kernel - Make VM fault waits in low memory the same as other low memory waits

[dragonfly.git] / sys / vm / vm_object.c

/*
 * Copyright (c) 1991, 1993
 *      The Regents of the University of California.  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_object.c   8.5 (Berkeley) 3/22/94
 *
 *
 * 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_object.c,v 1.171.2.8 2003/05/26 19:17:56 alc Exp $
 */

/*
 *      Virtual memory object module.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/proc.h>           /* for curproc, pageproc */
#include <sys/thread.h>
#include <sys/vnode.h>
#include <sys/vmmeter.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/refcount.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_pageout.h>
#include <vm/vm_pager.h>
#include <vm/swap_pager.h>
#include <vm/vm_kern.h>
#include <vm/vm_extern.h>
#include <vm/vm_zone.h>

#define EASY_SCAN_FACTOR        8

static void     vm_object_qcollapse(vm_object_t object,
                                    vm_object_t backing_object);
static void     vm_object_page_collect_flush(vm_object_t object, vm_page_t p,
                                             int pagerflags);
static void     vm_object_lock_init(vm_object_t);


/*
 *      Virtual memory objects maintain the actual data
 *      associated with allocated virtual memory.  A given
 *      page of memory exists within exactly one object.
 *
 *      An object is only deallocated when all "references"
 *      are given up.  Only one "reference" to a given
 *      region of an object should be writeable.
 *
 *      Associated with each object is a list of all resident
 *      memory pages belonging to that object; this list is
 *      maintained by the "vm_page" module, and locked by the object's
 *      lock.
 *
 *      Each object also records a "pager" routine which is
 *      used to retrieve (and store) pages to the proper backing
 *      storage.  In addition, objects may be backed by other
 *      objects from which they were virtual-copied.
 *
 *      The only items within the object structure which are
 *      modified after time of creation are:
 *              reference count         locked by object's lock
 *              pager routine           locked by object's lock
 *
 */

struct object_q vm_object_list;         /* locked by vmobj_token */
struct vm_object kernel_object;

static long vm_object_count;            /* locked by vmobj_token */
extern int vm_pageout_page_count;

static long object_collapses;
static long object_bypasses;
static int next_index;
static vm_zone_t obj_zone;
static struct vm_zone obj_zone_store;
#define VM_OBJECTS_INIT 256
static struct vm_object vm_objects_init[VM_OBJECTS_INIT];

/*
 * Misc low level routines
 */
static void
vm_object_lock_init(vm_object_t obj)
{
#if defined(DEBUG_LOCKS)
        int i;

        obj->debug_hold_bitmap = 0;
        obj->debug_hold_ovfl = 0;
        for (i = 0; i < VMOBJ_DEBUG_ARRAY_SIZE; i++) {
                obj->debug_hold_thrs[i] = NULL;
                obj->debug_hold_file[i] = NULL;
                obj->debug_hold_line[i] = 0;
        }
#endif
}

void
vm_object_lock_swap(void)
{
        lwkt_token_swap();
}

void
vm_object_lock(vm_object_t obj)
{
        lwkt_gettoken(&obj->token);
}

/*
 * Returns TRUE on sucesss
 */
static int
vm_object_lock_try(vm_object_t obj)
{
        return(lwkt_trytoken(&obj->token));
}

void
vm_object_lock_shared(vm_object_t obj)
{
        lwkt_gettoken_shared(&obj->token);
}

void
vm_object_unlock(vm_object_t obj)
{
        lwkt_reltoken(&obj->token);
}

static __inline void
vm_object_assert_held(vm_object_t obj)
{
        ASSERT_LWKT_TOKEN_HELD(&obj->token);
}

void
#ifndef DEBUG_LOCKS
vm_object_hold(vm_object_t obj)
#else
debugvm_object_hold(vm_object_t obj, char *file, int line)
#endif
{
        KKASSERT(obj != NULL);

        /*
         * Object must be held (object allocation is stable due to callers
         * context, typically already holding the token on a parent object)
         * prior to potentially blocking on the lock, otherwise the object
         * can get ripped away from us.
         */
        refcount_acquire(&obj->hold_count);
        vm_object_lock(obj);

#if defined(DEBUG_LOCKS)
        int i;
        u_int mask;

        for (;;) {
                mask = ~obj->debug_hold_bitmap;
                cpu_ccfence();
                if (mask == 0xFFFFFFFFU) {
                        if (obj->debug_hold_ovfl == 0)
                                obj->debug_hold_ovfl = 1;
                        break;
                }
                i = ffs(mask) - 1;
                if (atomic_cmpset_int(&obj->debug_hold_bitmap, ~mask,
                                      ~mask | (1 << i))) {
                        obj->debug_hold_bitmap |= (1 << i);
                        obj->debug_hold_thrs[i] = curthread;
                        obj->debug_hold_file[i] = file;
                        obj->debug_hold_line[i] = line;
                        break;
                }
        }
#endif
}

int
#ifndef DEBUG_LOCKS
vm_object_hold_try(vm_object_t obj)
#else
debugvm_object_hold_try(vm_object_t obj, char *file, int line)
#endif
{
        KKASSERT(obj != NULL);

        /*
         * Object must be held (object allocation is stable due to callers
         * context, typically already holding the token on a parent object)
         * prior to potentially blocking on the lock, otherwise the object
         * can get ripped away from us.
         */
        refcount_acquire(&obj->hold_count);
        if (vm_object_lock_try(obj) == 0) {
                if (refcount_release(&obj->hold_count)) {
                        if (obj->ref_count == 0 && (obj->flags & OBJ_DEAD))
                                zfree(obj_zone, obj);
                }
                return(0);
        }

#if defined(DEBUG_LOCKS)
        int i;
        u_int mask;

        for (;;) {
                mask = ~obj->debug_hold_bitmap;
                cpu_ccfence();
                if (mask == 0xFFFFFFFFU) {
                        if (obj->debug_hold_ovfl == 0)
                                obj->debug_hold_ovfl = 1;
                        break;
                }
                i = ffs(mask) - 1;
                if (atomic_cmpset_int(&obj->debug_hold_bitmap, ~mask,
                                      ~mask | (1 << i))) {
                        obj->debug_hold_bitmap |= (1 << i);
                        obj->debug_hold_thrs[i] = curthread;
                        obj->debug_hold_file[i] = file;
                        obj->debug_hold_line[i] = line;
                        break;
                }
        }
#endif
        return(1);
}

void
#ifndef DEBUG_LOCKS
vm_object_hold_shared(vm_object_t obj)
#else
debugvm_object_hold_shared(vm_object_t obj, char *file, int line)
#endif
{
        KKASSERT(obj != NULL);

        /*
         * Object must be held (object allocation is stable due to callers
         * context, typically already holding the token on a parent object)
         * prior to potentially blocking on the lock, otherwise the object
         * can get ripped away from us.
         */
        refcount_acquire(&obj->hold_count);
        vm_object_lock_shared(obj);

#if defined(DEBUG_LOCKS)
        int i;
        u_int mask;

        for (;;) {
                mask = ~obj->debug_hold_bitmap;
                cpu_ccfence();
                if (mask == 0xFFFFFFFFU) {
                        if (obj->debug_hold_ovfl == 0)
                                obj->debug_hold_ovfl = 1;
                        break;
                }
                i = ffs(mask) - 1;
                if (atomic_cmpset_int(&obj->debug_hold_bitmap, ~mask,
                                      ~mask | (1 << i))) {
                        obj->debug_hold_bitmap |= (1 << i);
                        obj->debug_hold_thrs[i] = curthread;
                        obj->debug_hold_file[i] = file;
                        obj->debug_hold_line[i] = line;
                        break;
                }
        }
#endif
}

/*
 * Obtain either a shared or exclusive lock on VM object
 * based on whether this is a terminal vnode object or not.
 */
int
#ifndef DEBUG_LOCKS
vm_object_hold_maybe_shared(vm_object_t obj)
#else
debugvm_object_hold_maybe_shared(vm_object_t obj, char *file, int line)
#endif
{
        if (vm_shared_fault &&
            obj->type == OBJT_VNODE &&
            obj->backing_object == NULL) {
                vm_object_hold_shared(obj);
                return(1);
        } else {
                vm_object_hold(obj);
                return(0);
        }
}

/*
 * Drop the token and hold_count on the object.
 */
void
vm_object_drop(vm_object_t obj)
{
        if (obj == NULL)
                return;

#if defined(DEBUG_LOCKS)
        int found = 0;
        int i;

        for (i = 0; i < VMOBJ_DEBUG_ARRAY_SIZE; i++) {
                if ((obj->debug_hold_bitmap & (1 << i)) &&
                    (obj->debug_hold_thrs[i] == curthread)) {
                        obj->debug_hold_bitmap &= ~(1 << i);
                        obj->debug_hold_thrs[i] = NULL;
                        obj->debug_hold_file[i] = NULL;
                        obj->debug_hold_line[i] = 0;
                        found = 1;
                        break;
                }
        }

        if (found == 0 && obj->debug_hold_ovfl == 0)
                panic("vm_object: attempt to drop hold on non-self-held obj");
#endif

        /*
         * No new holders should be possible once we drop hold_count 1->0 as
         * there is no longer any way to reference the object.
         */
        KKASSERT(obj->hold_count > 0);
        if (refcount_release(&obj->hold_count)) {
                if (obj->ref_count == 0 && (obj->flags & OBJ_DEAD)) {
                        vm_object_unlock(obj);
                        zfree(obj_zone, obj);
                } else {
                        vm_object_unlock(obj);
                }
        } else {
                vm_object_unlock(obj);
        }
}

/*
 * Initialize a freshly allocated object, returning a held object.
 *
 * Used only by vm_object_allocate() and zinitna().
 *
 * No requirements.
 */
void
_vm_object_allocate(objtype_t type, vm_pindex_t size, vm_object_t object)
{
        int incr;

