FreeBSD/Linux Kernel Cross Reference
sys/vm/vm_pageout.c

     /*-
      * Copyright (c) 1991 Regents of the University of California.
      * All rights reserved.
      * Copyright (c) 1994 John S. Dyson
      * All rights reserved.
      * Copyright (c) 1994 David Greenman
      * All rights reserved.
      * Copyright (c) 2005 Yahoo! Technologies Norway AS
      * All rights reserved.
     *
     * This code is derived from software contributed to Berkeley by
     * The Mach Operating System project at Carnegie-Mellon University.
     *
     * 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.
     *
     *      from: @(#)vm_pageout.c  7.4 (Berkeley) 5/7/91
     *
     *
     * Copyright (c) 1987, 1990 Carnegie-Mellon University.
     * All rights reserved.
     *
     * Authors: Avadis Tevanian, Jr., Michael Wayne Young
     *
     * Permission to use, copy, modify and distribute this software and
     * its documentation is hereby granted, provided that both the copyright
     * notice and this permission notice appear in all copies of the
     * software, derivative works or modified versions, and any portions
     * thereof, and that both notices appear in supporting documentation.
     *
     * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
     * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
     * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
     *
     * Carnegie Mellon requests users of this software to return to
     *
     *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
     *  School of Computer Science
     *  Carnegie Mellon University
     *  Pittsburgh PA 15213-3890
     *
     * any improvements or extensions that they make and grant Carnegie the
     * rights to redistribute these changes.
     */
    
    /*
     *      The proverbial page-out daemon.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_vm.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/eventhandler.h>
    #include <sys/lock.h>
    #include <sys/mutex.h>
    #include <sys/proc.h>
    #include <sys/kthread.h>
    #include <sys/ktr.h>
    #include <sys/mount.h>
    #include <sys/racct.h>
    #include <sys/resourcevar.h>
    #include <sys/sched.h>
    #include <sys/sdt.h>
    #include <sys/signalvar.h>
    #include <sys/smp.h>
    #include <sys/time.h>
    #include <sys/vnode.h>
    #include <sys/vmmeter.h>
    #include <sys/rwlock.h>
   #include <sys/sx.h>
   #include <sys/sysctl.h>
   
   #include <vm/vm.h>
   #include <vm/vm_param.h>
   #include <vm/vm_object.h>
   #include <vm/vm_page.h>
   #include <vm/vm_map.h>
   #include <vm/vm_pageout.h>
   #include <vm/vm_pager.h>
   #include <vm/vm_phys.h>
   #include <vm/swap_pager.h>
   #include <vm/vm_extern.h>
   #include <vm/uma.h>
   
   /*
    * System initialization
    */
   
   /* the kernel process "vm_pageout"*/
   static void vm_pageout(void);
   static void vm_pageout_init(void);
   static int vm_pageout_clean(vm_page_t m, int *numpagedout);
   static int vm_pageout_cluster(vm_page_t m);
   static bool vm_pageout_scan(struct vm_domain *vmd, int pass);
   static void vm_pageout_mightbe_oom(struct vm_domain *vmd, int page_shortage,
       int starting_page_shortage);
   
   SYSINIT(pagedaemon_init, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, vm_pageout_init,
       NULL);
   
   struct proc *pageproc;
   
   static struct kproc_desc page_kp = {
           "pagedaemon",
           vm_pageout,
           &pageproc
   };
   SYSINIT(pagedaemon, SI_SUB_KTHREAD_PAGE, SI_ORDER_SECOND, kproc_start,
       &page_kp);
   
   SDT_PROVIDER_DEFINE(vm);
   SDT_PROBE_DEFINE(vm, , , vm__lowmem_scan);
   
   /* Pagedaemon activity rates, in subdivisions of one second. */
   #define VM_LAUNDER_RATE         10
   #define VM_INACT_SCAN_RATE      2
   
   int vm_pageout_deficit;         /* Estimated number of pages deficit */
   u_int vm_pageout_wakeup_thresh;
   static int vm_pageout_oom_seq = 12;
   bool vm_pageout_wanted;         /* Event on which pageout daemon sleeps */
   bool vm_pages_needed;           /* Are threads waiting for free pages? */
   
   /* Pending request for dirty page laundering. */
   static enum {
           VM_LAUNDRY_IDLE,
           VM_LAUNDRY_BACKGROUND,
           VM_LAUNDRY_SHORTFALL
   } vm_laundry_request = VM_LAUNDRY_IDLE;
   
   static int vm_pageout_update_period;
   static int disable_swap_pageouts;
   static int lowmem_period = 10;
   static time_t lowmem_uptime;
   
   static int vm_panic_on_oom = 0;
   
   SYSCTL_INT(_vm, OID_AUTO, panic_on_oom,
           CTLFLAG_RWTUN, &vm_panic_on_oom, 0,
           "panic on out of memory instead of killing the largest process");
   
   SYSCTL_INT(_vm, OID_AUTO, pageout_wakeup_thresh,
           CTLFLAG_RWTUN, &vm_pageout_wakeup_thresh, 0,
           "free page threshold for waking up the pageout daemon");
   
   SYSCTL_INT(_vm, OID_AUTO, pageout_update_period,
           CTLFLAG_RWTUN, &vm_pageout_update_period, 0,
           "Maximum active LRU update period");
     
   SYSCTL_INT(_vm, OID_AUTO, lowmem_period, CTLFLAG_RWTUN, &lowmem_period, 0,
           "Low memory callback period");
   
   SYSCTL_INT(_vm, OID_AUTO, disable_swapspace_pageouts,
           CTLFLAG_RWTUN, &disable_swap_pageouts, 0, "Disallow swapout of dirty pages");
   
   static int pageout_lock_miss;
   SYSCTL_INT(_vm, OID_AUTO, pageout_lock_miss,
           CTLFLAG_RD, &pageout_lock_miss, 0, "vget() lock misses during pageout");
   
   SYSCTL_INT(_vm, OID_AUTO, pageout_oom_seq,
           CTLFLAG_RWTUN, &vm_pageout_oom_seq, 0,
           "back-to-back calls to oom detector to start OOM");
   
   static int act_scan_laundry_weight = 3;
   SYSCTL_INT(_vm, OID_AUTO, act_scan_laundry_weight, CTLFLAG_RWTUN,
       &act_scan_laundry_weight, 0,
       "weight given to clean vs. dirty pages in active queue scans");
   
   static u_int vm_background_launder_target;
   SYSCTL_UINT(_vm, OID_AUTO, background_launder_target, CTLFLAG_RWTUN,
       &vm_background_launder_target, 0,
       "background laundering target, in pages");
   
   static u_int vm_background_launder_rate = 4096;
   SYSCTL_UINT(_vm, OID_AUTO, background_launder_rate, CTLFLAG_RWTUN,
       &vm_background_launder_rate, 0,
       "background laundering rate, in kilobytes per second");
   
   static u_int vm_background_launder_max = 20 * 1024;
   SYSCTL_UINT(_vm, OID_AUTO, background_launder_max, CTLFLAG_RWTUN,
       &vm_background_launder_max, 0, "background laundering cap, in kilobytes");
   
   int vm_pageout_page_count = 32;
   
   int vm_page_max_wired;          /* XXX max # of wired pages system-wide */
   SYSCTL_INT(_vm, OID_AUTO, max_wired,
           CTLFLAG_RW, &vm_page_max_wired, 0, "System-wide limit to wired page count");
   
   static u_int isqrt(u_int num);
   static boolean_t vm_pageout_fallback_object_lock(vm_page_t, vm_page_t *);
   static int vm_pageout_launder(struct vm_domain *vmd, int launder,
       bool in_shortfall);
   static void vm_pageout_laundry_worker(void *arg);
   static boolean_t vm_pageout_page_lock(vm_page_t, vm_page_t *);
   
   /*
    * Initialize a dummy page for marking the caller's place in the specified
    * paging queue.  In principle, this function only needs to set the flag
    * PG_MARKER.  Nonetheless, it write busies and initializes the hold count
    * to one as safety precautions.
    */ 
   static void
   vm_pageout_init_marker(vm_page_t marker, u_short queue)
   {
   
           bzero(marker, sizeof(*marker));
           marker->flags = PG_MARKER;
           marker->busy_lock = VPB_SINGLE_EXCLUSIVER;
           marker->queue = queue;
           marker->hold_count = 1;
   }
   
