FreeBSD/Linux Kernel Cross Reference
sys/vm/vm_object.c

     /*
      * Copyright (c) 1991, 1993, 2013
      *      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.
     *
     * $FreeBSD: src/sys/vm/vm_object.c,v 1.171.2.8 2003/05/26 19:17:56 alc Exp $
     */
    
    /*
     *      Virtual memory object module.
     */
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/proc.h>           /* for curproc, pageproc */
    #include <sys/thread.h>
    #include <sys/vnode.h>
    #include <sys/vmmeter.h>
    #include <sys/mman.h>
    #include <sys/mount.h>
    #include <sys/kernel.h>
    #include <sys/sysctl.h>
    #include <sys/refcount.h>
    
    #include <vm/vm.h>
    #include <vm/vm_param.h>
    #include <vm/pmap.h>
    #include <vm/vm_map.h>
    #include <vm/vm_object.h>
    #include <vm/vm_page.h>
    #include <vm/vm_pageout.h>
    #include <vm/vm_pager.h>
    #include <vm/swap_pager.h>
    #include <vm/vm_kern.h>
    #include <vm/vm_extern.h>
    #include <vm/vm_zone.h>
    
    #include <vm/vm_page2.h>
    
    #include <machine/specialreg.h>
    
    #define EASY_SCAN_FACTOR        8
    
    static void     vm_object_qcollapse(vm_object_t object,
                                        vm_object_t backing_object);
   static void     vm_object_page_collect_flush(vm_object_t object, vm_page_t p,
                                                int pagerflags);
   static void     vm_object_lock_init(vm_object_t);
   
   
   /*
    *      Virtual memory objects maintain the actual data
    *      associated with allocated virtual memory.  A given
    *      page of memory exists within exactly one object.
    *
    *      An object is only deallocated when all "references"
    *      are given up.  Only one "reference" to a given
    *      region of an object should be writeable.
    *
    *      Associated with each object is a list of all resident
    *      memory pages belonging to that object; this list is
    *      maintained by the "vm_page" module, and locked by the object's
    *      lock.
    *
    *      Each object also records a "pager" routine which is
    *      used to retrieve (and store) pages to the proper backing
    *      storage.  In addition, objects may be backed by other
    *      objects from which they were virtual-copied.
    *
    *      The only items within the object structure which are
    *      modified after time of creation are:
    *              reference count         locked by object's lock
    *              pager routine           locked by object's lock
    *
    */
   
   struct vm_object kernel_object;
   
   static long vm_object_count;
   
   static long object_collapses;
   static long object_bypasses;
   static int next_index;
   static vm_zone_t obj_zone;
   static struct vm_zone obj_zone_store;
   #define VM_OBJECTS_INIT 256
   static struct vm_object vm_objects_init[VM_OBJECTS_INIT];
   
   struct object_q vm_object_lists[VMOBJ_HSIZE];
   struct lwkt_token vmobj_tokens[VMOBJ_HSIZE];
   
   /*
    * Misc low level routines
    */
   static void
   vm_object_lock_init(vm_object_t obj)
   {
   #if defined(DEBUG_LOCKS)
           int i;
   
           obj->debug_hold_bitmap = 0;
           obj->debug_hold_ovfl = 0;
           for (i = 0; i < VMOBJ_DEBUG_ARRAY_SIZE; i++) {
                   obj->debug_hold_thrs[i] = NULL;
                   obj->debug_hold_file[i] = NULL;
                   obj->debug_hold_line[i] = 0;
           }
   #endif
   }
   
   void
   vm_object_lock_swap(void)
   {
           lwkt_token_swap();
   }
   
   void
   vm_object_lock(vm_object_t obj)
   {
           lwkt_gettoken(&obj->token);
   }
   
   /*
    * Returns TRUE on sucesss
    */
   static int
   vm_object_lock_try(vm_object_t obj)
   {
           return(lwkt_trytoken(&obj->token));
   }
   
   void
   vm_object_lock_shared(vm_object_t obj)
   {
           lwkt_gettoken_shared(&obj->token);
   }
   
   void
   vm_object_unlock(vm_object_t obj)
   {
           lwkt_reltoken(&obj->token);
   }
   
   void
   vm_object_upgrade(vm_object_t obj)
   {
           lwkt_reltoken(&obj->token);
           lwkt_gettoken(&obj->token);
   }
   
   void
   vm_object_downgrade(vm_object_t obj)
   {
           lwkt_reltoken(&obj->token);
           lwkt_gettoken_shared(&obj->token);
   }
   
   static __inline void
   vm_object_assert_held(vm_object_t obj)
   {
           ASSERT_LWKT_TOKEN_HELD(&obj->token);
   }
   
   void
   #ifndef DEBUG_LOCKS
   vm_object_hold(vm_object_t obj)
   #else
   debugvm_object_hold(vm_object_t obj, char *file, int line)
   #endif
   {
           KKASSERT(obj != NULL);
   
           /*
            * Object must be held (object allocation is stable due to callers
            * context, typically already holding the token on a parent object)
            * prior to potentially blocking on the lock, otherwise the object
            * can get ripped away from us.
            */
           refcount_acquire(&obj->hold_count);
           vm_object_lock(obj);
   
