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.
      *
      * 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.
     *
     * $FreeBSD: releng/5.0/sys/vm/vm_pageout.c 107436 2002-12-01 05:40:18Z alc $
     */
    
    /*
     *      The proverbial page-out daemon.
     */
    
    #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/resourcevar.h>
    #include <sys/sched.h>
    #include <sys/signalvar.h>
    #include <sys/vnode.h>
    #include <sys/vmmeter.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/swap_pager.h>
   #include <vm/vm_extern.h>
   #include <vm/uma.h>
   
   #include <machine/mutex.h>
   
   /*
    * System initialization
    */
   
   /* the kernel process "vm_pageout"*/
   static void vm_pageout(void);
   static int vm_pageout_clean(vm_page_t);
   static void vm_pageout_pmap_collect(void);
   static void vm_pageout_scan(int pass);
   static int vm_pageout_free_page_calc(vm_size_t count);
   struct proc *pageproc;
   
   static struct kproc_desc page_kp = {
           "pagedaemon",
           vm_pageout,
           &pageproc
   };
   SYSINIT(pagedaemon, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, kproc_start, &page_kp)
   
   #if !defined(NO_SWAPPING)
   /* the kernel process "vm_daemon"*/
   static void vm_daemon(void);
   static struct   proc *vmproc;
   
   static struct kproc_desc vm_kp = {
           "vmdaemon",
           vm_daemon,
           &vmproc
   };
   SYSINIT(vmdaemon, SI_SUB_KTHREAD_VM, SI_ORDER_FIRST, kproc_start, &vm_kp)
   #endif
   
   
   int vm_pages_needed=0;          /* Event on which pageout daemon sleeps */
   int vm_pageout_deficit=0;       /* Estimated number of pages deficit */
   int vm_pageout_pages_needed=0;  /* flag saying that the pageout daemon needs pages */
   
   #if !defined(NO_SWAPPING)
   static int vm_pageout_req_swapout;      /* XXX */
   static int vm_daemon_needed;
   #endif
   extern int vm_swap_size;
   static int vm_max_launder = 32;
   static int vm_pageout_stats_max=0, vm_pageout_stats_interval = 0;
   static int vm_pageout_full_stats_interval = 0;
   static int vm_pageout_stats_free_max=0, vm_pageout_algorithm=0;
   static int defer_swap_pageouts=0;
   static int disable_swap_pageouts=0;
   
   #if defined(NO_SWAPPING)
   static int vm_swap_enabled=0;
   static int vm_swap_idle_enabled=0;
   #else
   static int vm_swap_enabled=1;
   static int vm_swap_idle_enabled=0;
   #endif
   
   SYSCTL_INT(_vm, VM_PAGEOUT_ALGORITHM, pageout_algorithm,
           CTLFLAG_RW, &vm_pageout_algorithm, 0, "LRU page mgmt");
   
   SYSCTL_INT(_vm, OID_AUTO, max_launder,
           CTLFLAG_RW, &vm_max_launder, 0, "Limit dirty flushes in pageout");
   
   SYSCTL_INT(_vm, OID_AUTO, pageout_stats_max,
           CTLFLAG_RW, &vm_pageout_stats_max, 0, "Max pageout stats scan length");
   
   SYSCTL_INT(_vm, OID_AUTO, pageout_full_stats_interval,
           CTLFLAG_RW, &vm_pageout_full_stats_interval, 0, "Interval for full stats scan");
   
   SYSCTL_INT(_vm, OID_AUTO, pageout_stats_interval,
           CTLFLAG_RW, &vm_pageout_stats_interval, 0, "Interval for partial stats scan");
   
   SYSCTL_INT(_vm, OID_AUTO, pageout_stats_free_max,
           CTLFLAG_RW, &vm_pageout_stats_free_max, 0, "Not implemented");
   
   #if defined(NO_SWAPPING)
   SYSCTL_INT(_vm, VM_SWAPPING_ENABLED, swap_enabled,
           CTLFLAG_RD, &vm_swap_enabled, 0, "");
   SYSCTL_INT(_vm, OID_AUTO, swap_idle_enabled,
           CTLFLAG_RD, &vm_swap_idle_enabled, 0, "");
   #else
   SYSCTL_INT(_vm, VM_SWAPPING_ENABLED, swap_enabled,
           CTLFLAG_RW, &vm_swap_enabled, 0, "Enable entire process swapout");
   SYSCTL_INT(_vm, OID_AUTO, swap_idle_enabled,
           CTLFLAG_RW, &vm_swap_idle_enabled, 0, "Allow swapout on idle criteria");
   #endif
   
   SYSCTL_INT(_vm, OID_AUTO, defer_swapspace_pageouts,
           CTLFLAG_RW, &defer_swap_pageouts, 0, "Give preference to dirty pages in mem");
   
   SYSCTL_INT(_vm, OID_AUTO, disable_swapspace_pageouts,
           CTLFLAG_RW, &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");
   
   #define VM_PAGEOUT_PAGE_COUNT 16
   int vm_pageout_page_count = VM_PAGEOUT_PAGE_COUNT;
   
   int vm_page_max_wired;          /* XXX max # of wired pages system-wide */
   
   #if !defined(NO_SWAPPING)
   typedef void freeer_fcn_t(vm_map_t, vm_object_t, vm_pindex_t, int);
   static void vm_pageout_map_deactivate_pages(vm_map_t, vm_pindex_t);
   static freeer_fcn_t vm_pageout_object_deactivate_pages;
   static void vm_req_vmdaemon(void);
   #endif
   static void vm_pageout_page_stats(void);
   
   /*
    * vm_pageout_clean:
    *
    * Clean the page and remove it from the laundry.
    * 
    * We set the busy bit to cause potential page faults on this page to
    * block.  Note the careful timing, however, the busy bit isn't set till
    * late and we cannot do anything that will mess with the page.
    */
   static int
   vm_pageout_clean(m)
           vm_page_t m;
   {
           vm_object_t object;
           vm_page_t mc[2*vm_pageout_page_count];
           int pageout_count;
           int ib, is, page_base;
           vm_pindex_t pindex = m->pindex;
   
           mtx_assert(&vm_page_queue_mtx, MA_OWNED);
   
           object = m->object;
   
