  return (NULL);
          }
          TAILQ_INSERT_AFTER(&mp->mnt_nvnodelist, vp, *mvp, v_nmntvnodes);
          MNT_IUNLOCK(mp);
          return (vp);
  }
  
  void
  __mnt_vnode_markerfree_all(struct vnode **mvp, struct mount *mp)
  {
  
          if (*mvp == NULL) {
                  MNT_IUNLOCK(mp);
                  return;
          }
  
          mtx_assert(MNT_MTX(mp), MA_OWNED);
  
          KASSERT((*mvp)->v_mount == mp, ("marker vnode mount list mismatch"));
          TAILQ_REMOVE(&mp->mnt_nvnodelist, *mvp, v_nmntvnodes);
          MNT_REL(mp);
          MNT_IUNLOCK(mp);
          free(*mvp, M_VNODE_MARKER);
          *mvp = NULL;
  }
  
  /*
   * These are helper functions for filesystems to traverse their
   * active vnodes.  See MNT_VNODE_FOREACH_ACTIVE() in sys/mount.h
   */
  static void
  mnt_vnode_markerfree_active(struct vnode **mvp, struct mount *mp)
  {
  
          KASSERT((*mvp)->v_mount == mp, ("marker vnode mount list mismatch"));
  
          MNT_ILOCK(mp);
          MNT_REL(mp);
          MNT_IUNLOCK(mp);
          free(*mvp, M_VNODE_MARKER);
          *mvp = NULL;
  }
  
  /*
   * Relock the mp mount vnode list lock with the vp vnode interlock in the
   * conventional lock order during mnt_vnode_next_active iteration.
   *
   * On entry, the mount vnode list lock is held and the vnode interlock is not.
   * The list lock is dropped and reacquired.  On success, both locks are held.
   * On failure, the mount vnode list lock is held but the vnode interlock is
   * not, and the procedure may have yielded.
   */
  static bool
  mnt_vnode_next_active_relock(struct vnode *mvp, struct mount *mp,
      struct vnode *vp)
  {
          const struct vnode *tmp;
          bool held, ret;
  
          VNASSERT(mvp->v_mount == mp && mvp->v_type == VMARKER &&
              TAILQ_NEXT(mvp, v_actfreelist) != NULL, mvp,
              ("%s: bad marker", __func__));
          VNASSERT(vp->v_mount == mp && vp->v_type != VMARKER, vp,
              ("%s: inappropriate vnode", __func__));
          ASSERT_VI_UNLOCKED(vp, __func__);
          mtx_assert(&mp->mnt_listmtx, MA_OWNED);
  
          ret = false;
  
          TAILQ_REMOVE(&mp->mnt_activevnodelist, mvp, v_actfreelist);
          TAILQ_INSERT_BEFORE(vp, mvp, v_actfreelist);
  
          /*
           * Use a hold to prevent vp from disappearing while the mount vnode
           * list lock is dropped and reacquired.  Normally a hold would be
           * acquired with vhold(), but that might try to acquire the vnode
           * interlock, which would be a LOR with the mount vnode list lock.
           */
          held = refcount_acquire_if_not_zero(&vp->v_holdcnt);
          mtx_unlock(&mp->mnt_listmtx);
          if (!held)
                  goto abort;
          VI_LOCK(vp);
          if (!refcount_release_if_not_last(&vp->v_holdcnt)) {
                  vdropl(vp);
                  goto abort;
          }
          mtx_lock(&mp->mnt_listmtx);
  
          /*
           * Determine whether the vnode is still the next one after the marker,
           * excepting any other markers.  If the vnode has not been doomed by
           * vgone() then the hold should have ensured that it remained on the
           * active list.  If it has been doomed but is still on the active list,
           * don't abort, but rather skip over it (avoid spinning on doomed
           * vnodes).
           */
          tmp = mvp;
          do {
                  tmp = TAILQ_NEXT(tmp, v_actfreelist);
          } while (tmp != NULL && tmp->v_type == VMARKER);
          if (tmp != vp) {
                  mtx_unlock(&mp->mnt_listmtx);
                  VI_UNLOCK(vp);
                  goto abort;
          }
  
