The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/kern/vfs_sync.c

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    1 /*
    2  * Copyright (c) 1989, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  * (c) UNIX System Laboratories, Inc.
    5  * All or some portions of this file are derived from material licensed
    6  * to the University of California by American Telephone and Telegraph
    7  * Co. or Unix System Laboratories, Inc. and are reproduced herein with
    8  * the permission of UNIX System Laboratories, Inc.
    9  *
   10  * Redistribution and use in source and binary forms, with or without
   11  * modification, are permitted provided that the following conditions
   12  * are met:
   13  * 1. Redistributions of source code must retain the above copyright
   14  *    notice, this list of conditions and the following disclaimer.
   15  * 2. Redistributions in binary form must reproduce the above copyright
   16  *    notice, this list of conditions and the following disclaimer in the
   17  *    documentation and/or other materials provided with the distribution.
   18  * 3. Neither the name of the University nor the names of its contributors
   19  *    may be used to endorse or promote products derived from this software
   20  *    without specific prior written permission.
   21  *
   22  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   23  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   24  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   25  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   26  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   27  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   28  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   29  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   31  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   32  * SUCH DAMAGE.
   33  *
   34  *      @(#)vfs_subr.c  8.31 (Berkeley) 5/26/95
   35  * $FreeBSD: src/sys/kern/vfs_subr.c,v 1.249.2.30 2003/04/04 20:35:57 tegge Exp $
   36  */
   37 
   38 /*
   39  * External virtual filesystem routines
   40  */
   41 
   42 #include <sys/param.h>
   43 #include <sys/systm.h>
   44 #include <sys/buf.h>
   45 #include <sys/conf.h>
   46 #include <sys/dirent.h>
   47 #include <sys/domain.h>
   48 #include <sys/eventhandler.h>
   49 #include <sys/fcntl.h>
   50 #include <sys/kernel.h>
   51 #include <sys/kthread.h>
   52 #include <sys/malloc.h>
   53 #include <sys/mbuf.h>
   54 #include <sys/mount.h>
   55 #include <sys/proc.h>
   56 #include <sys/namei.h>
   57 #include <sys/reboot.h>
   58 #include <sys/socket.h>
   59 #include <sys/stat.h>
   60 #include <sys/sysctl.h>
   61 #include <sys/syslog.h>
   62 #include <sys/vmmeter.h>
   63 #include <sys/vnode.h>
   64 
   65 #include <machine/limits.h>
   66 
   67 #include <vm/vm.h>
   68 #include <vm/vm_object.h>
   69 #include <vm/vm_extern.h>
   70 #include <vm/vm_kern.h>
   71 #include <vm/pmap.h>
   72 #include <vm/vm_map.h>
   73 #include <vm/vm_page.h>
   74 #include <vm/vm_pager.h>
   75 #include <vm/vnode_pager.h>
   76 
   77 #include <sys/buf2.h>
   78 #include <sys/thread2.h>
   79 
   80 /*
   81  * The workitem queue.