           /*
            * It doesn't cost us anything to pageout OBJT_DEFAULT or OBJT_SWAP
            * with the new swapper, but we could have serious problems paging
            * out other object types if there is insufficient memory.  
            *
            * Unfortunately, checking free memory here is far too late, so the
            * check has been moved up a procedural level.
            */
   
           /*
            * Don't mess with the page if it's busy, held, or special
            */
           if ((m->hold_count != 0) ||
               ((m->busy != 0) || (m->flags & (PG_BUSY|PG_UNMANAGED)))) {
                   return 0;
           }
   
           mc[vm_pageout_page_count] = m;
           pageout_count = 1;
           page_base = vm_pageout_page_count;
           ib = 1;
           is = 1;
   
           /*
            * Scan object for clusterable pages.
            *
            * We can cluster ONLY if: ->> the page is NOT
            * clean, wired, busy, held, or mapped into a
            * buffer, and one of the following:
            * 1) The page is inactive, or a seldom used
            *    active page.
            * -or-
            * 2) we force the issue.
            *
            * During heavy mmap/modification loads the pageout
            * daemon can really fragment the underlying file
            * due to flushing pages out of order and not trying
            * align the clusters (which leave sporatic 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 && pageout_count < vm_pageout_page_count) {
                   vm_page_t p;
   
                   if (ib > pindex) {
                           ib = 0;
                           break;
                   }
   
                   if ((p = vm_page_lookup(object, pindex - ib)) == NULL) {
                           ib = 0;
                           break;
                   }
                   if (((p->queue - p->pc) == PQ_CACHE) ||
                       (p->flags & (PG_BUSY|PG_UNMANAGED)) || p->busy) {
                           ib = 0;
                           break;
                   }
                   vm_page_test_dirty(p);
                   if ((p->dirty & p->valid) == 0 ||
                       p->queue != PQ_INACTIVE ||
                       p->wire_count != 0 ||       /* may be held by buf cache */
                       p->hold_count != 0) {       /* may be undergoing I/O */
                           ib = 0;
                           break;
                   }
                   mc[--page_base] = p;
                   ++pageout_count;
                   ++ib;
                   /*
                    * alignment boundry, 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) {
                   vm_page_t p;
   
                   if ((p = vm_page_lookup(object, pindex + is)) == NULL)
                           break;
                   if (((p->queue - p->pc) == PQ_CACHE) ||
                       (p->flags & (PG_BUSY|PG_UNMANAGED)) || p->busy) {
                           break;
                   }
                   vm_page_test_dirty(p);
                   if ((p->dirty & p->valid) == 0 ||
                       p->queue != PQ_INACTIVE ||
                       p->wire_count != 0 ||       /* may be held by buf cache */
                       p->hold_count != 0) {       /* may be undergoing I/O */
                           break;
                   }
                   mc[page_base + pageout_count] = p;
                   ++pageout_count;
                   ++is;
           }
   
           /*
            * If we exhausted our forward scan, continue with the reverse scan
            * when possible, even past a page boundry.  This catches boundry
            * conditions.
            */
           if (ib && pageout_count < vm_pageout_page_count)
                   goto more;
   
           /*
            * we allow reads during pageouts...
            */
           return vm_pageout_flush(&mc[page_base], pageout_count, 0);
   }
   
   /*
    * 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.
    */
   int
   vm_pageout_flush(mc, count, flags)
           vm_page_t *mc;
           int count;
           int flags;
   {
           vm_object_t object;
           int pageout_status[count];
           int numpagedout = 0;
           int i;
   
           mtx_assert(&vm_page_queue_mtx, MA_OWNED);
           /*
            * Initiate I/O.  Bump the vm_page_t->busy counter and
            * mark the pages 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 page %p index %d/%d: partially invalid page", mc[i], i, count));
                   vm_page_io_start(mc[i]);
                   pmap_page_protect(mc[i], VM_PROT_READ);
           }
           object = mc[0]->object;
           vm_page_unlock_queues();
           vm_object_pip_add(object, count);
   
           vm_pager_put_pages(object, mc, count,
               (flags | ((object == kernel_object) ? OBJPC_SYNC : 0)),
               pageout_status);
   
           vm_page_lock_queues();
           for (i = 0; i < count; i++) {
                   vm_page_t mt = mc[i];
   
                   switch (pageout_status[i]) {
                   case VM_PAGER_OK:
                           numpagedout++;
                           break;
                   case VM_PAGER_PEND:
                           numpagedout++;
                           break;
                   case VM_PAGER_BAD:
                           /*
                            * Page outside of range of object. Right now we
                            * essentially lose the changes by pretending it
                            * worked.
                            */
                           pmap_clear_modify(mt);
                           vm_page_undirty(mt);
                           break;
                   case VM_PAGER_ERROR:
                   case VM_PAGER_FAIL:
                           /*
                            * If page couldn't be paged out, then reactivate the
                            * page so it doesn't clog the inactive list.  (We
                            * will try paging out it again later).
                            */
                           vm_page_activate(mt);
                           break;
                   case VM_PAGER_AGAIN:
                           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_io_finish(mt);
                           if (!vm_page_count_severe() || !vm_page_try_to_cache(mt))
                                   pmap_page_protect(mt, VM_PROT_READ);
                   }
           }
           return numpagedout;
   }
   
   #if !defined(NO_SWAPPING)
   /*
    *      vm_pageout_object_deactivate_pages
    *
    *      deactivate enough pages to satisfy the inactive target
    *      requirements or if vm_page_proc_limit is set, then
    *      deactivate all of the pages in the object and its
    *      backing_objects.
    *
    *      The object and map must be locked.
    */
   static void
   vm_pageout_object_deactivate_pages(map, object, desired, map_remove_only)
           vm_map_t map;
           vm_object_t object;
           vm_pindex_t desired;
           int map_remove_only;
   {
           vm_page_t p, next;
           int actcount, rcount, remove_mode;
   