   #if defined(DEBUG_LOCKS)
           int i;
           u_int mask;
   
           for (;;) {
                   mask = ~obj->debug_hold_bitmap;
                   cpu_ccfence();
                   if (mask == 0xFFFFFFFFU) {
                           if (obj->debug_hold_ovfl == 0)
                                   obj->debug_hold_ovfl = 1;
                           break;
                   }
                   i = ffs(mask) - 1;
                   if (atomic_cmpset_int(&obj->debug_hold_bitmap, ~mask,
                                         ~mask | (1 << i))) {
                           obj->debug_hold_bitmap |= (1 << i);
                           obj->debug_hold_thrs[i] = curthread;
                           obj->debug_hold_file[i] = file;
                           obj->debug_hold_line[i] = line;
                           break;
                   }
           }
   #endif
   }
   
   int
   #ifndef DEBUG_LOCKS
   vm_object_hold_try(vm_object_t obj)
   #else
   debugvm_object_hold_try(vm_object_t obj, char *file, int line)
   #endif
   {
           KKASSERT(obj != NULL);
   
           /*
            * Object must be held (object allocation is stable due to callers
            * context, typically already holding the token on a parent object)
            * prior to potentially blocking on the lock, otherwise the object
            * can get ripped away from us.
            */
           refcount_acquire(&obj->hold_count);
           if (vm_object_lock_try(obj) == 0) {
                   if (refcount_release(&obj->hold_count)) {
                           if (obj->ref_count == 0 && (obj->flags & OBJ_DEAD))
                                   zfree(obj_zone, obj);
                   }
                   return(0);
           }
   
   #if defined(DEBUG_LOCKS)
           int i;
           u_int mask;
   
           for (;;) {
                   mask = ~obj->debug_hold_bitmap;
                   cpu_ccfence();
                   if (mask == 0xFFFFFFFFU) {
                           if (obj->debug_hold_ovfl == 0)
                                   obj->debug_hold_ovfl = 1;
                           break;
                   }
                   i = ffs(mask) - 1;
                   if (atomic_cmpset_int(&obj->debug_hold_bitmap, ~mask,
                                         ~mask | (1 << i))) {
                           obj->debug_hold_bitmap |= (1 << i);
                           obj->debug_hold_thrs[i] = curthread;
                           obj->debug_hold_file[i] = file;
                           obj->debug_hold_line[i] = line;
                           break;
                   }
           }
   #endif
           return(1);
   }
   
   void
   #ifndef DEBUG_LOCKS
   vm_object_hold_shared(vm_object_t obj)
   #else
   debugvm_object_hold_shared(vm_object_t obj, char *file, int line)
   #endif
   {
           KKASSERT(obj != NULL);
   
           /*
            * Object must be held (object allocation is stable due to callers
            * context, typically already holding the token on a parent object)
            * prior to potentially blocking on the lock, otherwise the object
            * can get ripped away from us.
            */
           refcount_acquire(&obj->hold_count);
           vm_object_lock_shared(obj);
   
   #if defined(DEBUG_LOCKS)
           int i;
           u_int mask;
   
           for (;;) {
                   mask = ~obj->debug_hold_bitmap;
                   cpu_ccfence();
                   if (mask == 0xFFFFFFFFU) {
                           if (obj->debug_hold_ovfl == 0)
                                   obj->debug_hold_ovfl = 1;
                           break;
                   }
                   i = ffs(mask) - 1;
                   if (atomic_cmpset_int(&obj->debug_hold_bitmap, ~mask,
                                         ~mask | (1 << i))) {
                           obj->debug_hold_bitmap |= (1 << i);
                           obj->debug_hold_thrs[i] = curthread;
                           obj->debug_hold_file[i] = file;
                           obj->debug_hold_line[i] = line;
                           break;
                   }
           }
   #endif
   }
   
   /*
    * Drop the token and hold_count on the object.
    *
    * WARNING! Token might be shared.
    */
   void
   vm_object_drop(vm_object_t obj)
   {
           if (obj == NULL)
                   return;
   
   #if defined(DEBUG_LOCKS)
           int found = 0;
           int i;
   
           for (i = 0; i < VMOBJ_DEBUG_ARRAY_SIZE; i++) {
                   if ((obj->debug_hold_bitmap & (1 << i)) &&
                       (obj->debug_hold_thrs[i] == curthread)) {
                           obj->debug_hold_bitmap &= ~(1 << i);
                           obj->debug_hold_thrs[i] = NULL;
                           obj->debug_hold_file[i] = NULL;
                           obj->debug_hold_line[i] = 0;
                           found = 1;
                           break;
                   }
           }
   
           if (found == 0 && obj->debug_hold_ovfl == 0)
                   panic("vm_object: attempt to drop hold on non-self-held obj");
   #endif
   
           /*
            * No new holders should be possible once we drop hold_count 1->0 as
            * there is no longer any way to reference the object.
            */
           KKASSERT(obj->hold_count > 0);
           if (refcount_release(&obj->hold_count)) {
                   if (obj->ref_count == 0 && (obj->flags & OBJ_DEAD)) {
                           vm_object_unlock(obj);
                           zfree(obj_zone, obj);
                   } else {
                           vm_object_unlock(obj);
                   }
           } else {
                   vm_object_unlock(obj);
           }
   }
   