   /*
    * vm_pageout_fallback_object_lock:
    * 
    * Lock vm object currently associated with `m'. VM_OBJECT_TRYWLOCK is
    * known to have failed and page queue must be either PQ_ACTIVE or
    * PQ_INACTIVE.  To avoid lock order violation, unlock the page queue
    * while locking the vm object.  Use marker page to detect page queue
    * changes and maintain notion of next page on page queue.  Return
    * TRUE if no changes were detected, FALSE otherwise.  vm object is
    * locked on return.
    * 
    * This function depends on both the lock portion of struct vm_object
    * and normal struct vm_page being type stable.
    */
   static boolean_t
   vm_pageout_fallback_object_lock(vm_page_t m, vm_page_t *next)
   {
           struct vm_page marker;
           struct vm_pagequeue *pq;
           boolean_t unchanged;
           u_short queue;
           vm_object_t object;
   
           queue = m->queue;
           vm_pageout_init_marker(&marker, queue);
           pq = vm_page_pagequeue(m);
           object = m->object;
           
           TAILQ_INSERT_AFTER(&pq->pq_pl, m, &marker, plinks.q);
           vm_pagequeue_unlock(pq);
           vm_page_unlock(m);
           VM_OBJECT_WLOCK(object);
           vm_page_lock(m);
           vm_pagequeue_lock(pq);
   
           /*
            * The page's object might have changed, and/or the page might
            * have moved from its original position in the queue.  If the
            * page's object has changed, then the caller should abandon
            * processing the page because the wrong object lock was
            * acquired.  Use the marker's plinks.q, not the page's, to
            * determine if the page has been moved.  The state of the
            * page's plinks.q can be indeterminate; whereas, the marker's
            * plinks.q must be valid.
            */
           *next = TAILQ_NEXT(&marker, plinks.q);
           unchanged = m->object == object &&
               m == TAILQ_PREV(&marker, pglist, plinks.q);
           KASSERT(!unchanged || m->queue == queue,
               ("page %p queue %d %d", m, queue, m->queue));
           TAILQ_REMOVE(&pq->pq_pl, &marker, plinks.q);
           return (unchanged);
   }
   
   /*
    * Lock the page while holding the page queue lock.  Use marker page
    * to detect page queue changes and maintain notion of next page on
    * page queue.  Return TRUE if no changes were detected, FALSE
    * otherwise.  The page is locked on return. The page queue lock might
    * be dropped and reacquired.
    *
    * This function depends on normal struct vm_page being type stable.
    */
   static boolean_t
   vm_pageout_page_lock(vm_page_t m, vm_page_t *next)
   {
           struct vm_page marker;
           struct vm_pagequeue *pq;
           boolean_t unchanged;
           u_short queue;
   
           vm_page_lock_assert(m, MA_NOTOWNED);
           if (vm_page_trylock(m))
                   return (TRUE);
   
           queue = m->queue;
           vm_pageout_init_marker(&marker, queue);
           pq = vm_page_pagequeue(m);
   
           TAILQ_INSERT_AFTER(&pq->pq_pl, m, &marker, plinks.q);
           vm_pagequeue_unlock(pq);
           vm_page_lock(m);
           vm_pagequeue_lock(pq);
   
           /* Page queue might have changed. */
           *next = TAILQ_NEXT(&marker, plinks.q);
           unchanged = m == TAILQ_PREV(&marker, pglist, plinks.q);
           KASSERT(!unchanged || m->queue == queue,
               ("page %p queue %d %d", m, queue, m->queue));
           TAILQ_REMOVE(&pq->pq_pl, &marker, plinks.q);
           return (unchanged);
   }
   
   /*
    * Scan for pages at adjacent offsets within the given page's object that are
    * eligible for laundering, form a cluster of these pages and the given page,
    * and launder that cluster.
    */
   static int
   vm_pageout_cluster(vm_page_t m)
   {
           vm_object_t object;
           vm_page_t mc[2 * vm_pageout_page_count], p, pb, ps;
           vm_pindex_t pindex;
           int ib, is, page_base, pageout_count;
   
           vm_page_assert_locked(m);
           object = m->object;
           VM_OBJECT_ASSERT_WLOCKED(object);
           pindex = m->pindex;
   
           /*
            * We can't clean the page if it is busy or held.
            */
           vm_page_assert_unbusied(m);
           KASSERT(m->hold_count == 0, ("page %p is held", m));
   
           pmap_remove_write(m);
           vm_page_unlock(m);
   
           mc[vm_pageout_page_count] = pb = ps = m;
           pageout_count = 1;
           page_base = vm_pageout_page_count;
           ib = 1;
           is = 1;
   
           /*
            * We can cluster only if the page is not clean, busy, or held, and
            * the page is in the laundry queue.
            *
            * During heavy mmap/modification loads the pageout
            * daemon can really fragment the underlying file
            * due to flushing pages out of order and not trying to
            * align the clusters (which leaves sporadic out-of-order
            * holes).  To solve this problem we do the reverse scan
            * first and attempt to align our cluster, then do a 
            * forward scan if room remains.
            */
   more:
           while (ib != 0 && pageout_count < vm_pageout_page_count) {
                   if (ib > pindex) {
                           ib = 0;
                           break;
                   }
                   if ((p = vm_page_prev(pb)) == NULL || vm_page_busied(p)) {
                           ib = 0;
                           break;
                   }
                   vm_page_test_dirty(p);
                   if (p->dirty == 0) {
                           ib = 0;
                           break;
                   }
                   vm_page_lock(p);
                   if (!vm_page_in_laundry(p) ||
                       p->hold_count != 0) {       /* may be undergoing I/O */
                           vm_page_unlock(p);
                           ib = 0;
                           break;
                   }
                   pmap_remove_write(p);
                   vm_page_unlock(p);
                   mc[--page_base] = pb = p;
                   ++pageout_count;
                   ++ib;
   
                   /*
                    * We are at an alignment boundary.  Stop here, and switch
                    * directions.  Do not clear ib.
                    */
                   if ((pindex - (ib - 1)) % vm_pageout_page_count == 0)
                           break;
           }
           while (pageout_count < vm_pageout_page_count && 
               pindex + is < object->size) {
                   if ((p = vm_page_next(ps)) == NULL || vm_page_busied(p))
                           break;
                   vm_page_test_dirty(p);
                   if (p->dirty == 0)
                           break;
                   vm_page_lock(p);
                   if (!vm_page_in_laundry(p) ||
                       p->hold_count != 0) {       /* may be undergoing I/O */
                           vm_page_unlock(p);
                           break;
                   }
                   pmap_remove_write(p);
                   vm_page_unlock(p);
                   mc[page_base + pageout_count] = ps = p;
                   ++pageout_count;
                   ++is;
           }
   
           /*
            * If we exhausted our forward scan, continue with the reverse scan
            * when possible, even past an alignment boundary.  This catches
            * boundary conditions.
            */
           if (ib != 0 && pageout_count < vm_pageout_page_count)
                   goto more;
   
           return (vm_pageout_flush(&mc[page_base], pageout_count,
               VM_PAGER_PUT_NOREUSE, 0, NULL, NULL));
   }
   
   /*
    * vm_pageout_flush() - launder the given pages
    *
    *      The given pages are laundered.  Note that we setup for the start of
    *      I/O ( i.e. busy the page ), mark it read-only, and bump the object
    *      reference count all in here rather then in the parent.  If we want
    *      the parent to do more sophisticated things we may have to change
    *      the ordering.
    *
    *      Returned runlen is the count of pages between mreq and first
    *      page after mreq with status VM_PAGER_AGAIN.
    *      *eio is set to TRUE if pager returned VM_PAGER_ERROR or VM_PAGER_FAIL
    *      for any page in runlen set.
    */
   int
   vm_pageout_flush(vm_page_t *mc, int count, int flags, int mreq, int *prunlen,
       boolean_t *eio)
   {
           vm_object_t object = mc[0]->object;
           int pageout_status[count];
           int numpagedout = 0;
           int i, runlen;
   
           VM_OBJECT_ASSERT_WLOCKED(object);
   
           /*
            * Initiate I/O.  Mark the pages busy and verify that they're valid
            * and read-only.
            *
            * We do not have to fixup the clean/dirty bits here... we can
            * allow the pager to do it after the I/O completes.
            *
            * NOTE! mc[i]->dirty may be partial or fragmented due to an
            * edge case with file fragments.
            */
           for (i = 0; i < count; i++) {
                   KASSERT(mc[i]->valid == VM_PAGE_BITS_ALL,
                       ("vm_pageout_flush: partially invalid page %p index %d/%d",
                           mc[i], i, count));
                   KASSERT((mc[i]->aflags & PGA_WRITEABLE) == 0,
                       ("vm_pageout_flush: writeable page %p", mc[i]));
                   vm_page_sbusy(mc[i]);
           }
           vm_object_pip_add(object, count);
   
           vm_pager_put_pages(object, mc, count, flags, pageout_status);
   
           runlen = count - mreq;
           if (eio != NULL)
                   *eio = FALSE;
           for (i = 0; i < count; i++) {
                   vm_page_t mt = mc[i];
   