           GIANT_REQUIRED;
           if (object->type == OBJT_DEVICE || object->type == OBJT_PHYS)
                   return;
   
           while (object) {
                   if (pmap_resident_count(vm_map_pmap(map)) <= desired)
                           return;
                   if (object->paging_in_progress)
                           return;
   
                   remove_mode = map_remove_only;
                   if (object->shadow_count > 1)
                           remove_mode = 1;
                   /*
                    * scan the objects entire memory queue
                    */
                   rcount = object->resident_page_count;
                   p = TAILQ_FIRST(&object->memq);
                   vm_page_lock_queues();
                   while (p && (rcount-- > 0)) {
                           if (pmap_resident_count(map->pmap) <= desired) {
                                   vm_page_unlock_queues();
                                   return;
                           }
                           next = TAILQ_NEXT(p, listq);
                           cnt.v_pdpages++;
                           if (p->wire_count != 0 ||
                               p->hold_count != 0 ||
                               p->busy != 0 ||
                               (p->flags & (PG_BUSY|PG_UNMANAGED)) ||
                               !pmap_page_exists_quick(vm_map_pmap(map), p)) {
                                   p = next;
                                   continue;
                           }
                           actcount = pmap_ts_referenced(p);
                           if (actcount) {
                                   vm_page_flag_set(p, PG_REFERENCED);
                           } else if (p->flags & PG_REFERENCED) {
                                   actcount = 1;
                           }
                           if ((p->queue != PQ_ACTIVE) &&
                                   (p->flags & PG_REFERENCED)) {
                                   vm_page_activate(p);
                                   p->act_count += actcount;
                                   vm_page_flag_clear(p, PG_REFERENCED);
                           } else if (p->queue == PQ_ACTIVE) {
                                   if ((p->flags & PG_REFERENCED) == 0) {
                                           p->act_count -= min(p->act_count, ACT_DECLINE);
                                           if (!remove_mode && (vm_pageout_algorithm || (p->act_count == 0))) {
                                                   pmap_remove_all(p);
                                                   vm_page_deactivate(p);
                                           } else {
                                                   vm_pageq_requeue(p);
                                           }
                                   } else {
                                           vm_page_activate(p);
                                           vm_page_flag_clear(p, PG_REFERENCED);
                                           if (p->act_count < (ACT_MAX - ACT_ADVANCE))
                                                   p->act_count += ACT_ADVANCE;
                                           vm_pageq_requeue(p);
                                   }
                           } else if (p->queue == PQ_INACTIVE) {
                                   pmap_remove_all(p);
                           }
                           p = next;
                   }
                   vm_page_unlock_queues();
                   object = object->backing_object;
           }
   }
   
   /*
    * deactivate some number of pages in a map, try to do it fairly, but
    * that is really hard to do.
    */
   static void
   vm_pageout_map_deactivate_pages(map, desired)
           vm_map_t map;
           vm_pindex_t desired;
   {
           vm_map_entry_t tmpe;
           vm_object_t obj, bigobj;
           int nothingwired;
   
           GIANT_REQUIRED;
           if (!vm_map_trylock(map))
                   return;
   
           bigobj = NULL;
           nothingwired = TRUE;
   
           /*
            * first, search out the biggest object, and try to free pages from
            * that.
            */
           tmpe = map->header.next;
           while (tmpe != &map->header) {
                   if ((tmpe->eflags & MAP_ENTRY_IS_SUB_MAP) == 0) {
                           obj = tmpe->object.vm_object;
                           if ((obj != NULL) && (obj->shadow_count <= 1) &&
                                   ((bigobj == NULL) ||
                                    (bigobj->resident_page_count < obj->resident_page_count))) {
                                   bigobj = obj;
                           }
                   }
                   if (tmpe->wired_count > 0)
                           nothingwired = FALSE;
                   tmpe = tmpe->next;
           }
   
           if (bigobj)
                   vm_pageout_object_deactivate_pages(map, bigobj, desired, 0);
   
           /*
            * Next, hunt around for other pages to deactivate.  We actually
            * do this search sort of wrong -- .text first is not the best idea.
            */
           tmpe = map->header.next;
           while (tmpe != &map->header) {
                   if (pmap_resident_count(vm_map_pmap(map)) <= desired)
                           break;
                   if ((tmpe->eflags & MAP_ENTRY_IS_SUB_MAP) == 0) {
                           obj = tmpe->object.vm_object;
                           if (obj)
                                   vm_pageout_object_deactivate_pages(map, obj, desired, 0);
                   }
                   tmpe = tmpe->next;
           }
   
           /*
            * Remove all mappings if a process is swapped out, this will free page
            * table pages.
            */
           if (desired == 0 && nothingwired) {
                   vm_page_lock_queues();
                   pmap_remove(vm_map_pmap(map), vm_map_min(map),
                       vm_map_max(map));
                   vm_page_unlock_queues();
           }
           vm_map_unlock(map);
   }
   #endif          /* !defined(NO_SWAPPING) */
   
   /*
    * Don't try to be fancy - being fancy can lead to VOP_LOCK's and therefore
    * to vnode deadlocks.  We only do it for OBJT_DEFAULT and OBJT_SWAP objects
    * which we know can be trivially freed.
    */
   void
   vm_pageout_page_free(vm_page_t m)
   {
           vm_object_t object = m->object;
           int type = object->type;
   
           mtx_assert(&vm_page_queue_mtx, MA_OWNED);
           if (type == OBJT_SWAP || type == OBJT_DEFAULT)
                   vm_object_reference(object);
           vm_page_busy(m);
           pmap_remove_all(m);
           vm_page_free(m);
           cnt.v_dfree++;
           if (type == OBJT_SWAP || type == OBJT_DEFAULT)
                   vm_object_deallocate(object);
   }
   