   /*
    * Initialize a freshly allocated object, returning a held object.
    *
    * Used only by vm_object_allocate() and zinitna().
    *
    * No requirements.
    */
   void
   _vm_object_allocate(objtype_t type, vm_pindex_t size, vm_object_t object)
   {
           int incr;
           int n;
   
           RB_INIT(&object->rb_memq);
           LIST_INIT(&object->shadow_head);
           lwkt_token_init(&object->token, "vmobj");
   
           object->type = type;
           object->size = size;
           object->ref_count = 1;
           object->memattr = VM_MEMATTR_DEFAULT;
           object->hold_count = 0;
           object->flags = 0;
           if ((object->type == OBJT_DEFAULT) || (object->type == OBJT_SWAP))
                   vm_object_set_flag(object, OBJ_ONEMAPPING);
           object->paging_in_progress = 0;
           object->resident_page_count = 0;
           object->agg_pv_list_count = 0;
           object->shadow_count = 0;
           /* cpu localization twist */
           object->pg_color = (int)(intptr_t)curthread;
           if ( size > (PQ_L2_SIZE / 3 + PQ_PRIME1))
                   incr = PQ_L2_SIZE / 3 + PQ_PRIME1;
           else
                   incr = size;
           next_index = (next_index + incr) & PQ_L2_MASK;
           object->handle = NULL;
           object->backing_object = NULL;
           object->backing_object_offset = (vm_ooffset_t)0;
   
           object->generation++;
           object->swblock_count = 0;
           RB_INIT(&object->swblock_root);
           vm_object_lock_init(object);
           pmap_object_init(object);
   
           vm_object_hold(object);
   
           n = VMOBJ_HASH(object);
           atomic_add_long(&vm_object_count, 1);
           lwkt_gettoken(&vmobj_tokens[n]);
           TAILQ_INSERT_TAIL(&vm_object_lists[n], object, object_list);
           lwkt_reltoken(&vmobj_tokens[n]);
   }
   
   /*
    * Initialize the VM objects module.
    *
    * Called from the low level boot code only.
    */
   void
   vm_object_init(void)
   {
           int i;
   
           for (i = 0; i < VMOBJ_HSIZE; ++i) {
                   TAILQ_INIT(&vm_object_lists[i]);
                   lwkt_token_init(&vmobj_tokens[i], "vmobjlst");
           }
           
           _vm_object_allocate(OBJT_DEFAULT, OFF_TO_IDX(KvaEnd),
                               &kernel_object);
           vm_object_drop(&kernel_object);
   
           obj_zone = &obj_zone_store;
           zbootinit(obj_zone, "VM OBJECT", sizeof (struct vm_object),
                   vm_objects_init, VM_OBJECTS_INIT);
   }
   
   void
   vm_object_init2(void)
   {
           zinitna(obj_zone, NULL, NULL, 0, 0, ZONE_PANICFAIL, 1);
   }
   
   /*
    * Allocate and return a new object of the specified type and size.
    *
    * No requirements.
    */
   vm_object_t
   vm_object_allocate(objtype_t type, vm_pindex_t size)
   {
           vm_object_t result;
   
           result = (vm_object_t) zalloc(obj_zone);
   
           _vm_object_allocate(type, size, result);
           vm_object_drop(result);
   
           return (result);
   }
   
   /*
    * This version returns a held object, allowing further atomic initialization
    * of the object.
    */
   vm_object_t
   vm_object_allocate_hold(objtype_t type, vm_pindex_t size)
   {
           vm_object_t result;
   
           result = (vm_object_t) zalloc(obj_zone);
   
           _vm_object_allocate(type, size, result);
   
           return (result);
   }
   
   /*
    * Add an additional reference to a vm_object.  The object must already be
    * held.  The original non-lock version is no longer supported.  The object
    * must NOT be chain locked by anyone at the time the reference is added.
    *
    * Referencing a chain-locked object can blow up the fairly sensitive
    * ref_count and shadow_count tests in the deallocator.  Most callers
    * will call vm_object_chain_wait() prior to calling
    * vm_object_reference_locked() to avoid the case.
    *
    * The object must be held, but may be held shared if desired (hence why
    * we use an atomic op).
    */
   void
   vm_object_reference_locked(vm_object_t object)
   {
           KKASSERT(object != NULL);
           ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
           KKASSERT((object->chainlk & (CHAINLK_EXCL | CHAINLK_MASK)) == 0);
           atomic_add_int(&object->ref_count, 1);
           if (object->type == OBJT_VNODE) {
                   vref(object->handle);
                   /* XXX what if the vnode is being destroyed? */
           }
   }
   
   /*
    * This version is only allowed for vnode objects.
    */
   void
   vm_object_reference_quick(vm_object_t object)
   {
           KKASSERT(object->type == OBJT_VNODE);
           atomic_add_int(&object->ref_count, 1);
           vref(object->handle);
   }
   
