Update comment describing struct vm_map

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

/*-
 * SPDX-License-Identifier: (BSD-3-Clause AND MIT-CMU)
 *
 * 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. 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.
 */

/*
 *      Virtual memory object module.
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#include "opt_vm.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/blockcount.h>
#include <sys/cpuset.h>
#include <sys/limits.h>
#include <sys/lock.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/kernel.h>
#include <sys/pctrie.h>
#include <sys/sysctl.h>
#include <sys/mutex.h>
#include <sys/proc.h>           /* for curproc, pageproc */
#include <sys/refcount.h>
#include <sys/socket.h>
#include <sys/resourcevar.h>
#include <sys/refcount.h>
#include <sys/rwlock.h>
#include <sys/user.h>
#include <sys/vnode.h>
#include <sys/vmmeter.h>
#include <sys/sx.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/vm_phys.h>
#include <vm/vm_pagequeue.h>
#include <vm/swap_pager.h>
#include <vm/vm_kern.h>
#include <vm/vm_extern.h>
#include <vm/vm_radix.h>
#include <vm/vm_reserv.h>
#include <vm/uma.h>

static int old_msync;
SYSCTL_INT(_vm, OID_AUTO, old_msync, CTLFLAG_RW, &old_msync, 0,
    "Use old (insecure) msync behavior");

static int      vm_object_page_collect_flush(vm_object_t object, vm_page_t p,
                    int pagerflags, int flags, boolean_t *allclean,
                    boolean_t *eio);
static boolean_t vm_object_page_remove_write(vm_page_t p, int flags,
                    boolean_t *allclean);
static void     vm_object_backing_remove(vm_object_t object);

/*
 *      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;
struct mtx vm_object_list_mtx;  /* lock for object list and count */

struct vm_object kernel_object_store;

static SYSCTL_NODE(_vm_stats, OID_AUTO, object, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
    "VM object stats");

static COUNTER_U64_DEFINE_EARLY(object_collapses);
SYSCTL_COUNTER_U64(_vm_stats_object, OID_AUTO, collapses, CTLFLAG_RD,
    &object_collapses,
    "VM object collapses");

static COUNTER_U64_DEFINE_EARLY(object_bypasses);
SYSCTL_COUNTER_U64(_vm_stats_object, OID_AUTO, bypasses, CTLFLAG_RD,
    &object_bypasses,
    "VM object bypasses");

static COUNTER_U64_DEFINE_EARLY(object_collapse_waits);
SYSCTL_COUNTER_U64(_vm_stats_object, OID_AUTO, collapse_waits, CTLFLAG_RD,
    &object_collapse_waits,
    "Number of sleeps for collapse");

static uma_zone_t obj_zone;

static int vm_object_zinit(void *mem, int size, int flags);

#ifdef INVARIANTS
static void vm_object_zdtor(void *mem, int size, void *arg);

static void
vm_object_zdtor(void *mem, int size, void *arg)
{
        vm_object_t object;

        object = (vm_object_t)mem;
        KASSERT(object->ref_count == 0,
            ("object %p ref_count = %d", object, object->ref_count));
        KASSERT(TAILQ_EMPTY(&object->memq),
            ("object %p has resident pages in its memq", object));
        KASSERT(vm_radix_is_empty(&object->rtree),
            ("object %p has resident pages in its trie", object));
#if VM_NRESERVLEVEL > 0
        KASSERT(LIST_EMPTY(&object->rvq),
            ("object %p has reservations",
            object));
#endif
        KASSERT(!vm_object_busied(object),
            ("object %p busy = %d", object, blockcount_read(&object->busy)));
        KASSERT(object->resident_page_count == 0,
            ("object %p resident_page_count = %d",
            object, object->resident_page_count));
        KASSERT(atomic_load_int(&object->shadow_count) == 0,
            ("object %p shadow_count = %d",
            object, atomic_load_int(&object->shadow_count)));
        KASSERT(object->type == OBJT_DEAD,
            ("object %p has non-dead type %d",
            object, object->type));
        KASSERT(object->charge == 0 && object->cred == NULL,
            ("object %p has non-zero charge %ju (%p)",
            object, (uintmax_t)object->charge, object->cred));
}
#endif

static int
vm_object_zinit(void *mem, int size, int flags)
{
        vm_object_t object;

        object = (vm_object_t)mem;
        rw_init_flags(&object->lock, "vm object", RW_DUPOK | RW_NEW);

        /* These are true for any object that has been freed */
        object->type = OBJT_DEAD;
        vm_radix_init(&object->rtree);
        refcount_init(&object->ref_count, 0);
        blockcount_init(&object->paging_in_progress);
        blockcount_init(&object->busy);
        object->resident_page_count = 0;
        atomic_store_int(&object->shadow_count, 0);
        object->flags = OBJ_DEAD;

        mtx_lock(&vm_object_list_mtx);
        TAILQ_INSERT_TAIL(&vm_object_list, object, object_list);
        mtx_unlock(&vm_object_list_mtx);
        return (0);
}

static void
_vm_object_allocate(objtype_t type, vm_pindex_t size, u_short flags,
    vm_object_t object, void *handle)
{

        TAILQ_INIT(&object->memq);
        LIST_INIT(&object->shadow_head);

        object->type = type;
        object->flags = flags;
        if ((flags & OBJ_SWAP) != 0)
                pctrie_init(&object->un_pager.swp.swp_blks);

        /*
         * Ensure that swap_pager_swapoff() iteration over object_list
         * sees up to date type and pctrie head if it observed
         * non-dead object.
         */
        atomic_thread_fence_rel();

        object->pg_color = 0;
        object->size = size;
        object->domain.dr_policy = NULL;
        object->generation = 1;
        object->cleangeneration = 1;
        refcount_init(&object->ref_count, 1);
        object->memattr = VM_MEMATTR_DEFAULT;
        object->cred = NULL;
        object->charge = 0;
        object->handle = handle;
        object->backing_object = NULL;
        object->backing_object_offset = (vm_ooffset_t) 0;
#if VM_NRESERVLEVEL > 0
        LIST_INIT(&object->rvq);
#endif
        umtx_shm_object_init(object);
}

/*
 *      vm_object_init:
 *
 *      Initialize the VM objects module.
 */
void
vm_object_init(void)
{
        TAILQ_INIT(&vm_object_list);
        mtx_init(&vm_object_list_mtx, "vm object_list", NULL, MTX_DEF);

        rw_init(&kernel_object->lock, "kernel vm object");
        _vm_object_allocate(OBJT_PHYS, atop(VM_MAX_KERNEL_ADDRESS -
            VM_MIN_KERNEL_ADDRESS), OBJ_UNMANAGED, kernel_object, NULL);
#if VM_NRESERVLEVEL > 0
        kernel_object->flags |= OBJ_COLORED;
        kernel_object->pg_color = (u_short)atop(VM_MIN_KERNEL_ADDRESS);
#endif
        kernel_object->un_pager.phys.ops = &default_phys_pg_ops;

        /*
         * The lock portion of struct vm_object must be type stable due
         * to vm_pageout_fallback_object_lock locking a vm object
         * without holding any references to it.
         *
         * paging_in_progress is valid always.  Lockless references to
         * the objects may acquire pip and then check OBJ_DEAD.
         */
        obj_zone = uma_zcreate("VM OBJECT", sizeof (struct vm_object), NULL,
#ifdef INVARIANTS
            vm_object_zdtor,
#else
            NULL,
#endif
            vm_object_zinit, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);

        vm_radix_zinit();
}

void
vm_object_clear_flag(vm_object_t object, u_short bits)
{

        VM_OBJECT_ASSERT_WLOCKED(object);
        object->flags &= ~bits;
}

/*
 *      Sets the default memory attribute for the specified object.  Pages
 *      that are allocated to this object are by default assigned this memory
 *      attribute.
 *
 *      Presently, this function must be called before any pages are allocated
 *      to the object.  In the future, this requirement may be relaxed for
 *      "default" and "swap" objects.
 */
int
vm_object_set_memattr(vm_object_t object, vm_memattr_t memattr)
{

        VM_OBJECT_ASSERT_WLOCKED(object);

        if (object->type == OBJT_DEAD)
                return (KERN_INVALID_ARGUMENT);
        if (!TAILQ_EMPTY(&object->memq))
                return (KERN_FAILURE);

        object->memattr = memattr;
        return (KERN_SUCCESS);
}

void
vm_object_pip_add(vm_object_t object, short i)
{

        if (i > 0)
                blockcount_acquire(&object->paging_in_progress, i);
}

void
vm_object_pip_wakeup(vm_object_t object)
{

        vm_object_pip_wakeupn(object, 1);
}

void
vm_object_pip_wakeupn(vm_object_t object, short i)
{

        if (i > 0)
                blockcount_release(&object->paging_in_progress, i);
}

/*
 * Atomically drop the object lock and wait for pip to drain.  This protects
 * from sleep/wakeup races due to identity changes.  The lock is not re-acquired
 * on return.
 */
static void
vm_object_pip_sleep(vm_object_t object, const char *waitid)
{

        (void)blockcount_sleep(&object->paging_in_progress, &object->lock,
            waitid, PVM | PDROP);
}

void
vm_object_pip_wait(vm_object_t object, const char *waitid)
{

        VM_OBJECT_ASSERT_WLOCKED(object);

        blockcount_wait(&object->paging_in_progress, &object->lock, waitid,
            PVM);
}

void
vm_object_pip_wait_unlocked(vm_object_t object, const char *waitid)
{

        VM_OBJECT_ASSERT_UNLOCKED(object);

        blockcount_wait(&object->paging_in_progress, NULL, waitid, PVM);
}

/*
 *      vm_object_allocate:
 *
 *      Returns a new object with the given size.
 */
vm_object_t
vm_object_allocate(objtype_t type, vm_pindex_t size)
{
        vm_object_t object;
        u_short flags;

        switch (type) {
        case OBJT_DEAD:
                panic("vm_object_allocate: can't create OBJT_DEAD");
        case OBJT_DEFAULT:
                flags = OBJ_COLORED;
                break;
        case OBJT_SWAP:
                flags = OBJ_COLORED | OBJ_SWAP;
                break;
        case OBJT_DEVICE:
        case OBJT_SG:
                flags = OBJ_FICTITIOUS | OBJ_UNMANAGED;
                break;
        case OBJT_MGTDEVICE:
                flags = OBJ_FICTITIOUS;
                break;
        case OBJT_PHYS:
                flags = OBJ_UNMANAGED;
                break;
        case OBJT_VNODE:
                flags = 0;
                break;
        default:
                panic("vm_object_allocate: type %d is undefined or dynamic",
                    type);
        }
        object = (vm_object_t)uma_zalloc(obj_zone, M_WAITOK);
        _vm_object_allocate(type, size, flags, object, NULL);

        return (object);
}

vm_object_t
vm_object_allocate_dyn(objtype_t dyntype, vm_pindex_t size, u_short flags)
{
        vm_object_t object;