   /*
    * This routine is very drastic, but can save the system
    * in a pinch.
    */
   static void
   vm_pageout_pmap_collect(void)
   {
           int i;
           vm_page_t m;
           static int warningdone;
   
           if (pmap_pagedaemon_waken == 0)
                   return;
           if (warningdone < 5) {
                   printf("collecting pv entries -- suggest increasing PMAP_SHPGPERPROC\n");
                   warningdone++;
           }
           vm_page_lock_queues();
           for (i = 0; i < vm_page_array_size; i++) {
                   m = &vm_page_array[i];
                   if (m->wire_count || m->hold_count || m->busy ||
                       (m->flags & (PG_BUSY | PG_UNMANAGED)))
                           continue;
                   pmap_remove_all(m);
           }
           vm_page_unlock_queues();
           pmap_pagedaemon_waken = 0;
   }
           
   /*
    *      vm_pageout_scan does the dirty work for the pageout daemon.
    */
   static void
   vm_pageout_scan(int pass)
   {
           vm_page_t m, next;
           struct vm_page marker;
           int save_page_shortage;
           int save_inactive_count;
           int page_shortage, maxscan, pcount;
           int addl_page_shortage, addl_page_shortage_init;
           struct proc *p, *bigproc;
           vm_offset_t size, bigsize;
           vm_object_t object;
           int actcount;
           int vnodes_skipped = 0;
           int maxlaunder;
           int s;
           struct thread *td;
   
           GIANT_REQUIRED;
           /*
            * Decrease registered cache sizes.
            */
           EVENTHANDLER_INVOKE(vm_lowmem, 0);
           /*
            * We do this explicitly after the caches have been drained above.
            */
           uma_reclaim();
           /*
            * Do whatever cleanup that the pmap code can.
            */
           vm_pageout_pmap_collect();
   
           addl_page_shortage_init = vm_pageout_deficit;
           vm_pageout_deficit = 0;
   
           /*
            * Calculate the number of pages we want to either free or move
            * to the cache.
            */
           page_shortage = vm_paging_target() + addl_page_shortage_init;
           save_page_shortage = page_shortage;
           save_inactive_count = cnt.v_inactive_count;
   
           /*
            * Initialize our marker
            */
           bzero(&marker, sizeof(marker));
           marker.flags = PG_BUSY | PG_FICTITIOUS | PG_MARKER;
           marker.queue = PQ_INACTIVE;
           marker.wire_count = 1;
   
           /*
            * Start scanning the inactive queue for pages we can move to the
            * cache or free.  The scan will stop when the target is reached or
            * we have scanned the entire inactive queue.  Note that m->act_count
            * is not used to form decisions for the inactive queue, only for the
            * active queue.
            *
            * maxlaunder limits the number of dirty pages we flush per scan.
            * For most systems a smaller value (16 or 32) is more robust under
            * extreme memory and disk pressure because any unnecessary writes
            * to disk can result in extreme performance degredation.  However,
            * systems with excessive dirty pages (especially when MAP_NOSYNC is
            * used) will die horribly with limited laundering.  If the pageout
            * daemon cannot clean enough pages in the first pass, we let it go
            * all out in succeeding passes.
            */
           if ((maxlaunder = vm_max_launder) <= 1)
                   maxlaunder = 1;
           if (pass)
                   maxlaunder = 10000;
   rescan0:
           addl_page_shortage = addl_page_shortage_init;
           maxscan = cnt.v_inactive_count;
   
           for (m = TAILQ_FIRST(&vm_page_queues[PQ_INACTIVE].pl);
                m != NULL && maxscan-- > 0 && page_shortage > 0;
                m = next) {
   
                   cnt.v_pdpages++;
   
                   if (m->queue != PQ_INACTIVE) {
                           goto rescan0;
                   }
   
                   next = TAILQ_NEXT(m, pageq);
   
                   /*
                    * skip marker pages
                    */
                   if (m->flags & PG_MARKER)
                           continue;
   
                   /*
                    * A held page may be undergoing I/O, so skip it.
                    */
                   if (m->hold_count) {
                           vm_pageq_requeue(m);
                           addl_page_shortage++;
                           continue;
                   }
                   /*
                    * Don't mess with busy pages, keep in the front of the
                    * queue, most likely are being paged out.
                    */
                   if (m->busy || (m->flags & PG_BUSY)) {
                           addl_page_shortage++;
                           continue;
                   }
   
                   vm_page_lock_queues();
                   /*
                    * If the object is not being used, we ignore previous 
                    * references.
                    */
                   if (m->object->ref_count == 0) {
                           vm_page_flag_clear(m, PG_REFERENCED);
                           pmap_clear_reference(m);
   
                   /*
                    * Otherwise, if the page has been referenced while in the 
                    * inactive queue, we bump the "activation count" upwards, 
                    * making it less likely that the page will be added back to 
                    * the inactive queue prematurely again.  Here we check the 
                    * page tables (or emulated bits, if any), given the upper 
                    * level VM system not knowing anything about existing 
                    * references.
                    */
                   } else if (((m->flags & PG_REFERENCED) == 0) &&
                           (actcount = pmap_ts_referenced(m))) {
                           vm_page_activate(m);
                           vm_page_unlock_queues();
                           m->act_count += (actcount + ACT_ADVANCE);
                           continue;
                   }
   
                   /*
                    * If the upper level VM system knows about any page 
                    * references, we activate the page.  We also set the 
                    * "activation count" higher than normal so that we will less 
                    * likely place pages back onto the inactive queue again.
                    */
                   if ((m->flags & PG_REFERENCED) != 0) {
                           vm_page_flag_clear(m, PG_REFERENCED);
                           actcount = pmap_ts_referenced(m);
                           vm_page_activate(m);
                           vm_page_unlock_queues();
                           m->act_count += (actcount + ACT_ADVANCE + 1);
                           continue;
                   }
   
                   /*
                    * If the upper level VM system doesn't know anything about 
                    * the page being dirty, we have to check for it again.  As 
                    * far as the VM code knows, any partially dirty pages are 
                    * fully dirty.
                    */
                   if (m->dirty == 0) {
                           vm_page_test_dirty(m);
                   } else {
                           vm_page_dirty(m);
                   }
                   vm_page_unlock_queues();
   