                   KASSERT(pageout_status[i] == VM_PAGER_PEND ||
                       !pmap_page_is_write_mapped(mt),
                       ("vm_pageout_flush: page %p is not write protected", mt));
                   switch (pageout_status[i]) {
                   case VM_PAGER_OK:
                           vm_page_lock(mt);
                           if (vm_page_in_laundry(mt))
                                   vm_page_deactivate_noreuse(mt);
                           vm_page_unlock(mt);
                           /* FALLTHROUGH */
                   case VM_PAGER_PEND:
                           numpagedout++;
                           break;
                   case VM_PAGER_BAD:
                           /*
                            * The page is outside the object's range.  We pretend
                            * that the page out worked and clean the page, so the
                            * changes will be lost if the page is reclaimed by
                            * the page daemon.
                            */
                           vm_page_undirty(mt);
                           vm_page_lock(mt);
                           if (vm_page_in_laundry(mt))
                                   vm_page_deactivate_noreuse(mt);
                           vm_page_unlock(mt);
                           break;
                   case VM_PAGER_ERROR:
                   case VM_PAGER_FAIL:
                           /*
                            * If the page couldn't be paged out, then reactivate
                            * it so that it doesn't clog the laundry and inactive
                            * queues.  (We will try paging it out again later).
                            */
                           vm_page_lock(mt);
                           vm_page_activate(mt);
                           vm_page_unlock(mt);
                           if (eio != NULL && i >= mreq && i - mreq < runlen)
                                   *eio = TRUE;
                           break;
                   case VM_PAGER_AGAIN:
                           if (i >= mreq && i - mreq < runlen)
                                   runlen = i - mreq;
                           break;
                   }
   
                   /*
                    * If the operation is still going, leave the page busy to
                    * block all other accesses. Also, leave the paging in
                    * progress indicator set so that we don't attempt an object
                    * collapse.
                    */
                   if (pageout_status[i] != VM_PAGER_PEND) {
                           vm_object_pip_wakeup(object);
                           vm_page_sunbusy(mt);
                   }
           }
           if (prunlen != NULL)
                   *prunlen = runlen;
           return (numpagedout);
   }
   
   /*
    * Attempt to acquire all of the necessary locks to launder a page and
    * then call through the clustering layer to PUTPAGES.  Wait a short
    * time for a vnode lock.
    *
    * Requires the page and object lock on entry, releases both before return.
    * Returns 0 on success and an errno otherwise.
    */
   static int
   vm_pageout_clean(vm_page_t m, int *numpagedout)
   {
           struct vnode *vp;
           struct mount *mp;
           vm_object_t object;
           vm_pindex_t pindex;
           int error, lockmode;
   
           vm_page_assert_locked(m);
           object = m->object;
           VM_OBJECT_ASSERT_WLOCKED(object);
           error = 0;
           vp = NULL;
           mp = NULL;
   
           /*
            * The object is already known NOT to be dead.   It
            * is possible for the vget() to block the whole
            * pageout daemon, but the new low-memory handling
            * code should prevent it.
            *
            * We can't wait forever for the vnode lock, we might
            * deadlock due to a vn_read() getting stuck in
            * vm_wait while holding this vnode.  We skip the 
            * vnode if we can't get it in a reasonable amount
            * of time.
            */
           if (object->type == OBJT_VNODE) {
                   vm_page_unlock(m);
                   vp = object->handle;
                   if (vp->v_type == VREG &&
                       vn_start_write(vp, &mp, V_NOWAIT) != 0) {
                           mp = NULL;
                           error = EDEADLK;
                           goto unlock_all;
                   }
                   KASSERT(mp != NULL,
                       ("vp %p with NULL v_mount", vp));
                   vm_object_reference_locked(object);
                   pindex = m->pindex;
                   VM_OBJECT_WUNLOCK(object);
                   lockmode = MNT_SHARED_WRITES(vp->v_mount) ?
                       LK_SHARED : LK_EXCLUSIVE;
                   if (vget(vp, lockmode | LK_TIMELOCK, curthread)) {
                           vp = NULL;
                           error = EDEADLK;
                           goto unlock_mp;
                   }
                   VM_OBJECT_WLOCK(object);
   
                   /*
                    * Ensure that the object and vnode were not disassociated
                    * while locks were dropped.
                    */
                   if (vp->v_object != object) {
                           error = ENOENT;
                           goto unlock_all;
                   }
                   vm_page_lock(m);
   
                   /*
                    * While the object and page were unlocked, the page
                    * may have been:
                    * (1) moved to a different queue,
                    * (2) reallocated to a different object,
                    * (3) reallocated to a different offset, or
                    * (4) cleaned.
                    */
                   if (!vm_page_in_laundry(m) || m->object != object ||
                       m->pindex != pindex || m->dirty == 0) {
                           vm_page_unlock(m);
                           error = ENXIO;
                           goto unlock_all;
                   }
   
                   /*
                    * The page may have been busied or held while the object
                    * and page locks were released.
                    */
                   if (vm_page_busied(m) || m->hold_count != 0) {
                           vm_page_unlock(m);
                           error = EBUSY;
                           goto unlock_all;
                   }
           }
   
           /*
            * If a page is dirty, then it is either being washed
            * (but not yet cleaned) or it is still in the
            * laundry.  If it is still in the laundry, then we
            * start the cleaning operation. 
            */
           if ((*numpagedout = vm_pageout_cluster(m)) == 0)
                   error = EIO;
   
   unlock_all:
           VM_OBJECT_WUNLOCK(object);
   
   unlock_mp:
           vm_page_lock_assert(m, MA_NOTOWNED);
           if (mp != NULL) {
                   if (vp != NULL)
                           vput(vp);
                   vm_object_deallocate(object);
                   vn_finished_write(mp);
           }
   
           return (error);
   }
   
   /*
    * Attempt to launder the specified number of pages.
    *
    * Returns the number of pages successfully laundered.
    */
   static int
   vm_pageout_launder(struct vm_domain *vmd, int launder, bool in_shortfall)
   {
           struct vm_pagequeue *pq;
           vm_object_t object;
           vm_page_t m, next;
           int act_delta, error, maxscan, numpagedout, starting_target;
           int vnodes_skipped;
           bool pageout_ok, queue_locked;
   
           starting_target = launder;
           vnodes_skipped = 0;
   
           /*
            * Scan the laundry queue for pages eligible to be laundered.  We stop
            * once the target number of dirty pages have been laundered, or once
            * we've reached the end of the queue.  A single iteration of this loop
            * may cause more than one page to be laundered because of clustering.
            *
            * maxscan ensures that we don't re-examine requeued pages.  Any
            * additional pages written as part of a cluster are subtracted from
            * maxscan since they must be taken from the laundry queue.
            */
           pq = &vmd->vmd_pagequeues[PQ_LAUNDRY];
           maxscan = pq->pq_cnt;
   
           vm_pagequeue_lock(pq);
           queue_locked = true;
           for (m = TAILQ_FIRST(&pq->pq_pl);
               m != NULL && maxscan-- > 0 && launder > 0;
               m = next) {
                   vm_pagequeue_assert_locked(pq);
                   KASSERT(queue_locked, ("unlocked laundry queue"));
                   KASSERT(vm_page_in_laundry(m),
                       ("page %p has an inconsistent queue", m));
                   next = TAILQ_NEXT(m, plinks.q);
                   if ((m->flags & PG_MARKER) != 0)
                           continue;
                   KASSERT((m->flags & PG_FICTITIOUS) == 0,
                       ("PG_FICTITIOUS page %p cannot be in laundry queue", m));
                   KASSERT((m->oflags & VPO_UNMANAGED) == 0,
                       ("VPO_UNMANAGED page %p cannot be in laundry queue", m));
                   if (!vm_pageout_page_lock(m, &next) || m->hold_count != 0) {
                           vm_page_unlock(m);
                           continue;
                   }
                   object = m->object;
                   if ((!VM_OBJECT_TRYWLOCK(object) &&
                       (!vm_pageout_fallback_object_lock(m, &next) ||
                       m->hold_count != 0)) || vm_page_busied(m)) {
                           VM_OBJECT_WUNLOCK(object);
                           vm_page_unlock(m);
                           continue;
                   }
   
