kernel - Attempt to fix a rare vm_page_queues panic

[dragonfly.git] / 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. 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: src/sys/vm/vm_pageout.c,v 1.151.2.15 2002/12/29 18:21:04 dillon Exp $
 */

/*
 *      The proverbial page-out daemon.
 */

#include "opt_vm.h"
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/kthread.h>
#include <sys/resourcevar.h>
#include <sys/signalvar.h>
#include <sys/vnode.h>
#include <sys/vmmeter.h>
#include <sys/sysctl.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <sys/lock.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 <sys/thread2.h>
#include <sys/spinlock2.h>
#include <vm/vm_page2.h>

/*
 * System initialization
 */

/* the kernel process "vm_pageout"*/
static int vm_pageout_clean (vm_page_t);
static int vm_pageout_free_page_calc (vm_size_t count);
struct thread *pagethread;

#if !defined(NO_SWAPPING)
/* the kernel process "vm_daemon"*/
static void vm_daemon (void);
static struct   thread *vmthread;

static struct kproc_desc vm_kp = {
        "vmdaemon",
        vm_daemon,
        &vmthread
};
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;  /* pageout daemon needs pages */
int vm_page_free_hysteresis = 16;

#if !defined(NO_SWAPPING)
static int vm_pageout_req_swapout;      /* XXX */
static int vm_daemon_needed;
#endif
static int vm_max_launder = 4096;
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;
static u_int vm_anonmem_decline = ACT_DECLINE;
static u_int vm_filemem_decline = ACT_DECLINE * 2;

#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_UINT(_vm, VM_PAGEOUT_ALGORITHM, anonmem_decline,
        CTLFLAG_RW, &vm_anonmem_decline, 0, "active->inactive anon memory");

SYSCTL_INT(_vm, VM_PAGEOUT_ALGORITHM, filemem_decline,
        CTLFLAG_RW, &vm_filemem_decline, 0, "active->inactive file cache");

SYSCTL_INT(_vm, OID_AUTO, page_free_hysteresis,
        CTLFLAG_RW, &vm_page_free_hysteresis, 0,
        "Free more pages than the minimum required");

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");

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(int q);

static __inline int
PQAVERAGE(int n)
{
        if (n >= 0)
                return((n + (PQ_L2_SIZE - 1)) / PQ_L2_SIZE + 1);
        else
                return((n - (PQ_L2_SIZE - 1)) / PQ_L2_SIZE - 1);
}

/*
 * vm_pageout_clean:
 *
 * Clean the page and remove it from the laundry.  The page must not be
 * busy on-call.
 * 
 * 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(vm_page_t m)
{
        vm_object_t object;
        vm_page_t mc[BLIST_MAX_ALLOC];
        int error;
        int ib, is, page_base;
        vm_pindex_t pindex = m->pindex;

        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
         *
         * XXX do we really need to check hold_count here?  hold_count
         * isn't supposed to mess with vm_page ops except prevent the
         * page from being reused.
         */
        if (m->hold_count != 0 || (m->flags & PG_UNMANAGED)) {
                vm_page_wakeup(m);
                return 0;
        }

        /*
         * Place page in cluster.  Align cluster for optimal swap space
         * allocation (whether it is swap or not).  This is typically ~16-32
         * pages, which also tends to align the cluster to multiples of the
         * filesystem block size if backed by a filesystem.
         */
        page_base = pindex % BLIST_MAX_ALLOC;
        mc[page_base] = m;
        ib = page_base - 1;
        is = page_base + 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.
         */

        vm_object_hold(object);
        while (ib >= 0) {
                vm_page_t p;

                p = vm_page_lookup_busy_try(object, pindex - page_base + ib,
                                            TRUE, &error);
                if (error || p == NULL)
                        break;
                if ((p->queue - p->pc) == PQ_CACHE ||
                    (p->flags & PG_UNMANAGED)) {
                        vm_page_wakeup(p);
                        break;
                }
                vm_page_test_dirty(p);
                if (((p->dirty & p->valid) == 0 &&
                     (p->flags & PG_NEED_COMMIT) == 0) ||
                    p->queue - p->pc != PQ_INACTIVE ||
                    p->wire_count != 0 ||       /* may be held by buf cache */
                    p->hold_count != 0) {       /* may be undergoing I/O */
                        vm_page_wakeup(p);
                        break;
                }
                mc[ib] = p;
                --ib;
        }
        ++ib;   /* fixup */

        while (is < BLIST_MAX_ALLOC &&
               pindex - page_base + is < object->size) {
                vm_page_t p;

                p = vm_page_lookup_busy_try(object, pindex - page_base + is,
                                            TRUE, &error);
                if (error || p == NULL)
                        break;
                if (((p->queue - p->pc) == PQ_CACHE) ||
                    (p->flags & (PG_BUSY|PG_UNMANAGED)) || p->busy) {
                        vm_page_wakeup(p);
                        break;
                }
                vm_page_test_dirty(p);
                if (((p->dirty & p->valid) == 0 &&
                     (p->flags & PG_NEED_COMMIT) == 0) ||
                    p->queue - p->pc != PQ_INACTIVE ||
                    p->wire_count != 0 ||       /* may be held by buf cache */
                    p->hold_count != 0) {       /* may be undergoing I/O */
                        vm_page_wakeup(p);
                        break;
                }
                mc[is] = p;
                ++is;
        }

        vm_object_drop(object);

        /*
         * we allow reads during pageouts...
         */
        return vm_pageout_flush(&mc[ib], is - ib, 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.
 *
 *      The pages in the array must be busied by the caller and will be
 *      unbusied by this function.
 */
int
vm_pageout_flush(vm_page_t *mc, int count, int flags)
{
        vm_object_t object;
        int pageout_status[count];
        int numpagedout = 0;
        int i;

        /*
         * Initiate I/O.  Bump the vm_page_t->busy counter.
         */
        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]);
        }

        /*
         * We must make the pages read-only.  This will also force the
         * modified bit in the related pmaps to be cleared.  The pager
         * cannot clear the bit for us since the I/O completion code
         * typically runs from an interrupt.  The act of making the page
         * read-only handles the case for us.
         *
         * Then we can unbusy the pages, we still hold a reference by virtue
         * of our soft-busy.
         */
        for (i = 0; i < count; i++) {
                vm_page_protect(mc[i], VM_PROT_READ);
                vm_page_wakeup(mc[i]);
        }

        object = mc[0]->object;
        vm_object_pip_add(object, count);

        vm_pager_put_pages(object, mc, count,
            (flags | ((object == &kernel_object) ? VM_PAGER_PUT_SYNC : 0)),
            pageout_status);