        RB_INIT(&object->rb_memq);
        LIST_INIT(&object->shadow_head);
        lwkt_token_init(&object->token, "vmobj");

        object->type = type;
        object->size = size;
        object->ref_count = 1;
        object->hold_count = 0;
        object->flags = 0;
        if ((object->type == OBJT_DEFAULT) || (object->type == OBJT_SWAP))
                vm_object_set_flag(object, OBJ_ONEMAPPING);
        object->paging_in_progress = 0;
        object->resident_page_count = 0;
        object->agg_pv_list_count = 0;
        object->shadow_count = 0;
        /* cpu localization twist */
        object->pg_color = (int)(intptr_t)curthread;
        if ( size > (PQ_L2_SIZE / 3 + PQ_PRIME1))
                incr = PQ_L2_SIZE / 3 + PQ_PRIME1;
        else
                incr = size;
        next_index = (next_index + incr) & PQ_L2_MASK;
        object->handle = NULL;
        object->backing_object = NULL;
        object->backing_object_offset = (vm_ooffset_t)0;

        object->generation++;
        object->swblock_count = 0;
        RB_INIT(&object->swblock_root);
        vm_object_lock_init(object);
        pmap_object_init(object);

        vm_object_hold(object);
        lwkt_gettoken(&vmobj_token);
        TAILQ_INSERT_TAIL(&vm_object_list, object, object_list);
        vm_object_count++;
        lwkt_reltoken(&vmobj_token);
}

/*
 * Initialize the VM objects module.
 *
 * Called from the low level boot code only.
 */
void
vm_object_init(void)
{
        TAILQ_INIT(&vm_object_list);
        
        _vm_object_allocate(OBJT_DEFAULT, OFF_TO_IDX(KvaEnd),
                            &kernel_object);
        vm_object_drop(&kernel_object);

        obj_zone = &obj_zone_store;
        zbootinit(obj_zone, "VM OBJECT", sizeof (struct vm_object),
                vm_objects_init, VM_OBJECTS_INIT);
}

void
vm_object_init2(void)
{
        zinitna(obj_zone, NULL, NULL, 0, 0, ZONE_PANICFAIL, 1);
}

/*
 * Allocate and return a new object of the specified type and size.
 *
 * No requirements.
 */
vm_object_t
vm_object_allocate(objtype_t type, vm_pindex_t size)
{
        vm_object_t result;

        result = (vm_object_t) zalloc(obj_zone);

        _vm_object_allocate(type, size, result);
        vm_object_drop(result);

        return (result);
}

/*
 * This version returns a held object, allowing further atomic initialization
 * of the object.
 */
vm_object_t
vm_object_allocate_hold(objtype_t type, vm_pindex_t size)
{
        vm_object_t result;

        result = (vm_object_t) zalloc(obj_zone);

        _vm_object_allocate(type, size, result);

        return (result);
}

/*
 * Add an additional reference to a vm_object.  The object must already be
 * held.  The original non-lock version is no longer supported.  The object
 * must NOT be chain locked by anyone at the time the reference is added.
 *
 * Referencing a chain-locked object can blow up the fairly sensitive
 * ref_count and shadow_count tests in the deallocator.  Most callers
 * will call vm_object_chain_wait() prior to calling
 * vm_object_reference_locked() to avoid the case.
 *
 * The object must be held, but may be held shared if desired (hence why
 * we use an atomic op).
 */
void
vm_object_reference_locked(vm_object_t object)
{
        KKASSERT(object != NULL);
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        KKASSERT((object->flags & OBJ_CHAINLOCK) == 0);
        atomic_add_int(&object->ref_count, 1);
        if (object->type == OBJT_VNODE) {
                vref(object->handle);
                /* XXX what if the vnode is being destroyed? */
        }
}

/*
 * Object OBJ_CHAINLOCK lock handling.
 *
 * The caller can chain-lock backing objects recursively and then
 * use vm_object_chain_release_all() to undo the whole chain.
 *
 * Chain locks are used to prevent collapses and are only applicable
 * to OBJT_DEFAULT and OBJT_SWAP objects.  Chain locking operations
 * on other object types are ignored.  This is also important because
 * it allows e.g. the vnode underlying a memory mapping to take concurrent
 * faults.
 *
 * The object must usually be held on entry, though intermediate
 * objects need not be held on release.
 */
void
vm_object_chain_wait(vm_object_t object)
{
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        while (object->flags & OBJ_CHAINLOCK) {
                vm_object_set_flag(object, OBJ_CHAINWANT);
                tsleep(object, 0, "objchain", 0);
        }
}

void
vm_object_chain_acquire(vm_object_t object)
{
        if (object->type == OBJT_DEFAULT || object->type == OBJT_SWAP) {
                vm_object_chain_wait(object);
                vm_object_set_flag(object, OBJ_CHAINLOCK);
        }
}

void
vm_object_chain_release(vm_object_t object)
{
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        if (object->type == OBJT_DEFAULT || object->type == OBJT_SWAP) {
                KKASSERT(object->flags & OBJ_CHAINLOCK);
                if (object->flags & OBJ_CHAINWANT) {
                        vm_object_clear_flag(object,
                                             OBJ_CHAINLOCK | OBJ_CHAINWANT);
                        wakeup(object);
                } else {
                        vm_object_clear_flag(object, OBJ_CHAINLOCK);
                }
        }
}

/*
 * This releases the entire chain of objects from first_object to and
 * including stopobj, flowing through object->backing_object.
 *
 * We release stopobj first as an optimization as this object is most
 * likely to be shared across multiple processes.
 */
void
vm_object_chain_release_all(vm_object_t first_object, vm_object_t stopobj)
{
        vm_object_t backing_object;
        vm_object_t object;

        vm_object_chain_release(stopobj);
        object = first_object;

        while (object != stopobj) {
                KKASSERT(object);
                if (object != first_object)
                        vm_object_hold(object);
                backing_object = object->backing_object;
                vm_object_chain_release(object);
                if (object != first_object)
                        vm_object_drop(object);
                object = backing_object;
        }
}

/*
 * Dereference an object and its underlying vnode.
 *
 * The object must be held exclusively and will remain held on return.
 * (We don't need an atomic op due to the exclusivity).
 */
static void
vm_object_vndeallocate(vm_object_t object)
{
        struct vnode *vp = (struct vnode *) object->handle;

        KASSERT(object->type == OBJT_VNODE,
            ("vm_object_vndeallocate: not a vnode object"));
        KASSERT(vp != NULL, ("vm_object_vndeallocate: missing vp"));
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
#ifdef INVARIANTS
        if (object->ref_count == 0) {
                vprint("vm_object_vndeallocate", vp);
                panic("vm_object_vndeallocate: bad object reference count");
        }
#endif
        object->ref_count--;
        if (object->ref_count == 0)
                vclrflags(vp, VTEXT);
        vrele(vp);
}

/*
 * Release a reference to the specified object, gained either through a
 * vm_object_allocate or a vm_object_reference call.  When all references
 * are gone, storage associated with this object may be relinquished.
 *
 * The caller does not have to hold the object locked but must have control
 * over the reference in question in order to guarantee that the object
 * does not get ripped out from under us.
 *
 * XXX Currently all deallocations require an exclusive lock.
 */
void
vm_object_deallocate(vm_object_t object)
{
        if (object) {
                vm_object_hold(object);
                vm_object_deallocate_locked(object);
                vm_object_drop(object);
        }
}

void
vm_object_deallocate_locked(vm_object_t object)
{
        struct vm_object_dealloc_list *dlist = NULL;
        struct vm_object_dealloc_list *dtmp;
        vm_object_t temp;
        int must_drop = 0;

        /*
         * We may chain deallocate object, but additional objects may
         * collect on the dlist which also have to be deallocated.  We
         * must avoid a recursion, vm_object chains can get deep.
         */
again:
        while (object != NULL) {
                ASSERT_LWKT_TOKEN_HELD_EXCL(&object->token);
#if 0
                /*
                 * Don't rip a ref_count out from under an object undergoing
                 * collapse, it will confuse the collapse code.
                 */
                vm_object_chain_wait(object);
#endif
                if (object->type == OBJT_VNODE) {
                        vm_object_vndeallocate(object);
                        break;
                }

                if (object->ref_count == 0) {
                        panic("vm_object_deallocate: object deallocated "
                              "too many times: %d", object->type);
                }
                if (object->ref_count > 2) {
                        object->ref_count--;
                        break;
                }

                /*
                 * Here on ref_count of one or two, which are special cases for
                 * objects.
                 *
                 * Nominal ref_count > 1 case if the second ref is not from
                 * a shadow.
                 *
                 * (ONEMAPPING only applies to DEFAULT AND SWAP objects)
                 */
                if (object->ref_count == 2 && object->shadow_count == 0) {
                        if (object->type == OBJT_DEFAULT ||
                            object->type == OBJT_SWAP) {
                                vm_object_set_flag(object, OBJ_ONEMAPPING);
                        }
                        object->ref_count--;
                        break;
                }

                /*
                 * If the second ref is from a shadow we chain along it
                 * upwards if object's handle is exhausted.
                 *
                 * We have to decrement object->ref_count before potentially
                 * collapsing the first shadow object or the collapse code
                 * will not be able to handle the degenerate case to remove
                 * object.  However, if we do it too early the object can
                 * get ripped out from under us.
                 */
                if (object->ref_count == 2 && object->shadow_count == 1 &&
                    object->handle == NULL && (object->type == OBJT_DEFAULT ||
                                               object->type == OBJT_SWAP)) {
                        temp = LIST_FIRST(&object->shadow_head);
                        KKASSERT(temp != NULL);
                        vm_object_hold(temp);

                        /*
                         * Wait for any paging to complete so the collapse
                         * doesn't (or isn't likely to) qcollapse.  pip
                         * waiting must occur before we acquire the
                         * chainlock.
                         */
                        while (
                                temp->paging_in_progress ||
                                object->paging_in_progress
                        ) {
                                vm_object_pip_wait(temp, "objde1");
                                vm_object_pip_wait(object, "objde2");
                        }

                        /*
                         * If the parent is locked we have to give up, as
                         * otherwise we would be acquiring locks in the
                         * wrong order and potentially deadlock.
                         */
                        if (temp->flags & OBJ_CHAINLOCK) {
                                vm_object_drop(temp);
                                goto skip;
                        }
                        vm_object_chain_acquire(temp);

                        /*
                         * Recheck/retry after the hold and the paging
                         * wait, both of which can block us.
                         */
                        if (object->ref_count != 2 ||
                            object->shadow_count != 1 ||
                            object->handle ||
                            LIST_FIRST(&object->shadow_head) != temp ||
                            (object->type != OBJT_DEFAULT &&
                             object->type != OBJT_SWAP)) {
                                vm_object_chain_release(temp);
                                vm_object_drop(temp);
                                continue;
                        }