        MPASS(dyntype >= OBJT_FIRST_DYN /* && dyntype < nitems(pagertab) */);
        object = (vm_object_t)uma_zalloc(obj_zone, M_WAITOK);
        _vm_object_allocate(dyntype, size, flags, object, NULL);

        return (object);
}

/*
 *      vm_object_allocate_anon:
 *
 *      Returns a new default object of the given size and marked as
 *      anonymous memory for special split/collapse handling.  Color
 *      to be initialized by the caller.
 */
vm_object_t
vm_object_allocate_anon(vm_pindex_t size, vm_object_t backing_object,
    struct ucred *cred, vm_size_t charge)
{
        vm_object_t handle, object;

        if (backing_object == NULL)
                handle = NULL;
        else if ((backing_object->flags & OBJ_ANON) != 0)
                handle = backing_object->handle;
        else
                handle = backing_object;
        object = uma_zalloc(obj_zone, M_WAITOK);
        _vm_object_allocate(OBJT_DEFAULT, size, OBJ_ANON | OBJ_ONEMAPPING,
            object, handle);
        object->cred = cred;
        object->charge = cred != NULL ? charge : 0;
        return (object);
}

static void
vm_object_reference_vnode(vm_object_t object)
{
        u_int old;

        /*
         * vnode objects need the lock for the first reference
         * to serialize with vnode_object_deallocate().
         */
        if (!refcount_acquire_if_gt(&object->ref_count, 0)) {
                VM_OBJECT_RLOCK(object);
                old = refcount_acquire(&object->ref_count);
                if (object->type == OBJT_VNODE && old == 0)
                        vref(object->handle);
                VM_OBJECT_RUNLOCK(object);
        }
}

/*
 *      vm_object_reference:
 *
 *      Acquires a reference to the given object.
 */
void
vm_object_reference(vm_object_t object)
{

        if (object == NULL)
                return;

        if (object->type == OBJT_VNODE)
                vm_object_reference_vnode(object);
        else
                refcount_acquire(&object->ref_count);
        KASSERT((object->flags & OBJ_DEAD) == 0,
            ("vm_object_reference: Referenced dead object."));
}

/*
 *      vm_object_reference_locked:
 *
 *      Gets another reference to the given object.
 *
 *      The object must be locked.
 */
void
vm_object_reference_locked(vm_object_t object)
{
        u_int old;

        VM_OBJECT_ASSERT_LOCKED(object);
        old = refcount_acquire(&object->ref_count);
        if (object->type == OBJT_VNODE && old == 0)
                vref(object->handle);
        KASSERT((object->flags & OBJ_DEAD) == 0,
            ("vm_object_reference: Referenced dead object."));
}

/*
 * Handle deallocating an object of type OBJT_VNODE.
 */
static void
vm_object_deallocate_vnode(vm_object_t object)
{
        struct vnode *vp = (struct vnode *) object->handle;
        bool last;

        KASSERT(object->type == OBJT_VNODE,
            ("vm_object_deallocate_vnode: not a vnode object"));
        KASSERT(vp != NULL, ("vm_object_deallocate_vnode: missing vp"));

        /* Object lock to protect handle lookup. */
        last = refcount_release(&object->ref_count);
        VM_OBJECT_RUNLOCK(object);

        if (!last)
                return;

        if (!umtx_shm_vnobj_persistent)
                umtx_shm_object_terminated(object);

        /* vrele may need the vnode lock. */
        vrele(vp);
}

/*
 * We dropped a reference on an object and discovered that it had a
 * single remaining shadow.  This is a sibling of the reference we
 * dropped.  Attempt to collapse the sibling and backing object.
 */
static vm_object_t
vm_object_deallocate_anon(vm_object_t backing_object)
{
        vm_object_t object;

        /* Fetch the final shadow.  */
        object = LIST_FIRST(&backing_object->shadow_head);
        KASSERT(object != NULL &&
            atomic_load_int(&backing_object->shadow_count) == 1,
            ("vm_object_anon_deallocate: ref_count: %d, shadow_count: %d",
            backing_object->ref_count,
            atomic_load_int(&backing_object->shadow_count)));
        KASSERT((object->flags & OBJ_ANON) != 0,
            ("invalid shadow object %p", object));

        if (!VM_OBJECT_TRYWLOCK(object)) {
                /*
                 * Prevent object from disappearing since we do not have a
                 * reference.
                 */
                vm_object_pip_add(object, 1);
                VM_OBJECT_WUNLOCK(backing_object);
                VM_OBJECT_WLOCK(object);
                vm_object_pip_wakeup(object);
        } else
                VM_OBJECT_WUNLOCK(backing_object);

        /*
         * Check for a collapse/terminate race with the last reference holder.
         */
        if ((object->flags & (OBJ_DEAD | OBJ_COLLAPSING)) != 0 ||
            !refcount_acquire_if_not_zero(&object->ref_count)) {
                VM_OBJECT_WUNLOCK(object);
                return (NULL);
        }
        backing_object = object->backing_object;
        if (backing_object != NULL && (backing_object->flags & OBJ_ANON) != 0)
                vm_object_collapse(object);
        VM_OBJECT_WUNLOCK(object);

        return (object);
}

/*
 *      vm_object_deallocate:
 *
 *      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.
 *
 *      No object may be locked.
 */
void
vm_object_deallocate(vm_object_t object)
{
        vm_object_t temp;
        bool released;

        while (object != NULL) {
                /*
                 * If the reference count goes to 0 we start calling
                 * vm_object_terminate() on the object chain.  A ref count
                 * of 1 may be a special case depending on the shadow count
                 * being 0 or 1.  These cases require a write lock on the
                 * object.
                 */
                if ((object->flags & OBJ_ANON) == 0)
                        released = refcount_release_if_gt(&object->ref_count, 1);
                else
                        released = refcount_release_if_gt(&object->ref_count, 2);
                if (released)
                        return;

                if (object->type == OBJT_VNODE) {
                        VM_OBJECT_RLOCK(object);
                        if (object->type == OBJT_VNODE) {
                                vm_object_deallocate_vnode(object);
                                return;
                        }
                        VM_OBJECT_RUNLOCK(object);
                }

                VM_OBJECT_WLOCK(object);
                KASSERT(object->ref_count > 0,
                    ("vm_object_deallocate: object deallocated too many times: %d",
                    object->type));

                /*
                 * If this is not the final reference to an anonymous
                 * object we may need to collapse the shadow chain.
                 */
                if (!refcount_release(&object->ref_count)) {
                        if (object->ref_count > 1 ||
                            atomic_load_int(&object->shadow_count) == 0) {
                                if ((object->flags & OBJ_ANON) != 0 &&
                                    object->ref_count == 1)
                                        vm_object_set_flag(object,
                                            OBJ_ONEMAPPING);
                                VM_OBJECT_WUNLOCK(object);
                                return;
                        }

                        /* Handle collapsing last ref on anonymous objects. */
                        object = vm_object_deallocate_anon(object);
                        continue;
                }

                /*
                 * Handle the final reference to an object.  We restart
                 * the loop with the backing object to avoid recursion.
                 */
                umtx_shm_object_terminated(object);
                temp = object->backing_object;
                if (temp != NULL) {
                        KASSERT(object->type == OBJT_DEFAULT ||
                            object->type == OBJT_SWAP,
                            ("shadowed tmpfs v_object 2 %p", object));
                        vm_object_backing_remove(object);
                }

                KASSERT((object->flags & OBJ_DEAD) == 0,
                    ("vm_object_deallocate: Terminating dead object."));
                vm_object_set_flag(object, OBJ_DEAD);
                vm_object_terminate(object);
                object = temp;
        }
}

/*
 *      vm_object_destroy removes the object from the global object list
 *      and frees the space for the object.
 */
void
vm_object_destroy(vm_object_t object)
{

        /*
         * Release the allocation charge.
         */
        if (object->cred != NULL) {
                swap_release_by_cred(object->charge, object->cred);
                object->charge = 0;
                crfree(object->cred);
                object->cred = NULL;
        }

        /*
         * Free the space for the object.
         */
        uma_zfree(obj_zone, object);
}

static void
vm_object_sub_shadow(vm_object_t object)
{
        KASSERT(object->shadow_count >= 1,
            ("object %p sub_shadow count zero", object));
        atomic_subtract_int(&object->shadow_count, 1);
}

static void
vm_object_backing_remove_locked(vm_object_t object)
{
        vm_object_t backing_object;

        backing_object = object->backing_object;
        VM_OBJECT_ASSERT_WLOCKED(object);
        VM_OBJECT_ASSERT_WLOCKED(backing_object);

        KASSERT((object->flags & OBJ_COLLAPSING) == 0,
            ("vm_object_backing_remove: Removing collapsing object."));

        vm_object_sub_shadow(backing_object);
        if ((object->flags & OBJ_SHADOWLIST) != 0) {
                LIST_REMOVE(object, shadow_list);
                vm_object_clear_flag(object, OBJ_SHADOWLIST);
        }
        object->backing_object = NULL;
}

static void
vm_object_backing_remove(vm_object_t object)
{
        vm_object_t backing_object;

        VM_OBJECT_ASSERT_WLOCKED(object);

        backing_object = object->backing_object;
        if ((object->flags & OBJ_SHADOWLIST) != 0) {
                VM_OBJECT_WLOCK(backing_object);
                vm_object_backing_remove_locked(object);
                VM_OBJECT_WUNLOCK(backing_object);
        } else {
                object->backing_object = NULL;
                vm_object_sub_shadow(backing_object);
        }
}

static void
vm_object_backing_insert_locked(vm_object_t object, vm_object_t backing_object)
{

        VM_OBJECT_ASSERT_WLOCKED(object);

        atomic_add_int(&backing_object->shadow_count, 1);
        if ((backing_object->flags & OBJ_ANON) != 0) {
                VM_OBJECT_ASSERT_WLOCKED(backing_object);
                LIST_INSERT_HEAD(&backing_object->shadow_head, object,
                    shadow_list);
                vm_object_set_flag(object, OBJ_SHADOWLIST);
        }
        object->backing_object = backing_object;
}

static void
vm_object_backing_insert(vm_object_t object, vm_object_t backing_object)
{

        VM_OBJECT_ASSERT_WLOCKED(object);

        if ((backing_object->flags & OBJ_ANON) != 0) {
                VM_OBJECT_WLOCK(backing_object);
                vm_object_backing_insert_locked(object, backing_object);
                VM_OBJECT_WUNLOCK(backing_object);
        } else {
                object->backing_object = backing_object;
                atomic_add_int(&backing_object->shadow_count, 1);
        }
}

/*
 * Insert an object into a backing_object's shadow list with an additional
 * reference to the backing_object added.
 */
static void
vm_object_backing_insert_ref(vm_object_t object, vm_object_t backing_object)
{