        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.
                         */
                        vm_page_busy_wait(mt, FALSE, "pgbad");
                        pmap_clear_modify(mt);
                        vm_page_undirty(mt);
                        vm_page_wakeup(mt);
                        break;
                case VM_PAGER_ERROR:
                case VM_PAGER_FAIL:
                        /*
                         * A page typically cannot be paged out when we
                         * have run out of swap.  We leave the page
                         * marked inactive and will try to page it out
                         * again later.
                         *
                         * Starvation of the active page list is used to
                         * determine when the system is massively memory
                         * starved.
                         */
                        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.
                 *
                 * For any pages which have completed synchronously, 
                 * deactivate the page if we are under a severe deficit.
                 * Do not try to enter them into the cache, though, they
                 * might still be read-heavy.
                 */
                if (pageout_status[i] != VM_PAGER_PEND) {
                        vm_page_busy_wait(mt, FALSE, "pgouw");
                        if (vm_page_count_severe())
                                vm_page_deactivate(mt);
#if 0
                        if (!vm_page_count_severe() || !vm_page_try_to_cache(mt))
                                vm_page_protect(mt, VM_PROT_READ);
#endif
                        vm_page_io_finish(mt);
                        vm_page_wakeup(mt);
                        vm_object_pip_wakeup(object);
                }
        }
        return numpagedout;
}

#if !defined(NO_SWAPPING)
/*
 * 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 map must be locked.
 * The caller must hold the vm_object.
 */
static int vm_pageout_object_deactivate_pages_callback(vm_page_t, void *);

static void
vm_pageout_object_deactivate_pages(vm_map_t map, vm_object_t object,
                                   vm_pindex_t desired, int map_remove_only)
{
        struct rb_vm_page_scan_info info;
        vm_object_t lobject;
        vm_object_t tobject;
        int remove_mode;

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        lobject = object;

        while (lobject) {
                if (pmap_resident_count(vm_map_pmap(map)) <= desired)
                        break;
                if (lobject->type == OBJT_DEVICE ||
                    lobject->type == OBJT_MGTDEVICE ||
                    lobject->type == OBJT_PHYS)
                        break;
                if (lobject->paging_in_progress)
                        break;

                remove_mode = map_remove_only;
                if (lobject->shadow_count > 1)
                        remove_mode = 1;

                /*
                 * scan the objects entire memory queue.  We hold the
                 * object's token so the scan should not race anything.
                 */
                info.limit = remove_mode;
                info.map = map;
                info.desired = desired;
                vm_page_rb_tree_RB_SCAN(&lobject->rb_memq, NULL,
                                vm_pageout_object_deactivate_pages_callback,
                                &info
                );
                while ((tobject = lobject->backing_object) != NULL) {
                        KKASSERT(tobject != object);
                        vm_object_hold(tobject);
                        if (tobject == lobject->backing_object)
                                break;
                        vm_object_drop(tobject);
                }
                if (lobject != object) {
                        if (tobject)
                                vm_object_lock_swap();
                        vm_object_drop(lobject);
                        /* leaves tobject locked & at top */
                }
                lobject = tobject;
        }
        if (lobject != object)
                vm_object_drop(lobject);        /* NULL ok */
}

/*
 * The caller must hold the vm_object.
 */
static int
vm_pageout_object_deactivate_pages_callback(vm_page_t p, void *data)
{
        struct rb_vm_page_scan_info *info = data;
        int actcount;

        if (pmap_resident_count(vm_map_pmap(info->map)) <= info->desired) {
                return(-1);
        }
        mycpu->gd_cnt.v_pdpages++;

        if (vm_page_busy_try(p, TRUE))
                return(0);
        if (p->wire_count || p->hold_count || (p->flags & PG_UNMANAGED)) {
                vm_page_wakeup(p);
                return(0);
        }
        if (!pmap_page_exists_quick(vm_map_pmap(info->map), p)) {
                vm_page_wakeup(p);
                return(0);
        }

        actcount = pmap_ts_referenced(p);
        if (actcount) {
                vm_page_flag_set(p, PG_REFERENCED);
        } else if (p->flags & PG_REFERENCED) {
                actcount = 1;
        }

        vm_page_and_queue_spin_lock(p);
        if (p->queue - p->pc != PQ_ACTIVE && (p->flags & PG_REFERENCED)) {
                vm_page_and_queue_spin_unlock(p);
                vm_page_activate(p);
                p->act_count += actcount;
                vm_page_flag_clear(p, PG_REFERENCED);
        } else if (p->queue - p->pc == PQ_ACTIVE) {
                if ((p->flags & PG_REFERENCED) == 0) {
                        p->act_count -= min(p->act_count, ACT_DECLINE);
                        if (!info->limit &&
                            (vm_pageout_algorithm || (p->act_count == 0))) {
                                vm_page_and_queue_spin_unlock(p);
                                vm_page_protect(p, VM_PROT_NONE);
                                vm_page_deactivate(p);
                        } else {
                                TAILQ_REMOVE(&vm_page_queues[p->queue].pl,
                                             p, pageq);
                                TAILQ_INSERT_TAIL(&vm_page_queues[p->queue].pl,
                                                  p, pageq);
                                vm_page_and_queue_spin_unlock(p);
                        }
                } else {
                        vm_page_and_queue_spin_unlock(p);
                        vm_page_activate(p);
                        vm_page_flag_clear(p, PG_REFERENCED);

                        vm_page_and_queue_spin_lock(p);
                        if (p->queue - p->pc == PQ_ACTIVE) {
                                if (p->act_count < (ACT_MAX - ACT_ADVANCE))
                                        p->act_count += ACT_ADVANCE;
                                TAILQ_REMOVE(&vm_page_queues[p->queue].pl,
                                             p, pageq);
                                TAILQ_INSERT_TAIL(&vm_page_queues[p->queue].pl,
                                                  p, pageq);
                        }
                        vm_page_and_queue_spin_unlock(p);
                }
        } else if (p->queue - p->pc == PQ_INACTIVE) {
                vm_page_and_queue_spin_unlock(p);
                vm_page_protect(p, VM_PROT_NONE);
        } else {
                vm_page_and_queue_spin_unlock(p);
        }
        vm_page_wakeup(p);
        return(0);
}

/*
 * 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(vm_map_t map, vm_pindex_t desired)
{
        vm_map_entry_t tmpe;
        vm_object_t obj, bigobj;
        int nothingwired;

        if (lockmgr(&map->lock, LK_EXCLUSIVE | LK_NOWAIT)) {
                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) {
                switch(tmpe->maptype) {
                case VM_MAPTYPE_NORMAL:
                case VM_MAPTYPE_VPAGETABLE:
                        obj = tmpe->object.vm_object;
                        if ((obj != NULL) && (obj->shadow_count <= 1) &&
                                ((bigobj == NULL) ||
                                 (bigobj->resident_page_count < obj->resident_page_count))) {
                                bigobj = obj;
                        }
                        break;
                default:
                        break;
                }
                if (tmpe->wired_count > 0)
                        nothingwired = FALSE;
                tmpe = tmpe->next;
        }

        if (bigobj)  {
                vm_object_hold(bigobj);
                vm_pageout_object_deactivate_pages(map, bigobj, desired, 0);
                vm_object_drop(bigobj);
        }