   82  */
   83 #define SYNCER_MAXDELAY         32
   84 static int sysctl_kern_syncdelay(SYSCTL_HANDLER_ARGS);
   85 time_t syncdelay = 30;          /* max time to delay syncing data */
   86 SYSCTL_PROC(_kern, OID_AUTO, syncdelay, CTLTYPE_INT | CTLFLAG_RW, 0, 0,
   87                 sysctl_kern_syncdelay, "I", "VFS data synchronization delay");
   88 time_t filedelay = 30;          /* time to delay syncing files */
   89 SYSCTL_INT(_kern, OID_AUTO, filedelay, CTLFLAG_RW,
   90                 &filedelay, 0, "File synchronization delay");
   91 time_t dirdelay = 29;           /* time to delay syncing directories */
   92 SYSCTL_INT(_kern, OID_AUTO, dirdelay, CTLFLAG_RW,
   93                 &dirdelay, 0, "Directory synchronization delay");
   94 time_t metadelay = 28;          /* time to delay syncing metadata */
   95 SYSCTL_INT(_kern, OID_AUTO, metadelay, CTLFLAG_RW,
   96                 &metadelay, 0, "VFS metadata synchronization delay");
   97 static int rushjob;                     /* number of slots to run ASAP */
   98 static int stat_rush_requests;  /* number of times I/O speeded up */
   99 SYSCTL_INT(_debug, OID_AUTO, rush_requests, CTLFLAG_RW,
  100                 &stat_rush_requests, 0, "");
  101 
  102 LIST_HEAD(synclist, vnode);
  103 
  104 #define SC_FLAG_EXIT            (0x1)           /* request syncer exit */
  105 #define SC_FLAG_DONE            (0x2)           /* syncer confirm exit */
  106 
  107 struct syncer_ctx {
  108         struct mount            *sc_mp;
  109         struct lwkt_token       sc_token;
  110         struct thread           *sc_thread;
  111         int                     sc_flags;
  112         struct synclist         *syncer_workitem_pending;
  113         long                    syncer_mask;
  114         int                     syncer_delayno;
  115         int                     syncer_forced;
  116         int                     syncer_rushjob;
  117 };
  118 
  119 static void syncer_thread(void *);
  120 
  121 static int
  122 sysctl_kern_syncdelay(SYSCTL_HANDLER_ARGS)
  123 {
  124         int error;
  125         int v = syncdelay;
  126 
  127         error = sysctl_handle_int(oidp, &v, 0, req);
  128         if (error || !req->newptr)
  129                 return (error);
  130         if (v < 1)
  131                 v = 1;
  132         if (v > SYNCER_MAXDELAY)
  133                 v = SYNCER_MAXDELAY;
  134         syncdelay = v;
  135 
  136         return(0);
  137 }
  138 
  139 /*
  140  * The workitem queue.
  141  * 
  142  * It is useful to delay writes of file data and filesystem metadata
  143  * for tens of seconds so that quickly created and deleted files need
  144  * not waste disk bandwidth being created and removed. To realize this,
  145  * we append vnodes to a "workitem" queue. When running with a soft
  146  * updates implementation, most pending metadata dependencies should
  147  * not wait for more than a few seconds. Thus, mounted on block devices
  148  * are delayed only about a half the time that file data is delayed.
  149  * Similarly, directory updates are more critical, so are only delayed
  150  * about a third the time that file data is delayed. Thus, there are
  151  * SYNCER_MAXDELAY queues that are processed round-robin at a rate of
  152  * one each second (driven off the filesystem syncer process). The
  153  * syncer_delayno variable indicates the next queue that is to be processed.
  154  * Items that need to be processed soon are placed in this queue:
  155  *
  156  *      syncer_workitem_pending[syncer_delayno]
  157  *
  158  * A delay of fifteen seconds is done by placing the request fifteen
  159  * entries later in the queue:
  160  *
  161  *      syncer_workitem_pending[(syncer_delayno + 15) & syncer_mask]
  162  *
  163  */
  164 
  165 /*
  166  * Add an item to the syncer work queue.
  167  *
  168  * WARNING: Cannot get vp->v_token here if not already held, we must
  169  *          depend on the syncer_token (which might already be held by
  170  *          the caller) to protect v_synclist and VONWORKLST.
  171  *
  172  * MPSAFE
  173  */
  174 void
  175 vn_syncer_add(struct vnode *vp, int delay)
  176 {
  177         struct syncer_ctx *ctx;
  178         int slot;
  179 
  180         ctx = vp->v_mount->mnt_syncer_ctx;
  181         lwkt_gettoken(&ctx->sc_token);
  182 
  183         if (vp->v_flag & VONWORKLST)
  184                 LIST_REMOVE(vp, v_synclist);
  185         if (delay <= 0) {
  186                 slot = -delay & ctx->syncer_mask;
  187         } else {
  188                 if (delay > SYNCER_MAXDELAY - 2)
  189                         delay = SYNCER_MAXDELAY - 2;
  190                 slot = (ctx->syncer_delayno + delay) & ctx->syncer_mask;
  191         }
  192 
  193         LIST_INSERT_HEAD(&ctx->syncer_workitem_pending[slot], vp, v_synclist);
  194         vsetflags(vp, VONWORKLST);
  195 
  196         lwkt_reltoken(&ctx->sc_token);
  197 }
  198 
  199 /*
  200  * Removes the vnode from the syncer list.  Since we might block while
  201  * acquiring the syncer_token we have to recheck conditions.