   /*
    * Object OBJ_CHAINLOCK lock handling.
    *
    * The caller can chain-lock backing objects recursively and then
    * use vm_object_chain_release_all() to undo the whole chain.
    *
    * Chain locks are used to prevent collapses and are only applicable
    * to OBJT_DEFAULT and OBJT_SWAP objects.  Chain locking operations
    * on other object types are ignored.  This is also important because
    * it allows e.g. the vnode underlying a memory mapping to take concurrent
    * faults.
    *
    * The object must usually be held on entry, though intermediate
    * objects need not be held on release.  The object must be held exclusively,
    * NOT shared.  Note that the prefault path checks the shared state and
    * avoids using the chain functions.
    */
   void
   vm_object_chain_wait(vm_object_t object, int shared)
   {
           ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
           for (;;) {
                   uint32_t chainlk = object->chainlk;
   
                   cpu_ccfence();
                   if (shared) {
                           if (chainlk & (CHAINLK_EXCL | CHAINLK_EXCLREQ)) {
                                   tsleep_interlock(object, 0);
                                   if (atomic_cmpset_int(&object->chainlk,
                                                         chainlk,
                                                         chainlk | CHAINLK_WAIT)) {
                                           tsleep(object, PINTERLOCKED,
                                                  "objchns", 0);
                                   }
                                   /* retry */
                           } else {
                                   break;
                           }
                           /* retry */
                   } else {
                           if (chainlk & (CHAINLK_MASK | CHAINLK_EXCL)) {
                                   tsleep_interlock(object, 0);
                                   if (atomic_cmpset_int(&object->chainlk,
                                                         chainlk,
                                                         chainlk | CHAINLK_WAIT))
                                   {
                                           tsleep(object, PINTERLOCKED,
                                                  "objchnx", 0);
                                   }
                                   /* retry */
                           } else {
                                   if (atomic_cmpset_int(&object->chainlk,
                                                         chainlk,
                                                         chainlk & ~CHAINLK_WAIT))
                                   {
                                           if (chainlk & CHAINLK_WAIT)
                                                   wakeup(object);
                                           break;
                                   }
                                   /* retry */
                           }
                   }
                   /* retry */
           }
   }
   
   void
   vm_object_chain_acquire(vm_object_t object, int shared)
   {
           if (object->type != OBJT_DEFAULT && object->type != OBJT_SWAP)
                   return;
           if (vm_shared_fault == 0)
                   shared = 0;
   
           for (;;) {
                   uint32_t chainlk = object->chainlk;
   
                   cpu_ccfence();
                   if (shared) {
                           if (chainlk & (CHAINLK_EXCL | CHAINLK_EXCLREQ)) {
                                   tsleep_interlock(object, 0);
                                   if (atomic_cmpset_int(&object->chainlk,
                                                         chainlk,
                                                         chainlk | CHAINLK_WAIT)) {
                                           tsleep(object, PINTERLOCKED,
                                                  "objchns", 0);
                                   }
                                   /* retry */
                           } else if (atomic_cmpset_int(&object->chainlk,
                                                 chainlk, chainlk + 1)) {
                                   break;
                           }
                           /* retry */
                   } else {
                           if (chainlk & (CHAINLK_MASK | CHAINLK_EXCL)) {
                                   tsleep_interlock(object, 0);
                                   if (atomic_cmpset_int(&object->chainlk,
                                                         chainlk,
                                                         chainlk |
                                                          CHAINLK_WAIT |
                                                          CHAINLK_EXCLREQ)) {
                                           tsleep(object, PINTERLOCKED,
                                                  "objchnx", 0);
                                   }
                                   /* retry */
                           } else {
                                   if (atomic_cmpset_int(&object->chainlk,
                                                         chainlk,
                                                         (chainlk | CHAINLK_EXCL) &
                                                         ~(CHAINLK_EXCLREQ |
                                                           CHAINLK_WAIT))) {
                                           if (chainlk & CHAINLK_WAIT)
                                                   wakeup(object);
                                           break;
                                   }
                                   /* retry */
                           }
                   }
                   /* retry */
           }
   }
   
   void
   vm_object_chain_release(vm_object_t object)
   {
           /*ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));*/
           if (object->type != OBJT_DEFAULT && object->type != OBJT_SWAP)
                   return;
           KKASSERT(object->chainlk & (CHAINLK_MASK | CHAINLK_EXCL));
           for (;;) {
                   uint32_t chainlk = object->chainlk;
   
                   cpu_ccfence();
                   if (chainlk & CHAINLK_MASK) {
                           if ((chainlk & CHAINLK_MASK) == 1 &&
                               atomic_cmpset_int(&object->chainlk,
                                                 chainlk,
                                                 (chainlk - 1) & ~CHAINLK_WAIT)) {
                                   if (chainlk & CHAINLK_WAIT)
                                           wakeup(object);
                                   break;
                           }
                           if ((chainlk & CHAINLK_MASK) > 1 &&
                               atomic_cmpset_int(&object->chainlk,
                                                 chainlk, chainlk - 1)) {
                                   break;
                           }
                           /* retry */
                   } else {
                           KKASSERT(chainlk & CHAINLK_EXCL);
                           if (atomic_cmpset_int(&object->chainlk,
                                                 chainlk,
                                                 chainlk & ~(CHAINLK_EXCL |
                                                             CHAINLK_WAIT))) {
                                   if (chainlk & CHAINLK_WAIT)
                                           wakeup(object);
                                   break;
                           }
                   }
           }
   }
   