        /*
         * 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;
                switch(tmpe->maptype) {
                case VM_MAPTYPE_NORMAL:
                case VM_MAPTYPE_VPAGETABLE:
                        obj = tmpe->object.vm_object;
                        if (obj) {
                                vm_object_hold(obj);
                                vm_pageout_object_deactivate_pages(map, obj, desired, 0);
                                vm_object_drop(obj);
                        }
                        break;
                default:
                        break;
                }
                tmpe = tmpe->next;
        }

        /*
         * Remove all mappings if a process is swapped out, this will free page
         * table pages.
         */
        if (desired == 0 && nothingwired)
                pmap_remove(vm_map_pmap(map),
                            VM_MIN_USER_ADDRESS, VM_MAX_USER_ADDRESS);
        vm_map_unlock(map);
}
#endif

/*
 * Called when the pageout scan wants to free a page.  We no longer
 * try to cycle the vm_object here with a reference & dealloc, which can
 * cause a non-trivial object collapse in a critical path.
 *
 * It is unclear why we cycled the ref_count in the past, perhaps to try
 * to optimize shadow chain collapses but I don't quite see why it would
 * be necessary.  An OBJ_DEAD object should terminate any and all vm_pages
 * synchronously and not have to be kicked-start.
 */
static void
vm_pageout_page_free(vm_page_t m) 
{
        vm_page_protect(m, VM_PROT_NONE);
        vm_page_free(m);
}

/*
 * vm_pageout_scan does the dirty work for the pageout daemon.
 */
struct vm_pageout_scan_info {
        struct proc *bigproc;
        vm_offset_t bigsize;
};

static int vm_pageout_scan_callback(struct proc *p, void *data);

static int
vm_pageout_scan_inactive(int pass, int q, int avail_shortage,
                         int *vnodes_skippedp)
{
        vm_page_t m;
        struct vm_page marker;
        struct vnode *vpfailed;         /* warning, allowed to be stale */
        int maxscan;
        int count;
        int delta = 0;
        vm_object_t object;
        int actcount;
        int maxlaunder;

        /*
         * 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;

        /*
         * Initialize our marker
         */
        bzero(&marker, sizeof(marker));
        marker.flags = PG_BUSY | PG_FICTITIOUS | PG_MARKER;
        marker.queue = PQ_INACTIVE + q;
        marker.pc = q;
        marker.wire_count = 1;

        /*
         * Inactive queue scan.
         *
         * NOTE: The vm_page must be spinlocked before the queue to avoid
         *       deadlocks, so it is easiest to simply iterate the loop
         *       with the queue unlocked at the top.
         */
        vpfailed = NULL;

        vm_page_queues_spin_lock(PQ_INACTIVE + q);
        TAILQ_INSERT_HEAD(&vm_page_queues[PQ_INACTIVE + q].pl, &marker, pageq);
        maxscan = vm_page_queues[PQ_INACTIVE + q].lcnt;

        /*
         * Queue locked at top of loop to avoid stack marker issues.
         */
        while ((m = TAILQ_NEXT(&marker, pageq)) != NULL &&
               maxscan-- > 0 && avail_shortage - delta > 0)
        {
                KKASSERT(m->queue - m->pc == PQ_INACTIVE);
                TAILQ_REMOVE(&vm_page_queues[PQ_INACTIVE + q].pl,
                             &marker, pageq);
                TAILQ_INSERT_AFTER(&vm_page_queues[PQ_INACTIVE + q].pl, m,
                                   &marker, pageq);
                mycpu->gd_cnt.v_pdpages++;

                /*
                 * Skip marker pages (atomic against other markers to avoid
                 * infinite hop-over scans).
                 */
                if (m->flags & PG_MARKER)
                        continue;

                /*
                 * Try to busy the page.  Don't mess with pages which are
                 * already busy or reorder them in the queue.
                 */
                if (vm_page_busy_try(m, TRUE))
                        continue;

                /*
                 * Remaining operations run with the page busy and neither
                 * the page or the queue will be spin-locked.
                 */
                vm_page_queues_spin_unlock(PQ_INACTIVE + q);
                KKASSERT(m->queue - m->pc == PQ_INACTIVE);
                lwkt_yield();

                /*
                 * It is possible for a page to be busied ad-hoc (e.g. the
                 * pmap_collect() code) and wired and race against the
                 * allocation of a new page.  vm_page_alloc() may be forced
                 * to deactivate the wired page in which case it winds up
                 * on the inactive queue and must be handled here.  We
                 * correct the problem simply by unqueuing the page.
                 */
                if (m->wire_count) {
                        vm_page_unqueue_nowakeup(m);
                        vm_page_wakeup(m);
                        kprintf("WARNING: pagedaemon: wired page on "
                                "inactive queue %p\n", m);
                        goto next;
                }

                /*
                 * A held page may be undergoing I/O, so skip it.
                 */
                if (m->hold_count) {
                        vm_page_and_queue_spin_lock(m);
                        if (m->queue - m->pc == PQ_INACTIVE) {
                                TAILQ_REMOVE(
                                        &vm_page_queues[PQ_INACTIVE + q].pl,
                                        m, pageq);
                                TAILQ_INSERT_TAIL(
                                        &vm_page_queues[PQ_INACTIVE + q].pl,
                                        m, pageq);
                                ++vm_swapcache_inactive_heuristic;
                        }
                        vm_page_and_queue_spin_unlock(m);
                        vm_page_wakeup(m);
                        goto next;
                }

                if (m->object == NULL || m->object->ref_count == 0) {
                        /*
                         * If the object is not being used, we ignore previous 
                         * references.
                         */
                        vm_page_flag_clear(m, PG_REFERENCED);
                        pmap_clear_reference(m);
                        /* fall through to end */
                } else if (((m->flags & PG_REFERENCED) == 0) &&
                            (actcount = pmap_ts_referenced(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.
                         */
                        vm_page_activate(m);
                        m->act_count += (actcount + ACT_ADVANCE);
                        vm_page_wakeup(m);
                        goto next;
                }

                /*
                 * (m) is still busied.
                 *
                 * 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);
                        m->act_count += (actcount + ACT_ADVANCE + 1);
                        vm_page_wakeup(m);
                        goto next;
                }

                /*
                 * 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.
                 *
                 * Pages marked PG_WRITEABLE may be mapped into the user
                 * address space of a process running on another cpu.  A
                 * user process (without holding the MP lock) running on
                 * another cpu may be able to touch the page while we are
                 * trying to remove it.  vm_page_cache() will handle this
                 * case for us.
                 */
                if (m->dirty == 0) {
                        vm_page_test_dirty(m);
                } else {
                        vm_page_dirty(m);
                }

                if (m->valid == 0 && (m->flags & PG_NEED_COMMIT) == 0) {
                        /*
                         * Invalid pages can be easily freed
                         */
                        vm_pageout_page_free(m);
                        mycpu->gd_cnt.v_dfree++;
                        ++delta;
                } else if (m->dirty == 0 && (m->flags & PG_NEED_COMMIT) == 0) {
                        /*
                         * Clean pages can be placed onto the cache queue.
                         * This effectively frees them.
                         */
                        vm_page_cache(m);
                        ++delta;
                } 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_page_and_queue_spin_lock(m);
                        if (m->queue - m->pc == PQ_INACTIVE) {
                                TAILQ_REMOVE(
                                        &vm_page_queues[PQ_INACTIVE + q].pl,
                                        m, pageq);
                                TAILQ_INSERT_TAIL(
                                        &vm_page_queues[PQ_INACTIVE + q].pl,
                                        m, pageq);
                                ++vm_swapcache_inactive_heuristic;
                        }
                        vm_page_and_queue_spin_unlock(m);
                        vm_page_wakeup(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;