  202  *
  203  * vp->v_token held on call
  204  */
  205 void
  206 vn_syncer_remove(struct vnode *vp)
  207 {
  208         struct syncer_ctx *ctx;
  209 
  210         ctx = vp->v_mount->mnt_syncer_ctx;
  211         lwkt_gettoken(&ctx->sc_token);
  212 
  213         if ((vp->v_flag & (VISDIRTY | VONWORKLST | VOBJDIRTY)) == VONWORKLST &&
  214             RB_EMPTY(&vp->v_rbdirty_tree)) {
  215                 vclrflags(vp, VONWORKLST);
  216                 LIST_REMOVE(vp, v_synclist);
  217         }
  218 
  219         lwkt_reltoken(&ctx->sc_token);
  220 }
  221 
  222 /*
  223  * vnode must be locked
  224  */
  225 void
  226 vclrisdirty(struct vnode *vp)
  227 {
  228         vclrflags(vp, VISDIRTY);
  229         if (vp->v_flag & VONWORKLST)
  230                 vn_syncer_remove(vp);
  231 }
  232 
  233 void
  234 vclrobjdirty(struct vnode *vp)
  235 {
  236         vclrflags(vp, VOBJDIRTY);
  237         if (vp->v_flag & VONWORKLST)
  238                 vn_syncer_remove(vp);
  239 }
  240 
  241 /*
  242  * vnode must be stable
  243  */
  244 void
  245 vsetisdirty(struct vnode *vp)
  246 {
  247         struct syncer_ctx *ctx;
  248 
  249         if ((vp->v_flag & VISDIRTY) == 0) {
  250                 ctx = vp->v_mount->mnt_syncer_ctx;
  251                 vsetflags(vp, VISDIRTY);
  252                 lwkt_gettoken(&ctx->sc_token);
  253                 if ((vp->v_flag & VONWORKLST) == 0)
  254                         vn_syncer_add(vp, syncdelay);
  255                 lwkt_reltoken(&ctx->sc_token);
  256         }
  257 }
  258 
  259 void
  260 vsetobjdirty(struct vnode *vp)
  261 {
  262         struct syncer_ctx *ctx;
  263 
  264         if ((vp->v_flag & VOBJDIRTY) == 0) {
  265                 ctx = vp->v_mount->mnt_syncer_ctx;
  266                 vsetflags(vp, VOBJDIRTY);
  267                 lwkt_gettoken(&ctx->sc_token);
  268                 if ((vp->v_flag & VONWORKLST) == 0)
  269                         vn_syncer_add(vp, syncdelay);
  270                 lwkt_reltoken(&ctx->sc_token);
  271         }
  272 }
  273 
  274 /*
  275  * Create per-filesystem syncer process
  276  */
  277 void
  278 vn_syncer_thr_create(struct mount *mp)
  279 {
  280         struct syncer_ctx *ctx;
  281         static int syncalloc = 0;
  282 
  283         ctx = kmalloc(sizeof(struct syncer_ctx), M_TEMP, M_WAITOK | M_ZERO);
  284         ctx->sc_mp = mp;
  285         ctx->sc_flags = 0;
  286         ctx->syncer_workitem_pending = hashinit(SYNCER_MAXDELAY, M_DEVBUF,
  287                                                 &ctx->syncer_mask);
  288         ctx->syncer_delayno = 0;
  289         lwkt_token_init(&ctx->sc_token, "syncer");
  290         mp->mnt_syncer_ctx = ctx;
  291         kthread_create(syncer_thread, ctx, &ctx->sc_thread,
  292                        "syncer%d", ++syncalloc & 0x7FFFFFFF);
  293 }
  294 
  295 /*
  296  * Stop per-filesystem syncer process
  297  */
  298 void
  299 vn_syncer_thr_stop(struct mount *mp)
  300 {
  301         struct syncer_ctx *ctx;
  302 
  303         ctx = mp->mnt_syncer_ctx;
  304         if (ctx == NULL)
  305                 return;
  306 
  307         lwkt_gettoken(&ctx->sc_token);
  308 
  309         /* Signal the syncer process to exit */
  310         ctx->sc_flags |= SC_FLAG_EXIT;
  311         wakeup(ctx);
  312         
  313         /* Wait till syncer process exits */
  314         while ((ctx->sc_flags & SC_FLAG_DONE) == 0) 
  315                 tsleep(&ctx->sc_flags, 0, "syncexit", hz);
  316 
  317         mp->mnt_syncer_ctx = NULL;
  318         lwkt_reltoken(&ctx->sc_token);
  319 
  320         hashdestroy(ctx->syncer_workitem_pending, M_DEVBUF, ctx->syncer_mask);
  321         kfree(ctx, M_TEMP);
  322 }
  323 
  324 struct  thread *updatethread;
  325 
  326 /*
  327  * System filesystem synchronizer daemon.