   /*
    * Release the chain from first_object through and including stopobj.
    * The caller is typically holding the first and last object locked
    * (shared or exclusive) to prevent destruction races.
    *
    * We release stopobj first as an optimization as this object is most
    * likely to be shared across multiple processes.
    */
   void
   vm_object_chain_release_all(vm_object_t first_object, vm_object_t stopobj)
   {
           vm_object_t backing_object;
           vm_object_t object;
   
           vm_object_chain_release(stopobj);
           object = first_object;
   
           while (object != stopobj) {
                   KKASSERT(object);
                   backing_object = object->backing_object;
                   vm_object_chain_release(object);
                   object = backing_object;
           }
   }
   
   /*
    * Dereference an object and its underlying vnode.  The object may be
    * held shared.  On return the object will remain held.
    *
    * This function may return a vnode in *vpp which the caller must release
    * after the caller drops its own lock.  If vpp is NULL, we assume that
    * the caller was holding an exclusive lock on the object and we vrele()
    * the vp ourselves.
    */
   static void
   vm_object_vndeallocate(vm_object_t object, struct vnode **vpp)
   {
           struct vnode *vp = (struct vnode *) object->handle;
   
           KASSERT(object->type == OBJT_VNODE,
               ("vm_object_vndeallocate: not a vnode object"));
           KASSERT(vp != NULL, ("vm_object_vndeallocate: missing vp"));
           ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
   #ifdef INVARIANTS
           if (object->ref_count == 0) {
                   vprint("vm_object_vndeallocate", vp);
                   panic("vm_object_vndeallocate: bad object reference count");
           }
   #endif
           for (;;) {
                   int count = object->ref_count;
                   cpu_ccfence();
                   if (count == 1) {
                           vm_object_upgrade(object);
                           if (atomic_cmpset_int(&object->ref_count, count, 0)) {
                                   vclrflags(vp, VTEXT);
                                   break;
                           }
                   } else {
                           if (atomic_cmpset_int(&object->ref_count,
                                                 count, count - 1)) {
                                   break;
                           }
                   }
                   /* retry */
           }
   
           /*
            * vrele or return the vp to vrele.  We can only safely vrele(vp)
            * if the object was locked exclusively.  But there are two races
            * here.
            *
            * We had to upgrade the object above to safely clear VTEXT
            * but the alternative path where the shared lock is retained
            * can STILL race to 0 in other paths and cause our own vrele()
            * to terminate the vnode.  We can't allow that if the VM object
            * is still locked shared.
            */
           if (vpp)
                   *vpp = vp;
           else
                   vrele(vp);
   }
   
   /*
    * Release a reference to the specified object, gained either through a
    * vm_object_allocate or a vm_object_reference call.  When all references
    * are gone, storage associated with this object may be relinquished.
    *
    * The caller does not have to hold the object locked but must have control
    * over the reference in question in order to guarantee that the object
    * does not get ripped out from under us.
    *
    * XXX Currently all deallocations require an exclusive lock.
    */
   void
   vm_object_deallocate(vm_object_t object)
   {
           struct vnode *vp;
           int count;
   
           if (object == NULL)
                   return;
           for (;;) {
                   count = object->ref_count;
                   cpu_ccfence();
   
                   /*
                    * If decrementing the count enters into special handling
                    * territory (0, 1, or 2) we have to do it the hard way.
                    * Fortunate though, objects with only a few refs like this
                    * are not likely to be heavily contended anyway.
                    *
                    * For vnode objects we only care about 1->0 transitions.
                    */
                   if (count <= 3 || (object->type == OBJT_VNODE && count <= 1)) {
                           vm_object_hold(object);
                           vm_object_deallocate_locked(object);
                           vm_object_drop(object);
                           break;
                   }
   
                   /*
                    * Try to decrement ref_count without acquiring a hold on
                    * the object.  This is particularly important for the exec*()
                    * and exit*() code paths because the program binary may
                    * have a great deal of sharing and an exclusive lock will
                    * crowbar performance in those circumstances.
                    */
                   if (object->type == OBJT_VNODE) {
                           vp = (struct vnode *)object->handle;
                           if (atomic_cmpset_int(&object->ref_count,
                                                 count, count - 1)) {
                                   vrele(vp);
                                   break;
                           }
                           /* retry */
                   } else {
                           if (atomic_cmpset_int(&object->ref_count,
                                                 count, count - 1)) {
                                   break;
                           }
                           /* retry */
                   }
                   /* retry */
           }
   }
   
   void
   vm_object_deallocate_locked(vm_object_t object)
   {
           struct vm_object_dealloc_list *dlist = NULL;
           struct vm_object_dealloc_list *dtmp;
           vm_object_t temp;
           int must_drop = 0;
   