                        swap_pageouts_ok = 0;
                        object = m->object;
                        if (object &&
                            (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(0));
                                                                                
                        }

                        /*
                         * We don't bother paging objects that are "dead".  
                         * Those objects are in a "rundown" state.
                         */
                        if (!swap_pageouts_ok || 
                            (object == NULL) ||
                            (object->flags & OBJ_DEAD)) {
                                vm_page_and_queue_spin_lock(m);
                                if (m->queue - m->pc == PQ_INACTIVE) {
                                        TAILQ_REMOVE(
                                            &vm_page_queues[PQ_INACTIVE + q].pl,
                                            m, pageq);
                                        TAILQ_INSERT_TAIL(
                                            &vm_page_queues[PQ_INACTIVE + q].pl,
                                            m, pageq);
                                        ++vm_swapcache_inactive_heuristic;
                                }
                                vm_page_and_queue_spin_unlock(m);
                                vm_page_wakeup(m);
                                goto next;
                        }

                        /*
                         * (m) is still busied.
                         *
                         * 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 because,
                         * quite often, a vm_fault has initiated an I/O and
                         * is holding a locked vnode at just the point where
                         * the pageout daemon is woken up.
                         *
                         * 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.
                         *
                         * vpfailed is used to (try to) avoid the case where
                         * a large number of pages are associated with a
                         * locked vnode, which could cause the pageout daemon
                         * to stall for an excessive amount of time.
                         */
                        if (object->type == OBJT_VNODE) {
                                int flags;

                                vp = object->handle;
                                flags = LK_EXCLUSIVE;
                                if (vp == vpfailed)
                                        flags |= LK_NOWAIT;
                                else
                                        flags |= LK_TIMELOCK;
                                vm_page_hold(m);
                                vm_page_wakeup(m);

                                /*
                                 * We have unbusied (m) temporarily so we can
                                 * acquire the vp lock without deadlocking.
                                 * (m) is held to prevent destruction.
                                 */
                                if (vget(vp, flags) != 0) {
                                        vpfailed = vp;
                                        ++pageout_lock_miss;
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                    ++*vnodes_skippedp;
                                        vm_page_unhold(m);
                                        goto next;
                                }

                                /*
                                 * 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 - m->pc != PQ_INACTIVE ||
                                    m->object != object ||
                                    object->handle != vp) {
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                ++*vnodes_skippedp;
                                        vput(vp);
                                        vm_page_unhold(m);
                                        goto next;
                                }
        
                                /*
                                 * 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 (vm_page_busy_try(m, TRUE)) {
                                        vput(vp);
                                        vm_page_unhold(m);
                                        goto next;
                                }
                                vm_page_unhold(m);

                                /*
                                 * (m) is busied again
                                 *
                                 * We own the busy bit and remove our hold
                                 * bit.  If the page is still held it
                                 * might be undergoing I/O, so skip it.
                                 */
                                if (m->hold_count) {
                                        vm_page_and_queue_spin_lock(m);
                                        if (m->queue - m->pc == PQ_INACTIVE) {
                                                TAILQ_REMOVE(&vm_page_queues[PQ_INACTIVE + q].pl, m, pageq);
                                                TAILQ_INSERT_TAIL(&vm_page_queues[PQ_INACTIVE + q].pl, m, pageq);
                                                ++vm_swapcache_inactive_heuristic;
                                        }
                                        vm_page_and_queue_spin_unlock(m);
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                ++*vnodes_skippedp;
                                        vm_page_wakeup(m);
                                        vput(vp);
                                        goto next;
                                }
                                /* (m) is left busied as we fall through */
                        }

                        /*
                         * page is busy and not held here.
                         *
                         * 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. 
                         *
                         * decrement inactive_shortage on success to account
                         * for the (future) cleaned page.  Otherwise we
                         * could wind up laundering or cleaning too many
                         * pages.
                         */
                        count = vm_pageout_clean(m);
                        delta += count;
                        maxlaunder -= count;

                        /*
                         * Clean ate busy, page no longer accessible
                         */
                        if (vp != NULL)
                                vput(vp);
                } else {
                        vm_page_wakeup(m);
                }

next:
                /*
                 * Systems with a ton of memory can wind up with huge
                 * deactivation counts.  Because the inactive scan is
                 * doing a lot of flushing, the combination can result
                 * in excessive paging even in situations where other
                 * unrelated threads free up sufficient VM.
                 *
                 * To deal with this we abort the nominal active->inactive
                 * scan before we hit the inactive target when free+cache
                 * levels have already reached their target.
                 *
                 * Note that nominally the inactive scan is not freeing or
                 * caching pages, it is deactivating active pages, so it
                 * will not by itself cause the abort condition.
                 */
                vm_page_queues_spin_lock(PQ_INACTIVE + q);
                if (vm_paging_target() < 0)
                        break;
        }

        /* page queue still spin-locked */
        TAILQ_REMOVE(&vm_page_queues[PQ_INACTIVE + q].pl, &marker, pageq);
        vm_page_queues_spin_unlock(PQ_INACTIVE + q);

        return (delta);
}

static int
vm_pageout_scan_active(int pass, int q,
                       int avail_shortage, int inactive_shortage,
                       int *recycle_countp)
{
        struct vm_page marker;
        vm_page_t m;
        int actcount;
        int delta = 0;
        int maxscan;

        /*
         * We want to move pages from the active queue to the inactive
         * queue to get the inactive queue to the inactive target.  If
         * we still have a page shortage from above we try to directly free
         * clean pages instead of moving them.
         *
         * If we do still have a shortage we keep track of the number of
         * pages we free or cache (recycle_count) as a measure of thrashing
         * between the active and inactive queues.
         *
         * If we were able to completely satisfy the free+cache targets
         * from the inactive pool we limit the number of pages we move
         * from the active pool to the inactive pool to 2x the pages we
         * had removed from the inactive pool (with a minimum of 1/5 the
         * inactive target).  If we were not able to completely satisfy
         * the free+cache targets we go for the whole target aggressively.
         *
         * NOTE: Both variables can end up negative.
         * NOTE: We are still in a critical section.
         */

        bzero(&marker, sizeof(marker));
        marker.flags = PG_BUSY | PG_FICTITIOUS | PG_MARKER;
        marker.queue = PQ_ACTIVE + q;
        marker.pc = q;
        marker.wire_count = 1;

        vm_page_queues_spin_lock(PQ_ACTIVE + q);
        TAILQ_INSERT_HEAD(&vm_page_queues[PQ_ACTIVE + q].pl, &marker, pageq);
        maxscan = vm_page_queues[PQ_ACTIVE + q].lcnt;