        VM_OBJECT_ASSERT_WLOCKED(object);

        if ((backing_object->flags & OBJ_ANON) != 0) {
                VM_OBJECT_WLOCK(backing_object);
                KASSERT((backing_object->flags & OBJ_DEAD) == 0,
                    ("shadowing dead anonymous object"));
                vm_object_reference_locked(backing_object);
                vm_object_backing_insert_locked(object, backing_object);
                vm_object_clear_flag(backing_object, OBJ_ONEMAPPING);
                VM_OBJECT_WUNLOCK(backing_object);
        } else {
                vm_object_reference(backing_object);
                atomic_add_int(&backing_object->shadow_count, 1);
                object->backing_object = backing_object;
        }
}

/*
 * Transfer a backing reference from backing_object to object.
 */
static void
vm_object_backing_transfer(vm_object_t object, vm_object_t backing_object)
{
        vm_object_t new_backing_object;

        /*
         * Note that the reference to backing_object->backing_object
         * moves from within backing_object to within object.
         */
        vm_object_backing_remove_locked(object);
        new_backing_object = backing_object->backing_object;
        if (new_backing_object == NULL)
                return;
        if ((new_backing_object->flags & OBJ_ANON) != 0) {
                VM_OBJECT_WLOCK(new_backing_object);
                vm_object_backing_remove_locked(backing_object);
                vm_object_backing_insert_locked(object, new_backing_object);
                VM_OBJECT_WUNLOCK(new_backing_object);
        } else {
                /*
                 * shadow_count for new_backing_object is left
                 * unchanged, its reference provided by backing_object
                 * is replaced by object.
                 */
                object->backing_object = new_backing_object;
                backing_object->backing_object = NULL;
        }
}

/*
 * Wait for a concurrent collapse to settle.
 */
static void
vm_object_collapse_wait(vm_object_t object)
{

        VM_OBJECT_ASSERT_WLOCKED(object);

        while ((object->flags & OBJ_COLLAPSING) != 0) {
                vm_object_pip_wait(object, "vmcolwait");
                counter_u64_add(object_collapse_waits, 1);
        }
}

/*
 * Waits for a backing object to clear a pending collapse and returns
 * it locked if it is an ANON object.
 */
static vm_object_t
vm_object_backing_collapse_wait(vm_object_t object)
{
        vm_object_t backing_object;

        VM_OBJECT_ASSERT_WLOCKED(object);

        for (;;) {
                backing_object = object->backing_object;
                if (backing_object == NULL ||
                    (backing_object->flags & OBJ_ANON) == 0)
                        return (NULL);
                VM_OBJECT_WLOCK(backing_object);
                if ((backing_object->flags & (OBJ_DEAD | OBJ_COLLAPSING)) == 0)
                        break;
                VM_OBJECT_WUNLOCK(object);
                vm_object_pip_sleep(backing_object, "vmbckwait");
                counter_u64_add(object_collapse_waits, 1);
                VM_OBJECT_WLOCK(object);
        }
        return (backing_object);
}

/*
 *      vm_object_terminate_pages removes any remaining pageable pages
 *      from the object and resets the object to an empty state.
 */
static void
vm_object_terminate_pages(vm_object_t object)
{
        vm_page_t p, p_next;

        VM_OBJECT_ASSERT_WLOCKED(object);

        /*
         * Free any remaining pageable pages.  This also removes them from the
         * paging queues.  However, don't free wired pages, just remove them
         * from the object.  Rather than incrementally removing each page from
         * the object, the page and object are reset to any empty state. 
         */
        TAILQ_FOREACH_SAFE(p, &object->memq, listq, p_next) {
                vm_page_assert_unbusied(p);
                KASSERT(p->object == object &&
                    (p->ref_count & VPRC_OBJREF) != 0,
                    ("vm_object_terminate_pages: page %p is inconsistent", p));

                p->object = NULL;
                if (vm_page_drop(p, VPRC_OBJREF) == VPRC_OBJREF) {
                        VM_CNT_INC(v_pfree);
                        vm_page_free(p);
                }
        }

        /*
         * If the object contained any pages, then reset it to an empty state.
         * None of the object's fields, including "resident_page_count", were
         * modified by the preceding loop.
         */
        if (object->resident_page_count != 0) {
                vm_radix_reclaim_allnodes(&object->rtree);
                TAILQ_INIT(&object->memq);
                object->resident_page_count = 0;
                if (object->type == OBJT_VNODE)
                        vdrop(object->handle);
        }
}

/*
 *      vm_object_terminate actually destroys the specified object, freeing
 *      up all previously used resources.
 *
 *      The object must be locked.
 *      This routine may block.
 */
void
vm_object_terminate(vm_object_t object)
{

        VM_OBJECT_ASSERT_WLOCKED(object);
        KASSERT((object->flags & OBJ_DEAD) != 0,
            ("terminating non-dead obj %p", object));
        KASSERT((object->flags & OBJ_COLLAPSING) == 0,
            ("terminating collapsing obj %p", object));
        KASSERT(object->backing_object == NULL,
            ("terminating shadow obj %p", object));

        /*
         * Wait for the pageout daemon and other current users to be
         * done with the object.  Note that new paging_in_progress
         * users can come after this wait, but they must check
         * OBJ_DEAD flag set (without unlocking the object), and avoid
         * the object being terminated.
         */
        vm_object_pip_wait(object, "objtrm");

        KASSERT(object->ref_count == 0,
            ("vm_object_terminate: object with references, ref_count=%d",
            object->ref_count));

        if ((object->flags & OBJ_PG_DTOR) == 0)
                vm_object_terminate_pages(object);

#if VM_NRESERVLEVEL > 0
        if (__predict_false(!LIST_EMPTY(&object->rvq)))
                vm_reserv_break_all(object);
#endif

        KASSERT(object->cred == NULL || object->type == OBJT_DEFAULT ||
            (object->flags & OBJ_SWAP) != 0,
            ("%s: non-swap obj %p has cred", __func__, object));

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

        vm_object_destroy(object);
}

/*
 * Make the page read-only so that we can clear the object flags.  However, if
 * this is a nosync mmap then the object is likely to stay dirty so do not
 * mess with the page and do not clear the object flags.  Returns TRUE if the
 * page should be flushed, and FALSE otherwise.
 */
static boolean_t
vm_object_page_remove_write(vm_page_t p, int flags, boolean_t *allclean)
{

        vm_page_assert_busied(p);

        /*
         * 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 so we do not have to set them.
         */
        if ((flags & OBJPC_NOSYNC) != 0 && (p->a.flags & PGA_NOSYNC) != 0) {
                *allclean = FALSE;
                return (FALSE);
        } else {
                pmap_remove_write(p);
                return (p->dirty != 0);
        }
}

/*
 *      vm_object_page_clean
 *
 *      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 PGA_NOSYNC set (originally comes from MAP_NOSYNC),
 *      leaving the object dirty.
 *
 *      For swap objects backing tmpfs regular files, do not flush anything,
 *      but remove write protection on the mapped pages to update mtime through
 *      mmaped writes.
 *
 *      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.
 *
 *      Returns FALSE if some page from the range was not written, as
 *      reported by the pager, and TRUE otherwise.
 */
boolean_t
vm_object_page_clean(vm_object_t object, vm_ooffset_t start, vm_ooffset_t end,
    int flags)
{
        vm_page_t np, p;
        vm_pindex_t pi, tend, tstart;
        int curgeneration, n, pagerflags;
        boolean_t eio, res, allclean;

        VM_OBJECT_ASSERT_WLOCKED(object);

        if (!vm_object_mightbedirty(object) || object->resident_page_count == 0)
                return (TRUE);

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

        tstart = OFF_TO_IDX(start);
        tend = (end == 0) ? object->size : OFF_TO_IDX(end + PAGE_MASK);
        allclean = tstart == 0 && tend >= object->size;
        res = TRUE;

rescan:
        curgeneration = object->generation;

        for (p = vm_page_find_least(object, tstart); p != NULL; p = np) {
                pi = p->pindex;
                if (pi >= tend)
                        break;
                np = TAILQ_NEXT(p, listq);
                if (vm_page_none_valid(p))
                        continue;
                if (vm_page_busy_acquire(p, VM_ALLOC_WAITFAIL) == 0) {
                        if (object->generation != curgeneration &&
                            (flags & OBJPC_SYNC) != 0)
                                goto rescan;
                        np = vm_page_find_least(object, pi);
                        continue;
                }
                if (!vm_object_page_remove_write(p, flags, &allclean)) {
                        vm_page_xunbusy(p);
                        continue;
                }
                if (object->type == OBJT_VNODE) {
                        n = vm_object_page_collect_flush(object, p, pagerflags,
                            flags, &allclean, &eio);
                        if (eio) {
                                res = FALSE;
                                allclean = FALSE;
                        }
                        if (object->generation != curgeneration &&
                            (flags & OBJPC_SYNC) != 0)
                                goto rescan;

                        /*
                         * If the VOP_PUTPAGES() did a truncated write, so
                         * that even the first page of the run is not fully
                         * written, vm_pageout_flush() returns 0 as the run
                         * length.  Since the condition that caused truncated
                         * write may be permanent, e.g. exhausted free space,
                         * accepting n == 0 would cause an infinite loop.
                         *
                         * Forwarding the iterator leaves the unwritten page
                         * behind, but there is not much we can do there if
                         * filesystem refuses to write it.
                         */
                        if (n == 0) {
                                n = 1;
                                allclean = FALSE;
                        }
                } else {
                        n = 1;
                        vm_page_xunbusy(p);
                }
                np = vm_page_find_least(object, pi + n);
        }
#if 0
        VOP_FSYNC(vp, (pagerflags & VM_PAGER_PUT_SYNC) ? MNT_WAIT : 0);
#endif

        /*
         * Leave updating cleangeneration for tmpfs objects to tmpfs
         * scan.  It needs to update mtime, which happens for other
         * filesystems during page writeouts.
         */
        if (allclean && object->type == OBJT_VNODE)
                object->cleangeneration = curgeneration;
        return (res);
}

static int
vm_object_page_collect_flush(vm_object_t object, vm_page_t p, int pagerflags,
    int flags, boolean_t *allclean, boolean_t *eio)
{
        vm_page_t ma[vm_pageout_page_count], p_first, tp;
        int count, i, mreq, runlen;

        vm_page_lock_assert(p, MA_NOTOWNED);
        vm_page_assert_xbusied(p);
        VM_OBJECT_ASSERT_WLOCKED(object);

        count = 1;
        mreq = 0;

        for (tp = p; count < vm_pageout_page_count; count++) {
                tp = vm_page_next(tp);
                if (tp == NULL || vm_page_tryxbusy(tp) == 0)
                        break;
                if (!vm_object_page_remove_write(tp, flags, allclean)) {
                        vm_page_xunbusy(tp);
                        break;
                }
        }

        for (p_first = p; count < vm_pageout_page_count; count++) {
                tp = vm_page_prev(p_first);
                if (tp == NULL || vm_page_tryxbusy(tp) == 0)
                        break;
                if (!vm_object_page_remove_write(tp, flags, allclean)) {
                        vm_page_xunbusy(tp);
                        break;
                }
                p_first = tp;
                mreq++;
        }

        for (tp = p_first, i = 0; i < count; tp = TAILQ_NEXT(tp, listq), i++)
                ma[i] = tp;

        vm_pageout_flush(ma, count, pagerflags, mreq, &runlen, eio);
        return (runlen);
}