                   /*
                    * Unlock the laundry queue, invalidating the 'next' pointer.
                    * Use a marker to remember our place in the laundry queue.
                    */
                   TAILQ_INSERT_AFTER(&pq->pq_pl, m, &vmd->vmd_laundry_marker,
                       plinks.q);
                   vm_pagequeue_unlock(pq);
                   queue_locked = false;
   
                   /*
                    * Invalid pages can be easily freed.  They cannot be
                    * mapped; vm_page_free() asserts this.
                    */
                   if (m->valid == 0)
                           goto free_page;
   
                   /*
                    * If the page has been referenced and the object is not dead,
                    * reactivate or requeue the page depending on whether the
                    * object is mapped.
                    */
                   if ((m->aflags & PGA_REFERENCED) != 0) {
                           vm_page_aflag_clear(m, PGA_REFERENCED);
                           act_delta = 1;
                   } else
                           act_delta = 0;
                   if (object->ref_count != 0)
                           act_delta += pmap_ts_referenced(m);
                   else {
                           KASSERT(!pmap_page_is_mapped(m),
                               ("page %p is mapped", m));
                   }
                   if (act_delta != 0) {
                           if (object->ref_count != 0) {
                                   PCPU_INC(cnt.v_reactivated);
                                   vm_page_activate(m);
   
                                   /*
                                    * Increase the activation count if the page
                                    * was referenced while in the laundry queue.
                                    * This makes it less likely that the page will
                                    * be returned prematurely to the inactive
                                    * queue.
                                    */
                                   m->act_count += act_delta + ACT_ADVANCE;
   
                                   /*
                                    * If this was a background laundering, count
                                    * activated pages towards our target.  The
                                    * purpose of background laundering is to ensure
                                    * that pages are eventually cycled through the
                                    * laundry queue, and an activation is a valid
                                    * way out.
                                    */
                                   if (!in_shortfall)
                                           launder--;
                                   goto drop_page;
                           } else if ((object->flags & OBJ_DEAD) == 0)
                                   goto requeue_page;
                   }
   
                   /*
                    * If the page appears to be clean at the machine-independent
                    * layer, then remove all of its mappings from the pmap in
                    * anticipation of freeing it.  If, however, any of the page's
                    * mappings allow write access, then the page may still be
                    * modified until the last of those mappings are removed.
                    */
                   if (object->ref_count != 0) {
                           vm_page_test_dirty(m);
                           if (m->dirty == 0)
                                   pmap_remove_all(m);
                   }
   
                   /*
                    * Clean pages are freed, and dirty pages are paged out unless
                    * they belong to a dead object.  Requeueing dirty pages from
                    * dead objects is pointless, as they are being paged out and
                    * freed by the thread that destroyed the object.
                    */
                   if (m->dirty == 0) {
   free_page:
                           vm_page_free(m);
                           PCPU_INC(cnt.v_dfree);
                   } else if ((object->flags & OBJ_DEAD) == 0) {
                           if (object->type != OBJT_SWAP &&
                               object->type != OBJT_DEFAULT)
                                   pageout_ok = true;
                           else if (disable_swap_pageouts)
                                   pageout_ok = false;
                           else
                                   pageout_ok = true;
                           if (!pageout_ok) {
   requeue_page:
                                   vm_pagequeue_lock(pq);
                                   queue_locked = true;
                                   vm_page_requeue_locked(m);
                                   goto drop_page;
                           }
   
                           /*
                            * Form a cluster with adjacent, dirty pages from the
                            * same object, and page out that entire cluster.
                            *
                            * The adjacent, dirty pages must also be in the
                            * laundry.  However, their mappings are not checked
                            * for new references.  Consequently, a recently
                            * referenced page may be paged out.  However, that
                            * page will not be prematurely reclaimed.  After page
                            * out, the page will be placed in the inactive queue,
                            * where any new references will be detected and the
                            * page reactivated.
                            */
                           error = vm_pageout_clean(m, &numpagedout);
                           if (error == 0) {
                                   launder -= numpagedout;
                                   maxscan -= numpagedout - 1;
                           } else if (error == EDEADLK) {
                                   pageout_lock_miss++;
                                   vnodes_skipped++;
                           }
                           goto relock_queue;
                   }
   drop_page:
                   vm_page_unlock(m);
                   VM_OBJECT_WUNLOCK(object);
   relock_queue:
                   if (!queue_locked) {
                           vm_pagequeue_lock(pq);
                           queue_locked = true;
                   }
                   next = TAILQ_NEXT(&vmd->vmd_laundry_marker, plinks.q);
                   TAILQ_REMOVE(&pq->pq_pl, &vmd->vmd_laundry_marker, plinks.q);
           }
           vm_pagequeue_unlock(pq);
   
           /*
            * Wakeup the sync daemon if we skipped a vnode in a writeable object
            * and we didn't launder enough pages.
            */
           if (vnodes_skipped > 0 && launder > 0)
                   (void)speedup_syncer();
   
           return (starting_target - launder);
   }
   
   /*
    * Compute the integer square root.
    */
   static u_int
   isqrt(u_int num)
   {
           u_int bit, root, tmp;
   
           bit = 1u << ((NBBY * sizeof(u_int)) - 2);
           while (bit > num)
                   bit >>= 2;
           root = 0;
           while (bit != 0) {
                   tmp = root + bit;
                   root >>= 1;
                   if (num >= tmp) {
                           num -= tmp;
                           root += bit;
                   }
                   bit >>= 2;
           }
           return (root);
   }
   
   /*
    * Perform the work of the laundry thread: periodically wake up and determine
    * whether any pages need to be laundered.  If so, determine the number of pages
    * that need to be laundered, and launder them.
    */
   static void
   vm_pageout_laundry_worker(void *arg)
   {
           struct vm_domain *domain;
           struct vm_pagequeue *pq;
           uint64_t nclean, ndirty;
           u_int last_launder, wakeups;
           int domidx, last_target, launder, shortfall, shortfall_cycle, target;
           bool in_shortfall;
   
           domidx = (uintptr_t)arg;
           domain = &vm_dom[domidx];
           pq = &domain->vmd_pagequeues[PQ_LAUNDRY];
           KASSERT(domain->vmd_segs != 0, ("domain without segments"));
           vm_pageout_init_marker(&domain->vmd_laundry_marker, PQ_LAUNDRY);
   
           shortfall = 0;
           in_shortfall = false;
           shortfall_cycle = 0;
           target = 0;
           last_launder = 0;
   
           /*
            * The pageout laundry worker is never done, so loop forever.
            */
           for (;;) {
                   KASSERT(target >= 0, ("negative target %d", target));
                   KASSERT(shortfall_cycle >= 0,
                       ("negative cycle %d", shortfall_cycle));
                   launder = 0;
                   wakeups = VM_METER_PCPU_CNT(v_pdwakeups);
   
                   /*
                    * First determine whether we need to launder pages to meet a
                    * shortage of free pages.
                    */
                   if (shortfall > 0) {
                           in_shortfall = true;
                           shortfall_cycle = VM_LAUNDER_RATE / VM_INACT_SCAN_RATE;
                           target = shortfall;
                   } else if (!in_shortfall)
                           goto trybackground;
                   else if (shortfall_cycle == 0 || vm_laundry_target() <= 0) {
                           /*
                            * We recently entered shortfall and began laundering
                            * pages.  If we have completed that laundering run
                            * (and we are no longer in shortfall) or we have met
                            * our laundry target through other activity, then we
                            * can stop laundering pages.
                            */
                           in_shortfall = false;
                           target = 0;
                           goto trybackground;
                   }
                   last_launder = wakeups;
                   launder = target / shortfall_cycle--;
                   goto dolaundry;
   
                   /*
                    * There's no immediate need to launder any pages; see if we
                    * meet the conditions to perform background laundering:
                    *
                    * 1. The ratio of dirty to clean inactive pages exceeds the
                    *    background laundering threshold and the pagedaemon has
                    *    been woken up to reclaim pages since our last
                    *    laundering, or
                    * 2. we haven't yet reached the target of the current
                    *    background laundering run.
                    *
                    * The background laundering threshold is not a constant.
                    * Instead, it is a slowly growing function of the number of
                    * page daemon wakeups since the last laundering.  Thus, as the
                    * ratio of dirty to clean inactive pages grows, the amount of
                    * memory pressure required to trigger laundering decreases.
                    */
   trybackground:
                   nclean = vm_cnt.v_inactive_count + vm_cnt.v_free_count;
                   ndirty = vm_cnt.v_laundry_count;
                   if (target == 0 && wakeups != last_launder &&
                       ndirty * isqrt(wakeups - last_launder) >= nclean) {
                           target = vm_background_launder_target;
                   }
   