        /*
         * Queue locked at top of loop to avoid stack marker issues.
         */
        while ((m = TAILQ_NEXT(&marker, pageq)) != NULL &&
               maxscan-- > 0 && (avail_shortage - delta > 0 ||
                                inactive_shortage > 0))
        {
                KKASSERT(m->queue - m->pc == PQ_ACTIVE);
                TAILQ_REMOVE(&vm_page_queues[PQ_ACTIVE + q].pl,
                             &marker, pageq);
                TAILQ_INSERT_AFTER(&vm_page_queues[PQ_ACTIVE + q].pl, m,
                                   &marker, pageq);

                /*
                 * Skip marker pages (atomic against other markers to avoid
                 * infinite hop-over scans).
                 */
                if (m->flags & PG_MARKER)
                        continue;

                /*
                 * Try to busy the page.  Don't mess with pages which are
                 * already busy or reorder them in the queue.
                 */
                if (vm_page_busy_try(m, TRUE))
                        continue;

                /*
                 * Remaining operations run with the page busy and neither
                 * the page or the queue will be spin-locked.
                 */
                vm_page_queues_spin_unlock(PQ_ACTIVE + q);
                KKASSERT(m->queue - m->pc == PQ_ACTIVE);
                lwkt_yield();

                /*
                 * Don't deactivate pages that are held, even if we can
                 * busy them.  (XXX why not?)
                 */
                if (m->hold_count != 0) {
                        vm_page_and_queue_spin_lock(m);
                        if (m->queue - m->pc == PQ_ACTIVE) {
                                TAILQ_REMOVE(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                                TAILQ_INSERT_TAIL(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                        }
                        vm_page_and_queue_spin_unlock(m);
                        vm_page_wakeup(m);
                        goto next;
                }

                /*
                 * The count for pagedaemon pages is done after checking the
                 * page for eligibility...
                 */
                mycpu->gd_cnt.v_pdpages++;

                /*
                 * Check to see "how much" the page has been used and clear
                 * the tracking access bits.  If the object has no references
                 * don't bother paying the expense.
                 */
                actcount = 0;
                if (m->object && m->object->ref_count != 0) {
                        if (m->flags & PG_REFERENCED)
                                ++actcount;
                        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_page_flag_clear(m, PG_REFERENCED);

                /*
                 * actcount is only valid if the object ref_count is non-zero.
                 * If the page does not have an object, actcount will be zero.
                 */
                if (actcount && m->object->ref_count != 0) {
                        vm_page_and_queue_spin_lock(m);
                        if (m->queue - m->pc == PQ_ACTIVE) {
                                TAILQ_REMOVE(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                                TAILQ_INSERT_TAIL(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                        }
                        vm_page_and_queue_spin_unlock(m);
                        vm_page_wakeup(m);
                } else {
                        switch(m->object->type) {
                        case OBJT_DEFAULT:
                        case OBJT_SWAP:
                                m->act_count -= min(m->act_count,
                                                    vm_anonmem_decline);
                                break;
                        default:
                                m->act_count -= min(m->act_count,
                                                    vm_filemem_decline);
                                break;
                        }
                        if (vm_pageout_algorithm ||
                            (m->object == NULL) ||
                            (m->object && (m->object->ref_count == 0)) ||
                            m->act_count < pass + 1
                        ) {
                                /*
                                 * Deactivate the page.  If we had a
                                 * shortage from our inactive scan try to
                                 * free (cache) the page instead.
                                 *
                                 * Don't just blindly cache the page if
                                 * we do not have a shortage from the
                                 * inactive scan, that could lead to
                                 * gigabytes being moved.
                                 */
                                --inactive_shortage;
                                if (avail_shortage - delta > 0 ||
                                    (m->object && (m->object->ref_count == 0)))
                                {
                                        if (avail_shortage - delta > 0)
                                                ++*recycle_countp;
                                        vm_page_protect(m, VM_PROT_NONE);
                                        if (m->dirty == 0 &&
                                            (m->flags & PG_NEED_COMMIT) == 0 &&
                                            avail_shortage - delta > 0) {
                                                vm_page_cache(m);
                                        } else {
                                                vm_page_deactivate(m);
                                                vm_page_wakeup(m);
                                        }
                                } else {
                                        vm_page_deactivate(m);
                                        vm_page_wakeup(m);
                                }
                                ++delta;
                        } else {
                                vm_page_and_queue_spin_lock(m);
                                if (m->queue - m->pc == PQ_ACTIVE) {
                                        TAILQ_REMOVE(
                                            &vm_page_queues[PQ_ACTIVE + q].pl,
                                            m, pageq);
                                        TAILQ_INSERT_TAIL(
                                            &vm_page_queues[PQ_ACTIVE + q].pl,
                                            m, pageq);
                                }
                                vm_page_and_queue_spin_unlock(m);
                                vm_page_wakeup(m);
                        }
                }
next:
                vm_page_queues_spin_lock(PQ_ACTIVE + q);
        }

        /*
         * Clean out our local marker.
         *
         * Page queue still spin-locked.
         */
        TAILQ_REMOVE(&vm_page_queues[PQ_ACTIVE + q].pl, &marker, pageq);
        vm_page_queues_spin_unlock(PQ_ACTIVE + q);

        return (delta);
}

/*
 * The number of actually free pages can drop down to v_free_reserved,
 * we try to build the free count back above v_free_min.  Note that
 * vm_paging_needed() also returns TRUE if v_free_count is not at
 * least v_free_min so that is the minimum we must build the free
 * count to.
 *
 * We use a slightly higher target to improve hysteresis,
 * ((v_free_target + v_free_min) / 2).  Since v_free_target
 * is usually the same as v_cache_min this maintains about
 * half the pages in the free queue as are in the cache queue,
 * providing pretty good pipelining for pageout operation.
 *
 * The system operator can manipulate vm.v_cache_min and
 * vm.v_free_target to tune the pageout demon.  Be sure
 * to keep vm.v_free_min < vm.v_free_target.
 *
 * Note that the original paging target is to get at least
 * (free_min + cache_min) into (free + cache).  The slightly
 * higher target will shift additional pages from cache to free
 * without effecting the original paging target in order to
 * maintain better hysteresis and not have the free count always
 * be dead-on v_free_min.
 *
 * NOTE: we are still in a critical section.
 *
 * Pages moved from PQ_CACHE to totally free are not counted in the
 * pages_freed counter.
 */
static void
vm_pageout_scan_cache(int avail_shortage, int vnodes_skipped, int recycle_count)
{
        struct vm_pageout_scan_info info;
        vm_page_t m;

        while (vmstats.v_free_count <
               (vmstats.v_free_min + vmstats.v_free_target) / 2) {
                /*
                 * This steals some code from vm/vm_page.c
                 */
                static int cache_rover = 0;

                m = vm_page_list_find(PQ_CACHE, cache_rover & PQ_L2_MASK, FALSE);
                if (m == NULL)
                        break;
                /* page is returned removed from its queue and spinlocked */
                if (vm_page_busy_try(m, TRUE)) {
                        vm_page_deactivate_locked(m);
                        vm_page_spin_unlock(m);
                        continue;
                }
                vm_page_spin_unlock(m);
                pagedaemon_wakeup();
                lwkt_yield();