/*
 * Note that there is absolutely no sense in writing out
 * anonymous objects, so we track down the vnode object
 * to write out.
 * We invalidate (remove) all pages from the address space
 * for semantic correctness.
 *
 * If the backing object is a device object with unmanaged pages, then any
 * mappings to the specified range of pages must be removed before this
 * function is called.
 *
 * Note: certain anonymous maps, such as MAP_NOSYNC maps,
 * may start out with a NULL object.
 */
boolean_t
vm_object_sync(vm_object_t object, vm_ooffset_t offset, vm_size_t size,
    boolean_t syncio, boolean_t invalidate)
{
        vm_object_t backing_object;
        struct vnode *vp;
        struct mount *mp;
        int error, flags, fsync_after;
        boolean_t res;

        if (object == NULL)
                return (TRUE);
        res = TRUE;
        error = 0;
        VM_OBJECT_WLOCK(object);
        while ((backing_object = object->backing_object) != NULL) {
                VM_OBJECT_WLOCK(backing_object);
                offset += object->backing_object_offset;
                VM_OBJECT_WUNLOCK(object);
                object = backing_object;
                if (object->size < OFF_TO_IDX(offset + size))
                        size = IDX_TO_OFF(object->size) - offset;
        }
        /*
         * Flush pages if writing is allowed, invalidate them
         * if invalidation requested.  Pages undergoing I/O
         * will be ignored by vm_object_page_remove().
         *
         * We cannot lock the vnode and then wait for paging
         * to complete without deadlocking against vm_fault.
         * Instead we simply call vm_object_page_remove() and
         * allow it to block internally on a page-by-page
         * basis when it encounters pages undergoing async
         * I/O.
         */
        if (object->type == OBJT_VNODE &&
            vm_object_mightbedirty(object) != 0 &&
            ((vp = object->handle)->v_vflag & VV_NOSYNC) == 0) {
                VM_OBJECT_WUNLOCK(object);
                (void)vn_start_write(vp, &mp, V_WAIT);
                vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
                if (syncio && !invalidate && offset == 0 &&
                    atop(size) == object->size) {
                        /*
                         * If syncing the whole mapping of the file,
                         * it is faster to schedule all the writes in
                         * async mode, also allowing the clustering,
                         * and then wait for i/o to complete.
                         */
                        flags = 0;
                        fsync_after = TRUE;
                } else {
                        flags = (syncio || invalidate) ? OBJPC_SYNC : 0;
                        flags |= invalidate ? (OBJPC_SYNC | OBJPC_INVAL) : 0;
                        fsync_after = FALSE;
                }
                VM_OBJECT_WLOCK(object);
                res = vm_object_page_clean(object, offset, offset + size,
                    flags);
                VM_OBJECT_WUNLOCK(object);
                if (fsync_after) {
                        for (;;) {
                                error = VOP_FSYNC(vp, MNT_WAIT, curthread);
                                if (error != ERELOOKUP)
                                        break;

                                /*
                                 * Allow SU/bufdaemon to handle more
                                 * dependencies in the meantime.
                                 */
                                VOP_UNLOCK(vp);
                                vn_finished_write(mp);

                                (void)vn_start_write(vp, &mp, V_WAIT);
                                vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
                        }
                }
                VOP_UNLOCK(vp);
                vn_finished_write(mp);
                if (error != 0)
                        res = FALSE;
                VM_OBJECT_WLOCK(object);
        }
        if ((object->type == OBJT_VNODE ||
             object->type == OBJT_DEVICE) && invalidate) {
                if (object->type == OBJT_DEVICE)
                        /*
                         * The option OBJPR_NOTMAPPED must be passed here
                         * because vm_object_page_remove() cannot remove
                         * unmanaged mappings.
                         */
                        flags = OBJPR_NOTMAPPED;
                else if (old_msync)
                        flags = 0;
                else
                        flags = OBJPR_CLEANONLY;
                vm_object_page_remove(object, OFF_TO_IDX(offset),
                    OFF_TO_IDX(offset + size + PAGE_MASK), flags);
        }
        VM_OBJECT_WUNLOCK(object);
        return (res);
}

/*
 * Determine whether the given advice can be applied to the object.  Advice is
 * not applied to unmanaged pages since they never belong to page queues, and
 * since MADV_FREE is destructive, it can apply only to anonymous pages that
 * have been mapped at most once.
 */
static bool
vm_object_advice_applies(vm_object_t object, int advice)
{

        if ((object->flags & OBJ_UNMANAGED) != 0)
                return (false);
        if (advice != MADV_FREE)
                return (true);
        return ((object->flags & (OBJ_ONEMAPPING | OBJ_ANON)) ==
            (OBJ_ONEMAPPING | OBJ_ANON));
}

static void
vm_object_madvise_freespace(vm_object_t object, int advice, vm_pindex_t pindex,
    vm_size_t size)
{

        if (advice == MADV_FREE)
                vm_pager_freespace(object, pindex, size);
}

/*
 *      vm_object_madvise:
 *
 *      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.
 */
void
vm_object_madvise(vm_object_t object, vm_pindex_t pindex, vm_pindex_t end,
    int advice)
{
        vm_pindex_t tpindex;
        vm_object_t backing_object, tobject;
        vm_page_t m, tm;

        if (object == NULL)
                return;

relookup:
        VM_OBJECT_WLOCK(object);
        if (!vm_object_advice_applies(object, advice)) {
                VM_OBJECT_WUNLOCK(object);
                return;
        }
        for (m = vm_page_find_least(object, pindex); pindex < end; pindex++) {
                tobject = object;

                /*
                 * If the next page isn't resident in the top-level object, we
                 * need to search the shadow chain.  When applying MADV_FREE, we
                 * take care to release any swap space used to store
                 * non-resident pages.
                 */
                if (m == NULL || pindex < m->pindex) {
                        /*
                         * Optimize a common case: if the top-level object has
                         * no backing object, we can skip over the non-resident
                         * range in constant time.
                         */
                        if (object->backing_object == NULL) {
                                tpindex = (m != NULL && m->pindex < end) ?
                                    m->pindex : end;
                                vm_object_madvise_freespace(object, advice,
                                    pindex, tpindex - pindex);
                                if ((pindex = tpindex) == end)
                                        break;
                                goto next_page;
                        }

                        tpindex = pindex;
                        do {
                                vm_object_madvise_freespace(tobject, advice,
                                    tpindex, 1);
                                /*
                                 * Prepare to search the next object in the
                                 * chain.
                                 */
                                backing_object = tobject->backing_object;
                                if (backing_object == NULL)
                                        goto next_pindex;
                                VM_OBJECT_WLOCK(backing_object);
                                tpindex +=
                                    OFF_TO_IDX(tobject->backing_object_offset);
                                if (tobject != object)
                                        VM_OBJECT_WUNLOCK(tobject);
                                tobject = backing_object;
                                if (!vm_object_advice_applies(tobject, advice))
                                        goto next_pindex;
                        } while ((tm = vm_page_lookup(tobject, tpindex)) ==
                            NULL);
                } else {
next_page:
                        tm = m;
                        m = TAILQ_NEXT(m, listq);
                }

                /*
                 * If the page is not in a normal state, skip it.  The page
                 * can not be invalidated while the object lock is held.
                 */
                if (!vm_page_all_valid(tm) || vm_page_wired(tm))
                        goto next_pindex;
                KASSERT((tm->flags & PG_FICTITIOUS) == 0,
                    ("vm_object_madvise: page %p is fictitious", tm));
                KASSERT((tm->oflags & VPO_UNMANAGED) == 0,
                    ("vm_object_madvise: page %p is not managed", tm));
                if (vm_page_tryxbusy(tm) == 0) {
                        if (object != tobject)
                                VM_OBJECT_WUNLOCK(object);
                        if (advice == MADV_WILLNEED) {
                                /*
                                 * Reference the page before unlocking and
                                 * sleeping so that the page daemon is less
                                 * likely to reclaim it.
                                 */
                                vm_page_aflag_set(tm, PGA_REFERENCED);
                        }
                        if (!vm_page_busy_sleep(tm, "madvpo", 0))
                                VM_OBJECT_WUNLOCK(tobject);
                        goto relookup;
                }
                vm_page_advise(tm, advice);
                vm_page_xunbusy(tm);
                vm_object_madvise_freespace(tobject, advice, tm->pindex, 1);
next_pindex:
                if (tobject != object)
                        VM_OBJECT_WUNLOCK(tobject);
        }
        VM_OBJECT_WUNLOCK(object);
}

/*
 *      vm_object_shadow:
 *
 *      Create a new object which is backed by the
 *      specified existing object range.  The source
 *      object reference is deallocated.
 *
 *      The new object and offset into that object
 *      are returned in the source parameters.
 */
void
vm_object_shadow(vm_object_t *object, vm_ooffset_t *offset, vm_size_t length,
    struct ucred *cred, bool shared)
{
        vm_object_t source;
        vm_object_t result;

        source = *object;

        /*
         * Don't create the new object if the old object isn't shared.
         *
         * If we hold the only reference we can guarantee that it won't
         * increase while we have the map locked.  Otherwise the race is
         * harmless and we will end up with an extra shadow object that
         * will be collapsed later.
         */
        if (source != NULL && source->ref_count == 1 &&
            (source->flags & OBJ_ANON) != 0)
                return;

        /*
         * Allocate a new object with the given length.
         */
        result = vm_object_allocate_anon(atop(length), source, cred, length);

        /*
         * Store the offset into the source object, and fix up the offset into
         * the new object.
         */
        result->backing_object_offset = *offset;

        if (shared || source != NULL) {
                VM_OBJECT_WLOCK(result);

                /*
                 * The new object shadows the source object, adding a
                 * reference to it.  Our caller changes his reference
                 * to point to the new object, removing a reference to
                 * the source object.  Net result: no change of
                 * reference count, unless the caller needs to add one
                 * more reference due to forking a shared map entry.
                 */
                if (shared) {
                        vm_object_reference_locked(result);
                        vm_object_clear_flag(result, OBJ_ONEMAPPING);
                }