                   /*
                   * We have a non-zero background laundering target.  If we've
                   * laundered up to our maximum without observing a page daemon
                   * wakeup, just stop.  This is a safety belt that ensures we
                   * don't launder an excessive amount if memory pressure is low
                   * and the ratio of dirty to clean pages is large.  Otherwise,
                   * proceed at the background laundering rate.
                   */
                  if (target > 0) {
                          if (wakeups != last_launder) {
                                  last_launder = wakeups;
                                  last_target = target;
                          } else if (last_target - target >=
                              vm_background_launder_max * PAGE_SIZE / 1024) {
                                  target = 0;
                          }
                          launder = vm_background_launder_rate * PAGE_SIZE / 1024;
                          launder /= VM_LAUNDER_RATE;
                          if (launder > target)
                                  launder = target;
                  }
  
  dolaundry:
                  if (launder > 0) {
                          /*
                           * Because of I/O clustering, the number of laundered
                           * pages could exceed "target" by the maximum size of
                           * a cluster minus one. 
                           */
                          target -= min(vm_pageout_launder(domain, launder,
                              in_shortfall), target);
                          pause("laundp", hz / VM_LAUNDER_RATE);
                  }
  
                  /*
                   * If we're not currently laundering pages and the page daemon
                   * hasn't posted a new request, sleep until the page daemon
                   * kicks us.
                   */
                  vm_pagequeue_lock(pq);
                  if (target == 0 && vm_laundry_request == VM_LAUNDRY_IDLE)
                          (void)mtx_sleep(&vm_laundry_request,
                              vm_pagequeue_lockptr(pq), PVM, "launds", 0);
  
                  /*
                   * If the pagedaemon has indicated that it's in shortfall, start
                   * a shortfall laundering unless we're already in the middle of
                   * one.  This may preempt a background laundering.
                   */
                  if (vm_laundry_request == VM_LAUNDRY_SHORTFALL &&
                      (!in_shortfall || shortfall_cycle == 0)) {
                          shortfall = vm_laundry_target() + vm_pageout_deficit;
                          target = 0;
                  } else
                          shortfall = 0;
  
                  if (target == 0)
                          vm_laundry_request = VM_LAUNDRY_IDLE;
                  vm_pagequeue_unlock(pq);
          }
  }
  
  /*
   *      vm_pageout_scan does the dirty work for the pageout daemon.
   *
   *      pass == 0: Update active LRU/deactivate pages
   *      pass >= 1: Free inactive pages
   *
   * Returns true if pass was zero or enough pages were freed by the inactive
   * queue scan to meet the target.
   */
  static bool
  vm_pageout_scan(struct vm_domain *vmd, int pass)
  {
          vm_page_t m, next;
          struct vm_pagequeue *pq;
          vm_object_t object;
          long min_scan;
          int act_delta, addl_page_shortage, deficit, inactq_shortage, maxscan;
          int page_shortage, scan_tick, scanned, starting_page_shortage;
          boolean_t queue_locked;
  
          /*
           * If we need to reclaim memory ask kernel caches to return
           * some.  We rate limit to avoid thrashing.
           */
          if (vmd == &vm_dom[0] && pass > 0 &&
              (time_uptime - lowmem_uptime) >= lowmem_period) {
                  /*
                   * Decrease registered cache sizes.
                   */
                  SDT_PROBE0(vm, , , vm__lowmem_scan);
                  EVENTHANDLER_INVOKE(vm_lowmem, VM_LOW_PAGES);
                  /*
                   * We do this explicitly after the caches have been
                   * drained above.
                   */
                  uma_reclaim();
                  lowmem_uptime = time_uptime;
          }
  
          /*
           * The addl_page_shortage is the number of temporarily
           * stuck pages in the inactive queue.  In other words, the
           * number of pages from the inactive count that should be
           * discounted in setting the target for the active queue scan.
           */
          addl_page_shortage = 0;
  
          /*
           * Calculate the number of pages that we want to free.  This number
           * can be negative if many pages are freed between the wakeup call to
           * the page daemon and this calculation.
           */
          if (pass > 0) {
                  deficit = atomic_readandclear_int(&vm_pageout_deficit);
                  page_shortage = vm_paging_target() + deficit;
          } else
                  page_shortage = deficit = 0;
          starting_page_shortage = page_shortage;
  
          /*
           * Start scanning the inactive queue for pages that we can free.  The
           * scan will stop when we reach the target or we have scanned the
           * entire queue.  (Note that m->act_count is not used to make
           * decisions for the inactive queue, only for the active queue.)
           */
          pq = &vmd->vmd_pagequeues[PQ_INACTIVE];
          maxscan = pq->pq_cnt;
          vm_pagequeue_lock(pq);
          queue_locked = TRUE;
          for (m = TAILQ_FIRST(&pq->pq_pl);
               m != NULL && maxscan-- > 0 && page_shortage > 0;
               m = next) {
                  vm_pagequeue_assert_locked(pq);
                  KASSERT(queue_locked, ("unlocked inactive queue"));
                  KASSERT(vm_page_inactive(m), ("Inactive queue %p", m));
  
                  PCPU_INC(cnt.v_pdpages);
                  next = TAILQ_NEXT(m, plinks.q);
  
                  /*
                   * skip marker pages
                   */
                  if (m->flags & PG_MARKER)
                          continue;
  
                  KASSERT((m->flags & PG_FICTITIOUS) == 0,
                      ("Fictitious page %p cannot be in inactive queue", m));
                  KASSERT((m->oflags & VPO_UNMANAGED) == 0,
                      ("Unmanaged page %p cannot be in inactive queue", m));
  
                  /*
                   * The page or object lock acquisitions fail if the
                   * page was removed from the queue or moved to a
                   * different position within the queue.  In either
                   * case, addl_page_shortage should not be incremented.
                   */
                  if (!vm_pageout_page_lock(m, &next))
                          goto unlock_page;
                  else if (m->hold_count != 0) {
                          /*
                           * Held pages are essentially stuck in the
                           * queue.  So, they ought to be discounted
                           * from the inactive count.  See the
                           * calculation of inactq_shortage before the
                           * loop over the active queue below.
                           */
                          addl_page_shortage++;
                          goto unlock_page;
                  }
                  object = m->object;
                  if (!VM_OBJECT_TRYWLOCK(object)) {
                          if (!vm_pageout_fallback_object_lock(m, &next))
                                  goto unlock_object;
                          else if (m->hold_count != 0) {
                                  addl_page_shortage++;
                                  goto unlock_object;
                          }
                  }
                  if (vm_page_busied(m)) {
                          /*
                           * Don't mess with busy pages.  Leave them at
                           * the front of the queue.  Most likely, they
                           * are being paged out and will leave the
                           * queue shortly after the scan finishes.  So,
                           * they ought to be discounted from the
                           * inactive count.
                           */
                          addl_page_shortage++;
  unlock_object:
                          VM_OBJECT_WUNLOCK(object);
  unlock_page:
                          vm_page_unlock(m);
                          continue;
                  }
                  KASSERT(m->hold_count == 0, ("Held page %p", m));
  
                  /*
                   * Dequeue the inactive page and unlock the inactive page
                   * queue, invalidating the 'next' pointer.  Dequeueing the
                   * page here avoids a later reacquisition (and release) of
                   * the inactive page queue lock when vm_page_activate(),
                   * vm_page_free(), or vm_page_launder() is called.  Use a
                   * marker to remember our place in the inactive queue.
                   */
                  TAILQ_INSERT_AFTER(&pq->pq_pl, m, &vmd->vmd_marker, plinks.q);
                  vm_page_dequeue_locked(m);
                  vm_pagequeue_unlock(pq);
                  queue_locked = FALSE;
  
                  /*
                   * Invalid pages can be easily freed. They cannot be
                   * mapped, vm_page_free() asserts this.
                   */
                  if (m->valid == 0)
                          goto free_page;
  