                /*
                 * Remaining operations run with the page busy and neither
                 * the page or the queue will be spin-locked.
                 */
                if ((m->flags & (PG_UNMANAGED | PG_NEED_COMMIT)) ||
                    m->hold_count ||
                    m->wire_count) {
                        vm_page_deactivate(m);
                        vm_page_wakeup(m);
                        continue;
                }
                KKASSERT((m->flags & PG_MAPPED) == 0);
                KKASSERT(m->dirty == 0);
                cache_rover += PQ_PRIME2;
                vm_pageout_page_free(m);
                mycpu->gd_cnt.v_dfree++;
        }

#if !defined(NO_SWAPPING)
        /*
         * Idle process swapout -- run once per second.
         */
        if (vm_swap_idle_enabled) {
                static time_t lsec;
                if (time_uptime != lsec) {
                        vm_pageout_req_swapout |= VM_SWAP_IDLE;
                        vm_req_vmdaemon();
                        lsec = time_uptime;
                }
        }
#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(0))
                        speedup_syncer(NULL);
#if !defined(NO_SWAPPING)
                if (vm_swap_enabled && vm_page_count_target()) {
                        vm_req_vmdaemon();
                        vm_pageout_req_swapout |= VM_SWAP_NORMAL;
                }
#endif
        }

        /*
         * Handle catastrophic conditions.  Under good conditions we should
         * be at the target, well beyond our minimum.  If we could not even
         * reach our minimum the system is under heavy stress.
         *
         * Determine whether we have run out of memory.  This occurs when
         * swap_pager_full is TRUE and the only pages left in the page
         * queues are dirty.  We will still likely have page shortages.
         *
         * - swap_pager_full is set if insufficient swap was
         *   available to satisfy a requested pageout.
         *
         * - the inactive queue is bloated (4 x size of active queue),
         *   meaning it is unable to get rid of dirty pages and.
         *
         * - vm_page_count_min() without counting pages recycled from the
         *   active queue (recycle_count) means we could not recover
         *   enough pages to meet bare minimum needs.  This test only
         *   works if the inactive queue is bloated.
         *
         * - due to a positive avail_shortage we shifted the remaining
         *   dirty pages from the active queue to the inactive queue
         *   trying to find clean ones to free.
         */
        if (swap_pager_full && vm_page_count_min(recycle_count))
                kprintf("Warning: system low on memory+swap!\n");
        if (swap_pager_full && vm_page_count_min(recycle_count) &&
            vmstats.v_inactive_count > vmstats.v_active_count * 4 &&
            avail_shortage > 0) {
                /*
                 * Kill something.
                 */
                info.bigproc = NULL;
                info.bigsize = 0;
                allproc_scan(vm_pageout_scan_callback, &info);
                if (info.bigproc != NULL) {
                        killproc(info.bigproc, "out of swap space");
                        info.bigproc->p_nice = PRIO_MIN;
                        info.bigproc->p_usched->resetpriority(
                                FIRST_LWP_IN_PROC(info.bigproc));
                        wakeup(&vmstats.v_free_count);
                        PRELE(info.bigproc);
                }
        }
}

static int
vm_pageout_scan_callback(struct proc *p, void *data)
{
        struct vm_pageout_scan_info *info = data;
        vm_offset_t size;

        /*
         * Never kill system processes or init.  If we have configured swap
         * then try to avoid killing low-numbered pids.
         */
        if ((p->p_flags & P_SYSTEM) || (p->p_pid == 1) ||
            ((p->p_pid < 48) && (vm_swap_size != 0))) {
                return (0);
        }

        lwkt_gettoken(&p->p_token);

        /*
         * if the process is in a non-running type state,
         * don't touch it.
         */
        if (p->p_stat != SACTIVE && p->p_stat != SSTOP) {
                lwkt_reltoken(&p->p_token);
                return (0);
        }

        /*
         * Get the approximate process size.  Note that anonymous pages
         * with backing swap will be counted twice, but there should not
         * be too many such pages due to the stress the VM system is
         * under at this point.
         */
        size = vmspace_anonymous_count(p->p_vmspace) +
                vmspace_swap_count(p->p_vmspace);

        /*
         * If the this process is bigger than the biggest one
         * remember it.
         */
        if (info->bigsize < size) {
                if (info->bigproc)
                        PRELE(info->bigproc);
                PHOLD(p);
                info->bigproc = p;
                info->bigsize = size;
        }
        lwkt_reltoken(&p->p_token);
        lwkt_yield();

        return(0);
}

/*
 * 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(int q)
{
        static int fullintervalcount = 0;
        struct vm_page marker;
        vm_page_t m;
        int pcount, tpcount;            /* Number of pages to check */
        int page_shortage;

        page_shortage = (vmstats.v_inactive_target + vmstats.v_cache_max +
                         vmstats.v_free_min) -
                        (vmstats.v_free_count + vmstats.v_inactive_count +
                         vmstats.v_cache_count);

        if (page_shortage <= 0)
                return;

        pcount = vm_page_queues[PQ_ACTIVE + q].lcnt;
        fullintervalcount += vm_pageout_stats_interval;
        if (fullintervalcount < vm_pageout_full_stats_interval) {
                tpcount = (vm_pageout_stats_max * pcount) /
                          vmstats.v_page_count + 1;
                if (pcount > tpcount)
                        pcount = tpcount;
        } else {
                fullintervalcount = 0;
        }

        bzero(&marker, sizeof(marker));
        marker.flags = PG_BUSY | PG_FICTITIOUS | PG_MARKER;
        marker.queue = PQ_ACTIVE + q;
        marker.pc = q;
        marker.wire_count = 1;

        vm_page_queues_spin_lock(PQ_ACTIVE + q);
        TAILQ_INSERT_HEAD(&vm_page_queues[PQ_ACTIVE + q].pl, &marker, pageq);

        /*
         * Queue locked at top of loop to avoid stack marker issues.
         */
        while ((m = TAILQ_NEXT(&marker, pageq)) != NULL &&
               pcount-- > 0)
        {
                int actcount;

                KKASSERT(m->queue - m->pc == PQ_ACTIVE);
                TAILQ_REMOVE(&vm_page_queues[PQ_ACTIVE + q].pl, &marker, pageq);
                TAILQ_INSERT_AFTER(&vm_page_queues[PQ_ACTIVE + q].pl, m,
                                   &marker, pageq);

                /*
                 * Skip marker pages (atomic against other markers to avoid
                 * infinite hop-over scans).
                 */
                if (m->flags & PG_MARKER)
                        continue;

                /*
                 * Ignore pages we can't busy
                 */
                if (vm_page_busy_try(m, TRUE))
                        continue;