                   /*
                    * Invalid pages can be easily freed
                    */
                   if (m->valid == 0) {
                           vm_page_lock_queues();
                           vm_pageout_page_free(m);
                           vm_page_unlock_queues();
                           --page_shortage;
   
                   /*
                    * Clean pages can be placed onto the cache queue.  This
                    * effectively frees them.
                    */
                   } else if (m->dirty == 0) {
                           vm_page_lock_queues();
                           vm_page_cache(m);
                           vm_page_unlock_queues();
                           --page_shortage;
                   } else if ((m->flags & PG_WINATCFLS) == 0 && pass == 0) {
                           /*
                            * Dirty pages need to be paged out, but flushing
                            * a page is extremely expensive verses freeing
                            * a clean page.  Rather then artificially limiting
                            * the number of pages we can flush, we instead give
                            * dirty pages extra priority on the inactive queue
                            * by forcing them to be cycled through the queue
                            * twice before being flushed, after which the
                            * (now clean) page will cycle through once more
                            * before being freed.  This significantly extends
                            * the thrash point for a heavily loaded machine.
                            */
                           vm_page_flag_set(m, PG_WINATCFLS);
                           vm_pageq_requeue(m);
                   } else if (maxlaunder > 0) {
                           /*
                            * We always want to try to flush some dirty pages if
                            * we encounter them, to keep the system stable.
                            * Normally this number is small, but under extreme
                            * pressure where there are insufficient clean pages
                            * on the inactive queue, we may have to go all out.
                            */
                           int swap_pageouts_ok;
                           struct vnode *vp = NULL;
                           struct mount *mp;
   
                           object = m->object;
   
                           if ((object->type != OBJT_SWAP) && (object->type != OBJT_DEFAULT)) {
                                   swap_pageouts_ok = 1;
                           } else {
                                   swap_pageouts_ok = !(defer_swap_pageouts || disable_swap_pageouts);
                                   swap_pageouts_ok |= (!disable_swap_pageouts && defer_swap_pageouts &&
                                   vm_page_count_min());
                                                                                   
                           }
   
                           /*
                            * We don't bother paging objects that are "dead".  
                            * Those objects are in a "rundown" state.
                            */
                           if (!swap_pageouts_ok || (object->flags & OBJ_DEAD)) {
                                   vm_pageq_requeue(m);
                                   continue;
                           }
   
                           /*
                            * 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.
                            *
                            * The previous code skipped locked vnodes and, worse,
                            * reordered pages in the queue.  This results in
                            * completely non-deterministic operation and, on a
                            * busy system, can lead to extremely non-optimal
                            * pageouts.  For example, it can cause clean pages
                            * to be freed and dirty pages to be moved to the end
                            * of the queue.  Since dirty pages are also moved to
                            * the end of the queue once-cleaned, this gives
                            * way too large a weighting to defering the freeing
                            * of dirty pages.
                            *
                            * 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) {
                                   vp = object->handle;
   
                                   mp = NULL;
                                   if (vp->v_type == VREG)
                                           vn_start_write(vp, &mp, V_NOWAIT);
                                   if (vget(vp, LK_EXCLUSIVE|LK_TIMELOCK, curthread)) {
                                           ++pageout_lock_miss;
                                           vn_finished_write(mp);
                                           if (object->flags & OBJ_MIGHTBEDIRTY)
                                                   vnodes_skipped++;
                                           continue;
                                   }
   
                                   /*
                                    * The page might have been moved to another
                                    * queue during potential blocking in vget()
                                    * above.  The page might have been freed and
                                    * reused for another vnode.  The object might
                                    * have been reused for another vnode.
                                    */
                                   if (m->queue != PQ_INACTIVE ||
                                       m->object != object ||
                                       object->handle != vp) {
                                           if (object->flags & OBJ_MIGHTBEDIRTY)
                                                   vnodes_skipped++;
                                           vput(vp);
                                           vn_finished_write(mp);
                                           continue;
                                   }
           
                                   /*
                                    * The page may have been busied during the
                                    * blocking in vput();  We don't move the
                                    * page back onto the end of the queue so that
                                    * statistics are more correct if we don't.
                                    */
                                   if (m->busy || (m->flags & PG_BUSY)) {
                                           vput(vp);
                                           vn_finished_write(mp);
                                           continue;
                                   }
   
                                   /*
                                    * If the page has become held it might
                                    * be undergoing I/O, so skip it
                                    */
                                   if (m->hold_count) {
                                           vm_pageq_requeue(m);
                                           if (object->flags & OBJ_MIGHTBEDIRTY)
                                                   vnodes_skipped++;
                                           vput(vp);
                                           vn_finished_write(mp);
                                           continue;
                                   }
                           }
   
                           /*
                            * 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. 
                            *
                            * This operation may cluster, invalidating the 'next'
                            * pointer.  To prevent an inordinate number of
                            * restarts we use our marker to remember our place.
                            *
                            * decrement page_shortage on success to account for
                            * the (future) cleaned page.  Otherwise we could wind
                            * up laundering or cleaning too many pages.
                            */
                           vm_page_lock_queues();
                           s = splvm();
                           TAILQ_INSERT_AFTER(&vm_page_queues[PQ_INACTIVE].pl, m, &marker, pageq);
                           splx(s);
                           if (vm_pageout_clean(m) != 0) {
                                   --page_shortage;
                                   --maxlaunder;
                           }
                           s = splvm();
                           next = TAILQ_NEXT(&marker, pageq);
                           TAILQ_REMOVE(&vm_page_queues[PQ_INACTIVE].pl, &marker, pageq);
                           splx(s);
                          vm_page_unlock_queues();
                          if (vp) {
                                  vput(vp);
                                  vn_finished_write(mp);
                          }
                  }
          }
  
