HAMMER Utilities: Sync with recent work.

[dragonfly.git] / sys / kern / vfs_cache.c

/*
 * Copyright (c) 2003,2004 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * 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 DragonFly Project 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 COPYRIGHT HOLDERS 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
 * COPYRIGHT HOLDERS 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.
 * 
 * Copyright (c) 1989, 1993, 1995
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Poul-Henning Kamp of the FreeBSD Project.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)vfs_cache.c 8.5 (Berkeley) 3/22/95
 * $FreeBSD: src/sys/kern/vfs_cache.c,v 1.42.2.6 2001/10/05 20:07:03 dillon Exp $
 * $DragonFly: src/sys/kern/vfs_cache.c,v 1.91 2008/06/14 05:34:06 dillon Exp $
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/mount.h>
#include <sys/vnode.h>
#include <sys/malloc.h>
#include <sys/sysproto.h>
#include <sys/proc.h>
#include <sys/namei.h>
#include <sys/nlookup.h>
#include <sys/filedesc.h>
#include <sys/fnv_hash.h>
#include <sys/globaldata.h>
#include <sys/kern_syscall.h>
#include <sys/dirent.h>
#include <ddb/ddb.h>

#include <sys/sysref2.h>

#define MAX_RECURSION_DEPTH     64

/*
 * Random lookups in the cache are accomplished with a hash table using
 * a hash key of (nc_src_vp, name).
 *
 * Negative entries may exist and correspond to structures where nc_vp
 * is NULL.  In a negative entry, NCF_WHITEOUT will be set if the entry
 * corresponds to a whited-out directory entry (verses simply not finding the
 * entry at all).
 *
 * Upon reaching the last segment of a path, if the reference is for DELETE,
 * or NOCACHE is set (rewrite), and the name is located in the cache, it
 * will be dropped.
 */

/*
 * Structures associated with name cacheing.
 */
#define NCHHASH(hash)   (&nchashtbl[(hash) & nchash])
#define MINNEG          1024

MALLOC_DEFINE(M_VFSCACHE, "vfscache", "VFS name cache entries");

static LIST_HEAD(nchashhead, namecache) *nchashtbl;     /* Hash Table */
static struct namecache_list    ncneglist;              /* instead of vnode */

/*
 * ncvp_debug - debug cache_fromvp().  This is used by the NFS server
 * to create the namecache infrastructure leading to a dangling vnode.
 *
 * 0    Only errors are reported
 * 1    Successes are reported
 * 2    Successes + the whole directory scan is reported
 * 3    Force the directory scan code run as if the parent vnode did not
 *      have a namecache record, even if it does have one.
 */
static int      ncvp_debug;
SYSCTL_INT(_debug, OID_AUTO, ncvp_debug, CTLFLAG_RW, &ncvp_debug, 0, "");

static u_long   nchash;                 /* size of hash table */
SYSCTL_ULONG(_debug, OID_AUTO, nchash, CTLFLAG_RD, &nchash, 0, "");

static u_long   ncnegfactor = 16;       /* ratio of negative entries */
SYSCTL_ULONG(_debug, OID_AUTO, ncnegfactor, CTLFLAG_RW, &ncnegfactor, 0, "");

static int      nclockwarn;             /* warn on locked entries in ticks */
SYSCTL_INT(_debug, OID_AUTO, nclockwarn, CTLFLAG_RW, &nclockwarn, 0, "");

static u_long   numneg;         /* number of cache entries allocated */
SYSCTL_ULONG(_debug, OID_AUTO, numneg, CTLFLAG_RD, &numneg, 0, "");

static u_long   numcache;               /* number of cache entries allocated */
SYSCTL_ULONG(_debug, OID_AUTO, numcache, CTLFLAG_RD, &numcache, 0, "");

static u_long   numunres;               /* number of unresolved entries */
SYSCTL_ULONG(_debug, OID_AUTO, numunres, CTLFLAG_RD, &numunres, 0, "");

SYSCTL_INT(_debug, OID_AUTO, vnsize, CTLFLAG_RD, 0, sizeof(struct vnode), "");
SYSCTL_INT(_debug, OID_AUTO, ncsize, CTLFLAG_RD, 0, sizeof(struct namecache), "");

static int cache_resolve_mp(struct mount *mp);
static struct vnode *cache_dvpref(struct namecache *ncp);
static void _cache_rehash(struct namecache *ncp);
static void _cache_lock(struct namecache *ncp);
static void _cache_setunresolved(struct namecache *ncp);

/*
 * The new name cache statistics
 */
SYSCTL_NODE(_vfs, OID_AUTO, cache, CTLFLAG_RW, 0, "Name cache statistics");
#define STATNODE(mode, name, var) \
        SYSCTL_ULONG(_vfs_cache, OID_AUTO, name, mode, var, 0, "");
STATNODE(CTLFLAG_RD, numneg, &numneg);
STATNODE(CTLFLAG_RD, numcache, &numcache);
static u_long numcalls; STATNODE(CTLFLAG_RD, numcalls, &numcalls);
static u_long dothits; STATNODE(CTLFLAG_RD, dothits, &dothits);
static u_long dotdothits; STATNODE(CTLFLAG_RD, dotdothits, &dotdothits);
static u_long numchecks; STATNODE(CTLFLAG_RD, numchecks, &numchecks);
static u_long nummiss; STATNODE(CTLFLAG_RD, nummiss, &nummiss);
static u_long nummisszap; STATNODE(CTLFLAG_RD, nummisszap, &nummisszap);
static u_long numposzaps; STATNODE(CTLFLAG_RD, numposzaps, &numposzaps);
static u_long numposhits; STATNODE(CTLFLAG_RD, numposhits, &numposhits);
static u_long numnegzaps; STATNODE(CTLFLAG_RD, numnegzaps, &numnegzaps);
static u_long numneghits; STATNODE(CTLFLAG_RD, numneghits, &numneghits);

struct nchstats nchstats[SMP_MAXCPU];
/*
 * Export VFS cache effectiveness statistics to user-land.
 *
 * The statistics are left for aggregation to user-land so
 * neat things can be achieved, like observing per-CPU cache
 * distribution.
 */
static int
sysctl_nchstats(SYSCTL_HANDLER_ARGS)
{
        struct globaldata *gd;
        int i, error;

        error = 0;
        for (i = 0; i < ncpus; ++i) {
                gd = globaldata_find(i);
                if ((error = SYSCTL_OUT(req, (void *)&(*gd->gd_nchstats),
                        sizeof(struct nchstats))))
                        break;
        }

        return (error);
}
SYSCTL_PROC(_vfs_cache, OID_AUTO, nchstats, CTLTYPE_OPAQUE|CTLFLAG_RD,
  0, 0, sysctl_nchstats, "S,nchstats", "VFS cache effectiveness statistics");

static void cache_zap(struct namecache *ncp);

/*
 * cache_hold() and cache_drop() prevent the premature deletion of a
 * namecache entry but do not prevent operations (such as zapping) on
 * that namecache entry.
 *
 * This routine may only be called from outside this source module if
 * nc_refs is already at least 1.
 *
 * This is a rare case where callers are allowed to hold a spinlock,
 * so we can't ourselves.
 */
static __inline
struct namecache *
_cache_hold(struct namecache *ncp)
{
        atomic_add_int(&ncp->nc_refs, 1);
        return(ncp);
}

/*
 * When dropping an entry, if only one ref remains and the entry has not
 * been resolved, zap it.  Since the one reference is being dropped the
 * entry had better not be locked.
 */
static __inline
void
_cache_drop(struct namecache *ncp)
{
        KKASSERT(ncp->nc_refs > 0);
        if (ncp->nc_refs == 1 && 
            (ncp->nc_flag & NCF_UNRESOLVED) && 
            TAILQ_EMPTY(&ncp->nc_list)
        ) {
                KKASSERT(ncp->nc_exlocks == 0);
                _cache_lock(ncp);
                cache_zap(ncp);
        } else {
                atomic_subtract_int(&ncp->nc_refs, 1);
        }
}

/*
 * Link a new namecache entry to its parent.  Be careful to avoid races
 * if vhold() blocks in the future.
 */
static void
cache_link_parent(struct namecache *ncp, struct namecache *par)
{
        KKASSERT(ncp->nc_parent == NULL);
        ncp->nc_parent = par;
        if (TAILQ_EMPTY(&par->nc_list)) {
                TAILQ_INSERT_HEAD(&par->nc_list, ncp, nc_entry);
                /*
                 * Any vp associated with an ncp which has children must
                 * be held to prevent it from being recycled.
                 */
                if (par->nc_vp)
                        vhold(par->nc_vp);
        } else {
                TAILQ_INSERT_HEAD(&par->nc_list, ncp, nc_entry);
        }
}

/*
 * Remove the parent association from a namecache structure.  If this is
 * the last child of the parent the cache_drop(par) will attempt to
 * recursively zap the parent.
 */
static void
cache_unlink_parent(struct namecache *ncp)
{
        struct namecache *par;

        if ((par = ncp->nc_parent) != NULL) {
                ncp->nc_parent = NULL;
                par = _cache_hold(par);
                TAILQ_REMOVE(&par->nc_list, ncp, nc_entry);
                if (par->nc_vp && TAILQ_EMPTY(&par->nc_list))
                        vdrop(par->nc_vp);
                _cache_drop(par);
        }
}

/*
 * Allocate a new namecache structure.  Most of the code does not require
 * zero-termination of the string but it makes vop_compat_ncreate() easier.
 */
static struct namecache *
cache_alloc(int nlen)
{
        struct namecache *ncp;

        ncp = kmalloc(sizeof(*ncp), M_VFSCACHE, M_WAITOK|M_ZERO);
        if (nlen)
                ncp->nc_name = kmalloc(nlen + 1, M_VFSCACHE, M_WAITOK);
        ncp->nc_nlen = nlen;
        ncp->nc_flag = NCF_UNRESOLVED;
        ncp->nc_error = ENOTCONN;       /* needs to be resolved */
        ncp->nc_refs = 1;

        /*
         * Construct a fake FSMID based on the time of day and a 32 bit
         * roller for uniqueness.  This is used to generate a useful
         * FSMID for filesystems which do not support it.
         */
        ncp->nc_fsmid = cache_getnewfsmid();
        TAILQ_INIT(&ncp->nc_list);
        _cache_lock(ncp);
        return(ncp);
}

static void
_cache_free(struct namecache *ncp)
{
        KKASSERT(ncp->nc_refs == 1 && ncp->nc_exlocks == 1);
        if (ncp->nc_name)
                kfree(ncp->nc_name, M_VFSCACHE);
        kfree(ncp, M_VFSCACHE);
}

void
cache_zero(struct nchandle *nch)
{
        nch->ncp = NULL;
        nch->mount = NULL;
}