                  /*
                   * If the page has been referenced and the object is not dead,
                   * reactivate or requeue the page depending on whether the
                   * object is mapped.
                   */
                  if ((m->aflags & PGA_REFERENCED) != 0) {
                          vm_page_aflag_clear(m, PGA_REFERENCED);
                          act_delta = 1;
                  } else
                          act_delta = 0;
                  if (object->ref_count != 0) {
                          act_delta += pmap_ts_referenced(m);
                  } else {
                          KASSERT(!pmap_page_is_mapped(m),
                              ("vm_pageout_scan: page %p is mapped", m));
                  }
                  if (act_delta != 0) {
                          if (object->ref_count != 0) {
                                  PCPU_INC(cnt.v_reactivated);
                                  vm_page_activate(m);
  
                                  /*
                                   * Increase the activation count if the page
                                   * was referenced while in the inactive queue.
                                   * This makes it less likely that the page will
                                   * be returned prematurely to the inactive
                                   * queue.
                                   */
                                  m->act_count += act_delta + ACT_ADVANCE;
                                  goto drop_page;
                          } else if ((object->flags & OBJ_DEAD) == 0) {
                                  vm_pagequeue_lock(pq);
                                  queue_locked = TRUE;
                                  m->queue = PQ_INACTIVE;
                                  TAILQ_INSERT_TAIL(&pq->pq_pl, m, plinks.q);
                                  vm_pagequeue_cnt_inc(pq);
                                  goto drop_page;
                          }
                  }
  
                  /*
                   * If the page appears to be clean at the machine-independent
                   * layer, then remove all of its mappings from the pmap in
                   * anticipation of freeing it.  If, however, any of the page's
                   * mappings allow write access, then the page may still be
                   * modified until the last of those mappings are removed.
                   */
                  if (object->ref_count != 0) {
                          vm_page_test_dirty(m);
                          if (m->dirty == 0)
                                  pmap_remove_all(m);
                  }
  
                  /*
                   * Clean pages can be freed, but dirty pages must be sent back
                   * to the laundry, unless they belong to a dead object.
                   * Requeueing dirty pages from dead objects is pointless, as
                   * they are being paged out and freed by the thread that
                   * destroyed the object.
                   */
                  if (m->dirty == 0) {
  free_page:
                          vm_page_free(m);
                          PCPU_INC(cnt.v_dfree);
                          --page_shortage;
                  } else if ((object->flags & OBJ_DEAD) == 0)
                          vm_page_launder(m);
  drop_page:
                  vm_page_unlock(m);
                  VM_OBJECT_WUNLOCK(object);
                  if (!queue_locked) {
                          vm_pagequeue_lock(pq);
                          queue_locked = TRUE;
                  }
                  next = TAILQ_NEXT(&vmd->vmd_marker, plinks.q);
                  TAILQ_REMOVE(&pq->pq_pl, &vmd->vmd_marker, plinks.q);
          }
          vm_pagequeue_unlock(pq);
  
          /*
           * Wake up the laundry thread so that it can perform any needed
           * laundering.  If we didn't meet our target, we're in shortfall and
           * need to launder more aggressively.
           */
          if (vm_laundry_request == VM_LAUNDRY_IDLE &&
              starting_page_shortage > 0) {
                  pq = &vm_dom[0].vmd_pagequeues[PQ_LAUNDRY];
                  vm_pagequeue_lock(pq);
                  if (page_shortage > 0) {
                          vm_laundry_request = VM_LAUNDRY_SHORTFALL;
                          PCPU_INC(cnt.v_pdshortfalls);
                  } else if (vm_laundry_request != VM_LAUNDRY_SHORTFALL)
                          vm_laundry_request = VM_LAUNDRY_BACKGROUND;
                  wakeup(&vm_laundry_request);
                  vm_pagequeue_unlock(pq);
          }
  
          /*
           * Wakeup the swapout daemon if we didn't free the targeted number of
           * pages.
           */
          if (page_shortage > 0)
                  vm_swapout_run();
  
          /*
           * If the inactive queue scan fails repeatedly to meet its
           * target, kill the largest process.
           */
          vm_pageout_mightbe_oom(vmd, page_shortage, starting_page_shortage);
  
          /*
           * Compute the number of pages we want to try to move from the
           * active queue to either the inactive or laundry queue.
           *
           * When scanning active pages, we make clean pages count more heavily
           * towards the page shortage than dirty pages.  This is because dirty
           * pages must be laundered before they can be reused and thus have less
           * utility when attempting to quickly alleviate a shortage.  However,
           * this weighting also causes the scan to deactivate dirty pages more
           * more aggressively, improving the effectiveness of clustering and
           * ensuring that they can eventually be reused.
           */
          inactq_shortage = vm_cnt.v_inactive_target - (vm_cnt.v_inactive_count +
              vm_cnt.v_laundry_count / act_scan_laundry_weight) +
              vm_paging_target() + deficit + addl_page_shortage;
          inactq_shortage *= act_scan_laundry_weight;
  
          pq = &vmd->vmd_pagequeues[PQ_ACTIVE];
          vm_pagequeue_lock(pq);
          maxscan = pq->pq_cnt;
  
          /*
           * If we're just idle polling attempt to visit every
           * active page within 'update_period' seconds.
           */
          scan_tick = ticks;
          if (vm_pageout_update_period != 0) {
                  min_scan = pq->pq_cnt;
                  min_scan *= scan_tick - vmd->vmd_last_active_scan;
                  min_scan /= hz * vm_pageout_update_period;
          } else
                  min_scan = 0;
          if (min_scan > 0 || (inactq_shortage > 0 && maxscan > 0))
                  vmd->vmd_last_active_scan = scan_tick;
  
          /*
           * Scan the active queue for pages that can be deactivated.  Update
           * the per-page activity counter and use it to identify deactivation
           * candidates.  Held pages may be deactivated.
           */
          for (m = TAILQ_FIRST(&pq->pq_pl), scanned = 0; m != NULL && (scanned <
              min_scan || (inactq_shortage > 0 && scanned < maxscan)); m = next,
              scanned++) {
                  KASSERT(m->queue == PQ_ACTIVE,
                      ("vm_pageout_scan: page %p isn't active", m));
                  next = TAILQ_NEXT(m, plinks.q);
                  if ((m->flags & PG_MARKER) != 0)
                          continue;
                  KASSERT((m->flags & PG_FICTITIOUS) == 0,
                      ("Fictitious page %p cannot be in active queue", m));
                  KASSERT((m->oflags & VPO_UNMANAGED) == 0,
                      ("Unmanaged page %p cannot be in active queue", m));
                  if (!vm_pageout_page_lock(m, &next)) {
                          vm_page_unlock(m);
                          continue;
                  }
  
                  /*
                   * The count for page daemon pages is updated after checking
                   * the page for eligibility.
                   */
                  PCPU_INC(cnt.v_pdpages);
  
                  /*
                   * Check to see "how much" the page has been used.
                   */
                  if ((m->aflags & PGA_REFERENCED) != 0) {
                          vm_page_aflag_clear(m, PGA_REFERENCED);
                          act_delta = 1;
                  } else
                          act_delta = 0;
  
                  /*
                   * Perform an unsynchronized object ref count check.  While
                   * the page lock ensures that the page is not reallocated to
                   * another object, in particular, one with unmanaged mappings
                   * that cannot support pmap_ts_referenced(), two races are,
                   * nonetheless, possible:
                   * 1) The count was transitioning to zero, but we saw a non-
                   *    zero value.  pmap_ts_referenced() will return zero
                   *    because the page is not mapped.
                   * 2) The count was transitioning to one, but we saw zero. 
                   *    This race delays the detection of a new reference.  At
                   *    worst, we will deactivate and reactivate the page.
                   */
                  if (m->object->ref_count != 0)
                          act_delta += pmap_ts_referenced(m);
  
                  /*
                   * Advance or decay the act_count based on recent usage.
                   */
                  if (act_delta != 0) {
                          m->act_count += ACT_ADVANCE + act_delta;
                          if (m->act_count > ACT_MAX)
                                  m->act_count = ACT_MAX;
                  } else
                          m->act_count -= min(m->act_count, ACT_DECLINE);
  
                  /*
                   * Move this page to the tail of the active, inactive or laundry
                   * queue depending on usage.
                   */
                  if (m->act_count == 0) {
                          /* Dequeue to avoid later lock recursion. */
                          vm_page_dequeue_locked(m);
  