                        /*
                         * We can safely drop object's ref_count now.
                         */
                        KKASSERT(object->ref_count == 2);
                        object->ref_count--;

                        /*
                         * If our single parent is not collapseable just
                         * decrement ref_count (2->1) and stop.
                         */
                        if (temp->handle || (temp->type != OBJT_DEFAULT &&
                                             temp->type != OBJT_SWAP)) {
                                vm_object_chain_release(temp);
                                vm_object_drop(temp);
                                break;
                        }

                        /*
                         * At this point we have already dropped object's
                         * ref_count so it is possible for a race to
                         * deallocate obj out from under us.  Any collapse
                         * will re-check the situation.  We must not block
                         * until we are able to collapse.
                         *
                         * Bump temp's ref_count to avoid an unwanted
                         * degenerate recursion (can't call
                         * vm_object_reference_locked() because it asserts
                         * that CHAINLOCK is not set).
                         */
                        temp->ref_count++;
                        KKASSERT(temp->ref_count > 1);

                        /*
                         * Collapse temp, then deallocate the extra ref
                         * formally.
                         */
                        vm_object_collapse(temp, &dlist);
                        vm_object_chain_release(temp);
                        if (must_drop) {
                                vm_object_lock_swap();
                                vm_object_drop(object);
                        }
                        object = temp;
                        must_drop = 1;
                        continue;
                }

                /*
                 * Drop the ref and handle termination on the 1->0 transition.
                 * We may have blocked above so we have to recheck.
                 */
skip:
                KKASSERT(object->ref_count != 0);
                if (object->ref_count >= 2) {
                        object->ref_count--;
                        break;
                }
                KKASSERT(object->ref_count == 1);

                /*
                 * 1->0 transition.  Chain through the backing_object.
                 * Maintain the ref until we've located the backing object,
                 * then re-check.
                 */
                while ((temp = object->backing_object) != NULL) {
                        vm_object_hold(temp);
                        if (temp == object->backing_object)
                                break;
                        vm_object_drop(temp);
                }

                /*
                 * 1->0 transition verified, retry if ref_count is no longer
                 * 1.  Otherwise disconnect the backing_object (temp) and
                 * clean up.
                 */
                if (object->ref_count != 1) {
                        vm_object_drop(temp);
                        continue;
                }

                /*
                 * It shouldn't be possible for the object to be chain locked
                 * if we're removing the last ref on it.
                 */
                KKASSERT((object->flags & OBJ_CHAINLOCK) == 0);

                if (temp) {
                        LIST_REMOVE(object, shadow_list);
                        temp->shadow_count--;
                        temp->generation++;
                        object->backing_object = NULL;
                }

                --object->ref_count;
                if ((object->flags & OBJ_DEAD) == 0)
                        vm_object_terminate(object);
                if (must_drop && temp)
                        vm_object_lock_swap();
                if (must_drop)
                        vm_object_drop(object);
                object = temp;
                must_drop = 1;
        }
        if (must_drop && object)
                vm_object_drop(object);

        /*
         * Additional tail recursion on dlist.  Avoid a recursion.  Objects
         * on the dlist have a hold count but are not locked.
         */
        if ((dtmp = dlist) != NULL) {
                dlist = dtmp->next;
                object = dtmp->object;
                kfree(dtmp, M_TEMP);

                vm_object_lock(object); /* already held, add lock */
                must_drop = 1;          /* and we're responsible for it */
                goto again;
        }
}

/*
 * Destroy the specified object, freeing up related resources.
 *
 * The object must have zero references.
 *
 * The object must held.  The caller is responsible for dropping the object
 * after terminate returns.  Terminate does NOT drop the object.
 */
static int vm_object_terminate_callback(vm_page_t p, void *data);

void
vm_object_terminate(vm_object_t object)
{
        /*
         * Make sure no one uses us.  Once we set OBJ_DEAD we should be
         * able to safely block.
         */
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        KKASSERT((object->flags & OBJ_DEAD) == 0);
        vm_object_set_flag(object, OBJ_DEAD);

        /*
         * Wait for the pageout daemon to be done with the object
         */
        vm_object_pip_wait(object, "objtrm1");

        KASSERT(!object->paging_in_progress,
                ("vm_object_terminate: pageout in progress"));

        /*
         * Clean and free the pages, as appropriate. All references to the
         * object are gone, so we don't need to lock it.
         */
        if (object->type == OBJT_VNODE) {
                struct vnode *vp;

                /*
                 * Clean pages and flush buffers.
                 *
                 * NOTE!  TMPFS buffer flushes do not typically flush the
                 *        actual page to swap as this would be highly
                 *        inefficient, and normal filesystems usually wrap
                 *        page flushes with buffer cache buffers.
                 *
                 *        To deal with this we have to call vinvalbuf() both
                 *        before and after the vm_object_page_clean().
                 */
                vp = (struct vnode *) object->handle;
                vinvalbuf(vp, V_SAVE, 0, 0);
                vm_object_page_clean(object, 0, 0, OBJPC_SYNC);
                vinvalbuf(vp, V_SAVE, 0, 0);
        }

        /*
         * Wait for any I/O to complete, after which there had better not
         * be any references left on the object.
         */
        vm_object_pip_wait(object, "objtrm2");

        if (object->ref_count != 0) {
                panic("vm_object_terminate: object with references, "
                      "ref_count=%d", object->ref_count);
        }

        /*
         * Cleanup any shared pmaps associated with this object.
         */
        pmap_object_free(object);

        /*
         * Now free any remaining pages. For internal objects, this also
         * removes them from paging queues. Don't free wired pages, just
         * remove them from the object. 
         */
        vm_page_rb_tree_RB_SCAN(&object->rb_memq, NULL,
                                vm_object_terminate_callback, NULL);

        /*
         * Let the pager know object is dead.
         */
        vm_pager_deallocate(object);

        /*
         * Wait for the object hold count to hit 1, clean out pages as
         * we go.  vmobj_token interlocks any race conditions that might
         * pick the object up from the vm_object_list after we have cleared
         * rb_memq.
         */
        for (;;) {
                if (RB_ROOT(&object->rb_memq) == NULL)
                        break;
                kprintf("vm_object_terminate: Warning, object %p "
                        "still has %d pages\n",
                        object, object->resident_page_count);
                vm_page_rb_tree_RB_SCAN(&object->rb_memq, NULL,
                                        vm_object_terminate_callback, NULL);
        }

        /*
         * There had better not be any pages left
         */
        KKASSERT(object->resident_page_count == 0);

        /*
         * Remove the object from the global object list.
         */
        lwkt_gettoken(&vmobj_token);
        TAILQ_REMOVE(&vm_object_list, object, object_list);
        vm_object_count--;
        lwkt_reltoken(&vmobj_token);
        vm_object_dead_wakeup(object);

        if (object->ref_count != 0) {
                panic("vm_object_terminate2: object with references, "
                      "ref_count=%d", object->ref_count);
        }

        /*
         * NOTE: The object hold_count is at least 1, so we cannot zfree()
         *       the object here.  See vm_object_drop().
         */
}

/*
 * The caller must hold the object.
 */
static int
vm_object_terminate_callback(vm_page_t p, void *data __unused)
{
        vm_object_t object;

        object = p->object;
        vm_page_busy_wait(p, TRUE, "vmpgtrm");
        if (object != p->object) {
                kprintf("vm_object_terminate: Warning: Encountered "
                        "busied page %p on queue %d\n", p, p->queue);
                vm_page_wakeup(p);
        } else if (p->wire_count == 0) {
                /*
                 * NOTE: p->dirty and PG_NEED_COMMIT are ignored.
                 */
                vm_page_free(p);
                mycpu->gd_cnt.v_pfree++;
        } else {
                if (p->queue != PQ_NONE)
                        kprintf("vm_object_terminate: Warning: Encountered "
                                "wired page %p on queue %d\n", p, p->queue);
                vm_page_remove(p);
                vm_page_wakeup(p);
        }
        lwkt_yield();
        return(0);
}

/*
 * The object is dead but still has an object<->pager association.  Sleep
 * and return.  The caller typically retests the association in a loop.
 *
 * The caller must hold the object.
 */
void
vm_object_dead_sleep(vm_object_t object, const char *wmesg)
{
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        if (object->handle) {
                vm_object_set_flag(object, OBJ_DEADWNT);
                tsleep(object, 0, wmesg, 0);
                /* object may be invalid after this point */
        }
}

/*
 * Wakeup anyone waiting for the object<->pager disassociation on
 * a dead object.
 *
 * The caller must hold the object.
 */
void
vm_object_dead_wakeup(vm_object_t object)
{
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        if (object->flags & OBJ_DEADWNT) {
                vm_object_clear_flag(object, OBJ_DEADWNT);
                wakeup(object);
        }
}

/*
 * Clean all dirty pages in the specified range of object.  Leaves page
 * on whatever queue it is currently on.   If NOSYNC is set then do not
 * write out pages with PG_NOSYNC set (originally comes from MAP_NOSYNC),
 * leaving the object dirty.
 *
 * When stuffing pages asynchronously, allow clustering.  XXX we need a
 * synchronous clustering mode implementation.
 *
 * Odd semantics: if start == end, we clean everything.
 *
 * The object must be locked? XXX
 */
static int vm_object_page_clean_pass1(struct vm_page *p, void *data);
static int vm_object_page_clean_pass2(struct vm_page *p, void *data);

void
vm_object_page_clean(vm_object_t object, vm_pindex_t start, vm_pindex_t end,
                     int flags)
{
        struct rb_vm_page_scan_info info;
        struct vnode *vp;
        int wholescan;
        int pagerflags;
        int generation;

        vm_object_hold(object);
        if (object->type != OBJT_VNODE ||
            (object->flags & OBJ_MIGHTBEDIRTY) == 0) {
                vm_object_drop(object);
                return;
        }

        pagerflags = (flags & (OBJPC_SYNC | OBJPC_INVAL)) ? 
                        VM_PAGER_PUT_SYNC : VM_PAGER_CLUSTER_OK;
        pagerflags |= (flags & OBJPC_INVAL) ? VM_PAGER_PUT_INVAL : 0;

        vp = object->handle;