          ret = true;
          goto out;
  abort:
          maybe_yield();
          mtx_lock(&mp->mnt_listmtx);
  out:
          if (ret)
                  ASSERT_VI_LOCKED(vp, __func__);
          else
                  ASSERT_VI_UNLOCKED(vp, __func__);
          mtx_assert(&mp->mnt_listmtx, MA_OWNED);
          return (ret);
  }
  
  static struct vnode *
  mnt_vnode_next_active(struct vnode **mvp, struct mount *mp)
  {
          struct vnode *vp, *nvp;
  
          mtx_assert(&mp->mnt_listmtx, MA_OWNED);
          KASSERT((*mvp)->v_mount == mp, ("marker vnode mount list mismatch"));
  restart:
          vp = TAILQ_NEXT(*mvp, v_actfreelist);
          while (vp != NULL) {
                  if (vp->v_type == VMARKER) {
                          vp = TAILQ_NEXT(vp, v_actfreelist);
                          continue;
                  }
                  /*
                   * Try-lock because this is the wrong lock order.  If that does
                   * not succeed, drop the mount vnode list lock and try to
                   * reacquire it and the vnode interlock in the right order.
                   */
                  if (!VI_TRYLOCK(vp) &&
                      !mnt_vnode_next_active_relock(*mvp, mp, vp))
                          goto restart;
                  KASSERT(vp->v_type != VMARKER, ("locked marker %p", vp));
                  KASSERT(vp->v_mount == mp || vp->v_mount == NULL,
                      ("alien vnode on the active list %p %p", vp, mp));
                  if (vp->v_mount == mp && (vp->v_iflag & VI_DOOMED) == 0)
                          break;
                  nvp = TAILQ_NEXT(vp, v_actfreelist);
                  VI_UNLOCK(vp);
                  vp = nvp;
          }
          TAILQ_REMOVE(&mp->mnt_activevnodelist, *mvp, v_actfreelist);
  
          /* Check if we are done */
          if (vp == NULL) {
                  mtx_unlock(&mp->mnt_listmtx);
                  mnt_vnode_markerfree_active(mvp, mp);
                  return (NULL);
          }
          TAILQ_INSERT_AFTER(&mp->mnt_activevnodelist, vp, *mvp, v_actfreelist);
          mtx_unlock(&mp->mnt_listmtx);
          ASSERT_VI_LOCKED(vp, "active iter");
          KASSERT((vp->v_iflag & VI_ACTIVE) != 0, ("Non-active vp %p", vp));
          return (vp);
  }
  
  struct vnode *
  __mnt_vnode_next_active(struct vnode **mvp, struct mount *mp)
  {
  
          if (should_yield())
                  kern_yield(PRI_USER);
          mtx_lock(&mp->mnt_listmtx);
          return (mnt_vnode_next_active(mvp, mp));
  }
  
  struct vnode *
  __mnt_vnode_first_active(struct vnode **mvp, struct mount *mp)
  {
          struct vnode *vp;
  
          *mvp = malloc(sizeof(struct vnode), M_VNODE_MARKER, M_WAITOK | M_ZERO);
          MNT_ILOCK(mp);
          MNT_REF(mp);
          MNT_IUNLOCK(mp);
          (*mvp)->v_type = VMARKER;
          (*mvp)->v_mount = mp;
  
          mtx_lock(&mp->mnt_listmtx);
          vp = TAILQ_FIRST(&mp->mnt_activevnodelist);
          if (vp == NULL) {
                  mtx_unlock(&mp->mnt_listmtx);
                  mnt_vnode_markerfree_active(mvp, mp);
                  return (NULL);
          }
          TAILQ_INSERT_BEFORE(vp, *mvp, v_actfreelist);
          return (mnt_vnode_next_active(mvp, mp));
  }
  
  void
  __mnt_vnode_markerfree_active(struct vnode **mvp, struct mount *mp)
  {
  
          if (*mvp == NULL)
                  return;
  
          mtx_lock(&mp->mnt_listmtx);
          TAILQ_REMOVE(&mp->mnt_activevnodelist, *mvp, v_actfreelist);
          mtx_unlock(&mp->mnt_listmtx);
          mnt_vnode_markerfree_active(mvp, mp);
  }