  328  */
  329 static void
  330 syncer_thread(void *_ctx)
  331 {
  332         struct syncer_ctx *ctx = _ctx;
  333         struct synclist *slp;
  334         struct vnode *vp;
  335         long starttime;
  336         int *sc_flagsp;
  337         int sc_flags;
  338         int vnodes_synced = 0;
  339         int delta;
  340         int dummy = 0;
  341 
  342         for (;;) {
  343                 kproc_suspend_loop();
  344 
  345                 starttime = time_uptime;
  346                 lwkt_gettoken(&ctx->sc_token);
  347 
  348                 /*
  349                  * Push files whose dirty time has expired.  Be careful
  350                  * of interrupt race on slp queue.
  351                  */
  352                 slp = &ctx->syncer_workitem_pending[ctx->syncer_delayno];
  353                 ctx->syncer_delayno = (ctx->syncer_delayno + 1) &
  354                                       ctx->syncer_mask;
  355 
  356                 while ((vp = LIST_FIRST(slp)) != NULL) {
  357                         if (ctx->syncer_forced) {
  358                                 if (vget(vp, LK_EXCLUSIVE) == 0) {
  359                                         VOP_FSYNC(vp, MNT_NOWAIT, 0);
  360                                         vput(vp);
  361                                         vnodes_synced++;
  362                                 }
  363                         } else {
  364                                 if (vget(vp, LK_EXCLUSIVE | LK_NOWAIT) == 0) {
  365                                         VOP_FSYNC(vp, MNT_LAZY, 0);
  366                                         vput(vp);
  367                                         vnodes_synced++;
  368                                 }
  369                         }
  370 
  371                         /*
  372                          * vp is stale but can still be used if we can
  373                          * verify that it remains at the head of the list.
  374                          * Be careful not to try to get vp->v_token as
  375                          * vp can become stale if this blocks.
  376                          *
  377                          * If the vp is still at the head of the list were
  378                          * unable to completely flush it and move it to
  379                          * a later slot to give other vnodes a fair shot.
  380                          *
  381                          * Note that v_tag VT_VFS vnodes can remain on the
  382                          * worklist with no dirty blocks, but sync_fsync()
  383                          * moves it to a later slot so we will never see it
  384                          * here.
  385                          *
  386                          * It is possible to race a vnode with no dirty
  387                          * buffers being removed from the list.  If this
  388                          * occurs we will move the vnode in the synclist
  389                          * and then the other thread will remove it.  Do
  390                          * not try to remove it here.
  391                          */
  392                         if (LIST_FIRST(slp) == vp)
  393                                 vn_syncer_add(vp, syncdelay);
  394                 }
  395 
  396                 sc_flags = ctx->sc_flags;
  397 
  398                 /* Exit on unmount */
  399                 if (sc_flags & SC_FLAG_EXIT)
  400                         break;
  401 
  402                 lwkt_reltoken(&ctx->sc_token);
  403 
  404                 /*
  405                  * Do sync processing for each mount.