        /*
         * Interlock other major object operations.  This allows us to 
         * temporarily clear OBJ_WRITEABLE and OBJ_MIGHTBEDIRTY.
         */
        vm_object_set_flag(object, OBJ_CLEANING);

        /*
         * Handle 'entire object' case
         */
        info.start_pindex = start;
        if (end == 0) {
                info.end_pindex = object->size - 1;
        } else {
                info.end_pindex = end - 1;
        }
        wholescan = (start == 0 && info.end_pindex == object->size - 1);
        info.limit = flags;
        info.pagerflags = pagerflags;
        info.object = object;

        /*
         * If cleaning the entire object do a pass to mark the pages read-only.
         * If everything worked out ok, clear OBJ_WRITEABLE and
         * OBJ_MIGHTBEDIRTY.
         */
        if (wholescan) {
                info.error = 0;
                vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                        vm_object_page_clean_pass1, &info);
                if (info.error == 0) {
                        vm_object_clear_flag(object,
                                             OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY);
                        if (object->type == OBJT_VNODE &&
                            (vp = (struct vnode *)object->handle) != NULL) {
                                if (vp->v_flag & VOBJDIRTY) 
                                        vclrflags(vp, VOBJDIRTY);
                        }
                }
        }

        /*
         * Do a pass to clean all the dirty pages we find.
         */
        do {
                info.error = 0;
                generation = object->generation;
                vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                        vm_object_page_clean_pass2, &info);
        } while (info.error || generation != object->generation);

        vm_object_clear_flag(object, OBJ_CLEANING);
        vm_object_drop(object);
}

/*
 * The caller must hold the object.
 */
static 
int
vm_object_page_clean_pass1(struct vm_page *p, void *data)
{
        struct rb_vm_page_scan_info *info = data;

        vm_page_flag_set(p, PG_CLEANCHK);
        if ((info->limit & OBJPC_NOSYNC) && (p->flags & PG_NOSYNC)) {
                info->error = 1;
        } else if (vm_page_busy_try(p, FALSE) == 0) {
                vm_page_protect(p, VM_PROT_READ);       /* must not block */
                vm_page_wakeup(p);
        } else {
                info->error = 1;
        }
        lwkt_yield();
        return(0);
}

/*
 * The caller must hold the object
 */
static 
int
vm_object_page_clean_pass2(struct vm_page *p, void *data)
{
        struct rb_vm_page_scan_info *info = data;
        int generation;

        /*
         * Do not mess with pages that were inserted after we started
         * the cleaning pass.
         */
        if ((p->flags & PG_CLEANCHK) == 0)
                goto done;

        generation = info->object->generation;
        vm_page_busy_wait(p, TRUE, "vpcwai");
        if (p->object != info->object ||
            info->object->generation != generation) {
                info->error = 1;
                vm_page_wakeup(p);
                goto done;
        }

        /*
         * Before wasting time traversing the pmaps, check for trivial
         * cases where the page cannot be dirty.
         */
        if (p->valid == 0 || (p->queue - p->pc) == PQ_CACHE) {
                KKASSERT((p->dirty & p->valid) == 0 &&
                         (p->flags & PG_NEED_COMMIT) == 0);
                vm_page_wakeup(p);
                goto done;
        }

        /*
         * Check whether the page is dirty or not.  The page has been set
         * to be read-only so the check will not race a user dirtying the
         * page.
         */
        vm_page_test_dirty(p);
        if ((p->dirty & p->valid) == 0 && (p->flags & PG_NEED_COMMIT) == 0) {
                vm_page_flag_clear(p, PG_CLEANCHK);
                vm_page_wakeup(p);
                goto done;
        }

        /*
         * If we have been asked to skip nosync pages and this is a
         * nosync page, skip it.  Note that the object flags were
         * not cleared in this case (because pass1 will have returned an
         * error), so we do not have to set them.
         */
        if ((info->limit & OBJPC_NOSYNC) && (p->flags & PG_NOSYNC)) {
                vm_page_flag_clear(p, PG_CLEANCHK);
                vm_page_wakeup(p);
                goto done;
        }

        /*
         * Flush as many pages as we can.  PG_CLEANCHK will be cleared on
         * the pages that get successfully flushed.  Set info->error if
         * we raced an object modification.
         */
        vm_object_page_collect_flush(info->object, p, info->pagerflags);
        vm_wait_nominal();
done:
        lwkt_yield();
        return(0);
}

/*
 * Collect the specified page and nearby pages and flush them out.
 * The number of pages flushed is returned.  The passed page is busied
 * by the caller and we are responsible for its disposition.
 *
 * The caller must hold the object.
 */
static void
vm_object_page_collect_flush(vm_object_t object, vm_page_t p, int pagerflags)
{
        int runlen;
        int error;
        int maxf;
        int chkb;
        int maxb;
        int i;
        vm_pindex_t pi;
        vm_page_t maf[vm_pageout_page_count];
        vm_page_t mab[vm_pageout_page_count];
        vm_page_t ma[vm_pageout_page_count];

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));

        pi = p->pindex;

        maxf = 0;
        for(i = 1; i < vm_pageout_page_count; i++) {
                vm_page_t tp;

                tp = vm_page_lookup_busy_try(object, pi + i, TRUE, &error);
                if (error)
                        break;
                if (tp == NULL)
                        break;
                if ((pagerflags & VM_PAGER_IGNORE_CLEANCHK) == 0 &&
                    (tp->flags & PG_CLEANCHK) == 0) {
                        vm_page_wakeup(tp);
                        break;
                }
                if ((tp->queue - tp->pc) == PQ_CACHE) {
                        vm_page_flag_clear(tp, PG_CLEANCHK);
                        vm_page_wakeup(tp);
                        break;
                }
                vm_page_test_dirty(tp);
                if ((tp->dirty & tp->valid) == 0 &&
                    (tp->flags & PG_NEED_COMMIT) == 0) {
                        vm_page_flag_clear(tp, PG_CLEANCHK);
                        vm_page_wakeup(tp);
                        break;
                }
                maf[i - 1] = tp;
                maxf++;
        }

        maxb = 0;
        chkb = vm_pageout_page_count -  maxf;
        /*
         * NOTE: chkb can be 0
         */
        for(i = 1; chkb && i < chkb; i++) {
                vm_page_t tp;

                tp = vm_page_lookup_busy_try(object, pi - i, TRUE, &error);
                if (error)
                        break;
                if (tp == NULL)
                        break;
                if ((pagerflags & VM_PAGER_IGNORE_CLEANCHK) == 0 &&
                    (tp->flags & PG_CLEANCHK) == 0) {
                        vm_page_wakeup(tp);
                        break;
                }
                if ((tp->queue - tp->pc) == PQ_CACHE) {
                        vm_page_flag_clear(tp, PG_CLEANCHK);
                        vm_page_wakeup(tp);
                        break;
                }
                vm_page_test_dirty(tp);
                if ((tp->dirty & tp->valid) == 0 &&
                    (tp->flags & PG_NEED_COMMIT) == 0) {
                        vm_page_flag_clear(tp, PG_CLEANCHK);
                        vm_page_wakeup(tp);
                        break;
                }
                mab[i - 1] = tp;
                maxb++;
        }

        /*
         * All pages in the maf[] and mab[] array are busied.
         */
        for (i = 0; i < maxb; i++) {
                int index = (maxb - i) - 1;
                ma[index] = mab[i];
                vm_page_flag_clear(ma[index], PG_CLEANCHK);
        }
        vm_page_flag_clear(p, PG_CLEANCHK);
        ma[maxb] = p;
        for(i = 0; i < maxf; i++) {
                int index = (maxb + i) + 1;
                ma[index] = maf[i];
                vm_page_flag_clear(ma[index], PG_CLEANCHK);
        }
        runlen = maxb + maxf + 1;

        for (i = 0; i < runlen; i++)    /* XXX need this any more? */
                vm_page_hold(ma[i]);

        vm_pageout_flush(ma, runlen, pagerflags);

        for (i = 0; i < runlen; i++)    /* XXX need this any more? */
                vm_page_unhold(ma[i]);
}

/*
 * Same as vm_object_pmap_copy, except range checking really
 * works, and is meant for small sections of an object.
 *
 * This code protects resident pages by making them read-only
 * and is typically called on a fork or split when a page
 * is converted to copy-on-write.  
 *
 * NOTE: If the page is already at VM_PROT_NONE, calling
 * vm_page_protect will have no effect.
 */
void
vm_object_pmap_copy_1(vm_object_t object, vm_pindex_t start, vm_pindex_t end)
{
        vm_pindex_t idx;
        vm_page_t p;

        if (object == NULL || (object->flags & OBJ_WRITEABLE) == 0)
                return;

        vm_object_hold(object);
        for (idx = start; idx < end; idx++) {
                p = vm_page_lookup(object, idx);
                if (p == NULL)
                        continue;
                vm_page_protect(p, VM_PROT_READ);
        }
        vm_object_drop(object);
}

/*
 * Removes all physical pages in the specified object range from all
 * physical maps.
 *
 * The object must *not* be locked.
 */

static int vm_object_pmap_remove_callback(vm_page_t p, void *data);

void
vm_object_pmap_remove(vm_object_t object, vm_pindex_t start, vm_pindex_t end)
{
        struct rb_vm_page_scan_info info;

        if (object == NULL)
                return;
        info.start_pindex = start;
        info.end_pindex = end - 1;

        vm_object_hold(object);
        vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                vm_object_pmap_remove_callback, &info);
        if (start == 0 && end == object->size)
                vm_object_clear_flag(object, OBJ_WRITEABLE);
        vm_object_drop(object);
}

/*
 * The caller must hold the object
 */
static int
vm_object_pmap_remove_callback(vm_page_t p, void *data __unused)
{
        vm_page_protect(p, VM_PROT_NONE);
        return(0);
}

