FreeBSD/Linux Kernel Cross Reference
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);
                   object->un_pager.swp.writemappings = 0;
           }
   
           /*
            * 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_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_SWAP, size,
               OBJ_ANON | OBJ_ONEMAPPING | OBJ_SWAP, 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_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;
           }
   }
   
   void
   vm_object_destroy(vm_object_t 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->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_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_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);
  
          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);
  #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
          }
  
          /*
           * 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);
  
          TAILQ_FOREACH(m, &new_object->memq, listq)
                  vm_page_xunbusy(m);
  
          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.
                           *
                           * swap_pager_copy() can sleep, in which case the
                           * backing_object's and object's locks are released and
                           * reacquired.
                           */
                          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_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_FLAGS(vmochk, vm_object_check, DB_CMD_MEMSAFE)
  {
          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_FLAGS(vmopag, vm_object_print_pages, DB_CMD_MEMSAFE)
  {
          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 */

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