  406                  */
  407                 if (ctx->sc_mp)
  408                         bio_ops_sync(ctx->sc_mp);
  409 
  410                 /*
  411                  * The variable rushjob allows the kernel to speed up the
  412                  * processing of the filesystem syncer process. A rushjob
  413                  * value of N tells the filesystem syncer to process the next
  414                  * N seconds worth of work on its queue ASAP. Currently rushjob
  415                  * is used by the soft update code to speed up the filesystem
  416                  * syncer process when the incore state is getting so far
  417                  * ahead of the disk that the kernel memory pool is being
  418                  * threatened with exhaustion.
  419                  */
  420                 delta = rushjob - ctx->syncer_rushjob;
  421                 if ((u_int)delta > syncdelay / 2) {
  422                         ctx->syncer_rushjob = rushjob - syncdelay / 2;
  423                         tsleep(&dummy, 0, "rush", 1);
  424                         continue;
  425                 }
  426                 if (delta) {
  427                         ++ctx->syncer_rushjob;
  428                         tsleep(&dummy, 0, "rush", 1);
  429                         continue;
  430                 }
  431 
  432                 /*
  433                  * If it has taken us less than a second to process the
  434                  * current work, then wait. Otherwise start right over
  435                  * again. We can still lose time if any single round
  436                  * takes more than two seconds, but it does not really
  437                  * matter as we are just trying to generally pace the
  438                  * filesystem activity.
  439                  */
  440                 if (time_uptime == starttime)
  441                         tsleep(ctx, 0, "syncer", hz);
  442         }
  443 
  444         /*
  445          * Unmount/exit path for per-filesystem syncers; sc_token held
  446          */
  447         ctx->sc_flags |= SC_FLAG_DONE;
  448         sc_flagsp = &ctx->sc_flags;
  449         lwkt_reltoken(&ctx->sc_token);
  450         wakeup(sc_flagsp);
  451 
  452         kthread_exit();
  453 }
  454 
  455 /*
  456  * Request that the syncer daemon for a specific mount speed up its work.
  457  * If mp is NULL the caller generally wants to speed up all syncers.
  458  */
  459 void
  460 speedup_syncer(struct mount *mp)
  461 {
  462         /*
  463          * Don't bother protecting the test.  unsleep_and_wakeup_thread()
  464          * will only do something real if the thread is in the right state.
  465          */
  466         atomic_add_int(&rushjob, 1);
  467         ++stat_rush_requests;
  468         if (mp)
  469                 wakeup(mp->mnt_syncer_ctx);
  470 }
  471 
  472 /*
  473  * Routine to create and manage a filesystem syncer vnode.
  474  */
  475 static int sync_close(struct vop_close_args *);
  476 static int sync_fsync(struct vop_fsync_args *);
  477 static int sync_inactive(struct vop_inactive_args *);
  478 static int sync_reclaim (struct vop_reclaim_args *);
  479 static int sync_print(struct vop_print_args *);
  480 
  481 static struct vop_ops sync_vnode_vops = {
  482         .vop_default =  vop_eopnotsupp,
  483         .vop_close =    sync_close,
  484         .vop_fsync =    sync_fsync,
  485         .vop_inactive = sync_inactive,
  486         .vop_reclaim =  sync_reclaim,
  487         .vop_print =    sync_print,
  488 };
  489 
  490 static struct vop_ops *sync_vnode_vops_p = &sync_vnode_vops;
  491 
  492 VNODEOP_SET(sync_vnode_vops);
  493 
  494 /*
  495  * Create a new filesystem syncer vnode for the specified mount point.
  496  * This vnode is placed on the worklist and is responsible for sync'ing
  497  * the filesystem.
  498  *
  499  * NOTE: read-only mounts are also placed on the worklist.  The filesystem
  500  * sync code is also responsible for cleaning up vnodes.
  501  */
  502 int
  503 vfs_allocate_syncvnode(struct mount *mp)
  504 {
  505         struct vnode *vp;
  506         static long start, incr, next;
  507         int error;
  508 
  509         /* Allocate a new vnode */
  510         error = getspecialvnode(VT_VFS, mp, &sync_vnode_vops_p, &vp, 0, 0);
  511         if (error) {
  512                 mp->mnt_syncer = NULL;
  513                 return (error);
  514         }
  515         vp->v_type = VNON;
  516         /*
  517          * Place the vnode onto the syncer worklist. We attempt to
  518          * scatter them about on the list so that they will go off
  519          * at evenly distributed times even if all the filesystems
  520          * are mounted at once.