                /*
                 * Remaining operations run with the page busy and neither
                 * the page or the queue will be spin-locked.
                 */
                vm_page_queues_spin_unlock(PQ_ACTIVE + q);
                KKASSERT(m->queue - m->pc == PQ_ACTIVE);

                /*
                 * We now have a safely busied page, the page and queue
                 * spinlocks have been released.
                 *
                 * Ignore held pages
                 */
                if (m->hold_count) {
                        vm_page_wakeup(m);
                        goto next;
                }

                /*
                 * Calculate activity
                 */
                actcount = 0;
                if (m->flags & PG_REFERENCED) {
                        vm_page_flag_clear(m, PG_REFERENCED);
                        actcount += 1;
                }
                actcount += pmap_ts_referenced(m);

                /*
                 * Update act_count and move page to end of queue.
                 */
                if (actcount) {
                        m->act_count += ACT_ADVANCE + actcount;
                        if (m->act_count > ACT_MAX)
                                m->act_count = ACT_MAX;
                        vm_page_and_queue_spin_lock(m);
                        if (m->queue - m->pc == PQ_ACTIVE) {
                                TAILQ_REMOVE(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                                TAILQ_INSERT_TAIL(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                        }
                        vm_page_and_queue_spin_unlock(m);
                        vm_page_wakeup(m);
                        goto next;
                }

                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.
                         *
                         * We use the marker to save our place so
                         * we can release the spin lock.  both (m)
                         * and (next) will be invalid.
                         */
                        vm_page_protect(m, VM_PROT_NONE);
                        vm_page_deactivate(m);
                } else {
                        m->act_count -= min(m->act_count, ACT_DECLINE);
                        vm_page_and_queue_spin_lock(m);
                        if (m->queue - m->pc == PQ_ACTIVE) {
                                TAILQ_REMOVE(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                                TAILQ_INSERT_TAIL(
                                        &vm_page_queues[PQ_ACTIVE + q].pl,
                                        m, pageq);
                        }
                        vm_page_and_queue_spin_unlock(m);
                }
                vm_page_wakeup(m);
next:
                vm_page_queues_spin_lock(PQ_ACTIVE + q);
        }

        /*
         * Remove our local marker
         *
         * Page queue still spin-locked.
         */
        TAILQ_REMOVE(&vm_page_queues[PQ_ACTIVE + q].pl, &marker, pageq);
        vm_page_queues_spin_unlock(PQ_ACTIVE + q);
}

static int
vm_pageout_free_page_calc(vm_size_t count)
{
        if (count < vmstats.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.
         *
         * v_free_min           normal allocations
         * v_free_reserved      system allocations
         * v_pageout_free_min   allocations by pageout daemon
         * v_interrupt_free_min low level allocations (e.g swap structures)
         */
        if (vmstats.v_page_count > 1024)
                vmstats.v_free_min = 64 + (vmstats.v_page_count - 1024) / 200;
        else
                vmstats.v_free_min = 64;
        vmstats.v_free_reserved = vmstats.v_free_min * 4 / 8 + 7;
        vmstats.v_free_severe = vmstats.v_free_min * 4 / 8 + 0;
        vmstats.v_pageout_free_min = vmstats.v_free_min * 2 / 8 + 7;
        vmstats.v_interrupt_free_min = vmstats.v_free_min * 1 / 8 + 7;

        return 1;
}


/*
 * vm_pageout is the high level pageout daemon.
 *
 * No requirements.
 */
static void
vm_pageout_thread(void)
{
        int pass;
        int q;
        int q1iterator = 0;
        int q2iterator = 0;

        /*
         * Initialize some paging parameters.
         */
        curthread->td_flags |= TDF_SYSTHREAD;

        vm_pageout_free_page_calc(vmstats.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 (vmstats.v_free_count > 6144)
                vmstats.v_free_target = 4 * vmstats.v_free_min + vmstats.v_free_reserved;
        else
                vmstats.v_free_target = 2 * vmstats.v_free_min + vmstats.v_free_reserved;

        /*
         * NOTE: With the new buffer cache b_act_count we want the default
         *       inactive target to be a percentage of available memory.
         *
         *       The inactive target essentially determines the minimum
         *       number of 'temporary' pages capable of caching one-time-use
         *       files when the VM system is otherwise full of pages
         *       belonging to multi-time-use files or active program data.
         *
         * NOTE: The inactive target is aggressively persued only if the
         *       inactive queue becomes too small.  If the inactive queue
         *       is large enough to satisfy page movement to free+cache
         *       then it is repopulated more slowly from the active queue.
         *       This allows a general inactive_target default to be set.
         *
         *       There is an issue here for processes which sit mostly idle
         *       'overnight', such as sshd, tcsh, and X.  Any movement from
         *       the active queue will eventually cause such pages to
         *       recycle eventually causing a lot of paging in the morning.
         *       To reduce the incidence of this pages cycled out of the
         *       buffer cache are moved directly to the inactive queue if
         *       they were only used once or twice.
         *
         *       The vfs.vm_cycle_point sysctl can be used to adjust this.
         *       Increasing the value (up to 64) increases the number of
         *       buffer recyclements which go directly to the inactive queue.
         */
        if (vmstats.v_free_count > 2048) {
                vmstats.v_cache_min = vmstats.v_free_target;
                vmstats.v_cache_max = 2 * vmstats.v_cache_min;
        } else {
                vmstats.v_cache_min = 0;
                vmstats.v_cache_max = 0;
        }
        vmstats.v_inactive_target = vmstats.v_free_count / 4;

        /* XXX does not really belong here */
        if (vm_page_max_wired == 0)
                vm_page_max_wired = vmstats.v_free_count / 3;

        if (vm_pageout_stats_max == 0)
                vm_pageout_stats_max = vmstats.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 avail_shortage;
                int inactive_shortage;
                int vnodes_skipped = 0;
                int recycle_count = 0;
                int tmp;

                /*
                 * Wait for an action request.  If we timeout check to
                 * see if paging is needed (in case the normal wakeup
                 * code raced us).
                 */
                if (vm_pages_needed == 0) {
                        error = tsleep(&vm_pages_needed,
                                       0, "psleep",
                                       vm_pageout_stats_interval * hz);
                        if (error &&
                            vm_paging_needed() == 0 &&
                            vm_pages_needed == 0) {
                                for (q = 0; q < PQ_L2_SIZE; ++q)
                                        vm_pageout_page_stats(q);
                                continue;
                        }
                        vm_pages_needed = 1;
                }

                mycpu->gd_cnt.v_pdwakeups++;

                /*
                 * Do whatever cleanup that the pmap code can.
                 */
                pmap_collect();