/*
 * Implements the madvise function at the object/page level.
 *
 * MADV_WILLNEED        (any object)
 *
 *      Activate the specified pages if they are resident.
 *
 * MADV_DONTNEED        (any object)
 *
 *      Deactivate the specified pages if they are resident.
 *
 * MADV_FREE    (OBJT_DEFAULT/OBJT_SWAP objects, OBJ_ONEMAPPING only)
 *
 *      Deactivate and clean the specified pages if they are
 *      resident.  This permits the process to reuse the pages
 *      without faulting or the kernel to reclaim the pages
 *      without I/O.
 *
 * No requirements.
 */
void
vm_object_madvise(vm_object_t object, vm_pindex_t pindex, int count, int advise)
{
        vm_pindex_t end, tpindex;
        vm_object_t tobject;
        vm_object_t xobj;
        vm_page_t m;
        int error;

        if (object == NULL)
                return;

        end = pindex + count;

        vm_object_hold(object);
        tobject = object;

        /*
         * Locate and adjust resident pages
         */
        for (; pindex < end; pindex += 1) {
relookup:
                if (tobject != object)
                        vm_object_drop(tobject);
                tobject = object;
                tpindex = pindex;
shadowlookup:
                /*
                 * MADV_FREE only operates on OBJT_DEFAULT or OBJT_SWAP pages
                 * and those pages must be OBJ_ONEMAPPING.
                 */
                if (advise == MADV_FREE) {
                        if ((tobject->type != OBJT_DEFAULT &&
                             tobject->type != OBJT_SWAP) ||
                            (tobject->flags & OBJ_ONEMAPPING) == 0) {
                                continue;
                        }
                }

                m = vm_page_lookup_busy_try(tobject, tpindex, TRUE, &error);

                if (error) {
                        vm_page_sleep_busy(m, TRUE, "madvpo");
                        goto relookup;
                }
                if (m == NULL) {
                        /*
                         * There may be swap even if there is no backing page
                         */
                        if (advise == MADV_FREE && tobject->type == OBJT_SWAP)
                                swap_pager_freespace(tobject, tpindex, 1);

                        /*
                         * next object
                         */
                        while ((xobj = tobject->backing_object) != NULL) {
                                KKASSERT(xobj != object);
                                vm_object_hold(xobj);
                                if (xobj == tobject->backing_object)
                                        break;
                                vm_object_drop(xobj);
                        }
                        if (xobj == NULL)
                                continue;
                        tpindex += OFF_TO_IDX(tobject->backing_object_offset);
                        if (tobject != object) {
                                vm_object_lock_swap();
                                vm_object_drop(tobject);
                        }
                        tobject = xobj;
                        goto shadowlookup;
                }

                /*
                 * If the page is not in a normal active state, we skip it.
                 * If the page is not managed there are no page queues to
                 * mess with.  Things can break if we mess with pages in
                 * any of the below states.
                 */
                if (m->wire_count ||
                    (m->flags & (PG_UNMANAGED | PG_NEED_COMMIT)) ||
                    m->valid != VM_PAGE_BITS_ALL
                ) {
                        vm_page_wakeup(m);
                        continue;
                }

                /*
                 * Theoretically once a page is known not to be busy, an
                 * interrupt cannot come along and rip it out from under us.
                 */

                if (advise == MADV_WILLNEED) {
                        vm_page_activate(m);
                } else if (advise == MADV_DONTNEED) {
                        vm_page_dontneed(m);
                } else if (advise == MADV_FREE) {
                        /*
                         * Mark the page clean.  This will allow the page
                         * to be freed up by the system.  However, such pages
                         * are often reused quickly by malloc()/free()
                         * so we do not do anything that would cause
                         * a page fault if we can help it.
                         *
                         * Specifically, we do not try to actually free
                         * the page now nor do we try to put it in the
                         * cache (which would cause a page fault on reuse).
                         *
                         * But we do make the page is freeable as we
                         * can without actually taking the step of unmapping
                         * it.
                         */
                        pmap_clear_modify(m);
                        m->dirty = 0;
                        m->act_count = 0;
                        vm_page_dontneed(m);
                        if (tobject->type == OBJT_SWAP)
                                swap_pager_freespace(tobject, tpindex, 1);
                }
                vm_page_wakeup(m);
        }       
        if (tobject != object)
                vm_object_drop(tobject);
        vm_object_drop(object);
}

/*
 * Create a new object which is backed by the specified existing object
 * range.  Replace the pointer and offset that was pointing at the existing
 * object with the pointer/offset for the new object.
 *
 * No other requirements.
 */
void
vm_object_shadow(vm_object_t *objectp, vm_ooffset_t *offset, vm_size_t length,
                 int addref)
{
        vm_object_t source;
        vm_object_t result;

        source = *objectp;

        /*
         * Don't create the new object if the old object isn't shared.
         * We have to chain wait before adding the reference to avoid
         * racing a collapse or deallocation.
         *
         * Add the additional ref to source here to avoid racing a later
         * collapse or deallocation. Clear the ONEMAPPING flag whether
         * addref is TRUE or not in this case because the original object
         * will be shadowed.
         */
        if (source) {
                vm_object_hold(source);
                vm_object_chain_wait(source);
                if (source->ref_count == 1 &&
                    source->handle == NULL &&
                    (source->type == OBJT_DEFAULT ||
                     source->type == OBJT_SWAP)) {
                        vm_object_drop(source);
                        if (addref) {
                                vm_object_reference_locked(source);
                                vm_object_clear_flag(source, OBJ_ONEMAPPING);
                        }
                        return;
                }
                vm_object_reference_locked(source);
                vm_object_clear_flag(source, OBJ_ONEMAPPING);
        }

        /*
         * Allocate a new object with the given length.  The new object
         * is returned referenced but we may have to add another one.
         * If we are adding a second reference we must clear OBJ_ONEMAPPING.
         * (typically because the caller is about to clone a vm_map_entry).
         *
         * The source object currently has an extra reference to prevent
         * collapses into it while we mess with its shadow list, which
         * we will remove later in this routine.
         */
        if ((result = vm_object_allocate(OBJT_DEFAULT, length)) == NULL)
                panic("vm_object_shadow: no object for shadowing");
        vm_object_hold(result);
        if (addref) {
                vm_object_reference_locked(result);
                vm_object_clear_flag(result, OBJ_ONEMAPPING);
        }

        /*
         * The new object shadows the source object.  Chain wait before
         * adjusting shadow_count or the shadow list to avoid races.
         *
         * Try to optimize the result object's page color when shadowing
         * in order to maintain page coloring consistency in the combined 
         * shadowed object.
         */
        KKASSERT(result->backing_object == NULL);
        result->backing_object = source;
        if (source) {
                vm_object_chain_wait(source);
                LIST_INSERT_HEAD(&source->shadow_head, result, shadow_list);
                source->shadow_count++;
                source->generation++;
                /* cpu localization twist */
                result->pg_color = (int)(intptr_t)curthread;
        }

        /*
         * Adjust the return storage.  Drop the ref on source before
         * returning.
         */
        result->backing_object_offset = *offset;
        vm_object_drop(result);
        *offset = 0;
        if (source) {
                vm_object_deallocate_locked(source);
                vm_object_drop(source);
        }

        /*
         * Return the new things
         */
        *objectp = result;
}

#define OBSC_TEST_ALL_SHADOWED  0x0001
#define OBSC_COLLAPSE_NOWAIT    0x0002
#define OBSC_COLLAPSE_WAIT      0x0004

static int vm_object_backing_scan_callback(vm_page_t p, void *data);

/*
 * The caller must hold the object.
 */
static __inline int
vm_object_backing_scan(vm_object_t object, vm_object_t backing_object, int op)
{
        struct rb_vm_page_scan_info info;

        vm_object_assert_held(object);
        vm_object_assert_held(backing_object);

        KKASSERT(backing_object == object->backing_object);
        info.backing_offset_index = OFF_TO_IDX(object->backing_object_offset);

        /*
         * Initial conditions
         */
        if (op & OBSC_TEST_ALL_SHADOWED) {
                /*
                 * We do not want to have to test for the existence of
                 * swap pages in the backing object.  XXX but with the
                 * new swapper this would be pretty easy to do.
                 *
                 * XXX what about anonymous MAP_SHARED memory that hasn't
                 * been ZFOD faulted yet?  If we do not test for this, the
                 * shadow test may succeed! XXX
                 */
                if (backing_object->type != OBJT_DEFAULT)
                        return(0);
        }
        if (op & OBSC_COLLAPSE_WAIT) {
                KKASSERT((backing_object->flags & OBJ_DEAD) == 0);
                vm_object_set_flag(backing_object, OBJ_DEAD);
                lwkt_gettoken(&vmobj_token);
                TAILQ_REMOVE(&vm_object_list, backing_object, object_list);
                vm_object_count--;
                lwkt_reltoken(&vmobj_token);
                vm_object_dead_wakeup(backing_object);
        }

        /*
         * Our scan.   We have to retry if a negative error code is returned,
         * otherwise 0 or 1 will be returned in info.error.  0 Indicates that
         * the scan had to be stopped because the parent does not completely
         * shadow the child.
         */
        info.object = object;
        info.backing_object = backing_object;
        info.limit = op;
        do {
                info.error = 1;
                vm_page_rb_tree_RB_SCAN(&backing_object->rb_memq, NULL,
                                        vm_object_backing_scan_callback,
                                        &info);
        } while (info.error < 0);

        return(info.error);
}

/*
 * The caller must hold the object.
 */
static int
vm_object_backing_scan_callback(vm_page_t p, void *data)
{
        struct rb_vm_page_scan_info *info = data;
        vm_object_t backing_object;
        vm_object_t object;
        vm_pindex_t pindex;
        vm_pindex_t new_pindex;
        vm_pindex_t backing_offset_index;
        int op;

        pindex = p->pindex;
        new_pindex = pindex - info->backing_offset_index;
        op = info->limit;
        object = info->object;
        backing_object = info->backing_object;
        backing_offset_index = info->backing_offset_index;

        if (op & OBSC_TEST_ALL_SHADOWED) {
                vm_page_t pp;

                /*
                 * Ignore pages outside the parent object's range
                 * and outside the parent object's mapping of the 
                 * backing object.
                 *
                 * note that we do not busy the backing object's
                 * page.
                 */
                if (pindex < backing_offset_index ||
                    new_pindex >= object->size
                ) {
                        return(0);
                }