  521          */
  522         next += incr;
  523         if (next == 0 || next > SYNCER_MAXDELAY) {
  524                 start /= 2;
  525                 incr /= 2;
  526                 if (start == 0) {
  527                         start = SYNCER_MAXDELAY / 2;
  528                         incr = SYNCER_MAXDELAY;
  529                 }
  530                 next = start;
  531         }
  532 
  533         /*
  534          * Only put the syncer vnode onto the syncer list if we have a
  535          * syncer thread.  Some VFS's (aka NULLFS) don't need a syncer
  536          * thread.
  537          */
  538         if (mp->mnt_syncer_ctx)
  539                 vn_syncer_add(vp, syncdelay > 0 ? next % syncdelay : 0);
  540 
  541         /*
  542          * The mnt_syncer field inherits the vnode reference, which is
  543          * held until later decomissioning.
  544          */
  545         mp->mnt_syncer = vp;
  546         vx_unlock(vp);
  547         return (0);
  548 }
  549 
  550 static int
  551 sync_close(struct vop_close_args *ap)
  552 {
  553         return (0);
  554 }
  555 
  556 /*
  557  * Do a lazy sync of the filesystem.
  558  *
  559  * sync_fsync { struct vnode *a_vp, int a_waitfor }
  560  */
  561 static int
  562 sync_fsync(struct vop_fsync_args *ap)
  563 {
  564         struct vnode *syncvp = ap->a_vp;
  565         struct mount *mp = syncvp->v_mount;
  566         int asyncflag;
  567 
  568         /*
  569          * We only need to do something if this is a lazy evaluation.
  570          */
  571         if ((ap->a_waitfor & MNT_LAZY) == 0)
  572                 return (0);
  573 
  574         /*
  575          * Move ourselves to the back of the sync list.
  576          */
  577         vn_syncer_add(syncvp, syncdelay);
  578 
  579         /*
  580          * Walk the list of vnodes pushing all that are dirty and
  581          * not already on the sync list, and freeing vnodes which have
  582          * no refs and whos VM objects are empty.  vfs_msync() handles
  583          * the VM issues and must be called whether the mount is readonly
  584          * or not.
  585          */
  586         if (vfs_busy(mp, LK_NOWAIT) != 0)
  587                 return (0);
  588         if (mp->mnt_flag & MNT_RDONLY) {
  589                 vfs_msync(mp, MNT_NOWAIT);
  590         } else {
  591                 asyncflag = mp->mnt_flag & MNT_ASYNC;
  592                 mp->mnt_flag &= ~MNT_ASYNC;     /* ZZZ hack */
  593                 vfs_msync(mp, MNT_NOWAIT);
  594                 VFS_SYNC(mp, MNT_NOWAIT | MNT_LAZY);
  595                 if (asyncflag)
  596                         mp->mnt_flag |= MNT_ASYNC;
  597         }
  598         vfs_unbusy(mp);
  599         return (0);
  600 }
  601 
  602 /*
  603  * The syncer vnode is no longer referenced.
  604  *
  605  * sync_inactive { struct vnode *a_vp, struct proc *a_p }
  606  */
  607 static int
  608 sync_inactive(struct vop_inactive_args *ap)
  609 {
  610         vgone_vxlocked(ap->a_vp);
  611         return (0);
  612 }
  613 
  614 /*
  615  * The syncer vnode is no longer needed and is being decommissioned.
  616  * This can only occur when the last reference has been released on
  617  * mp->mnt_syncer, so mp->mnt_syncer had better be NULL.
  618  *
  619  * Modifications to the worklist must be protected with a critical
  620  * section.