                /*
                 * Try to optimize the result object's page color when
                 * shadowing in order to maintain page coloring
                 * consistency in the combined shadowed object.
                 */
                if (source != NULL) {
                        vm_object_backing_insert(result, source);
                        result->domain = source->domain;
#if VM_NRESERVLEVEL > 0
                        vm_object_set_flag(result,
                            (source->flags & OBJ_COLORED));
                        result->pg_color = (source->pg_color +
                            OFF_TO_IDX(*offset)) & ((1 << (VM_NFREEORDER -
                            1)) - 1);
#endif
                }
                VM_OBJECT_WUNLOCK(result);
        }

        /*
         * Return the new things
         */
        *offset = 0;
        *object = result;
}

/*
 *      vm_object_split:
 *
 * Split the pages in a map entry into a new object.  This affords
 * easier removal of unused pages, and keeps object inheritance from
 * being a negative impact on memory usage.
 */
void
vm_object_split(vm_map_entry_t entry)
{
        vm_page_t m, m_busy, m_next;
        vm_object_t orig_object, new_object, backing_object;
        vm_pindex_t idx, offidxstart;
        vm_size_t size;

        orig_object = entry->object.vm_object;
        KASSERT((orig_object->flags & OBJ_ONEMAPPING) != 0,
            ("vm_object_split:  Splitting object with multiple mappings."));
        if ((orig_object->flags & OBJ_ANON) == 0)
                return;
        if (orig_object->ref_count <= 1)
                return;
        VM_OBJECT_WUNLOCK(orig_object);

        offidxstart = OFF_TO_IDX(entry->offset);
        size = atop(entry->end - entry->start);

        /*
         * If swap_pager_copy() is later called, it will convert new_object
         * into a swap object.
         */
        new_object = vm_object_allocate_anon(size, orig_object,
            orig_object->cred, ptoa(size));

        /*
         * We must wait for the orig_object to complete any in-progress
         * collapse so that the swap blocks are stable below.  The
         * additional reference on backing_object by new object will
         * prevent further collapse operations until split completes.
         */
        VM_OBJECT_WLOCK(orig_object);
        vm_object_collapse_wait(orig_object);

        /*
         * At this point, the new object is still private, so the order in
         * which the original and new objects are locked does not matter.
         */
        VM_OBJECT_WLOCK(new_object);
        new_object->domain = orig_object->domain;
        backing_object = orig_object->backing_object;
        if (backing_object != NULL) {
                vm_object_backing_insert_ref(new_object, backing_object);
                new_object->backing_object_offset = 
                    orig_object->backing_object_offset + entry->offset;
        }
        if (orig_object->cred != NULL) {
                crhold(orig_object->cred);
                KASSERT(orig_object->charge >= ptoa(size),
                    ("orig_object->charge < 0"));
                orig_object->charge -= ptoa(size);
        }

        /*
         * Mark the split operation so that swap_pager_getpages() knows
         * that the object is in transition.
         */
        vm_object_set_flag(orig_object, OBJ_SPLIT);
        m_busy = NULL;
#ifdef INVARIANTS
        idx = 0;
#endif
retry:
        m = vm_page_find_least(orig_object, offidxstart);
        KASSERT(m == NULL || idx <= m->pindex - offidxstart,
            ("%s: object %p was repopulated", __func__, orig_object));
        for (; m != NULL && (idx = m->pindex - offidxstart) < size;
            m = m_next) {
                m_next = TAILQ_NEXT(m, listq);

                /*
                 * We must wait for pending I/O to complete before we can
                 * rename the page.
                 *
                 * We do not have to VM_PROT_NONE the page as mappings should
                 * not be changed by this operation.
                 */
                if (vm_page_tryxbusy(m) == 0) {
                        VM_OBJECT_WUNLOCK(new_object);
                        if (vm_page_busy_sleep(m, "spltwt", 0))
                                VM_OBJECT_WLOCK(orig_object);
                        VM_OBJECT_WLOCK(new_object);
                        goto retry;
                }

                /*
                 * The page was left invalid.  Likely placed there by
                 * an incomplete fault.  Just remove and ignore.
                 */
                if (vm_page_none_valid(m)) {
                        if (vm_page_remove(m))
                                vm_page_free(m);
                        continue;
                }

                /* vm_page_rename() will dirty the page. */
                if (vm_page_rename(m, new_object, idx)) {
                        vm_page_xunbusy(m);
                        VM_OBJECT_WUNLOCK(new_object);
                        VM_OBJECT_WUNLOCK(orig_object);
                        vm_radix_wait();
                        VM_OBJECT_WLOCK(orig_object);
                        VM_OBJECT_WLOCK(new_object);
                        goto retry;
                }

#if VM_NRESERVLEVEL > 0
                /*
                 * If some of the reservation's allocated pages remain with
                 * the original object, then transferring the reservation to
                 * the new object is neither particularly beneficial nor
                 * particularly harmful as compared to leaving the reservation
                 * with the original object.  If, however, all of the
                 * reservation's allocated pages are transferred to the new
                 * object, then transferring the reservation is typically
                 * beneficial.  Determining which of these two cases applies
                 * would be more costly than unconditionally renaming the
                 * reservation.
                 */
                vm_reserv_rename(m, new_object, orig_object, offidxstart);
#endif

                /*
                 * orig_object's type may change while sleeping, so keep track
                 * of the beginning of the busied range.
                 */
                if (orig_object->type != OBJT_SWAP)
                        vm_page_xunbusy(m);
                else if (m_busy == NULL)
                        m_busy = m;
        }
        if ((orig_object->flags & OBJ_SWAP) != 0) {
                /*
                 * swap_pager_copy() can sleep, in which case the orig_object's
                 * and new_object's locks are released and reacquired. 
                 */
                swap_pager_copy(orig_object, new_object, offidxstart, 0);
                if (m_busy != NULL)
                        TAILQ_FOREACH_FROM(m_busy, &new_object->memq, listq)
                                vm_page_xunbusy(m_busy);
        }
        vm_object_clear_flag(orig_object, OBJ_SPLIT);
        VM_OBJECT_WUNLOCK(orig_object);
        VM_OBJECT_WUNLOCK(new_object);
        entry->object.vm_object = new_object;
        entry->offset = 0LL;
        vm_object_deallocate(orig_object);
        VM_OBJECT_WLOCK(new_object);
}

static vm_page_t
vm_object_collapse_scan_wait(vm_object_t object, vm_page_t p)
{
        vm_object_t backing_object;

        VM_OBJECT_ASSERT_WLOCKED(object);
        backing_object = object->backing_object;
        VM_OBJECT_ASSERT_WLOCKED(backing_object);

        KASSERT(p == NULL || p->object == object || p->object == backing_object,
            ("invalid ownership %p %p %p", p, object, backing_object));
        /* The page is only NULL when rename fails. */
        if (p == NULL) {
                VM_OBJECT_WUNLOCK(object);
                VM_OBJECT_WUNLOCK(backing_object);
                vm_radix_wait();
                VM_OBJECT_WLOCK(object);
        } else if (p->object == object) {
                VM_OBJECT_WUNLOCK(backing_object);
                if (vm_page_busy_sleep(p, "vmocol", 0))
                        VM_OBJECT_WLOCK(object);
        } else {
                VM_OBJECT_WUNLOCK(object);
                if (!vm_page_busy_sleep(p, "vmocol", 0))
                        VM_OBJECT_WUNLOCK(backing_object);
                VM_OBJECT_WLOCK(object);
        }
        VM_OBJECT_WLOCK(backing_object);
        return (TAILQ_FIRST(&backing_object->memq));
}

static bool
vm_object_scan_all_shadowed(vm_object_t object)
{
        vm_object_t backing_object;
        vm_page_t p, pp;
        vm_pindex_t backing_offset_index, new_pindex, pi, ps;

        VM_OBJECT_ASSERT_WLOCKED(object);
        VM_OBJECT_ASSERT_WLOCKED(object->backing_object);

        backing_object = object->backing_object;

        if ((backing_object->flags & OBJ_ANON) == 0)
                return (false);

        pi = backing_offset_index = OFF_TO_IDX(object->backing_object_offset);
        p = vm_page_find_least(backing_object, pi);
        ps = swap_pager_find_least(backing_object, pi);

        /*
         * Only check pages inside the parent object's range and
         * inside the parent object's mapping of the backing object.
         */
        for (;; pi++) {
                if (p != NULL && p->pindex < pi)
                        p = TAILQ_NEXT(p, listq);
                if (ps < pi)
                        ps = swap_pager_find_least(backing_object, pi);
                if (p == NULL && ps >= backing_object->size)
                        break;
                else if (p == NULL)
                        pi = ps;
                else
                        pi = MIN(p->pindex, ps);

                new_pindex = pi - backing_offset_index;
                if (new_pindex >= object->size)
                        break;

                if (p != NULL) {
                        /*
                         * If the backing object page is busy a
                         * grandparent or older page may still be
                         * undergoing CoW.  It is not safe to collapse
                         * the backing object until it is quiesced.
                         */
                        if (vm_page_tryxbusy(p) == 0)
                                return (false);

                        /*
                         * We raced with the fault handler that left
                         * newly allocated invalid page on the object
                         * queue and retried.
                         */
                        if (!vm_page_all_valid(p))
                                goto unbusy_ret;
                }

                /*
                 * 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);

                /*
                 * The valid check here is stable due to object lock
                 * being required to clear valid and initiate paging.
                 * Busy of p disallows fault handler to validate pp.
                 */
                if ((pp == NULL || vm_page_none_valid(pp)) &&
                    !vm_pager_has_page(object, new_pindex, NULL, NULL))
                        goto unbusy_ret;
                if (p != NULL)
                        vm_page_xunbusy(p);
        }
        return (true);

unbusy_ret:
        if (p != NULL)
                vm_page_xunbusy(p);
        return (false);
}

static void
vm_object_collapse_scan(vm_object_t object)
{
        vm_object_t backing_object;
        vm_page_t next, p, pp;
        vm_pindex_t backing_offset_index, new_pindex;

        VM_OBJECT_ASSERT_WLOCKED(object);
        VM_OBJECT_ASSERT_WLOCKED(object->backing_object);

        backing_object = object->backing_object;
        backing_offset_index = OFF_TO_IDX(object->backing_object_offset);

        /*
         * Our scan
         */
        for (p = TAILQ_FIRST(&backing_object->memq); p != NULL; p = next) {
                next = TAILQ_NEXT(p, listq);
                new_pindex = p->pindex - backing_offset_index;

                /*
                 * Check for busy page
                 */
                if (vm_page_tryxbusy(p) == 0) {
                        next = vm_object_collapse_scan_wait(object, p);
                        continue;
                }

                KASSERT(object->backing_object == backing_object,
                    ("vm_object_collapse_scan: backing object mismatch %p != %p",
                    object->backing_object, backing_object));
                KASSERT(p->object == backing_object,
                    ("vm_object_collapse_scan: object mismatch %p != %p",
                    p->object, backing_object));

                if (p->pindex < backing_offset_index ||
                    new_pindex >= object->size) {
                        vm_pager_freespace(backing_object, p->pindex, 1);