                /*
                 * Scan for pageout.  Try to avoid thrashing the system
                 * with activity.
                 *
                 * Calculate our target for the number of free+cache pages we
                 * want to get to.  This is higher then the number that causes
                 * allocations to stall (severe) in order to provide hysteresis,
                 * and if we don't make it all the way but get to the minimum
                 * we're happy.  Goose it a bit if there are multipler
                 * requests for memory.
                 */
                avail_shortage = vm_paging_target() + vm_pageout_deficit;
                vm_pageout_deficit = 0;

                if (avail_shortage > 0) {
                        for (q = 0; q < PQ_L2_SIZE; ++q) {
                                avail_shortage -=
                                        vm_pageout_scan_inactive(
                                            pass,
                                            (q + q1iterator) & PQ_L2_MASK,
                                            PQAVERAGE(avail_shortage),
                                            &vnodes_skipped);
                                if (avail_shortage <= 0)
                                        break;
                        }
                        q1iterator = q + 1;
                }

                /*
                 * Figure out how many active pages we must deactivate.  If
                 * we were able to reach our target with just the inactive
                 * scan above we limit the number of active pages we
                 * deactivate to reduce unnecessary work.
                 */
                inactive_shortage = vmstats.v_inactive_target -
                                    vmstats.v_inactive_count;

                /*
                 * If we were unable to free sufficient inactive pages to
                 * satisfy the free/cache queue requirements then simply
                 * reaching the inactive target may not be good enough.
                 * Try to deactivate pages in excess of the target based
                 * on the shortfall.
                 *
                 * However to prevent thrashing the VM system do not
                 * deactivate more than an additional 1/10 the inactive
                 * target's worth of active pages.
                 */
                if (avail_shortage > 0) {
                        tmp = avail_shortage * 2;
                        if (tmp > vmstats.v_inactive_target / 10)
                                tmp = vmstats.v_inactive_target / 10;
                        inactive_shortage += tmp;
                }

                /*
                 * Only trigger on inactive shortage.  Triggering on
                 * avail_shortage can starve the active queue with
                 * unnecessary active->inactive transitions and destroy
                 * performance.
                 */
                if (/*avail_shortage > 0 ||*/ inactive_shortage > 0) {
                        int delta;

                        for (q = 0; q < PQ_L2_SIZE; ++q) {
                                delta = vm_pageout_scan_active(
                                                pass,
                                                (q + q2iterator) & PQ_L2_MASK,
                                                PQAVERAGE(avail_shortage),
                                                PQAVERAGE(inactive_shortage),
                                                &recycle_count);
                                inactive_shortage -= delta;
                                avail_shortage -= delta;
                                if (inactive_shortage <= 0 &&
                                    avail_shortage <= 0) {
                                        break;
                                }
                        }
                        q2iterator = q + 1;
                }

                /*
                 * Finally free enough cache pages to meet our free page
                 * requirement and take more drastic measures if we are
                 * still in trouble.
                 */
                vm_pageout_scan_cache(avail_shortage, vnodes_skipped,
                                      recycle_count);

                /*
                 * Wait for more work.
                 */
                if (avail_shortage > 0) {
                        ++pass;
                        if (swap_pager_full) {
                                /*
                                 * Running out of memory, catastrophic back-off
                                 * to one-second intervals.
                                 */
                                tsleep(&vm_pages_needed, 0, "pdelay", hz);
                        } else if (pass < 10 && vm_pages_needed > 1) {
                                /*
                                 * Normal operation, additional processes
                                 * have already kicked us.  Retry immediately.
                                 */
                        } else if (pass < 10) {
                                /*
                                 * Normal operation, fewer processes.  Delay
                                 * a bit but allow wakeups.
                                 */
                                vm_pages_needed = 0;
                                tsleep(&vm_pages_needed, 0, "pdelay", hz / 10);
                                vm_pages_needed = 1;
                        } else {
                                /*
                                 * We've taken too many passes, forced delay.
                                 */
                                tsleep(&vm_pages_needed, 0, "pdelay", hz / 10);
                        }
                } else if (vm_pages_needed) {
                        /*
                         * Interlocked wakeup of waiters (non-optional).
                         *
                         * Similar to vm_page_free_wakeup() in vm_page.c,
                         * wake
                         */
                        pass = 0;
                        if (!vm_page_count_min(vm_page_free_hysteresis) ||
                            !vm_page_count_target()) {
                                vm_pages_needed = 0;
                                wakeup(&vmstats.v_free_count);
                        }
                } else {
                        pass = 0;
                }
        }
}

static struct kproc_desc page_kp = {
        "pagedaemon",
        vm_pageout_thread,
        &pagethread
};
SYSINIT(pagedaemon, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, kproc_start, &page_kp)


/*
 * Called after allocating a page out of the cache or free queue
 * to possibly wake the pagedaemon up to replentish our supply.
 *
 * We try to generate some hysteresis by waking the pagedaemon up
 * when our free+cache pages go below the free_min+cache_min level.
 * The pagedaemon tries to get the count back up to at least the
 * minimum, and through to the target level if possible.
 *
 * If the pagedaemon is already active bump vm_pages_needed as a hint
 * that there are even more requests pending.
 *
 * SMP races ok?
 * No requirements.
 */
void
pagedaemon_wakeup(void)
{
        if (vm_paging_needed() && curthread != pagethread) {
                if (vm_pages_needed == 0) {
                        vm_pages_needed = 1;    /* SMP race ok */
                        wakeup(&vm_pages_needed);
                } else if (vm_page_count_min(0)) {
                        ++vm_pages_needed;      /* SMP race ok */
                }
        }
}

#if !defined(NO_SWAPPING)

/*
 * SMP races ok?
 * No requirements.
 */
static void
vm_req_vmdaemon(void)
{
        static int lastrun = 0;

        if ((ticks > (lastrun + hz)) || (ticks < lastrun)) {
                wakeup(&vm_daemon_needed);
                lastrun = ticks;
        }
}

static int vm_daemon_callback(struct proc *p, void *data __unused);

/*
 * No requirements.
 */
static void
vm_daemon(void)
{
        /*
         * XXX vm_daemon_needed specific token?
         */
        while (TRUE) {
                tsleep(&vm_daemon_needed, 0, "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
                 */
                allproc_scan(vm_daemon_callback, NULL);
        }
}

static int
vm_daemon_callback(struct proc *p, void *data __unused)
{
        struct vmspace *vm;
        vm_pindex_t limit, size;

        /*
         * if this is a system process or if we have already
         * looked at this process, skip it.
         */
        lwkt_gettoken(&p->p_token);

        if (p->p_flags & (P_SYSTEM | P_WEXIT)) {
                lwkt_reltoken(&p->p_token);
                return (0);
        }

        /*
         * if the process is in a non-running type state,
         * don't touch it.
         */
        if (p->p_stat != SACTIVE && p->p_stat != SSTOP) {
                lwkt_reltoken(&p->p_token);
                return (0);
        }

        /*
         * 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 to get as
         * many pages out to swap as possible.
         */
        if (p->p_flags & P_SWAPPEDOUT)
                limit = 0;

        vm = p->p_vmspace;
        vmspace_hold(vm);
        size = vmspace_resident_count(vm);
        if (limit >= 0 && size >= limit) {
                vm_pageout_map_deactivate_pages(&vm->vm_map, limit);
        }
        vmspace_drop(vm);

        lwkt_reltoken(&p->p_token);

        return (0);
}

#endif