          /*
           * Compute the number of pages we want to try to move from the
           * active queue to the inactive queue.
           */
          page_shortage = vm_paging_target() +
                  cnt.v_inactive_target - cnt.v_inactive_count;
          page_shortage += addl_page_shortage;
  
          vm_page_lock_queues();
          /*
           * Scan the active queue for things we can deactivate. We nominally
           * track the per-page activity counter and use it to locate
           * deactivation candidates.
           */
          pcount = cnt.v_active_count;
          m = TAILQ_FIRST(&vm_page_queues[PQ_ACTIVE].pl);
  
          while ((m != NULL) && (pcount-- > 0) && (page_shortage > 0)) {
  
                  /*
                   * This is a consistency check, and should likely be a panic
                   * or warning.
                   */
                  if (m->queue != PQ_ACTIVE) {
                          break;
                  }
  
                  next = TAILQ_NEXT(m, pageq);
                  /*
                   * Don't deactivate pages that are busy.
                   */
                  if ((m->busy != 0) ||
                      (m->flags & PG_BUSY) ||
                      (m->hold_count != 0)) {
                          vm_pageq_requeue(m);
                          m = next;
                          continue;
                  }
  
                  /*
                   * The count for pagedaemon pages is done after checking the
                   * page for eligibility...
                   */
                  cnt.v_pdpages++;
  
                  /*
                   * Check to see "how much" the page has been used.
                   */
                  actcount = 0;
                  if (m->object->ref_count != 0) {
                          if (m->flags & PG_REFERENCED) {
                                  actcount += 1;
                          }
                          actcount += pmap_ts_referenced(m);
                          if (actcount) {
                                  m->act_count += ACT_ADVANCE + actcount;
                                  if (m->act_count > ACT_MAX)
                                          m->act_count = ACT_MAX;
                          }
                  }
  
                  /*
                   * Since we have "tested" this bit, we need to clear it now.
                   */
                  vm_page_flag_clear(m, PG_REFERENCED);
  
                  /*
                   * Only if an object is currently being used, do we use the
                   * page activation count stats.
                   */
                  if (actcount && (m->object->ref_count != 0)) {
                          vm_pageq_requeue(m);
                  } else {
                          m->act_count -= min(m->act_count, ACT_DECLINE);
                          if (vm_pageout_algorithm ||
                              m->object->ref_count == 0 ||
                              m->act_count == 0) {
                                  page_shortage--;
                                  if (m->object->ref_count == 0) {
                                          pmap_remove_all(m);
                                          if (m->dirty == 0)
                                                  vm_page_cache(m);
                                          else
                                                  vm_page_deactivate(m);
                                  } else {
                                          vm_page_deactivate(m);
                                  }
                          } else {
                                  vm_pageq_requeue(m);
                          }
                  }
                  m = next;
          }
          s = splvm();
  
          /*
           * We try to maintain some *really* free pages, this allows interrupt
           * code to be guaranteed space.  Since both cache and free queues 
           * are considered basically 'free', moving pages from cache to free
           * does not effect other calculations.
           */
          while (cnt.v_free_count < cnt.v_free_reserved) {
                  static int cache_rover = 0;
                  m = vm_pageq_find(PQ_CACHE, cache_rover, FALSE);
                  if (!m)
                          break;
                  if ((m->flags & (PG_BUSY|PG_UNMANAGED)) || 
                      m->busy || 
                      m->hold_count || 
                      m->wire_count) {
  #ifdef INVARIANTS
                          printf("Warning: busy page %p found in cache\n", m);
  #endif
                          vm_page_deactivate(m);
                          continue;
                  }
                  cache_rover = (cache_rover + PQ_PRIME2) & PQ_L2_MASK;
                  vm_pageout_page_free(m);
          }
          splx(s);
          vm_page_unlock_queues();
  #if !defined(NO_SWAPPING)
          /*
           * Idle process swapout -- run once per second.
           */
          if (vm_swap_idle_enabled) {
                  static long lsec;
                  if (time_second != lsec) {
                          vm_pageout_req_swapout |= VM_SWAP_IDLE;
                          vm_req_vmdaemon();
                          lsec = time_second;
                  }
          }
  #endif
                  
          /*
           * If we didn't get enough free pages, and we have skipped a vnode
           * in a writeable object, wakeup the sync daemon.  And kick swapout
           * if we did not get enough free pages.
           */
          if (vm_paging_target() > 0) {
                  if (vnodes_skipped && vm_page_count_min())
                          (void) speedup_syncer();
  #if !defined(NO_SWAPPING)
                  if (vm_swap_enabled && vm_page_count_target()) {
                          vm_req_vmdaemon();
                          vm_pageout_req_swapout |= VM_SWAP_NORMAL;
                  }
  #endif
          }
  
          /*
           * If we are out of swap and were not able to reach our paging
           * target, kill the largest process.
           *
           * 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.
           */
          if ((vm_swap_size < 64 && vm_page_count_min()) ||
              (swap_pager_full && vm_paging_target() > 0)) {
  #if 0
          if ((vm_swap_size < 64 || swap_pager_full) && vm_page_count_min()) {
  #endif
                  bigproc = NULL;
                  bigsize = 0;
                  sx_slock(&allproc_lock);
                  FOREACH_PROC_IN_SYSTEM(p) {
                          int breakout;
                          /*
                           * If this process is already locked, skip it.
                           */
                          if (PROC_TRYLOCK(p) == 0)
                                  continue;
                          /*
                           * if this is a system process, skip it
                           */
                          if ((p->p_flag & P_SYSTEM) || (p->p_pid == 1) ||
                              ((p->p_pid < 48) && (vm_swap_size != 0))) {
                                  PROC_UNLOCK(p);
                                  continue;
                          }
                          /*
                           * if the process is in a non-running type state,
                           * don't touch it. Check all the threads individually.
                           */
                          mtx_lock_spin(&sched_lock);
                          breakout = 0;
                          FOREACH_THREAD_IN_PROC(p, td) {
                                  if (!TD_ON_RUNQ(td) &&
                                      !TD_IS_RUNNING(td) &&
                                      !TD_IS_SLEEPING(td)) {
                                          breakout = 1;
                                          break;
                                  }
                          }
                          if (breakout) {
                                  mtx_unlock_spin(&sched_lock);
                                  PROC_UNLOCK(p);
                                  continue;
                          }
                          mtx_unlock_spin(&sched_lock);
                          /*
                           * get the process size
                           */
                          size = vmspace_resident_count(p->p_vmspace) +
                                  vmspace_swap_count(p->p_vmspace);
                          /*
                           * if the this process is bigger than the biggest one
                           * remember it.
                           */
                          if (size > bigsize) {
                                  if (bigproc != NULL)
                                          PROC_UNLOCK(bigproc);
                                  bigproc = p;
                                  bigsize = size;
                          } else
                                  PROC_UNLOCK(p);
                  }
                  sx_sunlock(&allproc_lock);
                  if (bigproc != NULL) {
                          struct ksegrp *kg;
                          killproc(bigproc, "out of swap space");
                          mtx_lock_spin(&sched_lock);
                          FOREACH_KSEGRP_IN_PROC(bigproc, kg) {
                                  sched_nice(kg, PRIO_MIN); /* XXXKSE ??? */
                          }
                          mtx_unlock_spin(&sched_lock);
                          PROC_UNLOCK(bigproc);
                          wakeup(&cnt.v_free_count);
                  }
          }
  }
  