/*
 * Ref and deref a namecache structure.
 *
 * Warning: caller may hold an unrelated read spinlock, which means we can't
 * use read spinlocks here.
 */
struct nchandle *
cache_hold(struct nchandle *nch)
{
        _cache_hold(nch->ncp);
        ++nch->mount->mnt_refs;
        return(nch);
}

void
cache_copy(struct nchandle *nch, struct nchandle *target)
{
        *target = *nch;
        _cache_hold(target->ncp);
        ++nch->mount->mnt_refs;
}

void
cache_changemount(struct nchandle *nch, struct mount *mp)
{
        --nch->mount->mnt_refs;
        nch->mount = mp;
        ++nch->mount->mnt_refs;
}

void
cache_drop(struct nchandle *nch)
{
        --nch->mount->mnt_refs;
        _cache_drop(nch->ncp);
        nch->ncp = NULL;
        nch->mount = NULL;
}

/*
 * Namespace locking.  The caller must already hold a reference to the
 * namecache structure in order to lock/unlock it.  This function prevents
 * the namespace from being created or destroyed by accessors other then
 * the lock holder.
 *
 * Note that holding a locked namecache structure prevents other threads
 * from making namespace changes (e.g. deleting or creating), prevents
 * vnode association state changes by other threads, and prevents the
 * namecache entry from being resolved or unresolved by other threads.
 *
 * The lock owner has full authority to associate/disassociate vnodes
 * and resolve/unresolve the locked ncp.
 *
 * WARNING!  Holding a locked ncp will prevent a vnode from being destroyed
 * or recycled, but it does NOT help you if the vnode had already initiated
 * a recyclement.  If this is important, use cache_get() rather then 
 * cache_lock() (and deal with the differences in the way the refs counter
 * is handled).  Or, alternatively, make an unconditional call to 
 * cache_validate() or cache_resolve() after cache_lock() returns.
 */
static
void
_cache_lock(struct namecache *ncp)
{
        thread_t td;
        int didwarn;

        KKASSERT(ncp->nc_refs != 0);
        didwarn = 0;
        td = curthread;

        for (;;) {
                if (ncp->nc_exlocks == 0) {
                        ncp->nc_exlocks = 1;
                        ncp->nc_locktd = td;
                        /* 
                         * The vp associated with a locked ncp must be held
                         * to prevent it from being recycled (which would
                         * cause the ncp to become unresolved).
                         *
                         * WARNING!  If VRECLAIMED is set the vnode could
                         * already be in the middle of a recycle.  Callers
                         * should not assume that nc_vp is usable when
                         * not NULL.  cache_vref() or cache_vget() must be
                         * called.
                         *
                         * XXX loop on race for later MPSAFE work.
                         */
                        if (ncp->nc_vp)
                                vhold(ncp->nc_vp);
                        break;
                }
                if (ncp->nc_locktd == td) {
                        ++ncp->nc_exlocks;
                        break;
                }
                ncp->nc_flag |= NCF_LOCKREQ;
                if (tsleep(ncp, 0, "clock", nclockwarn) == EWOULDBLOCK) {
                        if (didwarn)
                                continue;
                        didwarn = 1;
                        kprintf("[diagnostic] cache_lock: blocked on %p", ncp);
                        kprintf(" \"%*.*s\"\n",
                                ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name);
                }
        }

        if (didwarn == 1) {
                kprintf("[diagnostic] cache_lock: unblocked %*.*s\n",
                        ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name);
        }
}

void
cache_lock(struct nchandle *nch)
{
        _cache_lock(nch->ncp);
}

static
int
_cache_lock_nonblock(struct namecache *ncp)
{
        thread_t td;

        KKASSERT(ncp->nc_refs != 0);
        td = curthread;
        if (ncp->nc_exlocks == 0) {
                ncp->nc_exlocks = 1;
                ncp->nc_locktd = td;
                /* 
                 * The vp associated with a locked ncp must be held
                 * to prevent it from being recycled (which would
                 * cause the ncp to become unresolved).
                 *
                 * WARNING!  If VRECLAIMED is set the vnode could
                 * already be in the middle of a recycle.  Callers
                 * should not assume that nc_vp is usable when
                 * not NULL.  cache_vref() or cache_vget() must be
                 * called.
                 *
                 * XXX loop on race for later MPSAFE work.
                 */
                if (ncp->nc_vp)
                        vhold(ncp->nc_vp);
                return(0);
        } else {
                return(EWOULDBLOCK);
        }
}

int
cache_lock_nonblock(struct nchandle *nch)
{
        return(_cache_lock_nonblock(nch->ncp));
}

static
void
_cache_unlock(struct namecache *ncp)
{
        thread_t td = curthread;

        KKASSERT(ncp->nc_refs > 0);
        KKASSERT(ncp->nc_exlocks > 0);
        KKASSERT(ncp->nc_locktd == td);
        if (--ncp->nc_exlocks == 0) {
                if (ncp->nc_vp)
                        vdrop(ncp->nc_vp);
                ncp->nc_locktd = NULL;
                if (ncp->nc_flag & NCF_LOCKREQ) {
                        ncp->nc_flag &= ~NCF_LOCKREQ;
                        wakeup(ncp);
                }
        }
}

void
cache_unlock(struct nchandle *nch)
{
        _cache_unlock(nch->ncp);
}

/*
 * ref-and-lock, unlock-and-deref functions.
 *
 * This function is primarily used by nlookup.  Even though cache_lock
 * holds the vnode, it is possible that the vnode may have already
 * initiated a recyclement.  We want cache_get() to return a definitively
 * usable vnode or a definitively unresolved ncp.
 */
static
struct namecache *
_cache_get(struct namecache *ncp)
{
        _cache_hold(ncp);
        _cache_lock(ncp);
        if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                _cache_setunresolved(ncp);
        return(ncp);
}

/*
 * note: the same nchandle can be passed for both arguments.
 */
void
cache_get(struct nchandle *nch, struct nchandle *target)
{
        target->mount = nch->mount;
        target->ncp = _cache_get(nch->ncp);
        ++target->mount->mnt_refs;
}

static int
_cache_get_nonblock(struct namecache *ncp)
{
        /* XXX MP */
        if (ncp->nc_exlocks == 0 || ncp->nc_locktd == curthread) {
                _cache_hold(ncp);
                _cache_lock(ncp);
                if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                        _cache_setunresolved(ncp);
                return(0);
        }
        return(EWOULDBLOCK);
}

int
cache_get_nonblock(struct nchandle *nch)
{
        int error;

        if ((error = _cache_get_nonblock(nch->ncp)) == 0)
                ++nch->mount->mnt_refs;
        return (error);
}

static __inline
void
_cache_put(struct namecache *ncp)
{
        _cache_unlock(ncp);
        _cache_drop(ncp);
}

void
cache_put(struct nchandle *nch)
{
        --nch->mount->mnt_refs;
        _cache_put(nch->ncp);
        nch->ncp = NULL;
        nch->mount = NULL;
}

/*
 * Resolve an unresolved ncp by associating a vnode with it.  If the
 * vnode is NULL, a negative cache entry is created.
 *
 * The ncp should be locked on entry and will remain locked on return.
 */
static
void
_cache_setvp(struct namecache *ncp, struct vnode *vp)
{
        KKASSERT(ncp->nc_flag & NCF_UNRESOLVED);
        ncp->nc_vp = vp;
        if (vp != NULL) {
                /*
                 * Any vp associated with an ncp which has children must
                 * be held.  Any vp associated with a locked ncp must be held.
                 */
                if (!TAILQ_EMPTY(&ncp->nc_list))
                        vhold(vp);
                TAILQ_INSERT_HEAD(&vp->v_namecache, ncp, nc_vnode);
                if (ncp->nc_exlocks)
                        vhold(vp);

                /*
                 * Set auxiliary flags
                 */
                switch(vp->v_type) {
                case VDIR:
                        ncp->nc_flag |= NCF_ISDIR;
                        break;
                case VLNK:
                        ncp->nc_flag |= NCF_ISSYMLINK;
                        /* XXX cache the contents of the symlink */
                        break;
                default:
                        break;
                }
                ++numcache;
                ncp->nc_error = 0;
        } else {
                TAILQ_INSERT_TAIL(&ncneglist, ncp, nc_vnode);
                ++numneg;
                ncp->nc_error = ENOENT;
        }
        ncp->nc_flag &= ~NCF_UNRESOLVED;
}

void
cache_setvp(struct nchandle *nch, struct vnode *vp)
{
        _cache_setvp(nch->ncp, vp);
}

void
cache_settimeout(struct nchandle *nch, int nticks)
{
        struct namecache *ncp = nch->ncp;

        if ((ncp->nc_timeout = ticks + nticks) == 0)
                ncp->nc_timeout = 1;
}

/*
 * Disassociate the vnode or negative-cache association and mark a
 * namecache entry as unresolved again.  Note that the ncp is still
 * left in the hash table and still linked to its parent.
 *
 * The ncp should be locked and refd on entry and will remain locked and refd
 * on return.
 *
 * This routine is normally never called on a directory containing children.
 * However, NFS often does just that in its rename() code as a cop-out to
 * avoid complex namespace operations.  This disconnects a directory vnode
 * from its namecache and can cause the OLDAPI and NEWAPI to get out of
 * sync.
 *
 * NOTE: NCF_FSMID must be cleared so a refurbishment of the ncp, such as
 * in a create, properly propogates flag up the chain.
 */
static
void
_cache_setunresolved(struct namecache *ncp)
{
        struct vnode *vp;

        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                ncp->nc_flag |= NCF_UNRESOLVED;
                ncp->nc_timeout = 0;
                ncp->nc_error = ENOTCONN;
                ++numunres;
                if ((vp = ncp->nc_vp) != NULL) {
                        --numcache;
                        ncp->nc_vp = NULL;
                        TAILQ_REMOVE(&vp->v_namecache, ncp, nc_vnode);