                        KASSERT(!pmap_page_is_mapped(p),
                            ("freeing mapped page %p", p));
                        if (vm_page_remove(p))
                                vm_page_free(p);
                        continue;
                }

                if (!vm_page_all_valid(p)) {
                        KASSERT(!pmap_page_is_mapped(p),
                            ("freeing mapped page %p", p));
                        if (vm_page_remove(p))
                                vm_page_free(p);
                        continue;
                }

                pp = vm_page_lookup(object, new_pindex);
                if (pp != NULL && vm_page_tryxbusy(pp) == 0) {
                        vm_page_xunbusy(p);
                        /*
                         * The page in the parent is busy and possibly not
                         * (yet) valid.  Until its state is finalized by the
                         * busy bit owner, we can't tell whether it shadows the
                         * original page.
                         */
                        next = vm_object_collapse_scan_wait(object, pp);
                        continue;
                }

                if (pp != NULL && vm_page_none_valid(pp)) {
                        /*
                         * The page was invalid in the parent.  Likely placed
                         * there by an incomplete fault.  Just remove and
                         * ignore.  p can replace it.
                         */
                        if (vm_page_remove(pp))
                                vm_page_free(pp);
                        pp = NULL;
                }

                if (pp != NULL || vm_pager_has_page(object, new_pindex, NULL,
                        NULL)) {
                        /*
                         * The page already exists in the parent OR swap exists
                         * for this location in the parent.  Leave the parent's
                         * page alone.  Destroy the original page from the
                         * backing object.
                         */
                        vm_pager_freespace(backing_object, p->pindex, 1);
                        KASSERT(!pmap_page_is_mapped(p),
                            ("freeing mapped page %p", p));
                        if (vm_page_remove(p))
                                vm_page_free(p);
                        if (pp != NULL)
                                vm_page_xunbusy(pp);
                        continue;
                }

                /*
                 * 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.  vm_page_rename() will dirty the page.
                 */
                if (vm_page_rename(p, object, new_pindex)) {
                        vm_page_xunbusy(p);
                        next = vm_object_collapse_scan_wait(object, NULL);
                        continue;
                }

                /* Use the old pindex to free the right page. */
                vm_pager_freespace(backing_object, new_pindex +
                    backing_offset_index, 1);

#if VM_NRESERVLEVEL > 0
                /*
                 * Rename the reservation.
                 */
                vm_reserv_rename(p, object, backing_object,
                    backing_offset_index);
#endif
                vm_page_xunbusy(p);
        }
        return;
}

/*
 *      vm_object_collapse:
 *
 *      Collapse an object with the object backing it.
 *      Pages in the backing object are moved into the
 *      parent, and the backing object is deallocated.
 */
void
vm_object_collapse(vm_object_t object)
{
        vm_object_t backing_object, new_backing_object;

        VM_OBJECT_ASSERT_WLOCKED(object);

        while (TRUE) {
                KASSERT((object->flags & (OBJ_DEAD | OBJ_ANON)) == OBJ_ANON,
                    ("collapsing invalid object"));

                /*
                 * Wait for the backing_object to finish any pending
                 * collapse so that the caller sees the shortest possible
                 * shadow chain.
                 */
                backing_object = vm_object_backing_collapse_wait(object);
                if (backing_object == NULL)
                        return;

                KASSERT(object->ref_count > 0 &&
                    object->ref_count > atomic_load_int(&object->shadow_count),
                    ("collapse with invalid ref %d or shadow %d count.",
                    object->ref_count, atomic_load_int(&object->shadow_count)));
                KASSERT((backing_object->flags &
                    (OBJ_COLLAPSING | OBJ_DEAD)) == 0,
                    ("vm_object_collapse: Backing object already collapsing."));
                KASSERT((object->flags & (OBJ_COLLAPSING | OBJ_DEAD)) == 0,
                    ("vm_object_collapse: object is already collapsing."));

                /*
                 * 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.
                 */
                if (backing_object->ref_count == 1) {
                        KASSERT(atomic_load_int(&backing_object->shadow_count)
                            == 1,
                            ("vm_object_collapse: shadow_count: %d",
                            atomic_load_int(&backing_object->shadow_count)));
                        vm_object_pip_add(object, 1);
                        vm_object_set_flag(object, OBJ_COLLAPSING);
                        vm_object_pip_add(backing_object, 1);
                        vm_object_set_flag(backing_object, OBJ_DEAD);

                        /*
                         * If there is exactly one reference to the backing
                         * object, we can collapse it into the parent.
                         */
                        vm_object_collapse_scan(object);

#if VM_NRESERVLEVEL > 0
                        /*
                         * Break any reservations from backing_object.
                         */
                        if (__predict_false(!LIST_EMPTY(&backing_object->rvq)))
                                vm_reserv_break_all(backing_object);
#endif

                        /*
                         * Move the pager from backing_object to object.
                         */
                        if ((backing_object->flags & OBJ_SWAP) != 0) {
                                /*
                                 * swap_pager_copy() can sleep, in which case
                                 * the backing_object's and object's locks are
                                 * released and reacquired.
                                 * Since swap_pager_copy() is being asked to
                                 * destroy backing_object, it will change the
                                 * type to OBJT_DEFAULT.
                                 */
                                swap_pager_copy(
                                    backing_object,
                                    object,
                                    OFF_TO_IDX(object->backing_object_offset), TRUE);
                        }

                        /*
                         * Object now shadows whatever backing_object did.
                         */
                        vm_object_clear_flag(object, OBJ_COLLAPSING);
                        vm_object_backing_transfer(object, backing_object);
                        object->backing_object_offset +=
                            backing_object->backing_object_offset;
                        VM_OBJECT_WUNLOCK(object);
                        vm_object_pip_wakeup(object);

                        /*
                         * Discard backing_object.
                         *
                         * 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));
                        vm_object_pip_wakeup(backing_object);
                        (void)refcount_release(&backing_object->ref_count);
                        vm_object_terminate(backing_object);
                        counter_u64_add(object_collapses, 1);
                        VM_OBJECT_WLOCK(object);
                } else {
                        /*
                         * If we do not entirely shadow the backing object,
                         * there is nothing we can do so we give up.
                         *
                         * The object lock and backing_object lock must not
                         * be dropped during this sequence.
                         */
                        if (!vm_object_scan_all_shadowed(object)) {
                                VM_OBJECT_WUNLOCK(backing_object);
                                break;
                        }

                        /*
                         * Make the parent shadow the next object in the
                         * chain.  Deallocating backing_object will not remove
                         * it, since its reference count is at least 2.
                         */
                        vm_object_backing_remove_locked(object);
                        new_backing_object = backing_object->backing_object;
                        if (new_backing_object != NULL) {
                                vm_object_backing_insert_ref(object,
                                    new_backing_object);
                                object->backing_object_offset +=
                                    backing_object->backing_object_offset;
                        }

                        /*
                         * Drop the reference count on backing_object. Since
                         * its ref_count was at least 2, it will not vanish.
                         */
                        (void)refcount_release(&backing_object->ref_count);
                        KASSERT(backing_object->ref_count >= 1, (
"backing_object %p was somehow dereferenced during collapse!",
                            backing_object));
                        VM_OBJECT_WUNLOCK(backing_object);
                        counter_u64_add(object_bypasses, 1);
                }

                /*
                 * Try again with this object's new backing object.
                 */
        }
}

/*
 *      vm_object_page_remove:
 *
 *      For the given object, either frees or invalidates each of the
 *      specified pages.  In general, a page is freed.  However, if a page is
 *      wired for any reason other than the existence of a managed, wired
 *      mapping, then it may be invalidated but not removed from the object.
 *      Pages are specified by the given range ["start", "end") and the option
 *      OBJPR_CLEANONLY.  As a special case, if "end" is zero, then the range
 *      extends from "start" to the end of the object.  If the option
 *      OBJPR_CLEANONLY is specified, then only the non-dirty pages within the
 *      specified range are affected.  If the option OBJPR_NOTMAPPED is
 *      specified, then the pages within the specified range must have no
 *      mappings.  Otherwise, if this option is not specified, any mappings to
 *      the specified pages are removed before the pages are freed or
 *      invalidated.
 *
 *      In general, this operation should only be performed on objects that
 *      contain managed pages.  There are, however, two exceptions.  First, it
 *      is performed on the kernel and kmem objects by vm_map_entry_delete().
 *      Second, it is used by msync(..., MS_INVALIDATE) to invalidate device-
 *      backed pages.  In both of these cases, the option OBJPR_CLEANONLY must
 *      not be specified and the option OBJPR_NOTMAPPED must be specified.
 *
 *      The object must be locked.
 */
void
vm_object_page_remove(vm_object_t object, vm_pindex_t start, vm_pindex_t end,
    int options)
{
        vm_page_t p, next;

        VM_OBJECT_ASSERT_WLOCKED(object);
        KASSERT((object->flags & OBJ_UNMANAGED) == 0 ||
            (options & (OBJPR_CLEANONLY | OBJPR_NOTMAPPED)) == OBJPR_NOTMAPPED,
            ("vm_object_page_remove: illegal options for object %p", object));
        if (object->resident_page_count == 0)
                return;
        vm_object_pip_add(object, 1);
again:
        p = vm_page_find_least(object, start);

        /*
         * Here, the variable "p" is either (1) the page with the least pindex
         * greater than or equal to the parameter "start" or (2) NULL. 
         */
        for (; p != NULL && (p->pindex < end || end == 0); p = next) {
                next = TAILQ_NEXT(p, listq);

                /*
                 * Skip invalid pages if asked to do so.  Try to avoid acquiring
                 * the busy lock, as some consumers rely on this to avoid
                 * deadlocks.
                 *
                 * A thread may concurrently transition the page from invalid to
                 * valid using only the busy lock, so the result of this check
                 * is immediately stale.  It is up to consumers to handle this,
                 * for instance by ensuring that all invalid->valid transitions
                 * happen with a mutex held, as may be possible for a
                 * filesystem.
                 */
                if ((options & OBJPR_VALIDONLY) != 0 && vm_page_none_valid(p))
                        continue;