  /*
   * This routine tries to maintain the pseudo LRU active queue,
   * so that during long periods of time where there is no paging,
   * that some statistic accumulation still occurs.  This code
   * helps the situation where paging just starts to occur.
   */
  static void
  vm_pageout_page_stats()
  {
          vm_page_t m,next;
          int pcount,tpcount;             /* Number of pages to check */
          static int fullintervalcount = 0;
          int page_shortage;
          int s0;
  
          page_shortage = 
              (cnt.v_inactive_target + cnt.v_cache_max + cnt.v_free_min) -
              (cnt.v_free_count + cnt.v_inactive_count + cnt.v_cache_count);
  
          if (page_shortage <= 0)
                  return;
  
          s0 = splvm();
          vm_page_lock_queues();
          pcount = cnt.v_active_count;
          fullintervalcount += vm_pageout_stats_interval;
          if (fullintervalcount < vm_pageout_full_stats_interval) {
                  tpcount = (vm_pageout_stats_max * cnt.v_active_count) / cnt.v_page_count;
                  if (pcount > tpcount)
                          pcount = tpcount;
          } else {
                  fullintervalcount = 0;
          }
  
          m = TAILQ_FIRST(&vm_page_queues[PQ_ACTIVE].pl);
          while ((m != NULL) && (pcount-- > 0)) {
                  int actcount;
  
                  if (m->queue != PQ_ACTIVE) {
                          break;
                  }
  
                  next = TAILQ_NEXT(m, pageq);
                  /*
                   * Don't deactivate pages that are busy.
                   */
                  if ((m->busy != 0) ||
                      (m->flags & PG_BUSY) ||
                      (m->hold_count != 0)) {
                          vm_pageq_requeue(m);
                          m = next;
                          continue;
                  }
  
                  actcount = 0;
                  if (m->flags & PG_REFERENCED) {
                          vm_page_flag_clear(m, PG_REFERENCED);
                          actcount += 1;
                  }
  
                  actcount += pmap_ts_referenced(m);
                  if (actcount) {
                          m->act_count += ACT_ADVANCE + actcount;
                          if (m->act_count > ACT_MAX)
                                  m->act_count = ACT_MAX;
                          vm_pageq_requeue(m);
                  } else {
                          if (m->act_count == 0) {
                                  /*
                                   * We turn off page access, so that we have
                                   * more accurate RSS stats.  We don't do this
                                   * in the normal page deactivation when the
                                   * system is loaded VM wise, because the
                                   * cost of the large number of page protect
                                   * operations would be higher than the value
                                   * of doing the operation.
                                   */
                                  pmap_remove_all(m);
                                  vm_page_deactivate(m);
                          } else {
                                  m->act_count -= min(m->act_count, ACT_DECLINE);
                                  vm_pageq_requeue(m);
                          }
                  }
  
                  m = next;
          }
          vm_page_unlock_queues();
          splx(s0);
  }
  
  static int
  vm_pageout_free_page_calc(count)
  vm_size_t count;
  {
          if (count < cnt.v_page_count)
                   return 0;
          /*
           * free_reserved needs to include enough for the largest swap pager
           * structures plus enough for any pv_entry structs when paging.
           */
          if (cnt.v_page_count > 1024)
                  cnt.v_free_min = 4 + (cnt.v_page_count - 1024) / 200;
          else
                  cnt.v_free_min = 4;
          cnt.v_pageout_free_min = (2*MAXBSIZE)/PAGE_SIZE +
                  cnt.v_interrupt_free_min;
          cnt.v_free_reserved = vm_pageout_page_count +
                  cnt.v_pageout_free_min + (count / 768) + PQ_L2_SIZE;
          cnt.v_free_severe = cnt.v_free_min / 2;
          cnt.v_free_min += cnt.v_free_reserved;
          cnt.v_free_severe += cnt.v_free_reserved;
          return 1;
  }
  
  /*
   *      vm_pageout is the high level pageout daemon.
   */
  static void
  vm_pageout()
  {
          int pass;
  
          mtx_lock(&Giant);
  
          /*
           * Initialize some paging parameters.
           */
          cnt.v_interrupt_free_min = 2;
          if (cnt.v_page_count < 2000)
                  vm_pageout_page_count = 8;
  
          vm_pageout_free_page_calc(cnt.v_page_count);
          /*
           * v_free_target and v_cache_min control pageout hysteresis.  Note
           * that these are more a measure of the VM cache queue hysteresis
           * then the VM free queue.  Specifically, v_free_target is the
           * high water mark (free+cache pages).
           *
           * v_free_reserved + v_cache_min (mostly means v_cache_min) is the
           * low water mark, while v_free_min is the stop.  v_cache_min must
           * be big enough to handle memory needs while the pageout daemon
           * is signalled and run to free more pages.
           */
          if (cnt.v_free_count > 6144)
                  cnt.v_free_target = 4 * cnt.v_free_min + cnt.v_free_reserved;
          else
                  cnt.v_free_target = 2 * cnt.v_free_min + cnt.v_free_reserved;
  
          if (cnt.v_free_count > 2048) {
                  cnt.v_cache_min = cnt.v_free_target;
                  cnt.v_cache_max = 2 * cnt.v_cache_min;
                  cnt.v_inactive_target = (3 * cnt.v_free_target) / 2;
          } else {
                  cnt.v_cache_min = 0;
                  cnt.v_cache_max = 0;
                  cnt.v_inactive_target = cnt.v_free_count / 4;
          }
          if (cnt.v_inactive_target > cnt.v_free_count / 3)
                  cnt.v_inactive_target = cnt.v_free_count / 3;
  