                        /*
                         * Any vp associated with an ncp with children is
                         * held by that ncp.  Any vp associated with a locked
                         * ncp is held by that ncp.  These conditions must be
                         * undone when the vp is cleared out from the ncp.
                         */
                        if (ncp->nc_flag & NCF_FSMID)
                                vupdatefsmid(vp);
                        if (!TAILQ_EMPTY(&ncp->nc_list))
                                vdrop(vp);
                        if (ncp->nc_exlocks)
                                vdrop(vp);
                } else {
                        TAILQ_REMOVE(&ncneglist, ncp, nc_vnode);
                        --numneg;
                }
                ncp->nc_flag &= ~(NCF_WHITEOUT|NCF_ISDIR|NCF_ISSYMLINK|
                                  NCF_FSMID);
        }
}

void
cache_setunresolved(struct nchandle *nch)
{
        _cache_setunresolved(nch->ncp);
}

/*
 * Determine if we can clear NCF_ISMOUNTPT by scanning the mountlist
 * looking for matches.  This flag tells the lookup code when it must
 * check for a mount linkage and also prevents the directories in question
 * from being deleted or renamed.
 */
static
int
cache_clrmountpt_callback(struct mount *mp, void *data)
{
        struct nchandle *nch = data;

        if (mp->mnt_ncmounton.ncp == nch->ncp)
                return(1);
        if (mp->mnt_ncmountpt.ncp == nch->ncp)
                return(1);
        return(0);
}

void
cache_clrmountpt(struct nchandle *nch)
{
        int count;

        count = mountlist_scan(cache_clrmountpt_callback, nch,
                               MNTSCAN_FORWARD|MNTSCAN_NOBUSY);
        if (count == 0)
                nch->ncp->nc_flag &= ~NCF_ISMOUNTPT;
}

/*
 * Invalidate portions of the namecache topology given a starting entry.
 * The passed ncp is set to an unresolved state and:
 *
 * The passed ncp must be locked.
 *
 * CINV_DESTROY         - Set a flag in the passed ncp entry indicating
 *                        that the physical underlying nodes have been 
 *                        destroyed... as in deleted.  For example, when
 *                        a directory is removed.  This will cause record
 *                        lookups on the name to no longer be able to find
 *                        the record and tells the resolver to return failure
 *                        rather then trying to resolve through the parent.
 *
 *                        The topology itself, including ncp->nc_name,
 *                        remains intact.
 *
 *                        This only applies to the passed ncp, if CINV_CHILDREN
 *                        is specified the children are not flagged.
 *
 * CINV_CHILDREN        - Set all children (recursively) to an unresolved
 *                        state as well.
 *
 *                        Note that this will also have the side effect of
 *                        cleaning out any unreferenced nodes in the topology
 *                        from the leaves up as the recursion backs out.
 *
 * Note that the topology for any referenced nodes remains intact.
 *
 * It is possible for cache_inval() to race a cache_resolve(), meaning that
 * the namecache entry may not actually be invalidated on return if it was
 * revalidated while recursing down into its children.  This code guarentees
 * that the node(s) will go through an invalidation cycle, but does not 
 * guarentee that they will remain in an invalidated state. 
 *
 * Returns non-zero if a revalidation was detected during the invalidation
 * recursion, zero otherwise.  Note that since only the original ncp is
 * locked the revalidation ultimately can only indicate that the original ncp
 * *MIGHT* no have been reresolved.
 *
 * DEEP RECURSION HANDLING - If a recursive invalidation recurses deeply we
 * have to avoid blowing out the kernel stack.  We do this by saving the
 * deep namecache node and aborting the recursion, then re-recursing at that
 * node using a depth-first algorithm in order to allow multiple deep
 * recursions to chain through each other, then we restart the invalidation
 * from scratch.
 */

struct cinvtrack {
        struct namecache *resume_ncp;
        int depth;
};

static int _cache_inval_internal(struct namecache *, int, struct cinvtrack *);

static
int
_cache_inval(struct namecache *ncp, int flags)
{
        struct cinvtrack track;
        struct namecache *ncp2;
        int r;

        track.depth = 0;
        track.resume_ncp = NULL;

        for (;;) {
                r = _cache_inval_internal(ncp, flags, &track);
                if (track.resume_ncp == NULL)
                        break;
                kprintf("Warning: deep namecache recursion at %s\n",
                        ncp->nc_name);
                _cache_unlock(ncp);
                while ((ncp2 = track.resume_ncp) != NULL) {
                        track.resume_ncp = NULL;
                        _cache_lock(ncp2);
                        _cache_inval_internal(ncp2, flags & ~CINV_DESTROY,
                                             &track);
                        _cache_put(ncp2);
                }
                _cache_lock(ncp);
        }
        return(r);
}

int
cache_inval(struct nchandle *nch, int flags)
{
        return(_cache_inval(nch->ncp, flags));
}

static int
_cache_inval_internal(struct namecache *ncp, int flags, struct cinvtrack *track)
{
        struct namecache *kid;
        struct namecache *nextkid;
        int rcnt = 0;

        KKASSERT(ncp->nc_exlocks);

        _cache_setunresolved(ncp);
        if (flags & CINV_DESTROY)
                ncp->nc_flag |= NCF_DESTROYED;

        if ((flags & CINV_CHILDREN) && 
            (kid = TAILQ_FIRST(&ncp->nc_list)) != NULL
        ) {
                if (++track->depth > MAX_RECURSION_DEPTH) {
                        track->resume_ncp = ncp;
                        _cache_hold(ncp);
                        ++rcnt;
                }
                _cache_hold(kid);
                _cache_unlock(ncp);
                while (kid) {
                        if (track->resume_ncp) {
                                _cache_drop(kid);
                                break;
                        }
                        if ((nextkid = TAILQ_NEXT(kid, nc_entry)) != NULL)
                                _cache_hold(nextkid);
                        if ((kid->nc_flag & NCF_UNRESOLVED) == 0 ||
                            TAILQ_FIRST(&kid->nc_list)
                        ) {
                                _cache_lock(kid);
                                rcnt += _cache_inval_internal(kid, flags & ~CINV_DESTROY, track);
                                _cache_unlock(kid);
                        }
                        _cache_drop(kid);
                        kid = nextkid;
                }
                --track->depth;
                _cache_lock(ncp);
        }

        /*
         * Someone could have gotten in there while ncp was unlocked,
         * retry if so.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0)
                ++rcnt;
        return (rcnt);
}

/*
 * Invalidate a vnode's namecache associations.  To avoid races against
 * the resolver we do not invalidate a node which we previously invalidated
 * but which was then re-resolved while we were in the invalidation loop.
 *
 * Returns non-zero if any namecache entries remain after the invalidation
 * loop completed.
 *
 * NOTE: unlike the namecache topology which guarentees that ncp's will not
 * be ripped out of the topology while held, the vnode's v_namecache list
 * has no such restriction.  NCP's can be ripped out of the list at virtually
 * any time if not locked, even if held.
 */
int
cache_inval_vp(struct vnode *vp, int flags)
{
        struct namecache *ncp;
        struct namecache *next;

restart:
        ncp = TAILQ_FIRST(&vp->v_namecache);
        if (ncp)
                _cache_hold(ncp);
        while (ncp) {
                /* loop entered with ncp held */
                if ((next = TAILQ_NEXT(ncp, nc_vnode)) != NULL)
                        _cache_hold(next);
                _cache_lock(ncp);
                if (ncp->nc_vp != vp) {
                        kprintf("Warning: cache_inval_vp: race-A detected on "
                                "%s\n", ncp->nc_name);
                        _cache_put(ncp);
                        if (next)
                                _cache_drop(next);
                        goto restart;
                }
                _cache_inval(ncp, flags);
                _cache_put(ncp);                /* also releases reference */
                ncp = next;
                if (ncp && ncp->nc_vp != vp) {
                        kprintf("Warning: cache_inval_vp: race-B detected on "
                                "%s\n", ncp->nc_name);
                        _cache_drop(ncp);
                        goto restart;
                }
        }
        return(TAILQ_FIRST(&vp->v_namecache) != NULL);
}

/*
 * This routine is used instead of the normal cache_inval_vp() when we
 * are trying to recycle otherwise good vnodes.
 *
 * Return 0 on success, non-zero if not all namecache records could be
 * disassociated from the vnode (for various reasons).
 */
int
cache_inval_vp_nonblock(struct vnode *vp)
{
        struct namecache *ncp;
        struct namecache *next;

        ncp = TAILQ_FIRST(&vp->v_namecache);
        if (ncp)
                _cache_hold(ncp);
        while (ncp) {
                /* loop entered with ncp held */
                if ((next = TAILQ_NEXT(ncp, nc_vnode)) != NULL)
                        _cache_hold(next);
                if (_cache_lock_nonblock(ncp)) {
                        _cache_drop(ncp);
                        if (next)
                                _cache_drop(next);
                        break;
                }
                if (ncp->nc_vp != vp) {
                        kprintf("Warning: cache_inval_vp: race-A detected on "
                                "%s\n", ncp->nc_name);
                        _cache_put(ncp);
                        if (next)
                                _cache_drop(next);
                        break;
                }
                _cache_inval(ncp, 0);
                _cache_put(ncp);                /* also releases reference */
                ncp = next;
                if (ncp && ncp->nc_vp != vp) {
                        kprintf("Warning: cache_inval_vp: race-B detected on "
                                "%s\n", ncp->nc_name);
                        _cache_drop(ncp);
                        break;
                }
        }
        return(TAILQ_FIRST(&vp->v_namecache) != NULL);
}

/*
 * The source ncp has been renamed to the target ncp.  Both fncp and tncp
 * must be locked.  The target ncp is destroyed (as a normal rename-over
 * would destroy the target file or directory).
 *
 * Because there may be references to the source ncp we cannot copy its
 * contents to the target.  Instead the source ncp is relinked as the target
 * and the target ncp is removed from the namecache topology.
 */
void
cache_rename(struct nchandle *fnch, struct nchandle *tnch)
{
        struct namecache *fncp = fnch->ncp;
        struct namecache *tncp = tnch->ncp;
        char *oname;

        _cache_setunresolved(tncp);
        cache_unlink_parent(fncp);
        cache_link_parent(fncp, tncp->nc_parent);
        cache_unlink_parent(tncp);
        oname = fncp->nc_name;
        fncp->nc_name = tncp->nc_name;
        fncp->nc_nlen = tncp->nc_nlen;
        tncp->nc_name = NULL;
        tncp->nc_nlen = 0;
        if (fncp->nc_flag & NCF_HASHED)
                _cache_rehash(fncp);
        if (tncp->nc_flag & NCF_HASHED)
                _cache_rehash(tncp);
        if (oname)
                kfree(oname, M_VFSCACHE);
}