           /*
            * We may chain deallocate object, but additional objects may
            * collect on the dlist which also have to be deallocated.  We
            * must avoid a recursion, vm_object chains can get deep.
            */
   
   again:
           while (object != NULL) {
                   /*
                    * vnode case, caller either locked the object exclusively
                    * or this is a recursion with must_drop != 0 and the vnode
                    * object will be locked shared.
                    *
                    * If locked shared we have to drop the object before we can
                    * call vrele() or risk a shared/exclusive livelock.
                    */
                   if (object->type == OBJT_VNODE) {
                           ASSERT_LWKT_TOKEN_HELD(&object->token);
                           if (must_drop) {
                                   struct vnode *tmp_vp;
   
                                   vm_object_vndeallocate(object, &tmp_vp);
                                   vm_object_drop(object);
                                   must_drop = 0;
                                   object = NULL;
                                   vrele(tmp_vp);
                           } else {
                                   vm_object_vndeallocate(object, NULL);
                           }
                           break;
                   }
                   ASSERT_LWKT_TOKEN_HELD_EXCL(&object->token);
   
                   /*
                    * Normal case (object is locked exclusively)
                    */
                   if (object->ref_count == 0) {
                           panic("vm_object_deallocate: object deallocated "
                                 "too many times: %d", object->type);
                   }
                   if (object->ref_count > 2) {
                           atomic_add_int(&object->ref_count, -1);
                           break;
                   }
   
                   /*
                    * Here on ref_count of one or two, which are special cases for
                    * objects.
                    *
                    * Nominal ref_count > 1 case if the second ref is not from
                    * a shadow.
                    *
                    * (ONEMAPPING only applies to DEFAULT AND SWAP objects)
                    */
                   if (object->ref_count == 2 && object->shadow_count == 0) {
                           if (object->type == OBJT_DEFAULT ||
                               object->type == OBJT_SWAP) {
                                   vm_object_set_flag(object, OBJ_ONEMAPPING);
                           }
                           atomic_add_int(&object->ref_count, -1);
                           break;
                   }
   
                   /*
                    * If the second ref is from a shadow we chain along it
                    * upwards if object's handle is exhausted.
                    *
                    * We have to decrement object->ref_count before potentially
                    * collapsing the first shadow object or the collapse code
                    * will not be able to handle the degenerate case to remove
                    * object.  However, if we do it too early the object can
                    * get ripped out from under us.
                    */
                   if (object->ref_count == 2 && object->shadow_count == 1 &&
                       object->handle == NULL && (object->type == OBJT_DEFAULT ||
                                                  object->type == OBJT_SWAP)) {
                           temp = LIST_FIRST(&object->shadow_head);
                           KKASSERT(temp != NULL);
                           vm_object_hold(temp);
   
                           /*
                            * Wait for any paging to complete so the collapse
                            * doesn't (or isn't likely to) qcollapse.  pip
                            * waiting must occur before we acquire the
                            * chainlock.
                            */
                           while (
                                   temp->paging_in_progress ||
                                   object->paging_in_progress
                           ) {
                                   vm_object_pip_wait(temp, "objde1");
                                   vm_object_pip_wait(object, "objde2");
                           }
   
                           /*
                            * If the parent is locked we have to give up, as
                            * otherwise we would be acquiring locks in the
                            * wrong order and potentially deadlock.
                            */
                           if (temp->chainlk & (CHAINLK_EXCL | CHAINLK_MASK)) {
                                   vm_object_drop(temp);
                                   goto skip;
                           }
                           vm_object_chain_acquire(temp, 0);
   
                           /*
                            * Recheck/retry after the hold and the paging
                            * wait, both of which can block us.
                            */
                           if (object->ref_count != 2 ||
                               object->shadow_count != 1 ||
                               object->handle ||
                               LIST_FIRST(&object->shadow_head) != temp ||
                               (object->type != OBJT_DEFAULT &&
                                object->type != OBJT_SWAP)) {
                                   vm_object_chain_release(temp);
                                   vm_object_drop(temp);
                                   continue;
                           }
   
                           /*
                            * We can safely drop object's ref_count now.
                            */
                           KKASSERT(object->ref_count == 2);
                          atomic_add_int(&object->ref_count, -1);
  
                          /*
                           * If our single parent is not collapseable just
                           * decrement ref_count (2->1) and stop.
                           */
                          if (temp->handle || (temp->type != OBJT_DEFAULT &&
                                               temp->type != OBJT_SWAP)) {
                                  vm_object_chain_release(temp);
                                  vm_object_drop(temp);
                                  break;
                          }
  
                          /*
                           * At this point we have already dropped object's
                           * ref_count so it is possible for a race to
                           * deallocate obj out from under us.  Any collapse
                           * will re-check the situation.  We must not block
                           * until we are able to collapse.
                           *
                           * Bump temp's ref_count to avoid an unwanted
                           * degenerate recursion (can't call
                           * vm_object_reference_locked() because it asserts
                           * that CHAINLOCK is not set).
                           */
                          atomic_add_int(&temp->ref_count, 1);
                          KKASSERT(temp->ref_count > 1);
  