          /* XXX does not really belong here */
          if (vm_page_max_wired == 0)
                  vm_page_max_wired = cnt.v_free_count / 3;
  
          if (vm_pageout_stats_max == 0)
                  vm_pageout_stats_max = cnt.v_free_target;
  
          /*
           * Set interval in seconds for stats scan.
           */
          if (vm_pageout_stats_interval == 0)
                  vm_pageout_stats_interval = 5;
          if (vm_pageout_full_stats_interval == 0)
                  vm_pageout_full_stats_interval = vm_pageout_stats_interval * 4;
  
          /*
           * Set maximum free per pass
           */
          if (vm_pageout_stats_free_max == 0)
                  vm_pageout_stats_free_max = 5;
  
          swap_pager_swap_init();
          pass = 0;
          /*
           * The pageout daemon is never done, so loop forever.
           */
          while (TRUE) {
                  int error;
                  int s = splvm();
  
                  /*
                   * If we have enough free memory, wakeup waiters.  Do
                   * not clear vm_pages_needed until we reach our target,
                   * otherwise we may be woken up over and over again and
                   * waste a lot of cpu.
                   */
                  if (vm_pages_needed && !vm_page_count_min()) {
                          if (vm_paging_needed() <= 0)
                                  vm_pages_needed = 0;
                          wakeup(&cnt.v_free_count);
                  }
                  if (vm_pages_needed) {
                          /*
                           * Still not done, take a second pass without waiting
                           * (unlimited dirty cleaning), otherwise sleep a bit
                           * and try again.
                           */
                          ++pass;
                          if (pass > 1)
                                  tsleep(&vm_pages_needed, PVM,
                                         "psleep", hz/2);
                  } else {
                          /*
                           * Good enough, sleep & handle stats.  Prime the pass
                           * for the next run.
                           */
                          if (pass > 1)
                                  pass = 1;
                          else
                                  pass = 0;
                          error = tsleep(&vm_pages_needed, PVM,
                                      "psleep", vm_pageout_stats_interval * hz);
                          if (error && !vm_pages_needed) {
                                  splx(s);
                                  pass = 0;
                                  vm_pageout_page_stats();
                                  continue;
                          }
                  }
  
                  if (vm_pages_needed)
                          cnt.v_pdwakeups++;
                  splx(s);
                  vm_pageout_scan(pass);
                  vm_pageout_deficit = 0;
          }
  }
  
  void
  pagedaemon_wakeup()
  {
          if (!vm_pages_needed && curthread->td_proc != pageproc) {
                  vm_pages_needed++;
                  wakeup(&vm_pages_needed);
          }
  }
  
  #if !defined(NO_SWAPPING)
  static void
  vm_req_vmdaemon()
  {
          static int lastrun = 0;
  
          if ((ticks > (lastrun + hz)) || (ticks < lastrun)) {
                  wakeup(&vm_daemon_needed);
                  lastrun = ticks;
          }
  }
  
  static void
  vm_daemon()
  {
          struct proc *p;
          int breakout;
          struct thread *td;
  
          mtx_lock(&Giant);
          while (TRUE) {
                  tsleep(&vm_daemon_needed, PPAUSE, "psleep", 0);
                  if (vm_pageout_req_swapout) {
                          swapout_procs(vm_pageout_req_swapout);
                          vm_pageout_req_swapout = 0;
                  }
                  /*
                   * scan the processes for exceeding their rlimits or if
                   * process is swapped out -- deactivate pages
                   */
                  sx_slock(&allproc_lock);
                  LIST_FOREACH(p, &allproc, p_list) {
                          vm_pindex_t limit, size;
  
                          /*
                           * if this is a system process or if we have already
                           * looked at this process, skip it.
                           */
                          if (p->p_flag & (P_SYSTEM | P_WEXIT)) {
                                  continue;
                          }
                          /*
                           * if the process is in a non-running type state,
                           * don't touch it.
                           */
                          mtx_lock_spin(&sched_lock);
                          breakout = 0;
                          FOREACH_THREAD_IN_PROC(p, td) {
                                  if (!TD_ON_RUNQ(td) &&
                                      !TD_IS_RUNNING(td) &&
                                      !TD_IS_SLEEPING(td)) {
                                          breakout = 1;
                                          break;
                                  }
                          }
                          if (breakout) {
                                  mtx_unlock_spin(&sched_lock);
                                  continue;
                          }
                          /*
                           * get a limit
                           */
                          limit = OFF_TO_IDX(
                              qmin(p->p_rlimit[RLIMIT_RSS].rlim_cur,
                                  p->p_rlimit[RLIMIT_RSS].rlim_max));
  
                          /*
                           * let processes that are swapped out really be
                           * swapped out set the limit to nothing (will force a
                           * swap-out.)
                           */
                          if ((p->p_sflag & PS_INMEM) == 0)
                                  limit = 0;      /* XXX */
                          mtx_unlock_spin(&sched_lock);
  
                          size = vmspace_resident_count(p->p_vmspace);
                          if (limit >= 0 && size >= limit) {
                                  vm_pageout_map_deactivate_pages(
                                      &p->p_vmspace->vm_map, limit);
                          }
                  }
                  sx_sunlock(&allproc_lock);
          }
  }
  #endif                  /* !defined(NO_SWAPPING) */

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