/*
 * vget the vnode associated with the namecache entry.  Resolve the namecache
 * entry if necessary and deal with namecache/vp races.  The passed ncp must
 * be referenced and may be locked.  The ncp's ref/locking state is not 
 * effected by this call.
 *
 * lk_type may be LK_SHARED, LK_EXCLUSIVE.  A ref'd, possibly locked
 * (depending on the passed lk_type) will be returned in *vpp with an error
 * of 0, or NULL will be returned in *vpp with a non-0 error code.  The
 * most typical error is ENOENT, meaning that the ncp represents a negative
 * cache hit and there is no vnode to retrieve, but other errors can occur
 * too.
 *
 * The main race we have to deal with are namecache zaps.  The ncp itself
 * will not disappear since it is referenced, and it turns out that the
 * validity of the vp pointer can be checked simply by rechecking the
 * contents of ncp->nc_vp.
 */
int
cache_vget(struct nchandle *nch, struct ucred *cred,
           int lk_type, struct vnode **vpp)
{
        struct namecache *ncp;
        struct vnode *vp;
        int error;

        ncp = nch->ncp;
again:
        vp = NULL;
        if (ncp->nc_flag & NCF_UNRESOLVED) {
                _cache_lock(ncp);
                error = cache_resolve(nch, cred);
                _cache_unlock(ncp);
        } else {
                error = 0;
        }
        if (error == 0 && (vp = ncp->nc_vp) != NULL) {
                /*
                 * Accessing the vnode from the namecache is a bit 
                 * dangerous.  Because there are no refs on the vnode, it
                 * could be in the middle of a reclaim.
                 */
                if (vp->v_flag & VRECLAIMED) {
                        kprintf("Warning: vnode reclaim race detected in cache_vget on %p (%s)\n", vp, ncp->nc_name);
                        _cache_lock(ncp);
                        _cache_setunresolved(ncp);
                        _cache_unlock(ncp);
                        goto again;
                }
                error = vget(vp, lk_type);
                if (error) {
                        if (vp != ncp->nc_vp)
                                goto again;
                        vp = NULL;
                } else if (vp != ncp->nc_vp) {
                        vput(vp);
                        goto again;
                } else if (vp->v_flag & VRECLAIMED) {
                        panic("vget succeeded on a VRECLAIMED node! vp %p", vp);
                }
        }
        if (error == 0 && vp == NULL)
                error = ENOENT;
        *vpp = vp;
        return(error);
}

int
cache_vref(struct nchandle *nch, struct ucred *cred, struct vnode **vpp)
{
        struct namecache *ncp;
        struct vnode *vp;
        int error;

        ncp = nch->ncp;

again:
        vp = NULL;
        if (ncp->nc_flag & NCF_UNRESOLVED) {
                _cache_lock(ncp);
                error = cache_resolve(nch, cred);
                _cache_unlock(ncp);
        } else {
                error = 0;
        }
        if (error == 0 && (vp = ncp->nc_vp) != NULL) {
                /*
                 * Since we did not obtain any locks, a cache zap 
                 * race can occur here if the vnode is in the middle
                 * of being reclaimed and has not yet been able to
                 * clean out its cache node.  If that case occurs,
                 * we must lock and unresolve the cache, then loop
                 * to retry.
                 */
                if ((error = vget(vp, LK_SHARED)) != 0) {
                        if (error == ENOENT) {
                                kprintf("Warning: vnode reclaim race detected on cache_vref %p (%s)\n", vp, ncp->nc_name);
                                _cache_lock(ncp);
                                _cache_setunresolved(ncp);
                                _cache_unlock(ncp);
                                goto again;
                        }
                        /* fatal error */
                } else {
                        /* caller does not want a lock */
                        vn_unlock(vp);
                }
        }
        if (error == 0 && vp == NULL)
                error = ENOENT;
        *vpp = vp;
        return(error);
}

/*
 * Return a referenced vnode representing the parent directory of
 * ncp.  Because the caller has locked the ncp it should not be possible for
 * the parent ncp to go away.
 *
 * However, we might race against the parent dvp and not be able to
 * reference it.  If we race, return NULL.
 */
static struct vnode *
cache_dvpref(struct namecache *ncp)
{
        struct namecache *par;
        struct vnode *dvp;

        dvp = NULL;
        if ((par = ncp->nc_parent) != NULL) {
                if ((par->nc_flag & NCF_UNRESOLVED) == 0) {
                        if ((dvp = par->nc_vp) != NULL) {
                                if (vget(dvp, LK_SHARED) == 0) {
                                        vn_unlock(dvp);
                                        /* return referenced, unlocked dvp */
                                } else {
                                        dvp = NULL;
                                }
                        }
                }
        }
        return(dvp);
}

/*
 * Recursively set the FSMID update flag for namecache nodes leading
 * to root.  This will cause the next getattr or reclaim to increment the
 * fsmid and mark the inode for lazy updating.
 *
 * Stop recursing when we hit a node whos NCF_FSMID flag is already set.
 * This makes FSMIDs work in an Einsteinian fashion - where the observation
 * effects the result.  In this case a program monitoring a higher level
 * node will have detected some prior change and started its scan (clearing
 * NCF_FSMID in higher level nodes), but since it has not yet observed the
 * node where we find NCF_FSMID still set, we can safely make the related
 * modification without interfering with the theorized program.
 *
 * This also means that FSMIDs cannot represent time-domain quantities
 * in a hierarchical sense.  But the main reason for doing it this way
 * is to reduce the amount of recursion that occurs in the critical path
 * when e.g. a program is writing to a file that sits deep in a directory
 * hierarchy.
 */
void
cache_update_fsmid(struct nchandle *nch)
{
        struct namecache *ncp;
        struct namecache *scan;
        struct vnode *vp;

        ncp = nch->ncp;

        /*
         * Warning: even if we get a non-NULL vp it could still be in the
         * middle of a recyclement.  Don't do anything fancy, just set
         * NCF_FSMID.
         */
        if ((vp = ncp->nc_vp) != NULL) {
                TAILQ_FOREACH(ncp, &vp->v_namecache, nc_vnode) {
                        for (scan = ncp; scan; scan = scan->nc_parent) {
                                if (scan->nc_flag & NCF_FSMID)
                                        break;
                                scan->nc_flag |= NCF_FSMID;
                        }
                }
        } else {
                while (ncp && (ncp->nc_flag & NCF_FSMID) == 0) {
                        ncp->nc_flag |= NCF_FSMID;
                        ncp = ncp->nc_parent;
                }
        }
}

void
cache_update_fsmid_vp(struct vnode *vp)
{
        struct namecache *ncp;
        struct namecache *scan;

        TAILQ_FOREACH(ncp, &vp->v_namecache, nc_vnode) {
                for (scan = ncp; scan; scan = scan->nc_parent) {
                        if (scan->nc_flag & NCF_FSMID)
                                break;
                        scan->nc_flag |= NCF_FSMID;
                }
        }
}

/*
 * If getattr is called on a vnode (e.g. a stat call), the filesystem
 * may call this routine to determine if the namecache has the hierarchical
 * change flag set, requiring the fsmid to be updated.
 *
 * Since 0 indicates no support, make sure the filesystem fsmid is at least
 * 1.
 */
int
cache_check_fsmid_vp(struct vnode *vp, int64_t *fsmid)
{
        struct namecache *ncp;
        int changed = 0;

        TAILQ_FOREACH(ncp, &vp->v_namecache, nc_vnode) {
                if (ncp->nc_flag & NCF_FSMID) {
                        ncp->nc_flag &= ~NCF_FSMID;
                        changed = 1;
                }
        }
        if (*fsmid == 0)
                ++*fsmid;
        if (changed)
                ++*fsmid;
        return(changed);
}

/*
 * Obtain the FSMID for a vnode for filesystems which do not support
 * a built-in FSMID.
 */
int64_t
cache_sync_fsmid_vp(struct vnode *vp)
{
        struct namecache *ncp;

        if ((ncp = TAILQ_FIRST(&vp->v_namecache)) != NULL) {
                if (ncp->nc_flag & NCF_FSMID) {
                        ncp->nc_flag &= ~NCF_FSMID;
                        ++ncp->nc_fsmid;
                }
                return(ncp->nc_fsmid);
        }
        return(VNOVAL);
}

/*
 * Convert a directory vnode to a namecache record without any other 
 * knowledge of the topology.  This ONLY works with directory vnodes and
 * is ONLY used by the NFS server.  dvp must be refd but unlocked, and the
 * returned ncp (if not NULL) will be held and unlocked.
 *
 * If 'makeit' is 0 and dvp has no existing namecache record, NULL is returned.
 * If 'makeit' is 1 we attempt to track-down and create the namecache topology
 * for dvp.  This will fail only if the directory has been deleted out from
 * under the caller.  
 *
 * Callers must always check for a NULL return no matter the value of 'makeit'.
 *
 * To avoid underflowing the kernel stack each recursive call increments
 * the makeit variable.
 */

static int cache_inefficient_scan(struct nchandle *nch, struct ucred *cred,
                                  struct vnode *dvp, char *fakename);
static int cache_fromdvp_try(struct vnode *dvp, struct ucred *cred, 
                                  struct vnode **saved_dvp);

int
cache_fromdvp(struct vnode *dvp, struct ucred *cred, int makeit,
              struct nchandle *nch)
{
        struct vnode *saved_dvp;
        struct vnode *pvp;
        char *fakename;
        int error;

        nch->ncp = NULL;
        nch->mount = dvp->v_mount;
        saved_dvp = NULL;
        fakename = NULL;

        /*
         * Temporary debugging code to force the directory scanning code
         * to be exercised.
         */
        if (ncvp_debug >= 3 && makeit && TAILQ_FIRST(&dvp->v_namecache)) {
                nch->ncp = TAILQ_FIRST(&dvp->v_namecache);
                kprintf("cache_fromdvp: forcing %s\n", nch->ncp->nc_name);
                goto force;
        }