                          /*
                           * Collapse temp, then deallocate the extra ref
                           * formally.
                           */
                          vm_object_collapse(temp, &dlist);
                          vm_object_chain_release(temp);
                          if (must_drop) {
                                  vm_object_lock_swap();
                                  vm_object_drop(object);
                          }
                          object = temp;
                          must_drop = 1;
                          continue;
                  }
  
                  /*
                   * Drop the ref and handle termination on the 1->0 transition.
                   * We may have blocked above so we have to recheck.
                   */
  skip:
                  KKASSERT(object->ref_count != 0);
                  if (object->ref_count >= 2) {
                          atomic_add_int(&object->ref_count, -1);
                          break;
                  }
                  KKASSERT(object->ref_count == 1);
  
                  /*
                   * 1->0 transition.  Chain through the backing_object.
                   * Maintain the ref until we've located the backing object,
                   * then re-check.
                   */
                  while ((temp = object->backing_object) != NULL) {
                          if (temp->type == OBJT_VNODE)
                                  vm_object_hold_shared(temp);
                          else
                                  vm_object_hold(temp);
                          if (temp == object->backing_object)
                                  break;
                          vm_object_drop(temp);
                  }
  
                  /*
                   * 1->0 transition verified, retry if ref_count is no longer
                   * 1.  Otherwise disconnect the backing_object (temp) and
                   * clean up.
                   */
                  if (object->ref_count != 1) {
                          vm_object_drop(temp);
                          continue;
                  }
  
                  /*
                   * It shouldn't be possible for the object to be chain locked
                   * if we're removing the last ref on it.
                   */
                  KKASSERT((object->chainlk & (CHAINLK_EXCL|CHAINLK_MASK)) == 0);
  
                  if (temp) {
                          if (object->flags & OBJ_ONSHADOW) {
                                  LIST_REMOVE(object, shadow_list);
                                  temp->shadow_count--;
                                  temp->generation++;
                                  vm_object_clear_flag(object, OBJ_ONSHADOW);
                          }
                          object->backing_object = NULL;
                  }
  
                  atomic_add_int(&object->ref_count, -1);
                  if ((object->flags & OBJ_DEAD) == 0)
                          vm_object_terminate(object);
                  if (must_drop && temp)
                          vm_object_lock_swap();
                  if (must_drop)
                          vm_object_drop(object);
                  object = temp;
                  must_drop = 1;
          }
  
          if (must_drop && object)
                  vm_object_drop(object);
  
          /*
           * Additional tail recursion on dlist.  Avoid a recursion.  Objects
           * on the dlist have a hold count but are not locked.
           */
          if ((dtmp = dlist) != NULL) {
                  dlist = dtmp->next;
                  object = dtmp->object;
                  kfree(dtmp, M_TEMP);
  
                  vm_object_lock(object); /* already held, add lock */
                  must_drop = 1;          /* and we're responsible for it */
                  goto again;
          }
  }
  
  /*
   * Destroy the specified object, freeing up related resources.
   *
   * The object must have zero references.
   *
   * The object must held.  The caller is responsible for dropping the object
   * after terminate returns.  Terminate does NOT drop the object.
   */
  static int vm_object_terminate_callback(vm_page_t p, void *data);
  
  void
  vm_object_terminate(vm_object_t object)
  {
          int n;
  
          /*
           * Make sure no one uses us.  Once we set OBJ_DEAD we should be
           * able to safely block.
           */
          ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
          KKASSERT((object->flags & OBJ_DEAD) == 0);
          vm_object_set_flag(object, OBJ_DEAD);
  
          /*
           * Wait for the pageout daemon to be done with the object
           */
          vm_object_pip_wait(object, "objtrm1");
  
          KASSERT(!object->paging_in_progress,
                  ("vm_object_terminate: pageout in progress"));
  
          /*
           * Clean and free the pages, as appropriate. All references to the
           * object are gone, so we don't need to lock it.
           */
          if (object->type == OBJT_VNODE) {
                  struct vnode *vp;
  
                  /*
                   * Clean pages and flush buffers.
                   *
                   * NOTE!  TMPFS buffer flushes do not typically flush the
                   *        actual page to swap as this would be highly
                   *        inefficient, and normal filesystems usually wrap
                   *        page flushes with buffer cache buffers.
                   *
                   *        To deal with this we have to call vinvalbuf() both
                   *        before and after the vm_object_page_clean().
                   */
                  vp = (struct vnode *) object->handle;
                  vinvalbuf(vp, V_SAVE, 0, 0);
                  vm_object_page_clean(object, 0, 0, OBJPC_SYNC);
                  vinvalbuf(vp, V_SAVE, 0, 0);
          }
  
          /*
           * Wait for any I/O to complete, after which there had better not
           * be any references left on the object.
           */
          vm_object_pip_wait(object, "objtrm2");
  
          if (object->ref_count != 0) {
                  panic("vm_object_terminate: object with references, "
                        "ref_count=%d", object->ref_count);
          }
  
          /*
           * Cleanup any shared pmaps associated with this object.
           */
          pmap_object_free(object);
  