                /*
                 * See if the parent has the page or if the parent's
                 * object pager has the page.  If the parent has the
                 * page but the page is not valid, the parent's
                 * object pager must have the page.
                 *
                 * If this fails, the parent does not completely shadow
                 * the object and we might as well give up now.
                 */
                pp = vm_page_lookup(object, new_pindex);
                if ((pp == NULL || pp->valid == 0) &&
                    !vm_pager_has_page(object, new_pindex)
                ) {
                        info->error = 0;        /* problemo */
                        return(-1);             /* stop the scan */
                }
        }

        /*
         * Check for busy page.  Note that we may have lost (p) when we
         * possibly blocked above.
         */
        if (op & (OBSC_COLLAPSE_WAIT | OBSC_COLLAPSE_NOWAIT)) {
                vm_page_t pp;

                if (vm_page_busy_try(p, TRUE)) {
                        if (op & OBSC_COLLAPSE_NOWAIT) {
                                return(0);
                        } else {
                                /*
                                 * If we slept, anything could have
                                 * happened.   Ask that the scan be restarted.
                                 *
                                 * Since the object is marked dead, the
                                 * backing offset should not have changed.  
                                 */
                                vm_page_sleep_busy(p, TRUE, "vmocol");
                                info->error = -1;
                                return(-1);
                        }
                }

                /*
                 * If (p) is no longer valid restart the scan.
                 */
                if (p->object != backing_object || p->pindex != pindex) {
                        kprintf("vm_object_backing_scan: Warning: page "
                                "%p ripped out from under us\n", p);
                        vm_page_wakeup(p);
                        info->error = -1;
                        return(-1);
                }

                if (op & OBSC_COLLAPSE_NOWAIT) {
                        if (p->valid == 0 ||
                            p->wire_count ||
                            (p->flags & PG_NEED_COMMIT)) {
                                vm_page_wakeup(p);
                                return(0);
                        }
                } else {
                        /* XXX what if p->valid == 0 , hold_count, etc? */
                }

                KASSERT(
                    p->object == backing_object,
                    ("vm_object_qcollapse(): object mismatch")
                );

                /*
                 * Destroy any associated swap
                 */
                if (backing_object->type == OBJT_SWAP)
                        swap_pager_freespace(backing_object, p->pindex, 1);

                if (
                    p->pindex < backing_offset_index ||
                    new_pindex >= object->size
                ) {
                        /*
                         * Page is out of the parent object's range, we 
                         * can simply destroy it. 
                         */
                        vm_page_protect(p, VM_PROT_NONE);
                        vm_page_free(p);
                        return(0);
                }

                pp = vm_page_lookup(object, new_pindex);
                if (pp != NULL || vm_pager_has_page(object, new_pindex)) {
                        /*
                         * page already exists in parent OR swap exists
                         * for this location in the parent.  Destroy 
                         * the original page from the backing object.
                         *
                         * Leave the parent's page alone
                         */
                        vm_page_protect(p, VM_PROT_NONE);
                        vm_page_free(p);
                        return(0);
                }

                /*
                 * Page does not exist in parent, rename the
                 * page from the backing object to the main object. 
                 *
                 * If the page was mapped to a process, it can remain 
                 * mapped through the rename.
                 */
                if ((p->queue - p->pc) == PQ_CACHE)
                        vm_page_deactivate(p);

                vm_page_rename(p, object, new_pindex);
                vm_page_wakeup(p);
                /* page automatically made dirty by rename */
        }
        return(0);
}

/*
 * This version of collapse allows the operation to occur earlier and
 * when paging_in_progress is true for an object...  This is not a complete
 * operation, but should plug 99.9% of the rest of the leaks.
 *
 * The caller must hold the object and backing_object and both must be
 * chainlocked.
 *
 * (only called from vm_object_collapse)
 */
static void
vm_object_qcollapse(vm_object_t object, vm_object_t backing_object)
{
        if (backing_object->ref_count == 1) {
                backing_object->ref_count += 2;
                vm_object_backing_scan(object, backing_object,
                                       OBSC_COLLAPSE_NOWAIT);
                backing_object->ref_count -= 2;
        }
}

/*
 * Collapse an object with the object backing it.  Pages in the backing
 * object are moved into the parent, and the backing object is deallocated.
 * Any conflict is resolved in favor of the parent's existing pages.
 *
 * object must be held and chain-locked on call.
 *
 * The caller must have an extra ref on object to prevent a race from
 * destroying it during the collapse.
 */
void
vm_object_collapse(vm_object_t object, struct vm_object_dealloc_list **dlistp)
{
        struct vm_object_dealloc_list *dlist = NULL;
        vm_object_t backing_object;

        /*
         * Only one thread is attempting a collapse at any given moment.
         * There are few restrictions for (object) that callers of this
         * function check so reentrancy is likely.
         */
        KKASSERT(object != NULL);
        vm_object_assert_held(object);
        KKASSERT(object->flags & OBJ_CHAINLOCK);

        for (;;) {
                vm_object_t bbobj;
                int dodealloc;

                /*
                 * We have to hold the backing object, check races.
                 */
                while ((backing_object = object->backing_object) != NULL) {
                        vm_object_hold(backing_object);
                        if (backing_object == object->backing_object)
                                break;
                        vm_object_drop(backing_object);
                }

                /*
                 * No backing object?  Nothing to collapse then.
                 */
                if (backing_object == NULL)
                        break;

                /*
                 * You can't collapse with a non-default/non-swap object.
                 */
                if (backing_object->type != OBJT_DEFAULT &&
                    backing_object->type != OBJT_SWAP) {
                        vm_object_drop(backing_object);
                        backing_object = NULL;
                        break;
                }

                /*
                 * Chain-lock the backing object too because if we
                 * successfully merge its pages into the top object we
                 * will collapse backing_object->backing_object as the
                 * new backing_object.  Re-check that it is still our
                 * backing object.
                 */
                vm_object_chain_acquire(backing_object);
                if (backing_object != object->backing_object) {
                        vm_object_chain_release(backing_object);
                        vm_object_drop(backing_object);
                        continue;
                }

                /*
                 * we check the backing object first, because it is most likely
                 * not collapsable.
                 */
                if (backing_object->handle != NULL ||
                    (backing_object->type != OBJT_DEFAULT &&
                     backing_object->type != OBJT_SWAP) ||
                    (backing_object->flags & OBJ_DEAD) ||
                    object->handle != NULL ||
                    (object->type != OBJT_DEFAULT &&
                     object->type != OBJT_SWAP) ||
                    (object->flags & OBJ_DEAD)) {
                        break;
                }

                /*
                 * If paging is in progress we can't do a normal collapse.
                 */
                if (
                    object->paging_in_progress != 0 ||
                    backing_object->paging_in_progress != 0
                ) {
                        vm_object_qcollapse(object, backing_object);
                        break;
                }

                /*
                 * We know that we can either collapse the backing object (if
                 * the parent is the only reference to it) or (perhaps) have
                 * the parent bypass the object if the parent happens to shadow
                 * all the resident pages in the entire backing object.
                 *
                 * This is ignoring pager-backed pages such as swap pages.
                 * vm_object_backing_scan fails the shadowing test in this
                 * case.
                 */
                if (backing_object->ref_count == 1) {
                        /*
                         * If there is exactly one reference to the backing
                         * object, we can collapse it into the parent.  
                         */
                        KKASSERT(object->backing_object == backing_object);
                        vm_object_backing_scan(object, backing_object,
                                               OBSC_COLLAPSE_WAIT);

                        /*
                         * Move the pager from backing_object to object.
                         */
                        if (backing_object->type == OBJT_SWAP) {
                                vm_object_pip_add(backing_object, 1);

                                /*
                                 * scrap the paging_offset junk and do a 
                                 * discrete copy.  This also removes major 
                                 * assumptions about how the swap-pager 
                                 * works from where it doesn't belong.  The
                                 * new swapper is able to optimize the
                                 * destroy-source case.
                                 */
                                vm_object_pip_add(object, 1);
                                swap_pager_copy(backing_object, object,
                                    OFF_TO_IDX(object->backing_object_offset),
                                    TRUE);
                                vm_object_pip_wakeup(object);
                                vm_object_pip_wakeup(backing_object);
                        }

                        /*
                         * Object now shadows whatever backing_object did.
                         * Remove object from backing_object's shadow_list.
                         */
                        LIST_REMOVE(object, shadow_list);
                        KKASSERT(object->backing_object == backing_object);
                        backing_object->shadow_count--;
                        backing_object->generation++;

                        /*
                         * backing_object->backing_object moves from within
                         * backing_object to within object.
                         */
                        while ((bbobj = backing_object->backing_object) != NULL) {
                                vm_object_hold(bbobj);
                                if (bbobj == backing_object->backing_object)
                                        break;
                                vm_object_drop(bbobj);
                        }
                        if (bbobj) {
                                LIST_REMOVE(backing_object, shadow_list);
                                bbobj->shadow_count--;
                                bbobj->generation++;
                                backing_object->backing_object = NULL;
                        }
                        object->backing_object = bbobj;
                        if (bbobj) {
                                LIST_INSERT_HEAD(&bbobj->shadow_head,
                                                 object, shadow_list);
                                bbobj->shadow_count++;
                                bbobj->generation++;
                        }

                        object->backing_object_offset +=
                                backing_object->backing_object_offset;

                        vm_object_drop(bbobj);

                        /*
                         * Discard the old backing_object.  Nothing should be
                         * able to ref it, other than a vm_map_split(),
                         * and vm_map_split() will stall on our chain lock.
                         * And we control the parent so it shouldn't be
                         * possible for it to go away either.
                         *
                         * Since the backing object has no pages, no pager
                         * left, and no object references within it, all
                         * that is necessary is to dispose of it.
                         */
                        KASSERT(backing_object->ref_count == 1,
                                ("backing_object %p was somehow "
                                 "re-referenced during collapse!",
                                 backing_object));
                        KASSERT(RB_EMPTY(&backing_object->rb_memq),
                                ("backing_object %p somehow has left "
                                 "over pages during collapse!",
                                 backing_object));