        /*
         * Loop until resolution, inside code will break out on error.
         */
        while ((nch->ncp = TAILQ_FIRST(&dvp->v_namecache)) == NULL && makeit) {
force:
                /*
                 * If dvp is the root of its filesystem it should already
                 * have a namecache pointer associated with it as a side 
                 * effect of the mount, but it may have been disassociated.
                 */
                if (dvp->v_flag & VROOT) {
                        nch->ncp = _cache_get(nch->mount->mnt_ncmountpt.ncp);
                        error = cache_resolve_mp(nch->mount);
                        _cache_put(nch->ncp);
                        if (ncvp_debug) {
                                kprintf("cache_fromdvp: resolve root of mount %p error %d", 
                                        dvp->v_mount, error);
                        }
                        if (error) {
                                if (ncvp_debug)
                                        kprintf(" failed\n");
                                nch->ncp = NULL;
                                break;
                        }
                        if (ncvp_debug)
                                kprintf(" succeeded\n");
                        continue;
                }

                /*
                 * If we are recursed too deeply resort to an O(n^2)
                 * algorithm to resolve the namecache topology.  The
                 * resolved pvp is left referenced in saved_dvp to
                 * prevent the tree from being destroyed while we loop.
                 */
                if (makeit > 20) {
                        error = cache_fromdvp_try(dvp, cred, &saved_dvp);
                        if (error) {
                                kprintf("lookupdotdot(longpath) failed %d "
                                       "dvp %p\n", error, dvp);
                                nch->ncp = NULL;
                                break;
                        }
                        continue;
                }

                /*
                 * Get the parent directory and resolve its ncp.
                 */
                if (fakename) {
                        kfree(fakename, M_TEMP);
                        fakename = NULL;
                }
                error = vop_nlookupdotdot(*dvp->v_ops, dvp, &pvp, cred,
                                          &fakename);
                if (error) {
                        kprintf("lookupdotdot failed %d dvp %p\n", error, dvp);
                        break;
                }
                vn_unlock(pvp);

                /*
                 * Reuse makeit as a recursion depth counter.  On success
                 * nch will be fully referenced.
                 */
                cache_fromdvp(pvp, cred, makeit + 1, nch);
                vrele(pvp);
                if (nch->ncp == NULL)
                        break;

                /*
                 * Do an inefficient scan of pvp (embodied by ncp) to look
                 * for dvp.  This will create a namecache record for dvp on
                 * success.  We loop up to recheck on success.
                 *
                 * ncp and dvp are both held but not locked.
                 */
                error = cache_inefficient_scan(nch, cred, dvp, fakename);
                if (error) {
                        kprintf("cache_fromdvp: scan %p (%s) failed on dvp=%p\n",
                                pvp, nch->ncp->nc_name, dvp);
                        cache_drop(nch);
                        /* nch was NULLed out, reload mount */
                        nch->mount = dvp->v_mount;
                        break;
                }
                if (ncvp_debug) {
                        kprintf("cache_fromdvp: scan %p (%s) succeeded\n",
                                pvp, nch->ncp->nc_name);
                }
                cache_drop(nch);
                /* nch was NULLed out, reload mount */
                nch->mount = dvp->v_mount;
        }

        if (fakename)
                kfree(fakename, M_TEMP);

        /*
         * hold it for real so the mount gets a ref
         */
        if (nch->ncp)
                cache_hold(nch);
        if (saved_dvp)
                vrele(saved_dvp);
        if (nch->ncp)
                return (0);
        return (EINVAL);
}

/*
 * Go up the chain of parent directories until we find something
 * we can resolve into the namecache.  This is very inefficient.
 */
static
int
cache_fromdvp_try(struct vnode *dvp, struct ucred *cred,
                  struct vnode **saved_dvp)
{
        struct nchandle nch;
        struct vnode *pvp;
        int error;
        static time_t last_fromdvp_report;
        char *fakename;

        /*
         * Loop getting the parent directory vnode until we get something we
         * can resolve in the namecache.
         */
        vref(dvp);
        nch.mount = dvp->v_mount;
        nch.ncp = NULL;
        fakename = NULL;

        for (;;) {
                if (fakename) {
                        kfree(fakename, M_TEMP);
                        fakename = NULL;
                }
                error = vop_nlookupdotdot(*dvp->v_ops, dvp, &pvp, cred,
                                          &fakename);
                if (error) {
                        vrele(dvp);
                        break;
                }
                vn_unlock(pvp);
                if ((nch.ncp = TAILQ_FIRST(&pvp->v_namecache)) != NULL) {
                        _cache_hold(nch.ncp);
                        vrele(pvp);
                        break;
                }
                if (pvp->v_flag & VROOT) {
                        nch.ncp = _cache_get(pvp->v_mount->mnt_ncmountpt.ncp);
                        error = cache_resolve_mp(nch.mount);
                        _cache_unlock(nch.ncp);
                        vrele(pvp);
                        if (error) {
                                _cache_drop(nch.ncp);
                                nch.ncp = NULL;
                                vrele(dvp);
                        }
                        break;
                }
                vrele(dvp);
                dvp = pvp;
        }
        if (error == 0) {
                if (last_fromdvp_report != time_second) {
                        last_fromdvp_report = time_second;
                        kprintf("Warning: extremely inefficient path "
                                "resolution on %s\n",
                                nch.ncp->nc_name);
                }
                error = cache_inefficient_scan(&nch, cred, dvp, fakename);

                /*
                 * Hopefully dvp now has a namecache record associated with
                 * it.  Leave it referenced to prevent the kernel from
                 * recycling the vnode.  Otherwise extremely long directory
                 * paths could result in endless recycling.
                 */
                if (*saved_dvp)
                    vrele(*saved_dvp);
                *saved_dvp = dvp;
                _cache_drop(nch.ncp);
        }
        if (fakename)
                kfree(fakename, M_TEMP);
        return (error);
}

/*
 * Do an inefficient scan of the directory represented by ncp looking for
 * the directory vnode dvp.  ncp must be held but not locked on entry and
 * will be held on return.  dvp must be refd but not locked on entry and
 * will remain refd on return.
 *
 * Why do this at all?  Well, due to its stateless nature the NFS server
 * converts file handles directly to vnodes without necessarily going through
 * the namecache ops that would otherwise create the namecache topology
 * leading to the vnode.  We could either (1) Change the namecache algorithms
 * to allow disconnect namecache records that are re-merged opportunistically,
 * or (2) Make the NFS server backtrack and scan to recover a connected
 * namecache topology in order to then be able to issue new API lookups.
 *
 * It turns out that (1) is a huge mess.  It takes a nice clean set of 
 * namecache algorithms and introduces a lot of complication in every subsystem
 * that calls into the namecache to deal with the re-merge case, especially
 * since we are using the namecache to placehold negative lookups and the
 * vnode might not be immediately assigned. (2) is certainly far less
 * efficient then (1), but since we are only talking about directories here
 * (which are likely to remain cached), the case does not actually run all
 * that often and has the supreme advantage of not polluting the namecache
 * algorithms.
 *
 * If a fakename is supplied just construct a namecache entry using the
 * fake name.
 */
static int
cache_inefficient_scan(struct nchandle *nch, struct ucred *cred, 
                       struct vnode *dvp, char *fakename)
{
        struct nlcomponent nlc;
        struct nchandle rncp;
        struct dirent *den;
        struct vnode *pvp;
        struct vattr vat;
        struct iovec iov;
        struct uio uio;
        int blksize;
        int eofflag;
        int bytes;
        char *rbuf;
        int error;

        vat.va_blocksize = 0;
        if ((error = VOP_GETATTR(dvp, &vat)) != 0)
                return (error);
        if ((error = cache_vref(nch, cred, &pvp)) != 0)
                return (error);
        if (ncvp_debug)
                kprintf("inefficient_scan: directory iosize %ld vattr fileid = %lld\n", vat.va_blocksize, vat.va_fileid);

        /*
         * Use the supplied fakename if not NULL.  Fake names are typically
         * not in the actual filesystem hierarchy.  This is used by HAMMER
         * to glue @@timestamp recursions together.
         */
        if (fakename) {
                nlc.nlc_nameptr = fakename;
                nlc.nlc_namelen = strlen(fakename);
                rncp = cache_nlookup(nch, &nlc);
                goto done;
        }

        if ((blksize = vat.va_blocksize) == 0)
                blksize = DEV_BSIZE;
        rbuf = kmalloc(blksize, M_TEMP, M_WAITOK);
        rncp.ncp = NULL;

        eofflag = 0;
        uio.uio_offset = 0;
again:
        iov.iov_base = rbuf;
        iov.iov_len = blksize;
        uio.uio_iov = &iov;
        uio.uio_iovcnt = 1;
        uio.uio_resid = blksize;
        uio.uio_segflg = UIO_SYSSPACE;
        uio.uio_rw = UIO_READ;
        uio.uio_td = curthread;

        if (ncvp_debug >= 2)
                kprintf("cache_inefficient_scan: readdir @ %08x\n", (int)uio.uio_offset);
        error = VOP_READDIR(pvp, &uio, cred, &eofflag, NULL, NULL);
        if (error == 0) {
                den = (struct dirent *)rbuf;
                bytes = blksize - uio.uio_resid;

                while (bytes > 0) {
                        if (ncvp_debug >= 2) {
                                kprintf("cache_inefficient_scan: %*.*s\n",
                                        den->d_namlen, den->d_namlen, 
                                        den->d_name);
                        }
                        if (den->d_type != DT_WHT &&
                            den->d_ino == vat.va_fileid) {
                                if (ncvp_debug) {
                                        kprintf("cache_inefficient_scan: "
                                               "MATCHED inode %lld path %s/%*.*s\n",
                                               vat.va_fileid, nch->ncp->nc_name,
                                               den->d_namlen, den->d_namlen,
                                               den->d_name);
                                }
                                nlc.nlc_nameptr = den->d_name;
                                nlc.nlc_namelen = den->d_namlen;
                                rncp = cache_nlookup(nch, &nlc);
                                KKASSERT(rncp.ncp != NULL);
                                break;
                        }
                        bytes -= _DIRENT_DIRSIZ(den);
                        den = _DIRENT_NEXT(den);
                }
                if (rncp.ncp == NULL && eofflag == 0 && uio.uio_resid != blksize)
                        goto again;
        }
        kfree(rbuf, M_TEMP);
done:
        vrele(pvp);
        if (rncp.ncp) {
                if (rncp.ncp->nc_flag & NCF_UNRESOLVED) {
                        _cache_setvp(rncp.ncp, dvp);
                        if (ncvp_debug >= 2) {
                                kprintf("cache_inefficient_scan: setvp %s/%s = %p\n",
                                        nch->ncp->nc_name, rncp.ncp->nc_name, dvp);
                        }
                } else {
                        if (ncvp_debug >= 2) {
                                kprintf("cache_inefficient_scan: setvp %s/%s already set %p/%p\n", 
                                        nch->ncp->nc_name, rncp.ncp->nc_name, dvp,
                                        rncp.ncp->nc_vp);
                        }
                }
                if (rncp.ncp->nc_vp == NULL)
                        error = rncp.ncp->nc_error;
                /* 
                 * Release rncp after a successful nlookup.  rncp was fully
                 * referenced.
                 */
                cache_put(&rncp);
        } else {
                kprintf("cache_inefficient_scan: dvp %p NOT FOUND in %s\n",
                        dvp, nch->ncp->nc_name);
                error = ENOENT;
        }
        return (error);
}