                /*
                 * If the page is wired for any reason besides the existence
                 * of managed, wired mappings, then it cannot be freed.  For
                 * example, fictitious pages, which represent device memory,
                 * are inherently wired and cannot be freed.  They can,
                 * however, be invalidated if the option OBJPR_CLEANONLY is
                 * not specified.
                 */
                if (vm_page_tryxbusy(p) == 0) {
                        if (vm_page_busy_sleep(p, "vmopar", 0))
                                VM_OBJECT_WLOCK(object);
                        goto again;
                }
                if ((options & OBJPR_VALIDONLY) != 0 && vm_page_none_valid(p)) {
                        vm_page_xunbusy(p);
                        continue;
                }
                if (vm_page_wired(p)) {
wired:
                        if ((options & OBJPR_NOTMAPPED) == 0 &&
                            object->ref_count != 0)
                                pmap_remove_all(p);
                        if ((options & OBJPR_CLEANONLY) == 0) {
                                vm_page_invalid(p);
                                vm_page_undirty(p);
                        }
                        vm_page_xunbusy(p);
                        continue;
                }
                KASSERT((p->flags & PG_FICTITIOUS) == 0,
                    ("vm_object_page_remove: page %p is fictitious", p));
                if ((options & OBJPR_CLEANONLY) != 0 &&
                    !vm_page_none_valid(p)) {
                        if ((options & OBJPR_NOTMAPPED) == 0 &&
                            object->ref_count != 0 &&
                            !vm_page_try_remove_write(p))
                                goto wired;
                        if (p->dirty != 0) {
                                vm_page_xunbusy(p);
                                continue;
                        }
                }
                if ((options & OBJPR_NOTMAPPED) == 0 &&
                    object->ref_count != 0 && !vm_page_try_remove_all(p))
                        goto wired;
                vm_page_free(p);
        }
        vm_object_pip_wakeup(object);

        vm_pager_freespace(object, start, (end == 0 ? object->size : end) -
            start);
}

/*
 *      vm_object_page_noreuse:
 *
 *      For the given object, attempt to move the specified pages to
 *      the head of the inactive queue.  This bypasses regular LRU
 *      operation and allows the pages to be reused quickly under memory
 *      pressure.  If a page is wired for any reason, then it will not
 *      be queued.  Pages are specified by the range ["start", "end").
 *      As a special case, if "end" is zero, then the range extends from
 *      "start" to the end of the object.
 *
 *      This operation should only be performed on objects that
 *      contain non-fictitious, managed pages.
 *
 *      The object must be locked.
 */
void
vm_object_page_noreuse(vm_object_t object, vm_pindex_t start, vm_pindex_t end)
{
        vm_page_t p, next;

        VM_OBJECT_ASSERT_LOCKED(object);
        KASSERT((object->flags & (OBJ_FICTITIOUS | OBJ_UNMANAGED)) == 0,
            ("vm_object_page_noreuse: illegal object %p", object));
        if (object->resident_page_count == 0)
                return;
        p = vm_page_find_least(object, start);

        /*
         * Here, the variable "p" is either (1) the page with the least pindex
         * greater than or equal to the parameter "start" or (2) NULL. 
         */
        for (; p != NULL && (p->pindex < end || end == 0); p = next) {
                next = TAILQ_NEXT(p, listq);
                vm_page_deactivate_noreuse(p);
        }
}

/*
 *      Populate the specified range of the object with valid pages.  Returns
 *      TRUE if the range is successfully populated and FALSE otherwise.
 *
 *      Note: This function should be optimized to pass a larger array of
 *      pages to vm_pager_get_pages() before it is applied to a non-
 *      OBJT_DEVICE object.
 *
 *      The object must be locked.
 */
boolean_t
vm_object_populate(vm_object_t object, vm_pindex_t start, vm_pindex_t end)
{
        vm_page_t m;
        vm_pindex_t pindex;
        int rv;

        VM_OBJECT_ASSERT_WLOCKED(object);
        for (pindex = start; pindex < end; pindex++) {
                rv = vm_page_grab_valid(&m, object, pindex, VM_ALLOC_NORMAL);
                if (rv != VM_PAGER_OK)
                        break;

                /*
                 * Keep "m" busy because a subsequent iteration may unlock
                 * the object.
                 */
        }
        if (pindex > start) {
                m = vm_page_lookup(object, start);
                while (m != NULL && m->pindex < pindex) {
                        vm_page_xunbusy(m);
                        m = TAILQ_NEXT(m, listq);
                }
        }
        return (pindex == end);
}

/*
 *      Routine:        vm_object_coalesce
 *      Function:       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
 *              prev_size       Size of reference to prev_object
 *              next_size       Size of reference to the second object
 *              reserved        Indicator that extension region has
 *                              swap accounted for
 *
 *      Conditions:
 *      The object must *not* be locked.
 */
boolean_t
vm_object_coalesce(vm_object_t prev_object, vm_ooffset_t prev_offset,
    vm_size_t prev_size, vm_size_t next_size, boolean_t reserved)
{
        vm_pindex_t next_pindex;

        if (prev_object == NULL)
                return (TRUE);
        if ((prev_object->flags & OBJ_ANON) == 0)
                return (FALSE);

        VM_OBJECT_WLOCK(prev_object);
        /*
         * Try to collapse the object first.
         */
        vm_object_collapse(prev_object);

        /*
         * 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_WUNLOCK(prev_object);
                return (FALSE);
        }

        prev_size >>= PAGE_SHIFT;
        next_size >>= PAGE_SHIFT;
        next_pindex = OFF_TO_IDX(prev_offset) + prev_size;

        if (prev_object->ref_count > 1 &&
            prev_object->size != next_pindex &&
            (prev_object->flags & OBJ_ONEMAPPING) == 0) {
                VM_OBJECT_WUNLOCK(prev_object);
                return (FALSE);
        }

        /*
         * Account for the charge.
         */
        if (prev_object->cred != NULL) {
                /*
                 * If prev_object was charged, then this mapping,
                 * although not charged now, may become writable
                 * later. Non-NULL cred in the object would prevent
                 * swap reservation during enabling of the write
                 * access, so reserve swap now. Failed reservation
                 * cause allocation of the separate object for the map
                 * entry, and swap reservation for this entry is
                 * managed in appropriate time.
                 */
                if (!reserved && !swap_reserve_by_cred(ptoa(next_size),
                    prev_object->cred)) {
                        VM_OBJECT_WUNLOCK(prev_object);
                        return (FALSE);
                }
                prev_object->charge += ptoa(next_size);
        }

        /*
         * 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, 0);
#if 0
                if (prev_object->cred != NULL) {
                        KASSERT(prev_object->charge >=
                            ptoa(prev_object->size - next_pindex),
                            ("object %p overcharged 1 %jx %jx", prev_object,
                                (uintmax_t)next_pindex, (uintmax_t)next_size));
                        prev_object->charge -= ptoa(prev_object->size -
                            next_pindex);
                }
#endif
        }

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

        VM_OBJECT_WUNLOCK(prev_object);
        return (TRUE);
}

void
vm_object_set_writeable_dirty_(vm_object_t object)
{
        atomic_add_int(&object->generation, 1);
}

bool
vm_object_mightbedirty_(vm_object_t object)
{
        return (object->generation != object->cleangeneration);
}

/*
 *      vm_object_unwire:
 *
 *      For each page offset within the specified range of the given object,
 *      find the highest-level page in the shadow chain and unwire it.  A page
 *      must exist at every page offset, and the highest-level page must be
 *      wired.
 */
void
vm_object_unwire(vm_object_t object, vm_ooffset_t offset, vm_size_t length,
    uint8_t queue)
{
        vm_object_t tobject, t1object;
        vm_page_t m, tm;
        vm_pindex_t end_pindex, pindex, tpindex;
        int depth, locked_depth;

        KASSERT((offset & PAGE_MASK) == 0,
            ("vm_object_unwire: offset is not page aligned"));
        KASSERT((length & PAGE_MASK) == 0,
            ("vm_object_unwire: length is not a multiple of PAGE_SIZE"));
        /* The wired count of a fictitious page never changes. */
        if ((object->flags & OBJ_FICTITIOUS) != 0)
                return;
        pindex = OFF_TO_IDX(offset);
        end_pindex = pindex + atop(length);
again:
        locked_depth = 1;
        VM_OBJECT_RLOCK(object);
        m = vm_page_find_least(object, pindex);
        while (pindex < end_pindex) {
                if (m == NULL || pindex < m->pindex) {
                        /*
                         * The first object in the shadow chain doesn't
                         * contain a page at the current index.  Therefore,
                         * the page must exist in a backing object.
                         */
                        tobject = object;
                        tpindex = pindex;
                        depth = 0;
                        do {
                                tpindex +=
                                    OFF_TO_IDX(tobject->backing_object_offset);
                                tobject = tobject->backing_object;
                                KASSERT(tobject != NULL,
                                    ("vm_object_unwire: missing page"));
                                if ((tobject->flags & OBJ_FICTITIOUS) != 0)
                                        goto next_page;
                                depth++;
                                if (depth == locked_depth) {
                                        locked_depth++;
                                        VM_OBJECT_RLOCK(tobject);
                                }
                        } while ((tm = vm_page_lookup(tobject, tpindex)) ==
                            NULL);
                } else {
                        tm = m;
                        m = TAILQ_NEXT(m, listq);
                }
                if (vm_page_trysbusy(tm) == 0) {
                        for (tobject = object; locked_depth >= 1;
                            locked_depth--) {
                                t1object = tobject->backing_object;
                                if (tm->object != tobject)
                                        VM_OBJECT_RUNLOCK(tobject);
                                tobject = t1object;
                        }
                        tobject = tm->object;
                        if (!vm_page_busy_sleep(tm, "unwbo",
                            VM_ALLOC_IGN_SBUSY))
                                VM_OBJECT_RUNLOCK(tobject);
                        goto again;
                }
                vm_page_unwire(tm, queue);
                vm_page_sunbusy(tm);
next_page:
                pindex++;
        }
        /* Release the accumulated object locks. */
        for (tobject = object; locked_depth >= 1; locked_depth--) {
                t1object = tobject->backing_object;
                VM_OBJECT_RUNLOCK(tobject);
                tobject = t1object;
        }
}

/*
 * Return the vnode for the given object, or NULL if none exists.
 * For tmpfs objects, the function may return NULL if there is
 * no vnode allocated at the time of the call.
 */
struct vnode *
vm_object_vnode(vm_object_t object)
{
        struct vnode *vp;

        VM_OBJECT_ASSERT_LOCKED(object);
        vm_pager_getvp(object, &vp, NULL);
        return (vp);
}

/*
 * Busy the vm object.  This prevents new pages belonging to the object from
 * becoming busy.  Existing pages persist as busy.  Callers are responsible
 * for checking page state before proceeding.
 */
void
vm_object_busy(vm_object_t obj)
{

        VM_OBJECT_ASSERT_LOCKED(obj);

        blockcount_acquire(&obj->busy, 1);
        /* The fence is required to order loads of page busy. */
        atomic_thread_fence_acq_rel();
}

void
vm_object_unbusy(vm_object_t obj)
{

        blockcount_release(&obj->busy, 1);
}

void
vm_object_busy_wait(vm_object_t obj, const char *wmesg)
{

        VM_OBJECT_ASSERT_UNLOCKED(obj);