                          /*
                           * When not short for inactive pages, let dirty pages go
                           * through the inactive queue before moving to the
                           * laundry queues.  This gives them some extra time to
                           * be reactivated, potentially avoiding an expensive
                           * pageout.  During a page shortage, the inactive queue
                           * is necessarily small, so we may move dirty pages
                           * directly to the laundry queue.
                           */
                          if (inactq_shortage <= 0)
                                  vm_page_deactivate(m);
                          else {
                                  /*
                                   * Calling vm_page_test_dirty() here would
                                   * require acquisition of the object's write
                                   * lock.  However, during a page shortage,
                                   * directing dirty pages into the laundry
                                   * queue is only an optimization and not a
                                   * requirement.  Therefore, we simply rely on
                                   * the opportunistic updates to the page's
                                   * dirty field by the pmap.
                                   */
                                  if (m->dirty == 0) {
                                          vm_page_deactivate(m);
                                          inactq_shortage -=
                                              act_scan_laundry_weight;
                                  } else {
                                          vm_page_launder(m);
                                          inactq_shortage--;
                                  }
                          }
                  } else
                          vm_page_requeue_locked(m);
                  vm_page_unlock(m);
          }
          vm_pagequeue_unlock(pq);
          if (pass > 0)
                  vm_swapout_run_idle();
          return (page_shortage <= 0);
  }
  
  static int vm_pageout_oom_vote;
  
  /*
   * The pagedaemon threads randlomly select one to perform the
   * OOM.  Trying to kill processes before all pagedaemons
   * failed to reach free target is premature.
   */
  static void
  vm_pageout_mightbe_oom(struct vm_domain *vmd, int page_shortage,
      int starting_page_shortage)
  {
          int old_vote;
  
          if (starting_page_shortage <= 0 || starting_page_shortage !=
              page_shortage)
                  vmd->vmd_oom_seq = 0;
          else
                  vmd->vmd_oom_seq++;
          if (vmd->vmd_oom_seq < vm_pageout_oom_seq) {
                  if (vmd->vmd_oom) {
                          vmd->vmd_oom = FALSE;
                          atomic_subtract_int(&vm_pageout_oom_vote, 1);
                  }
                  return;
          }
  
          /*
           * Do not follow the call sequence until OOM condition is
           * cleared.
           */
          vmd->vmd_oom_seq = 0;
  
          if (vmd->vmd_oom)
                  return;
  
          vmd->vmd_oom = TRUE;
          old_vote = atomic_fetchadd_int(&vm_pageout_oom_vote, 1);
          if (old_vote != vm_ndomains - 1)
                  return;
  
          /*
           * The current pagedaemon thread is the last in the quorum to
           * start OOM.  Initiate the selection and signaling of the
           * victim.
           */
          vm_pageout_oom(VM_OOM_MEM);
  
          /*
           * After one round of OOM terror, recall our vote.  On the
           * next pass, current pagedaemon would vote again if the low
           * memory condition is still there, due to vmd_oom being
           * false.
           */
          vmd->vmd_oom = FALSE;
          atomic_subtract_int(&vm_pageout_oom_vote, 1);
  }
  
  /*
   * The OOM killer is the page daemon's action of last resort when
   * memory allocation requests have been stalled for a prolonged period
   * of time because it cannot reclaim memory.  This function computes
   * the approximate number of physical pages that could be reclaimed if
   * the specified address space is destroyed.
   *
   * Private, anonymous memory owned by the address space is the
   * principal resource that we expect to recover after an OOM kill.
   * Since the physical pages mapped by the address space's COW entries
   * are typically shared pages, they are unlikely to be released and so
   * they are not counted.
   *
   * To get to the point where the page daemon runs the OOM killer, its
   * efforts to write-back vnode-backed pages may have stalled.  This
   * could be caused by a memory allocation deadlock in the write path
   * that might be resolved by an OOM kill.  Therefore, physical pages
   * belonging to vnode-backed objects are counted, because they might
   * be freed without being written out first if the address space holds
   * the last reference to an unlinked vnode.
   *
   * Similarly, physical pages belonging to OBJT_PHYS objects are
   * counted because the address space might hold the last reference to
   * the object.
   */
  static long
  vm_pageout_oom_pagecount(struct vmspace *vmspace)
  {
          vm_map_t map;
          vm_map_entry_t entry;
          vm_object_t obj;
          long res;
  
          map = &vmspace->vm_map;
          KASSERT(!map->system_map, ("system map"));
          sx_assert(&map->lock, SA_LOCKED);
          res = 0;
          for (entry = map->header.next; entry != &map->header;
              entry = entry->next) {
                  if ((entry->eflags & MAP_ENTRY_IS_SUB_MAP) != 0)
                          continue;
                  obj = entry->object.vm_object;
                  if (obj == NULL)
                          continue;
                  if ((entry->eflags & MAP_ENTRY_NEEDS_COPY) != 0 &&
                      obj->ref_count != 1)
                          continue;
                  switch (obj->type) {
                  case OBJT_DEFAULT:
                  case OBJT_SWAP:
                  case OBJT_PHYS:
                  case OBJT_VNODE:
                          res += obj->resident_page_count;
                          break;
                  }
          }
          return (res);
  }
  
  void
  vm_pageout_oom(int shortage)
  {
          struct proc *p, *bigproc;
          vm_offset_t size, bigsize;
          struct thread *td;
          struct vmspace *vm;
          bool breakout;
  
          /*
           * We keep the process bigproc locked once we find it to keep anyone
           * from messing with it; however, there is a possibility of
           * deadlock if process B is bigproc and one of it's child processes
           * attempts to propagate a signal to B while we are waiting for A's
           * lock while walking this list.  To avoid this, we don't block on
           * the process lock but just skip a process if it is already locked.
           */
          bigproc = NULL;
          bigsize = 0;
          sx_slock(&allproc_lock);
          FOREACH_PROC_IN_SYSTEM(p) {
                  PROC_LOCK(p);
  
                  /*
                   * If this is a system, protected or killed process, skip it.
                   */
                  if (p->p_state != PRS_NORMAL || (p->p_flag & (P_INEXEC |
                      P_PROTECTED | P_SYSTEM | P_WEXIT)) != 0 ||
                      p->p_pid == 1 || P_KILLED(p) ||
                      (p->p_pid < 48 && swap_pager_avail != 0)) {
                          PROC_UNLOCK(p);
                          continue;
                  }
                  /*
                   * If the process is in a non-running type state,
                   * don't touch it.  Check all the threads individually.
                   */
                  breakout = false;
                  FOREACH_THREAD_IN_PROC(p, td) {
                          thread_lock(td);
                          if (!TD_ON_RUNQ(td) &&
                              !TD_IS_RUNNING(td) &&
                              !TD_IS_SLEEPING(td) &&
                              !TD_IS_SUSPENDED(td) &&
                              !TD_IS_SWAPPED(td)) {
                                  thread_unlock(td);
                                  breakout = true;
                                  break;
                          }
                          thread_unlock(td);
                  }
                  if (breakout) {
                          PROC_UNLOCK(p);
                          continue;
                  }
                  /*
                   * get the process size
                   */
                  vm = vmspace_acquire_ref(p);
                  if (vm == NULL) {
                          PROC_UNLOCK(p);
                          continue;
                  }
                  _PHOLD_LITE(p);
                  PROC_UNLOCK(p);
                  sx_sunlock(&allproc_lock);
                  if (!vm_map_trylock_read(&vm->vm_map)) {
                          vmspace_free(vm);
                          sx_slock(&allproc_lock);
                          PRELE(p);
                          continue;
                  }
                  size = vmspace_swap_count(vm);
                  if (shortage == VM_OOM_MEM)
                          size += vm_pageout_oom_pagecount(vm);
                  vm_map_unlock_read(&vm->vm_map);
                  vmspace_free(vm);
                  sx_slock(&allproc_lock);
  
                  /*
                   * If this process is bigger than the biggest one,
                   * remember it.
                   */
                  if (size > bigsize) {
                          if (bigproc != NULL)
                                  PRELE(bigproc);
                          bigproc = p;
                          bigsize = size;
                  } else {
                          PRELE(p);
                  }
          }
          sx_sunlock(&allproc_lock);
          if (bigproc != NULL) {
                  if (vm_panic_on_oom != 0)
                          panic("out of swap space");
                  PROC_LOCK(bigproc);
                  killproc(bigproc, "out of swap space");
                  sched_nice(bigproc, PRIO_MIN);
                  _PRELE(bigproc);
                  PROC_UNLOCK(bigproc);
                  wakeup(&vm_cnt.v_free_count);
          }
  }
  
  static void
  vm_pageout_worker(void *arg)
  {
          struct vm_domain *domain;
          int domidx, pass;
          bool target_met;
  
          domidx = (uintptr_t)arg;
          domain = &vm_dom[domidx];
          pass = 0;
          target_met = true;
  