                        /*
                         * The object can be destroyed.
                         *
                         * XXX just fall through and dodealloc instead
                         *     of forcing destruction?
                         */
                        --backing_object->ref_count;
                        if ((backing_object->flags & OBJ_DEAD) == 0)
                                vm_object_terminate(backing_object);
                        object_collapses++;
                        dodealloc = 0;
                } else {
                        /*
                         * If we do not entirely shadow the backing object,
                         * there is nothing we can do so we give up.
                         */
                        if (vm_object_backing_scan(object, backing_object,
                                                OBSC_TEST_ALL_SHADOWED) == 0) {
                                break;
                        }

                        /*
                         * bbobj is backing_object->backing_object.  Since
                         * object completely shadows backing_object we can
                         * bypass it and become backed by bbobj instead.
                         */
                        while ((bbobj = backing_object->backing_object) != NULL) {
                                vm_object_hold(bbobj);
                                if (bbobj == backing_object->backing_object)
                                        break;
                                vm_object_drop(bbobj);
                        }

                        /*
                         * Make object shadow bbobj instead of backing_object.
                         * Remove object from backing_object's shadow list.
                         *
                         * Deallocating backing_object will not remove
                         * it, since its reference count is at least 2.
                         */
                        KKASSERT(object->backing_object == backing_object);
                        LIST_REMOVE(object, shadow_list);
                        backing_object->shadow_count--;
                        backing_object->generation++;

                        /*
                         * Add a ref to bbobj, bbobj now shadows object.
                         *
                         * NOTE: backing_object->backing_object still points
                         *       to bbobj.  That relationship remains intact
                         *       because backing_object has > 1 ref, so
                         *       someone else is pointing to it (hence why
                         *       we can't collapse it into object and can
                         *       only handle the all-shadowed bypass case).
                         */
                        if (bbobj) {
                                vm_object_chain_wait(bbobj);
                                vm_object_reference_locked(bbobj);
                                LIST_INSERT_HEAD(&bbobj->shadow_head,
                                                 object, shadow_list);
                                bbobj->shadow_count++;
                                bbobj->generation++;
                                object->backing_object_offset +=
                                        backing_object->backing_object_offset;
                                object->backing_object = bbobj;
                                vm_object_drop(bbobj);
                        } else {
                                object->backing_object = NULL;
                        }

                        /*
                         * Drop the reference count on backing_object.  To
                         * handle ref_count races properly we can't assume
                         * that the ref_count is still at least 2 so we
                         * have to actually call vm_object_deallocate()
                         * (after clearing the chainlock).
                         */
                        object_bypasses++;
                        dodealloc = 1;
                }

                /*
                 * Ok, we want to loop on the new object->bbobj association,
                 * possibly collapsing it further.  However if dodealloc is
                 * non-zero we have to deallocate the backing_object which
                 * itself can potentially undergo a collapse, creating a
                 * recursion depth issue with the LWKT token subsystem.
                 *
                 * In the case where we must deallocate the backing_object
                 * it is possible now that the backing_object has a single
                 * shadow count on some other object (not represented here
                 * as yet), since it no longer shadows us.  Thus when we
                 * call vm_object_deallocate() it may attempt to collapse
                 * itself into its remaining parent.
                 */
                if (dodealloc) {
                        struct vm_object_dealloc_list *dtmp;

                        vm_object_chain_release(backing_object);
                        vm_object_unlock(backing_object);
                        /* backing_object remains held */

                        /*
                         * Auto-deallocation list for caller convenience.
                         */
                        if (dlistp == NULL)
                                dlistp = &dlist;

                        dtmp = kmalloc(sizeof(*dtmp), M_TEMP, M_WAITOK);
                        dtmp->object = backing_object;
                        dtmp->next = *dlistp;
                        *dlistp = dtmp;
                } else {
                        vm_object_chain_release(backing_object);
                        vm_object_drop(backing_object);
                }
                /* backing_object = NULL; not needed */
                /* loop */
        }

        /*
         * Clean up any left over backing_object
         */
        if (backing_object) {
                vm_object_chain_release(backing_object);
                vm_object_drop(backing_object);
        }

        /*
         * Clean up any auto-deallocation list.  This is a convenience
         * for top-level callers so they don't have to pass &dlist.
         * Do not clean up any caller-passed dlistp, the caller will
         * do that.
         */
        if (dlist)
                vm_object_deallocate_list(&dlist);

}

/*
 * vm_object_collapse() may collect additional objects in need of
 * deallocation.  This routine deallocates these objects.  The
 * deallocation itself can trigger additional collapses (which the
 * deallocate function takes care of).  This procedure is used to
 * reduce procedural recursion since these vm_object shadow chains
 * can become quite long.
 */
void
vm_object_deallocate_list(struct vm_object_dealloc_list **dlistp)
{
        struct vm_object_dealloc_list *dlist;

        while ((dlist = *dlistp) != NULL) {
                *dlistp = dlist->next;
                vm_object_lock(dlist->object);
                vm_object_deallocate_locked(dlist->object);
                vm_object_drop(dlist->object);
                kfree(dlist, M_TEMP);
        }
}

/*
 * Removes all physical pages in the specified object range from the
 * object's list of pages.
 *
 * No requirements.
 */
static int vm_object_page_remove_callback(vm_page_t p, void *data);

void
vm_object_page_remove(vm_object_t object, vm_pindex_t start, vm_pindex_t end,
                      boolean_t clean_only)
{
        struct rb_vm_page_scan_info info;
        int all;

        /*
         * Degenerate cases and assertions
         */
        vm_object_hold(object);
        if (object == NULL ||
            (object->resident_page_count == 0 && object->swblock_count == 0)) {
                vm_object_drop(object);
                return;
        }
        KASSERT(object->type != OBJT_PHYS, 
                ("attempt to remove pages from a physical object"));

        /*
         * Indicate that paging is occuring on the object
         */
        vm_object_pip_add(object, 1);

        /*
         * Figure out the actual removal range and whether we are removing
         * the entire contents of the object or not.  If removing the entire
         * contents, be sure to get all pages, even those that might be 
         * beyond the end of the object.
         */
        info.start_pindex = start;
        if (end == 0)
                info.end_pindex = (vm_pindex_t)-1;
        else
                info.end_pindex = end - 1;
        info.limit = clean_only;
        all = (start == 0 && info.end_pindex >= object->size - 1);

        /*
         * Loop until we are sure we have gotten them all.
         */
        do {
                info.error = 0;
                vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                        vm_object_page_remove_callback, &info);
        } while (info.error);

        /*
         * Remove any related swap if throwing away pages, or for
         * non-swap objects (the swap is a clean copy in that case).
         */
        if (object->type != OBJT_SWAP || clean_only == FALSE) {
                if (all)
                        swap_pager_freespace_all(object);
                else
                        swap_pager_freespace(object, info.start_pindex,
                             info.end_pindex - info.start_pindex + 1);
        }

        /*
         * Cleanup
         */
        vm_object_pip_wakeup(object);
        vm_object_drop(object);
}

/*
 * The caller must hold the object
 */
static int
vm_object_page_remove_callback(vm_page_t p, void *data)
{
        struct rb_vm_page_scan_info *info = data;

        if (vm_page_busy_try(p, TRUE)) {
                vm_page_sleep_busy(p, TRUE, "vmopar");
                info->error = 1;
                return(0);
        }

        /*
         * Wired pages cannot be destroyed, but they can be invalidated
         * and we do so if clean_only (limit) is not set.
         *
         * WARNING!  The page may be wired due to being part of a buffer
         *           cache buffer, and the buffer might be marked B_CACHE.
         *           This is fine as part of a truncation but VFSs must be
         *           sure to fix the buffer up when re-extending the file.
         *
         * NOTE!     PG_NEED_COMMIT is ignored.
         */
        if (p->wire_count != 0) {
                vm_page_protect(p, VM_PROT_NONE);
                if (info->limit == 0)
                        p->valid = 0;
                vm_page_wakeup(p);
                return(0);
        }

        /*
         * limit is our clean_only flag.  If set and the page is dirty or
         * requires a commit, do not free it.  If set and the page is being
         * held by someone, do not free it.
         */
        if (info->limit && p->valid) {
                vm_page_test_dirty(p);
                if ((p->valid & p->dirty) || (p->flags & PG_NEED_COMMIT)) {
                        vm_page_wakeup(p);
                        return(0);
                }
#if 0
                if (p->hold_count) {
                        vm_page_wakeup(p);
                        return(0);
                }
#endif
        }

        /*
         * Destroy the page
         */
        vm_page_protect(p, VM_PROT_NONE);
        vm_page_free(p);
        return(0);
}

/*
 * Coalesces two objects backing up adjoining regions of memory into a
 * single object.
 *
 * returns TRUE if objects were combined.
 *
 * NOTE: Only works at the moment if the second object is NULL -
 *       if it's not, which object do we lock first?
 *
 * Parameters:
 *      prev_object     First object to coalesce
 *      prev_offset     Offset into prev_object
 *      next_object     Second object into coalesce
 *      next_offset     Offset into next_object
 *
 *      prev_size       Size of reference to prev_object
 *      next_size       Size of reference to next_object
 *
 * The caller does not need to hold (prev_object) but must have a stable
 * pointer to it (typically by holding the vm_map locked).
 */
boolean_t
vm_object_coalesce(vm_object_t prev_object, vm_pindex_t prev_pindex,
                   vm_size_t prev_size, vm_size_t next_size)
{
        vm_pindex_t next_pindex;

        if (prev_object == NULL)
                return (TRUE);

        vm_object_hold(prev_object);

        if (prev_object->type != OBJT_DEFAULT &&
            prev_object->type != OBJT_SWAP) {
                vm_object_drop(prev_object);
                return (FALSE);
        }

        /*
         * Try to collapse the object first
         */
        vm_object_chain_acquire(prev_object);
        vm_object_collapse(prev_object, NULL);

        /*
         * Can't coalesce if: . more than one reference . paged out . shadows
         * another object . has a copy elsewhere (any of which mean that the
         * pages not mapped to prev_entry may be in use anyway)
         */

        if (prev_object->backing_object != NULL) {
                vm_object_chain_release(prev_object);
                vm_object_drop(prev_object);
                return (FALSE);
        }

        prev_size >>= PAGE_SHIFT;
        next_size >>= PAGE_SHIFT;
        next_pindex = prev_pindex + prev_size;

        if ((prev_object->ref_count > 1) &&
            (prev_object->size != next_pindex)) {
                vm_object_chain_release(prev_object);
                vm_object_drop(prev_object);
                return (FALSE);
        }