        (void)blockcount_sleep(&obj->busy, NULL, wmesg, PVM);
}

/*
 * This function aims to determine if the object is mapped,
 * specifically, if it is referenced by a vm_map_entry.  Because
 * objects occasionally acquire transient references that do not
 * represent a mapping, the method used here is inexact.  However, it
 * has very low overhead and is good enough for the advisory
 * vm.vmtotal sysctl.
 */
bool
vm_object_is_active(vm_object_t obj)
{

        return (obj->ref_count > atomic_load_int(&obj->shadow_count));
}

static int
vm_object_list_handler(struct sysctl_req *req, bool swap_only)
{
        struct kinfo_vmobject *kvo;
        char *fullpath, *freepath;
        struct vnode *vp;
        struct vattr va;
        vm_object_t obj;
        vm_page_t m;
        u_long sp;
        int count, error;

        if (req->oldptr == NULL) {
                /*
                 * If an old buffer has not been provided, generate an
                 * estimate of the space needed for a subsequent call.
                 */
                mtx_lock(&vm_object_list_mtx);
                count = 0;
                TAILQ_FOREACH(obj, &vm_object_list, object_list) {
                        if (obj->type == OBJT_DEAD)
                                continue;
                        count++;
                }
                mtx_unlock(&vm_object_list_mtx);
                return (SYSCTL_OUT(req, NULL, sizeof(struct kinfo_vmobject) *
                    count * 11 / 10));
        }

        kvo = malloc(sizeof(*kvo), M_TEMP, M_WAITOK | M_ZERO);
        error = 0;

        /*
         * VM objects are type stable and are never removed from the
         * list once added.  This allows us to safely read obj->object_list
         * after reacquiring the VM object lock.
         */
        mtx_lock(&vm_object_list_mtx);
        TAILQ_FOREACH(obj, &vm_object_list, object_list) {
                if (obj->type == OBJT_DEAD ||
                    (swap_only && (obj->flags & (OBJ_ANON | OBJ_SWAP)) == 0))
                        continue;
                VM_OBJECT_RLOCK(obj);
                if (obj->type == OBJT_DEAD ||
                    (swap_only && (obj->flags & (OBJ_ANON | OBJ_SWAP)) == 0)) {
                        VM_OBJECT_RUNLOCK(obj);
                        continue;
                }
                mtx_unlock(&vm_object_list_mtx);
                kvo->kvo_size = ptoa(obj->size);
                kvo->kvo_resident = obj->resident_page_count;
                kvo->kvo_ref_count = obj->ref_count;
                kvo->kvo_shadow_count = atomic_load_int(&obj->shadow_count);
                kvo->kvo_memattr = obj->memattr;
                kvo->kvo_active = 0;
                kvo->kvo_inactive = 0;
                if (!swap_only) {
                        TAILQ_FOREACH(m, &obj->memq, listq) {
                                /*
                                 * A page may belong to the object but be
                                 * dequeued and set to PQ_NONE while the
                                 * object lock is not held.  This makes the
                                 * reads of m->queue below racy, and we do not
                                 * count pages set to PQ_NONE.  However, this
                                 * sysctl is only meant to give an
                                 * approximation of the system anyway.
                                 */
                                if (m->a.queue == PQ_ACTIVE)
                                        kvo->kvo_active++;
                                else if (m->a.queue == PQ_INACTIVE)
                                        kvo->kvo_inactive++;
                        }
                }

                kvo->kvo_vn_fileid = 0;
                kvo->kvo_vn_fsid = 0;
                kvo->kvo_vn_fsid_freebsd11 = 0;
                freepath = NULL;
                fullpath = "";
                vp = NULL;
                kvo->kvo_type = vm_object_kvme_type(obj, swap_only ? NULL : &vp);
                if (vp != NULL) {
                        vref(vp);
                } else if ((obj->flags & OBJ_ANON) != 0) {
                        MPASS(kvo->kvo_type == KVME_TYPE_DEFAULT ||
                            kvo->kvo_type == KVME_TYPE_SWAP);
                        kvo->kvo_me = (uintptr_t)obj;
                        /* tmpfs objs are reported as vnodes */
                        kvo->kvo_backing_obj = (uintptr_t)obj->backing_object;
                        sp = swap_pager_swapped_pages(obj);
                        kvo->kvo_swapped = sp > UINT32_MAX ? UINT32_MAX : sp;
                }
                VM_OBJECT_RUNLOCK(obj);
                if (vp != NULL) {
                        vn_fullpath(vp, &fullpath, &freepath);
                        vn_lock(vp, LK_SHARED | LK_RETRY);
                        if (VOP_GETATTR(vp, &va, curthread->td_ucred) == 0) {
                                kvo->kvo_vn_fileid = va.va_fileid;
                                kvo->kvo_vn_fsid = va.va_fsid;
                                kvo->kvo_vn_fsid_freebsd11 = va.va_fsid;
                                                                /* truncate */
                        }
                        vput(vp);
                }

                strlcpy(kvo->kvo_path, fullpath, sizeof(kvo->kvo_path));
                if (freepath != NULL)
                        free(freepath, M_TEMP);

                /* Pack record size down */
                kvo->kvo_structsize = offsetof(struct kinfo_vmobject, kvo_path)
                    + strlen(kvo->kvo_path) + 1;
                kvo->kvo_structsize = roundup(kvo->kvo_structsize,
                    sizeof(uint64_t));
                error = SYSCTL_OUT(req, kvo, kvo->kvo_structsize);
                maybe_yield();
                mtx_lock(&vm_object_list_mtx);
                if (error)
                        break;
        }
        mtx_unlock(&vm_object_list_mtx);
        free(kvo, M_TEMP);
        return (error);
}

static int
sysctl_vm_object_list(SYSCTL_HANDLER_ARGS)
{
        return (vm_object_list_handler(req, false));
}

SYSCTL_PROC(_vm, OID_AUTO, objects, CTLTYPE_STRUCT | CTLFLAG_RW | CTLFLAG_SKIP |
    CTLFLAG_MPSAFE, NULL, 0, sysctl_vm_object_list, "S,kinfo_vmobject",
    "List of VM objects");

static int
sysctl_vm_object_list_swap(SYSCTL_HANDLER_ARGS)
{
        return (vm_object_list_handler(req, true));
}

/*
 * This sysctl returns list of the anonymous or swap objects. Intent
 * is to provide stripped optimized list useful to analyze swap use.
 * Since technically non-swap (default) objects participate in the
 * shadow chains, and are converted to swap type as needed by swap
 * pager, we must report them.
 */
SYSCTL_PROC(_vm, OID_AUTO, swap_objects,
    CTLTYPE_STRUCT | CTLFLAG_RW | CTLFLAG_SKIP | CTLFLAG_MPSAFE, NULL, 0,
    sysctl_vm_object_list_swap, "S,kinfo_vmobject",
    "List of swap VM objects");

#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)
{
        vm_map_t tmpm;
        vm_map_entry_t tmpe;
        vm_object_t obj;

        if (map == 0)
                return 0;

        if (entry == 0) {
                VM_MAP_ENTRY_FOREACH(tmpe, map) {
                        if (_vm_object_in_map(map, object, tmpe)) {
                                return 1;
                        }
                }
        } else if (entry->eflags & MAP_ENTRY_IS_SUB_MAP) {
                tmpm = entry->object.sub_map;
                VM_MAP_ENTRY_FOREACH(tmpe, tmpm) {
                        if (_vm_object_in_map(tmpm, object, tmpe)) {
                                return 1;
                        }
                }
        } else if ((obj = entry->object.vm_object) != NULL) {
                for (; obj; obj = obj->backing_object)
                        if (obj == object) {
                                return 1;
                        }
        }
        return 0;
}

static int
vm_object_in_map(vm_object_t object)
{
        struct proc *p;

        /* sx_slock(&allproc_lock); */
        FOREACH_PROC_IN_SYSTEM(p) {
                if (!p->p_vmspace /* || (p->p_flag & (P_SYSTEM|P_WEXIT)) */)
                        continue;
                if (_vm_object_in_map(&p->p_vmspace->vm_map, object, 0)) {
                        /* sx_sunlock(&allproc_lock); */
                        return 1;
                }
        }
        /* sx_sunlock(&allproc_lock); */
        if (_vm_object_in_map(kernel_map, object, 0))
                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.
         */
        TAILQ_FOREACH(object, &vm_object_list, object_list) {
                if ((object->flags & OBJ_ANON) != 0) {
                        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);
                        }
                }
                if (db_pager_quit)
                        return;
        }
}

/*
 *      vm_object_print:        [ debug ]
 */
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%jx, res=%d, ref=%d, flags=0x%x ruid %d charge %jx\n",
            object, (int)object->type, (uintmax_t)object->size,
            object->resident_page_count, object->ref_count, object->flags,
            object->cred ? object->cred->cr_ruid : -1, (uintmax_t)object->charge);
        db_iprintf(" sref=%d, backing_object(%d)=(%p)+0x%jx\n",
            atomic_load_int(&object->shadow_count),
            object->backing_object ? object->backing_object->ref_count : 0,
            object->backing_object, (uintmax_t)object->backing_object_offset);

        if (!full)
                return;

        db_indent += 2;
        count = 0;
        TAILQ_FOREACH(p, &object->memq, listq) {
                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%jx,page=0x%jx)",
                    (uintmax_t)p->pindex, (uintmax_t)VM_PAGE_TO_PHYS(p));

                if (db_pager_quit)
                        break;
        }
        if (count != 0)
                db_printf("\n");
        db_indent -= 2;
}

/* XXX. */
#undef count

/* XXX need this non-static entry for calling from vm_map_print. */
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);
}

DB_SHOW_COMMAND(vmopag, vm_object_print_pages)
{
        vm_object_t object;
        vm_pindex_t fidx;
        vm_paddr_t pa;
        vm_page_t m, prev_m;
        int rcount;

        TAILQ_FOREACH(object, &vm_object_list, object_list) {
                db_printf("new object: %p\n", (void *)object);
                if (db_pager_quit)
                        return;

                rcount = 0;
                fidx = 0;
                pa = -1;
                TAILQ_FOREACH(m, &object->memq, listq) {
                        if ((prev_m = TAILQ_PREV(m, pglist, listq)) != NULL &&
                            prev_m->pindex + 1 != m->pindex) {
                                if (rcount) {
                                        db_printf(" index(%ld)run(%d)pa(0x%lx)\n",
                                                (long)fidx, rcount, (long)pa);
                                        if (db_pager_quit)
                                                return;
                                        rcount = 0;
                                }
                        }                               
                        if (rcount &&
                                (VM_PAGE_TO_PHYS(m) == pa + rcount * PAGE_SIZE)) {
                                ++rcount;
                                continue;
                        }
                        if (rcount) {
                                db_printf(" index(%ld)run(%d)pa(0x%lx)\n",
                                        (long)fidx, rcount, (long)pa);
                                if (db_pager_quit)
                                        return;
                        }
                        fidx = m->pindex;
                        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 (db_pager_quit)
                                return;
                }
        }
}
#endif /* DDB */