          /*
           * XXXKIB It could be useful to bind pageout daemon threads to
           * the cores belonging to the domain, from which vm_page_array
           * is allocated.
           */
  
          KASSERT(domain->vmd_segs != 0, ("domain without segments"));
          domain->vmd_last_active_scan = ticks;
          vm_pageout_init_marker(&domain->vmd_marker, PQ_INACTIVE);
          vm_pageout_init_marker(&domain->vmd_inacthead, PQ_INACTIVE);
          TAILQ_INSERT_HEAD(&domain->vmd_pagequeues[PQ_INACTIVE].pq_pl,
              &domain->vmd_inacthead, plinks.q);
  
          /*
           * The pageout daemon worker is never done, so loop forever.
           */
          while (TRUE) {
                  mtx_lock(&vm_page_queue_free_mtx);
  
                  /*
                   * Generally, after a level >= 1 scan, if there are enough
                   * free pages to wakeup the waiters, then they are already
                   * awake.  A call to vm_page_free() during the scan awakened
                   * them.  However, in the following case, this wakeup serves
                   * to bound the amount of time that a thread might wait.
                   * Suppose a thread's call to vm_page_alloc() fails, but
                   * before that thread calls VM_WAIT, enough pages are freed by
                   * other threads to alleviate the free page shortage.  The
                   * thread will, nonetheless, wait until another page is freed
                   * or this wakeup is performed.
                   */
                  if (vm_pages_needed && !vm_page_count_min()) {
                          vm_pages_needed = false;
                          wakeup(&vm_cnt.v_free_count);
                  }
  
                  /*
                   * Do not clear vm_pageout_wanted until we reach our free page
                   * target.  Otherwise, we may be awakened over and over again,
                   * wasting CPU time.
                   */
                  if (vm_pageout_wanted && target_met)
                          vm_pageout_wanted = false;
  
                  /*
                   * Might the page daemon receive a wakeup call?
                   */
                  if (vm_pageout_wanted) {
                          /*
                           * No.  Either vm_pageout_wanted was set by another
                           * thread during the previous scan, which must have
                           * been a level 0 scan, or vm_pageout_wanted was
                           * already set and the scan failed to free enough
                           * pages.  If we haven't yet performed a level >= 1
                           * (page reclamation) scan, then increase the level
                           * and scan again now.  Otherwise, sleep a bit and
                           * try again later.
                           */
                          mtx_unlock(&vm_page_queue_free_mtx);
                          if (pass >= 1)
                                  pause("pwait", hz / VM_INACT_SCAN_RATE);
                          pass++;
                  } else {
                          /*
                           * Yes.  If threads are still sleeping in VM_WAIT
                           * then we immediately start a new scan.  Otherwise,
                           * sleep until the next wakeup or until pages need to
                           * have their reference stats updated.
                           */
                          if (vm_pages_needed) {
                                  mtx_unlock(&vm_page_queue_free_mtx);
                                  if (pass == 0)
                                          pass++;
                          } else if (mtx_sleep(&vm_pageout_wanted,
                              &vm_page_queue_free_mtx, PDROP | PVM, "psleep",
                              hz) == 0) {
                                  PCPU_INC(cnt.v_pdwakeups);
                                  pass = 1;
                          } else
                                  pass = 0;
                  }
  
                  target_met = vm_pageout_scan(domain, pass);
          }
  }
  
  /*
   *      vm_pageout_init initialises basic pageout daemon settings.
   */
  static void
  vm_pageout_init(void)
  {
          /*
           * Initialize some paging parameters.
           */
          vm_cnt.v_interrupt_free_min = 2;
          if (vm_cnt.v_page_count < 2000)
                  vm_pageout_page_count = 8;
  
          /*
           * v_free_reserved needs to include enough for the largest
           * swap pager structures plus enough for any pv_entry structs
           * when paging. 
           */
          if (vm_cnt.v_page_count > 1024)
                  vm_cnt.v_free_min = 4 + (vm_cnt.v_page_count - 1024) / 200;
          else
                  vm_cnt.v_free_min = 4;
          vm_cnt.v_pageout_free_min = (2*MAXBSIZE)/PAGE_SIZE +
              vm_cnt.v_interrupt_free_min;
          vm_cnt.v_free_reserved = vm_pageout_page_count +
              vm_cnt.v_pageout_free_min + (vm_cnt.v_page_count / 768);
          vm_cnt.v_free_severe = vm_cnt.v_free_min / 2;
          vm_cnt.v_free_target = 4 * vm_cnt.v_free_min + vm_cnt.v_free_reserved;
          vm_cnt.v_free_min += vm_cnt.v_free_reserved;
          vm_cnt.v_free_severe += vm_cnt.v_free_reserved;
          vm_cnt.v_inactive_target = (3 * vm_cnt.v_free_target) / 2;
          if (vm_cnt.v_inactive_target > vm_cnt.v_free_count / 3)
                  vm_cnt.v_inactive_target = vm_cnt.v_free_count / 3;
  
          /*
           * Set the default wakeup threshold to be 10% above the minimum
           * page limit.  This keeps the steady state out of shortfall.
           */
          vm_pageout_wakeup_thresh = (vm_cnt.v_free_min / 10) * 11;
  
          /*
           * Set interval in seconds for active scan.  We want to visit each
           * page at least once every ten minutes.  This is to prevent worst
           * case paging behaviors with stale active LRU.
           */
          if (vm_pageout_update_period == 0)
                  vm_pageout_update_period = 600;
  
          /* XXX does not really belong here */
          if (vm_page_max_wired == 0)
                  vm_page_max_wired = vm_cnt.v_free_count / 3;
  
          /*
           * Target amount of memory to move out of the laundry queue during a
           * background laundering.  This is proportional to the amount of system
           * memory.
           */
          vm_background_launder_target = (vm_cnt.v_free_target -
              vm_cnt.v_free_min) / 10;
  }
  
  /*
   *     vm_pageout is the high level pageout daemon.
   */
  static void
  vm_pageout(void)
  {
          int error;
  #ifdef VM_NUMA_ALLOC
          int i;
  #endif
  
          swap_pager_swap_init();
          snprintf(curthread->td_name, sizeof(curthread->td_name), "dom0");
          error = kthread_add(vm_pageout_laundry_worker, NULL, curproc, NULL,
              0, 0, "laundry: dom0");
          if (error != 0)
                  panic("starting laundry for domain 0, error %d", error);
  #ifdef VM_NUMA_ALLOC
          for (i = 1; i < vm_ndomains; i++) {
                  error = kthread_add(vm_pageout_worker, (void *)(uintptr_t)i,
                      curproc, NULL, 0, 0, "dom%d", i);
                  if (error != 0) {
                          panic("starting pageout for domain %d, error %d\n",
                              i, error);
                  }
          }
  #endif
          error = kthread_add(uma_reclaim_worker, NULL, curproc, NULL,
              0, 0, "uma");
          if (error != 0)
                  panic("starting uma_reclaim helper, error %d\n", error);
          vm_pageout_worker((void *)(uintptr_t)0);
  }
  
  /*
   * Perform an advisory wakeup of the page daemon.
   */
  void
  pagedaemon_wakeup(void)
  {
  
          mtx_assert(&vm_page_queue_free_mtx, MA_NOTOWNED);
  
          if (!vm_pageout_wanted && curthread->td_proc != pageproc) {
                  vm_pageout_wanted = true;
                  wakeup(&vm_pageout_wanted);
          }
  }
  
  /*
   * Wake up the page daemon and wait for it to reclaim free pages.
   *
   * This function returns with the free queues mutex unlocked.
   */
  void
  pagedaemon_wait(int pri, const char *wmesg)
  {
  
          mtx_assert(&vm_page_queue_free_mtx, MA_OWNED);
  
          /*
           * vm_pageout_wanted may have been set by an advisory wakeup, but if the
           * page daemon is running on a CPU, the wakeup will have been lost.
           * Thus, deliver a potentially spurious wakeup to ensure that the page
           * daemon has been notified of the shortage.
           */
          if (!vm_pageout_wanted || !vm_pages_needed) {
                  vm_pageout_wanted = true;
                  wakeup(&vm_pageout_wanted);
          }
          vm_pages_needed = true;
          msleep(&vm_cnt.v_free_count, &vm_page_queue_free_mtx, PDROP | pri,
              wmesg, 0);
  }

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