/*
 * Zap a namecache entry.  The ncp is unconditionally set to an unresolved
 * state, which disassociates it from its vnode or ncneglist.
 *
 * Then, if there are no additional references to the ncp and no children,
 * the ncp is removed from the topology and destroyed.  This function will
 * also run through the nc_parent chain and destroy parent ncps if possible.
 * As a side benefit, it turns out the only conditions that allow running
 * up the chain are also the conditions to ensure no deadlock will occur.
 *
 * References and/or children may exist if the ncp is in the middle of the
 * topology, preventing the ncp from being destroyed.
 *
 * This function must be called with the ncp held and locked and will unlock
 * and drop it during zapping.
 */
static void
cache_zap(struct namecache *ncp)
{
        struct namecache *par;

        /*
         * Disassociate the vnode or negative cache ref and set NCF_UNRESOLVED.
         */
        _cache_setunresolved(ncp);

        /*
         * Try to scrap the entry and possibly tail-recurse on its parent.
         * We only scrap unref'd (other then our ref) unresolved entries,
         * we do not scrap 'live' entries.
         */
        while (ncp->nc_flag & NCF_UNRESOLVED) {
                /*
                 * Someone other then us has a ref, stop.
                 */
                if (ncp->nc_refs > 1)
                        goto done;

                /*
                 * We have children, stop.
                 */
                if (!TAILQ_EMPTY(&ncp->nc_list))
                        goto done;

                /*
                 * Remove ncp from the topology: hash table and parent linkage.
                 */
                if (ncp->nc_flag & NCF_HASHED) {
                        ncp->nc_flag &= ~NCF_HASHED;
                        LIST_REMOVE(ncp, nc_hash);
                }
                if ((par = ncp->nc_parent) != NULL) {
                        par = _cache_hold(par);
                        TAILQ_REMOVE(&par->nc_list, ncp, nc_entry);
                        ncp->nc_parent = NULL;
                        if (par->nc_vp && TAILQ_EMPTY(&par->nc_list))
                                vdrop(par->nc_vp);
                }

                /*
                 * ncp should not have picked up any refs.  Physically
                 * destroy the ncp.
                 */
                KKASSERT(ncp->nc_refs == 1);
                --numunres;
                /* _cache_unlock(ncp) not required */
                ncp->nc_refs = -1;      /* safety */
                if (ncp->nc_name)
                        kfree(ncp->nc_name, M_VFSCACHE);
                kfree(ncp, M_VFSCACHE);

                /*
                 * Loop on the parent (it may be NULL).  Only bother looping
                 * if the parent has a single ref (ours), which also means
                 * we can lock it trivially.
                 */
                ncp = par;
                if (ncp == NULL)
                        return;
                if (ncp->nc_refs != 1) {
                        _cache_drop(ncp);
                        return;
                }
                KKASSERT(par->nc_exlocks == 0);
                _cache_lock(ncp);
        }
done:
        _cache_unlock(ncp);
        atomic_subtract_int(&ncp->nc_refs, 1);
}

static enum { CHI_LOW, CHI_HIGH } cache_hysteresis_state = CHI_LOW;

static __inline
void
cache_hysteresis(void)
{
        /*
         * Don't cache too many negative hits.  We use hysteresis to reduce
         * the impact on the critical path.
         */
        switch(cache_hysteresis_state) {
        case CHI_LOW:
                if (numneg > MINNEG && numneg * ncnegfactor > numcache) {
                        cache_cleanneg(10);
                        cache_hysteresis_state = CHI_HIGH;
                }
                break;
        case CHI_HIGH:
                if (numneg > MINNEG * 9 / 10 && 
                    numneg * ncnegfactor * 9 / 10 > numcache
                ) {
                        cache_cleanneg(10);
                } else {
                        cache_hysteresis_state = CHI_LOW;
                }
                break;
        }
}

/*
 * NEW NAMECACHE LOOKUP API
 *
 * Lookup an entry in the cache.  A locked, referenced, non-NULL 
 * entry is *always* returned, even if the supplied component is illegal.
 * The resulting namecache entry should be returned to the system with
 * cache_put() or _cache_unlock() + cache_drop().
 *
 * namecache locks are recursive but care must be taken to avoid lock order
 * reversals.
 *
 * Nobody else will be able to manipulate the associated namespace (e.g.
 * create, delete, rename, rename-target) until the caller unlocks the
 * entry.
 *
 * The returned entry will be in one of three states:  positive hit (non-null
 * vnode), negative hit (null vnode), or unresolved (NCF_UNRESOLVED is set).
 * Unresolved entries must be resolved through the filesystem to associate the
 * vnode and/or determine whether a positive or negative hit has occured.
 *
 * It is not necessary to lock a directory in order to lock namespace under
 * that directory.  In fact, it is explicitly not allowed to do that.  A
 * directory is typically only locked when being created, renamed, or
 * destroyed.
 *
 * The directory (par) may be unresolved, in which case any returned child
 * will likely also be marked unresolved.  Likely but not guarenteed.  Since
 * the filesystem lookup requires a resolved directory vnode the caller is
 * responsible for resolving the namecache chain top-down.  This API 
 * specifically allows whole chains to be created in an unresolved state.
 */
struct nchandle
cache_nlookup(struct nchandle *par_nch, struct nlcomponent *nlc)
{
        struct nchandle nch;
        struct namecache *ncp;
        struct namecache *new_ncp;
        struct nchashhead *nchpp;
        u_int32_t hash;
        globaldata_t gd;

        numcalls++;
        gd = mycpu;

        /*
         * Try to locate an existing entry
         */
        hash = fnv_32_buf(nlc->nlc_nameptr, nlc->nlc_namelen, FNV1_32_INIT);
        hash = fnv_32_buf(&par_nch->ncp, sizeof(par_nch->ncp), hash);
        new_ncp = NULL;
restart:
        LIST_FOREACH(ncp, (NCHHASH(hash)), nc_hash) {
                numchecks++;

                /*
                 * Try to zap entries that have timed out.  We have
                 * to be careful here because locked leafs may depend
                 * on the vnode remaining intact in a parent, so only
                 * do this under very specific conditions.
                 */
                if (ncp->nc_timeout && 
                    (int)(ncp->nc_timeout - ticks) < 0 &&
                    (ncp->nc_flag & NCF_UNRESOLVED) == 0 &&
                    ncp->nc_exlocks == 0 &&
                    TAILQ_EMPTY(&ncp->nc_list)
                ) {
                        cache_zap(_cache_get(ncp));
                        goto restart;
                }

                /*
                 * Break out if we find a matching entry.  Note that
                 * UNRESOLVED entries may match, but DESTROYED entries
                 * do not.
                 */
                if (ncp->nc_parent == par_nch->ncp &&
                    ncp->nc_nlen == nlc->nlc_namelen &&
                    bcmp(ncp->nc_name, nlc->nlc_nameptr, ncp->nc_nlen) == 0 &&
                    (ncp->nc_flag & NCF_DESTROYED) == 0
                ) {
                        if (_cache_get_nonblock(ncp) == 0) {
                                if (new_ncp)
                                        _cache_free(new_ncp);
                                goto found;
                        }
                        _cache_get(ncp);
                        _cache_put(ncp);
                        goto restart;
                }
        }

        /*
         * We failed to locate an entry, create a new entry and add it to
         * the cache.  We have to relookup after possibly blocking in
         * malloc.
         */
        if (new_ncp == NULL) {
                new_ncp = cache_alloc(nlc->nlc_namelen);
                goto restart;
        }

        ncp = new_ncp;

        /*
         * Initialize as a new UNRESOLVED entry, lock (non-blocking),
         * and link to the parent.  The mount point is usually inherited
         * from the parent unless this is a special case such as a mount
         * point where nlc_namelen is 0.   If nlc_namelen is 0 nc_name will
         * be NULL.
         */
        if (nlc->nlc_namelen) {
                bcopy(nlc->nlc_nameptr, ncp->nc_name, nlc->nlc_namelen);
                ncp->nc_name[nlc->nlc_namelen] = 0;
        }
        nchpp = NCHHASH(hash);
        LIST_INSERT_HEAD(nchpp, ncp, nc_hash);
        ncp->nc_flag |= NCF_HASHED;
        cache_link_parent(ncp, par_nch->ncp);
found:
        /*
         * stats and namecache size management
         */
        if (ncp->nc_flag & NCF_UNRESOLVED)
                ++gd->gd_nchstats->ncs_miss;
        else if (ncp->nc_vp)
                ++gd->gd_nchstats->ncs_goodhits;
        else
                ++gd->gd_nchstats->ncs_neghits;
        cache_hysteresis();
        nch.mount = par_nch->mount;
        nch.ncp = ncp;
        ++nch.mount->mnt_refs;
        return(nch);
}

/*
 * The namecache entry is marked as being used as a mount point. 
 * Locate the mount if it is visible to the caller.
 */
struct findmount_info {
        struct mount *result;
        struct mount *nch_mount;
        struct namecache *nch_ncp;
};

static
int
cache_findmount_callback(struct mount *mp, void *data)
{
        struct findmount_info *info = data;

        /*
         * Check the mount's mounted-on point against the passed nch.
         */
        if (mp->mnt_ncmounton.mount == info->nch_mount &&
            mp->mnt_ncmounton.ncp == info->nch_ncp
        ) {
            info->result = mp;
            return(-1);
        }
        return(0);
}

struct mount *
cache_findmount(struct nchandle *nch)
{
        struct findmount_info info;

        info.result = NULL;
        info.nch_mount = nch->mount;
        info.nch_ncp = nch->ncp;
        mountlist_scan(cache_findmount_callback, &info,
                               MNTSCAN_FORWARD|MNTSCAN_NOBUSY);
        return(info.result);
}