        /*
         * Remove any pages that may still be in the object from a previous
         * deallocation.
         */
        if (next_pindex < prev_object->size) {
                vm_object_page_remove(prev_object,
                                      next_pindex,
                                      next_pindex + next_size, FALSE);
                if (prev_object->type == OBJT_SWAP)
                        swap_pager_freespace(prev_object,
                                             next_pindex, next_size);
        }

        /*
         * Extend the object if necessary.
         */
        if (next_pindex + next_size > prev_object->size)
                prev_object->size = next_pindex + next_size;

        vm_object_chain_release(prev_object);
        vm_object_drop(prev_object);
        return (TRUE);
}

/*
 * Make the object writable and flag is being possibly dirty.
 *
 * The caller must hold the object. XXX called from vm_page_dirty(),
 * There is currently no requirement to hold the object.
 */
void
vm_object_set_writeable_dirty(vm_object_t object)
{
        struct vnode *vp;

        /*vm_object_assert_held(object);*/
        /*
         * Avoid contention in vm fault path by checking the state before
         * issuing an atomic op on it.
         */
        if ((object->flags & (OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY)) !=
            (OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY)) {
                vm_object_set_flag(object, OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY);
        }
        if (object->type == OBJT_VNODE &&
            (vp = (struct vnode *)object->handle) != NULL) {
                if ((vp->v_flag & VOBJDIRTY) == 0) {
                        vsetflags(vp, VOBJDIRTY);
                }
        }
}

#include "opt_ddb.h"
#ifdef DDB
#include <sys/kernel.h>

#include <sys/cons.h>

#include <ddb/ddb.h>

static int      _vm_object_in_map (vm_map_t map, vm_object_t object,
                                       vm_map_entry_t entry);
static int      vm_object_in_map (vm_object_t object);

/*
 * The caller must hold the object.
 */
static int
_vm_object_in_map(vm_map_t map, vm_object_t object, vm_map_entry_t entry)
{
        vm_map_t tmpm;
        vm_map_entry_t tmpe;
        vm_object_t obj, nobj;
        int entcount;

        if (map == 0)
                return 0;
        if (entry == 0) {
                tmpe = map->header.next;
                entcount = map->nentries;
                while (entcount-- && (tmpe != &map->header)) {
                        if( _vm_object_in_map(map, object, tmpe)) {
                                return 1;
                        }
                        tmpe = tmpe->next;
                }
                return (0);
        }
        switch(entry->maptype) {
        case VM_MAPTYPE_SUBMAP:
                tmpm = entry->object.sub_map;
                tmpe = tmpm->header.next;
                entcount = tmpm->nentries;
                while (entcount-- && tmpe != &tmpm->header) {
                        if( _vm_object_in_map(tmpm, object, tmpe)) {
                                return 1;
                        }
                        tmpe = tmpe->next;
                }
                break;
        case VM_MAPTYPE_NORMAL:
        case VM_MAPTYPE_VPAGETABLE:
                obj = entry->object.vm_object;
                while (obj) {
                        if (obj == object) {
                                if (obj != entry->object.vm_object)
                                        vm_object_drop(obj);
                                return 1;
                        }
                        while ((nobj = obj->backing_object) != NULL) {
                                vm_object_hold(nobj);
                                if (nobj == obj->backing_object)
                                        break;
                                vm_object_drop(nobj);
                        }
                        if (obj != entry->object.vm_object) {
                                if (nobj)
                                        vm_object_lock_swap();
                                vm_object_drop(obj);
                        }
                        obj = nobj;
                }
                break;
        default:
                break;
        }
        return 0;
}

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

struct vm_object_in_map_info {
        vm_object_t object;
        int rv;
};

/*
 * Debugging only
 */
static int
vm_object_in_map(vm_object_t object)
{
        struct vm_object_in_map_info info;

        info.rv = 0;
        info.object = object;

        allproc_scan(vm_object_in_map_callback, &info);
        if (info.rv)
                return 1;
        if( _vm_object_in_map(&kernel_map, object, 0))
                return 1;
        if( _vm_object_in_map(&pager_map, object, 0))
                return 1;
        if( _vm_object_in_map(&buffer_map, object, 0))
                return 1;
        return 0;
}

/*
 * Debugging only
 */
static int
vm_object_in_map_callback(struct proc *p, void *data)
{
        struct vm_object_in_map_info *info = data;

        if (p->p_vmspace) {
                if (_vm_object_in_map(&p->p_vmspace->vm_map, info->object, 0)) {
                        info->rv = 1;
                        return -1;
                }
        }
        return (0);
}

DB_SHOW_COMMAND(vmochk, vm_object_check)
{
        vm_object_t object;

        /*
         * make sure that internal objs are in a map somewhere
         * and none have zero ref counts.
         */
        for (object = TAILQ_FIRST(&vm_object_list);
                        object != NULL;
                        object = TAILQ_NEXT(object, object_list)) {
                if (object->type == OBJT_MARKER)
                        continue;
                if (object->handle == NULL &&
                    (object->type == OBJT_DEFAULT || object->type == OBJT_SWAP)) {
                        if (object->ref_count == 0) {
                                db_printf("vmochk: internal obj has zero ref count: %ld\n",
                                        (long)object->size);
                        }
                        if (!vm_object_in_map(object)) {
                                db_printf(
                        "vmochk: internal obj is not in a map: "
                        "ref: %d, size: %lu: 0x%lx, backing_object: %p\n",
                                    object->ref_count, (u_long)object->size, 
                                    (u_long)object->size,
                                    (void *)object->backing_object);
                        }
                }
        }
}

/*
 * Debugging only
 */
DB_SHOW_COMMAND(object, vm_object_print_static)
{
        /* XXX convert args. */
        vm_object_t object = (vm_object_t)addr;
        boolean_t full = have_addr;

        vm_page_t p;

        /* XXX count is an (unused) arg.  Avoid shadowing it. */
#define count   was_count

        int count;

        if (object == NULL)
                return;

        db_iprintf(
            "Object %p: type=%d, size=0x%lx, res=%d, ref=%d, flags=0x%x\n",
            object, (int)object->type, (u_long)object->size,
            object->resident_page_count, object->ref_count, object->flags);
        /*
         * XXX no %qd in kernel.  Truncate object->backing_object_offset.
         */
        db_iprintf(" sref=%d, backing_object(%d)=(%p)+0x%lx\n",
            object->shadow_count, 
            object->backing_object ? object->backing_object->ref_count : 0,
            object->backing_object, (long)object->backing_object_offset);

        if (!full)
                return;

        db_indent += 2;
        count = 0;
        RB_FOREACH(p, vm_page_rb_tree, &object->rb_memq) {
                if (count == 0)
                        db_iprintf("memory:=");
                else if (count == 6) {
                        db_printf("\n");
                        db_iprintf(" ...");
                        count = 0;
                } else
                        db_printf(",");
                count++;

                db_printf("(off=0x%lx,page=0x%lx)",
                    (u_long) p->pindex, (u_long) VM_PAGE_TO_PHYS(p));
        }
        if (count != 0)
                db_printf("\n");
        db_indent -= 2;
}

/* XXX. */
#undef count

/*
 * XXX need this non-static entry for calling from vm_map_print.
 *
 * Debugging only
 */
void
vm_object_print(/* db_expr_t */ long addr,
                boolean_t have_addr,
                /* db_expr_t */ long count,
                char *modif)
{
        vm_object_print_static(addr, have_addr, count, modif);
}

/*
 * Debugging only
 */
DB_SHOW_COMMAND(vmopag, vm_object_print_pages)
{
        vm_object_t object;
        int nl = 0;
        int c;
        for (object = TAILQ_FIRST(&vm_object_list);
                        object != NULL;
                        object = TAILQ_NEXT(object, object_list)) {
                vm_pindex_t idx, fidx;
                vm_pindex_t osize;
                vm_paddr_t pa = -1, padiff;
                int rcount;
                vm_page_t m;

                if (object->type == OBJT_MARKER)
                        continue;
                db_printf("new object: %p\n", (void *)object);
                if ( nl > 18) {
                        c = cngetc();
                        if (c != ' ')
                                return;
                        nl = 0;
                }
                nl++;
                rcount = 0;
                fidx = 0;
                osize = object->size;
                if (osize > 128)
                        osize = 128;
                for (idx = 0; idx < osize; idx++) {
                        m = vm_page_lookup(object, idx);
                        if (m == NULL) {
                                if (rcount) {
                                        db_printf(" index(%ld)run(%d)pa(0x%lx)\n",
                                                (long)fidx, rcount, (long)pa);
                                        if ( nl > 18) {
                                                c = cngetc();
                                                if (c != ' ')
                                                        return;
                                                nl = 0;
                                        }
                                        nl++;
                                        rcount = 0;
                                }
                                continue;
                        }

                                
                        if (rcount &&
                                (VM_PAGE_TO_PHYS(m) == pa + rcount * PAGE_SIZE)) {
                                ++rcount;
                                continue;
                        }
                        if (rcount) {
                                padiff = pa + rcount * PAGE_SIZE - VM_PAGE_TO_PHYS(m);
                                padiff >>= PAGE_SHIFT;
                                padiff &= PQ_L2_MASK;
                                if (padiff == 0) {
                                        pa = VM_PAGE_TO_PHYS(m) - rcount * PAGE_SIZE;
                                        ++rcount;
                                        continue;
                                }
                                db_printf(" index(%ld)run(%d)pa(0x%lx)",
                                        (long)fidx, rcount, (long)pa);
                                db_printf("pd(%ld)\n", (long)padiff);
                                if ( nl > 18) {
                                        c = cngetc();
                                        if (c != ' ')
                                                return;
                                        nl = 0;
                                }
                                nl++;
                        }
                        fidx = idx;
                        pa = VM_PAGE_TO_PHYS(m);
                        rcount = 1;
                }
                if (rcount) {
                        db_printf(" index(%ld)run(%d)pa(0x%lx)\n",
                                (long)fidx, rcount, (long)pa);
                        if ( nl > 18) {
                                c = cngetc();
                                if (c != ' ')
                                        return;
                                nl = 0;
                        }
                        nl++;
                }
        }
}
#endif /* DDB */