  621  *
  622  *      sync_reclaim { struct vnode *a_vp }
  623  */
  624 static int
  625 sync_reclaim(struct vop_reclaim_args *ap)
  626 {
  627         struct vnode *vp = ap->a_vp;
  628         struct syncer_ctx *ctx;
  629 
  630         ctx = vp->v_mount->mnt_syncer_ctx;
  631         if (ctx) {
  632                 lwkt_gettoken(&ctx->sc_token);
  633                 KKASSERT(vp->v_mount->mnt_syncer != vp);
  634                 if (vp->v_flag & VONWORKLST) {
  635                         LIST_REMOVE(vp, v_synclist);
  636                         vclrflags(vp, VONWORKLST);
  637                 }
  638                 lwkt_reltoken(&ctx->sc_token);
  639         } else {
  640                 KKASSERT((vp->v_flag & VONWORKLST) == 0);
  641         }
  642 
  643         return (0);
  644 }
  645 
  646 /*
  647  * This is very similar to vmntvnodescan() but it only scans the
  648  * vnodes on the syncer list.  VFS's which support faster VFS_SYNC
  649  * operations use the VISDIRTY flag on the vnode to ensure that vnodes
  650  * with dirty inodes are added to the syncer in addition to vnodes
  651  * with dirty buffers, and can use this function instead of nmntvnodescan().
  652  * 
  653  * This is important when a system has millions of vnodes.
  654  */
  655 int
  656 vsyncscan(
  657     struct mount *mp,
  658     int vmsc_flags,
  659     int (*slowfunc)(struct mount *mp, struct vnode *vp, void *data),
  660     void *data
  661 ) {
  662         struct syncer_ctx *ctx;
  663         struct synclist *slp;
  664         struct vnode *vp;
  665         int b;
  666         int i;
  667         int lkflags;
  668 
  669         if (vmsc_flags & VMSC_NOWAIT)
  670                 lkflags = LK_NOWAIT;
  671         else
  672                 lkflags = 0;
  673 
  674         /*
  675          * Syncer list context.  This API requires a dedicated syncer thread.
  676          * (MNTK_THR_SYNC).
  677          */
  678         KKASSERT(mp->mnt_kern_flag & MNTK_THR_SYNC);
  679         ctx = mp->mnt_syncer_ctx;
  680         lwkt_gettoken(&ctx->sc_token);
  681 
  682         /*
  683          * Setup for loop.  Allow races against the syncer thread but
  684          * require that the syncer thread no be lazy if we were told
  685          * not to be lazy.
  686          */
  687         b = ctx->syncer_delayno & ctx->syncer_mask;
  688         i = b;
  689         if ((vmsc_flags & VMSC_NOWAIT) == 0)
  690                 ++ctx->syncer_forced;
  691 
  692         do {
  693                 slp = &ctx->syncer_workitem_pending[i];
  694 
  695                 while ((vp = LIST_FIRST(slp)) != NULL) {
  696                         KKASSERT(vp->v_mount == mp);
  697                         if (vmsc_flags & VMSC_GETVP) {
  698                                 if (vget(vp, LK_EXCLUSIVE | lkflags) == 0) {
  699                                         slowfunc(mp, vp, data);
  700                                         vput(vp);
  701                                 }
  702                         } else if (vmsc_flags & VMSC_GETVX) {
  703                                 vx_get(vp);
  704                                 slowfunc(mp, vp, data);
  705                                 vx_put(vp);
  706                         } else {
  707                                 vhold(vp);
  708                                 slowfunc(mp, vp, data);
  709                                 vdrop(vp);
  710                         }
  711                         if (LIST_FIRST(slp) == vp)
  712                                 vn_syncer_add(vp, -(i + syncdelay));
  713                 }
  714                 i = (i + 1) & ctx->syncer_mask;
  715         } while (i != b);
  716 
  717         if ((vmsc_flags & VMSC_NOWAIT) == 0)
  718                 --ctx->syncer_forced;
  719         lwkt_reltoken(&ctx->sc_token);
  720         return(0);
  721 }
  722 
  723 /*
  724  * Print out a syncer vnode.
  725  *
  726  *      sync_print { struct vnode *a_vp }
  727  */
  728 static int
  729 sync_print(struct vop_print_args *ap)
  730 {
  731         struct vnode *vp = ap->a_vp;
  732 
  733         kprintf("syncer vnode");
  734         lockmgr_printinfo(&vp->v_lock);
  735         kprintf("\n");
  736         return (0);
  737 }
  738 

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