/*
 * Resolve an unresolved namecache entry, generally by looking it up.
 * The passed ncp must be locked and refd. 
 *
 * Theoretically since a vnode cannot be recycled while held, and since
 * the nc_parent chain holds its vnode as long as children exist, the
 * direct parent of the cache entry we are trying to resolve should
 * have a valid vnode.  If not then generate an error that we can 
 * determine is related to a resolver bug.
 *
 * However, if a vnode was in the middle of a recyclement when the NCP
 * got locked, ncp->nc_vp might point to a vnode that is about to become
 * invalid.  cache_resolve() handles this case by unresolving the entry
 * and then re-resolving it.
 *
 * Note that successful resolution does not necessarily return an error
 * code of 0.  If the ncp resolves to a negative cache hit then ENOENT
 * will be returned.
 */
int
cache_resolve(struct nchandle *nch, struct ucred *cred)
{
        struct namecache *par;
        struct namecache *ncp;
        struct nchandle nctmp;
        struct mount *mp;
        struct vnode *dvp;
        int error;

        ncp = nch->ncp;
        mp = nch->mount;
restart:
        /*
         * If the ncp is already resolved we have nothing to do.  However,
         * we do want to guarentee that a usable vnode is returned when
         * a vnode is present, so make sure it hasn't been reclaimed.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                        _cache_setunresolved(ncp);
                if ((ncp->nc_flag & NCF_UNRESOLVED) == 0)
                        return (ncp->nc_error);
        }

        /*
         * Mount points need special handling because the parent does not
         * belong to the same filesystem as the ncp.
         */
        if (ncp == mp->mnt_ncmountpt.ncp)
                return (cache_resolve_mp(mp));

        /*
         * We expect an unbroken chain of ncps to at least the mount point,
         * and even all the way to root (but this code doesn't have to go
         * past the mount point).
         */
        if (ncp->nc_parent == NULL) {
                kprintf("EXDEV case 1 %p %*.*s\n", ncp,
                        ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name);
                ncp->nc_error = EXDEV;
                return(ncp->nc_error);
        }

        /*
         * The vp's of the parent directories in the chain are held via vhold()
         * due to the existance of the child, and should not disappear. 
         * However, there are cases where they can disappear:
         *
         *      - due to filesystem I/O errors.
         *      - due to NFS being stupid about tracking the namespace and
         *        destroys the namespace for entire directories quite often.
         *      - due to forced unmounts.
         *      - due to an rmdir (parent will be marked DESTROYED)
         *
         * When this occurs we have to track the chain backwards and resolve
         * it, looping until the resolver catches up to the current node.  We
         * could recurse here but we might run ourselves out of kernel stack
         * so we do it in a more painful manner.  This situation really should
         * not occur all that often, or if it does not have to go back too
         * many nodes to resolve the ncp.
         */
        while ((dvp = cache_dvpref(ncp)) == NULL) {
                /*
                 * This case can occur if a process is CD'd into a
                 * directory which is then rmdir'd.  If the parent is marked
                 * destroyed there is no point trying to resolve it.
                 */
                if (ncp->nc_parent->nc_flag & NCF_DESTROYED)
                        return(ENOENT);

                par = ncp->nc_parent;
                while (par->nc_parent && par->nc_parent->nc_vp == NULL)
                        par = par->nc_parent;
                if (par->nc_parent == NULL) {
                        kprintf("EXDEV case 2 %*.*s\n",
                                par->nc_nlen, par->nc_nlen, par->nc_name);
                        return (EXDEV);
                }
                kprintf("[diagnostic] cache_resolve: had to recurse on %*.*s\n",
                        par->nc_nlen, par->nc_nlen, par->nc_name);
                /*
                 * The parent is not set in stone, ref and lock it to prevent
                 * it from disappearing.  Also note that due to renames it
                 * is possible for our ncp to move and for par to no longer
                 * be one of its parents.  We resolve it anyway, the loop 
                 * will handle any moves.
                 */
                _cache_get(par);
                if (par == nch->mount->mnt_ncmountpt.ncp) {
                        cache_resolve_mp(nch->mount);
                } else if ((dvp = cache_dvpref(par)) == NULL) {
                        kprintf("[diagnostic] cache_resolve: raced on %*.*s\n", par->nc_nlen, par->nc_nlen, par->nc_name);
                        _cache_put(par);
                        continue;
                } else {
                        if (par->nc_flag & NCF_UNRESOLVED) {
                                nctmp.mount = mp;
                                nctmp.ncp = par;
                                par->nc_error = VOP_NRESOLVE(&nctmp, dvp, cred);
                        }
                        vrele(dvp);
                }
                if ((error = par->nc_error) != 0) {
                        if (par->nc_error != EAGAIN) {
                                kprintf("EXDEV case 3 %*.*s error %d\n",
                                    par->nc_nlen, par->nc_nlen, par->nc_name,
                                    par->nc_error);
                                _cache_put(par);
                                return(error);
                        }
                        kprintf("[diagnostic] cache_resolve: EAGAIN par %p %*.*s\n",
                                par, par->nc_nlen, par->nc_nlen, par->nc_name);
                }
                _cache_put(par);
                /* loop */
        }

        /*
         * Call VOP_NRESOLVE() to get the vp, then scan for any disconnected
         * ncp's and reattach them.  If this occurs the original ncp is marked
         * EAGAIN to force a relookup.
         *
         * NOTE: in order to call VOP_NRESOLVE(), the parent of the passed
         * ncp must already be resolved.
         */
        if (dvp) {
                nctmp.mount = mp;
                nctmp.ncp = ncp;
                ncp->nc_error = VOP_NRESOLVE(&nctmp, dvp, cred);
                vrele(dvp);
        } else {
                ncp->nc_error = EPERM;
        }
        if (ncp->nc_error == EAGAIN) {
                kprintf("[diagnostic] cache_resolve: EAGAIN ncp %p %*.*s\n",
                        ncp, ncp->nc_nlen, ncp->nc_nlen, ncp->nc_name);
                goto restart;
        }
        return(ncp->nc_error);
}

/*
 * Resolve the ncp associated with a mount point.  Such ncp's almost always
 * remain resolved and this routine is rarely called.  NFS MPs tends to force
 * re-resolution more often due to its mac-truck-smash-the-namecache
 * method of tracking namespace changes.
 *
 * The semantics for this call is that the passed ncp must be locked on
 * entry and will be locked on return.  However, if we actually have to
 * resolve the mount point we temporarily unlock the entry in order to
 * avoid race-to-root deadlocks due to e.g. dead NFS mounts.  Because of
 * the unlock we have to recheck the flags after we relock.
 */
static int
cache_resolve_mp(struct mount *mp)
{
        struct namecache *ncp = mp->mnt_ncmountpt.ncp;
        struct vnode *vp;
        int error;

        KKASSERT(mp != NULL);

        /*
         * If the ncp is already resolved we have nothing to do.  However,
         * we do want to guarentee that a usable vnode is returned when
         * a vnode is present, so make sure it hasn't been reclaimed.
         */
        if ((ncp->nc_flag & NCF_UNRESOLVED) == 0) {
                if (ncp->nc_vp && (ncp->nc_vp->v_flag & VRECLAIMED))
                        _cache_setunresolved(ncp);
        }

        if (ncp->nc_flag & NCF_UNRESOLVED) {
                _cache_unlock(ncp);
                while (vfs_busy(mp, 0))
                        ;
                error = VFS_ROOT(mp, &vp);
                _cache_lock(ncp);

                /*
                 * recheck the ncp state after relocking.
                 */
                if (ncp->nc_flag & NCF_UNRESOLVED) {
                        ncp->nc_error = error;
                        if (error == 0) {
                                _cache_setvp(ncp, vp);
                                vput(vp);
                        } else {
                                kprintf("[diagnostic] cache_resolve_mp: failed to resolve mount %p\n", mp);
                                _cache_setvp(ncp, NULL);
                        }
                } else if (error == 0) {
                        vput(vp);
                }
                vfs_unbusy(mp);
        }
        return(ncp->nc_error);
}

void
cache_cleanneg(int count)
{
        struct namecache *ncp;

        /*
         * Automode from the vnlru proc - clean out 10% of the negative cache
         * entries.
         */
        if (count == 0)
                count = numneg / 10 + 1;

        /*
         * Attempt to clean out the specified number of negative cache
         * entries.
         */
        while (count) {
                ncp = TAILQ_FIRST(&ncneglist);
                if (ncp == NULL) {
                        KKASSERT(numneg == 0);
                        break;
                }
                TAILQ_REMOVE(&ncneglist, ncp, nc_vnode);
                TAILQ_INSERT_TAIL(&ncneglist, ncp, nc_vnode);
                if (_cache_get_nonblock(ncp) == 0)
                        cache_zap(ncp);
                --count;
        }
}

/*
 * Rehash a ncp.  Rehashing is typically required if the name changes (should
 * not generally occur) or the parent link changes.  This function will
 * unhash the ncp if the ncp is no longer hashable.
 */
static void
_cache_rehash(struct namecache *ncp)
{
        struct nchashhead *nchpp;
        u_int32_t hash;

        if (ncp->nc_flag & NCF_HASHED) {
                ncp->nc_flag &= ~NCF_HASHED;
                LIST_REMOVE(ncp, nc_hash);
        }
        if (ncp->nc_nlen && ncp->nc_parent) {
                hash = fnv_32_buf(ncp->nc_name, ncp->nc_nlen, FNV1_32_INIT);
                hash = fnv_32_buf(&ncp->nc_parent, 
                                        sizeof(ncp->nc_parent), hash);
                nchpp = NCHHASH(hash);
                LIST_INSERT_HEAD(nchpp, ncp, nc_hash);
                ncp->nc_flag |= NCF_HASHED;
        }
}

/*
 * Name cache initialization, from vfsinit() when we are booting
 */
void
nchinit(void)
{
        int i;
        globaldata_t gd;

        /* initialise per-cpu namecache effectiveness statistics. */
        for (i = 0; i < ncpus; ++i) {
                gd = globaldata_find(i);
                gd->gd_nchstats = &nchstats[i];
        }
        TAILQ_INIT(&ncneglist);
        nchashtbl = hashinit(desiredvnodes*2, M_VFSCACHE, &nchash);
        nclockwarn = 5 * hz;
}

/*
 * Called from start_init() to bootstrap the root filesystem.  Returns
 * a referenced, unlocked namecache record.
 */
void
cache_allocroot(struct nchandle *nch, struct mount *mp, struct vnode *vp)
{
        nch->ncp = cache_alloc(0);
        nch->mount = mp;
        ++mp->mnt_refs;
        if (vp)
                _cache_setvp(nch->ncp, vp);
}

/*
 * vfs_cache_setroot()
 *
 *      Create an association between the root of our namecache and
 *      the root vnode.  This routine may be called several times during
 *      booting.
 *
 *      If the caller intends to save the returned namecache pointer somewhere
 *      it must cache_hold() it.
 */
void
vfs_cache_setroot(struct vnode *nvp, struct nchandle *nch)
{
        struct vnode *ovp;
        struct nchandle onch;

        ovp = rootvnode;
        onch = rootnch;
        rootvnode = nvp;
        if (nch)
                rootnch = *nch;
        else
                cache_zero(&rootnch);
        if (ovp)
                vrele(ovp);
        if (onch.ncp)
                cache_drop(&onch);
}

/*
 * XXX OLD API COMPAT FUNCTION.  This really messes up the new namecache
 * topology and is being removed as quickly as possible.  The new VOP_N*()
 * API calls are required to make specific adjustments using the supplied
 * ncp pointers rather then just bogusly purging random vnodes.
 *
 * Invalidate all namecache entries to a particular vnode as well as 
 * any direct children of that vnode in the namecache.  This is a 
 * 'catch all' purge used by filesystems that do not know any better.
 *
 * Note that the linkage between the vnode and its namecache entries will
 * be removed, but the namecache entries themselves might stay put due to
 * active references from elsewhere in the system or due to the existance of
 * the children.   The namecache topology is left intact even if we do not
 * know what the vnode association is.  Such entries will be marked
 * NCF_UNRESOLVED.
 */
void
cache_purge(struct vnode *vp)
{
        cache_inval_vp(vp, CINV_DESTROY | CINV_CHILDREN);
}

/*
 * Flush all entries referencing a particular filesystem.
 *
 * Since we need to check it anyway, we will flush all the invalid
 * entries at the same time.
 */
#if 0

void
cache_purgevfs(struct mount *mp)
{
        struct nchashhead *nchpp;
        struct namecache *ncp, *nnp;

        /*
         * Scan hash tables for applicable entries.
         */
        for (nchpp = &nchashtbl[nchash]; nchpp >= nchashtbl; nchpp--) {
                ncp = LIST_FIRST(nchpp);
                if (ncp)
                        _cache_hold(ncp);
                while (ncp) {
                        nnp = LIST_NEXT(ncp, nc_hash);
                        if (nnp)
                                _cache_hold(nnp);
                        if (ncp->nc_mount == mp) {
                                _cache_lock(ncp);
                                cache_zap(ncp);
                        } else {
                                _cache_drop(ncp);
                        }
                        ncp = nnp;
                }
        }
}

#endif

/*
 * Create a new (theoretically) unique fsmid
 */
int64_t
cache_getnewfsmid(void)
{
        static int fsmid_roller;
        int64_t fsmid;

        ++fsmid_roller;
        fsmid = ((int64_t)time_second << 32) |
                        (fsmid_roller & 0x7FFFFFFF);
        return (fsmid);
}


static int disablecwd;
SYSCTL_INT(_debug, OID_AUTO, disablecwd, CTLFLAG_RW, &disablecwd, 0, "");

static u_long numcwdcalls; STATNODE(CTLFLAG_RD, numcwdcalls, &numcwdcalls);
static u_long numcwdfail1; STATNODE(CTLFLAG_RD, numcwdfail1, &numcwdfail1);
static u_long numcwdfail2; STATNODE(CTLFLAG_RD, numcwdfail2, &numcwdfail2);
static u_long numcwdfail3; STATNODE(CTLFLAG_RD, numcwdfail3, &numcwdfail3);
static u_long numcwdfail4; STATNODE(CTLFLAG_RD, numcwdfail4, &numcwdfail4);
static u_long numcwdfound; STATNODE(CTLFLAG_RD, numcwdfound, &numcwdfound);

int
sys___getcwd(struct __getcwd_args *uap)
{
        int buflen;
        int error;
        char *buf;
        char *bp;

        if (disablecwd)
                return (ENODEV);

        buflen = uap->buflen;
        if (buflen < 2)
                return (EINVAL);
        if (buflen > MAXPATHLEN)
                buflen = MAXPATHLEN;

        buf = kmalloc(buflen, M_TEMP, M_WAITOK);
        bp = kern_getcwd(buf, buflen, &error);
        if (error == 0)
                error = copyout(bp, uap->buf, strlen(bp) + 1);
        kfree(buf, M_TEMP);
        return (error);
}

char *
kern_getcwd(char *buf, size_t buflen, int *error)
{
        struct proc *p = curproc;
        char *bp;
        int i, slash_prefixed;
        struct filedesc *fdp;
        struct nchandle nch;

        numcwdcalls++;
        bp = buf;
        bp += buflen - 1;
        *bp = '\0';
        fdp = p->p_fd;
        slash_prefixed = 0;

        nch = fdp->fd_ncdir;
        while (nch.ncp && (nch.ncp != fdp->fd_nrdir.ncp || 
               nch.mount != fdp->fd_nrdir.mount)
        ) {
                /*
                 * While traversing upwards if we encounter the root
                 * of the current mount we have to skip to the mount point
                 * in the underlying filesystem.
                 */
                if (nch.ncp == nch.mount->mnt_ncmountpt.ncp) {
                        nch = nch.mount->mnt_ncmounton;
                        continue;
                }

                /*
                 * Prepend the path segment
                 */
                for (i = nch.ncp->nc_nlen - 1; i >= 0; i--) {
                        if (bp == buf) {
                                numcwdfail4++;
                                *error = ENOMEM;
                                return(NULL);
                        }
                        *--bp = nch.ncp->nc_name[i];
                }
                if (bp == buf) {
                        numcwdfail4++;
                        *error = ENOMEM;
                        return(NULL);
                }
                *--bp = '/';
                slash_prefixed = 1;

                /*
                 * Go up a directory.  This isn't a mount point so we don't
                 * have to check again.
                 */
                nch.ncp = nch.ncp->nc_parent;
        }
        if (nch.ncp == NULL) {
                numcwdfail2++;
                *error = ENOENT;
                return(NULL);
        }
        if (!slash_prefixed) {
                if (bp == buf) {
                        numcwdfail4++;
                        *error = ENOMEM;
                        return(NULL);
                }
                *--bp = '/';
        }
        numcwdfound++;
        *error = 0;
        return (bp);
}

/*
 * Thus begins the fullpath magic.
 */

#undef STATNODE
#define STATNODE(name)                                                  \
        static u_int name;                                              \
        SYSCTL_UINT(_vfs_cache, OID_AUTO, name, CTLFLAG_RD, &name, 0, "")

static int disablefullpath;
SYSCTL_INT(_debug, OID_AUTO, disablefullpath, CTLFLAG_RW,
    &disablefullpath, 0, "");

STATNODE(numfullpathcalls);
STATNODE(numfullpathfail1);
STATNODE(numfullpathfail2);
STATNODE(numfullpathfail3);
STATNODE(numfullpathfail4);
STATNODE(numfullpathfound);

int
cache_fullpath(struct proc *p, struct nchandle *nchp, char **retbuf, char **freebuf)
{
        char *bp, *buf;
        int i, slash_prefixed;
        struct nchandle fd_nrdir;
        struct nchandle nch;

        numfullpathcalls--;

        *retbuf = NULL; 
        *freebuf = NULL;

        buf = kmalloc(MAXPATHLEN, M_TEMP, M_WAITOK);
        bp = buf + MAXPATHLEN - 1;
        *bp = '\0';
        if (p != NULL)
                fd_nrdir = p->p_fd->fd_nrdir;
        else
                fd_nrdir = rootnch;
        slash_prefixed = 0;
        nch = *nchp;

        while (nch.ncp && 
               (nch.ncp != fd_nrdir.ncp || nch.mount != fd_nrdir.mount)
        ) {
                /*
                 * While traversing upwards if we encounter the root
                 * of the current mount we have to skip to the mount point.
                 */
                if (nch.ncp == nch.mount->mnt_ncmountpt.ncp) {
                        nch = nch.mount->mnt_ncmounton;
                        continue;
                }

                /*
                 * Prepend the path segment
                 */
                for (i = nch.ncp->nc_nlen - 1; i >= 0; i--) {
                        if (bp == buf) {
                                numfullpathfail4++;
                                kfree(buf, M_TEMP);
                                return(ENOMEM);
                        }
                        *--bp = nch.ncp->nc_name[i];
                }
                if (bp == buf) {
                        numfullpathfail4++;
                        kfree(buf, M_TEMP);
                        return(ENOMEM);
                }
                *--bp = '/';
                slash_prefixed = 1;

                /*
                 * Go up a directory.  This isn't a mount point so we don't
                 * have to check again.
                 */
                nch.ncp = nch.ncp->nc_parent;
        }
        if (nch.ncp == NULL) {
                numfullpathfail2++;
                kfree(buf, M_TEMP);
                return(ENOENT);
        }

        if (!slash_prefixed) {
                if (bp == buf) {
                        numfullpathfail4++;
                        kfree(buf, M_TEMP);
                        return(ENOMEM);
                }
                *--bp = '/';
        }
        numfullpathfound++;
        *retbuf = bp; 
        *freebuf = buf;

        return(0);
}

int
vn_fullpath(struct proc *p, struct vnode *vn, char **retbuf, char **freebuf) 
{
        struct namecache *ncp;
        struct nchandle nch;

        numfullpathcalls++;
        if (disablefullpath)
                return (ENODEV);

        if (p == NULL)
                return (EINVAL);

        /* vn is NULL, client wants us to use p->p_textvp */
        if (vn == NULL) {
                if ((vn = p->p_textvp) == NULL)
                        return (EINVAL);
        }
        TAILQ_FOREACH(ncp, &vn->v_namecache, nc_vnode) {
                if (ncp->nc_nlen)
                        break;
        }
        if (ncp == NULL)
                return (EINVAL);

        numfullpathcalls--;
        nch.ncp = ncp;;
        nch.mount = vn->v_mount;
        return(cache_fullpath(p, &nch, retbuf, freebuf));
}