          /*
           * Now free any remaining pages. For internal objects, this also
           * removes them from paging queues. Don't free wired pages, just
           * remove them from the object. 
           */
          vm_page_rb_tree_RB_SCAN(&object->rb_memq, NULL,
                                  vm_object_terminate_callback, NULL);
  
          /*
           * Let the pager know object is dead.
           */
          vm_pager_deallocate(object);
  
          /*
           * Wait for the object hold count to hit 1, clean out pages as
           * we go.  vmobj_token interlocks any race conditions that might
           * pick the object up from the vm_object_list after we have cleared
           * rb_memq.
           */
          for (;;) {
                  if (RB_ROOT(&object->rb_memq) == NULL)
                          break;
                  kprintf("vm_object_terminate: Warning, object %p "
                          "still has %d pages\n",
                          object, object->resident_page_count);
                  vm_page_rb_tree_RB_SCAN(&object->rb_memq, NULL,
                                          vm_object_terminate_callback, NULL);
          }
  
          /*
           * There had better not be any pages left
           */
          KKASSERT(object->resident_page_count == 0);
  
          /*
           * Remove the object from the global object list.
           */
          n = VMOBJ_HASH(object);
          lwkt_gettoken(&vmobj_tokens[n]);
          TAILQ_REMOVE(&vm_object_lists[n], object, object_list);
          lwkt_reltoken(&vmobj_tokens[n]);
          atomic_add_long(&vm_object_count, -1);
  
          if (object->ref_count != 0) {
                  panic("vm_object_terminate2: object with references, "
                        "ref_count=%d", object->ref_count);
          }
  
          /*
           * NOTE: The object hold_count is at least 1, so we cannot zfree()
           *       the object here.  See vm_object_drop().
           */
  }
  
  /*
   * The caller must hold the object.
   */
  static int
  vm_object_terminate_callback(vm_page_t p, void *data __unused)
  {
          vm_object_t object;
  
          object = p->object;
          vm_page_busy_wait(p, TRUE, "vmpgtrm");
          if (object != p->object) {
                  kprintf("vm_object_terminate: Warning: Encountered "
                          "busied page %p on queue %d\n", p, p->queue);
                  vm_page_wakeup(p);
          } else if (p->wire_count == 0) {
                  /*
                   * NOTE: p->dirty and PG_NEED_COMMIT are ignored.
                   */
                  vm_page_free(p);
                  mycpu->gd_cnt.v_pfree++;
          } else {
                  if (p->queue != PQ_NONE)
                          kprintf("vm_object_terminate: Warning: Encountered "
                                  "wired page %p on queue %d\n", p, p->queue);
                  vm_page_remove(p);
                  vm_page_wakeup(p);
          }
          lwkt_yield();
          return(0);
  }
  
  /*
   * Clean all dirty pages in the specified range of object.  Leaves page
   * on whatever queue it is currently on.   If NOSYNC is set then do not
   * write out pages with PG_NOSYNC set (originally comes from MAP_NOSYNC),
   * leaving the object dirty.
   *
   * When stuffing pages asynchronously, allow clustering.  XXX we need a
   * synchronous clustering mode implementation.
   *
   * Odd semantics: if start == end, we clean everything.
   *
   * The object must be locked? XXX
   */
  static int vm_object_page_clean_pass1(struct vm_page *p, void *data);
  static int vm_object_page_clean_pass2(struct vm_page *p, void *data);
  
  void
  vm_object_page_clean(vm_object_t object, vm_pindex_t start, vm_pindex_t end,
                       int flags)
  {
          struct rb_vm_page_scan_info info;
          struct vnode *vp;
          int wholescan;
          int pagerflags;
          int generation;
  
          vm_object_hold(object);
          if (object->type != OBJT_VNODE ||
              (object->flags & OBJ_MIGHTBEDIRTY) == 0) {
                  vm_object_drop(object);
                  return;
          }
  
          pagerflags = (flags & (OBJPC_SYNC | OBJPC_INVAL)) ? 
                          VM_PAGER_PUT_SYNC : VM_PAGER_CLUSTER_OK;
          pagerflags |= (flags & OBJPC_INVAL) ? VM_PAGER_PUT_INVAL : 0;
  
          vp = object->handle;
  
          /*
           * Interlock other major object operations.  This allows us to 
           * temporarily clear OBJ_WRITEABLE and OBJ_MIGHTBEDIRTY.
           */
          vm_object_set_flag(object, OBJ_CLEANING);
  
          /*
           * Handle 'entire object' case
           */
          info.start_pindex = start;
          if (end == 0) {
                  info.end_pindex = object->size - 1;
          } else {
                  info.end_pindex = end - 1;
          }
          wholescan = (start == 0 && info.end_pindex == object->size - 1);
          info.limit = flags;
          info.pagerflags = pagerflags;
          info.object = object;
  
          /*
           * If cleaning the entire object do a pass to mark the pages read-only.
           * If everything worked out ok, clear OBJ_WRITEABLE and
           * OBJ_MIGHTBEDIRTY.
           */
          if (wholescan) {
                  info.error = 0;
                  vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                          vm_object_page_clean_pass1, &info);
                  if (info.error == 0) {
                          vm_object_clear_flag(object,
                                               OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY);
                          if (object->type == OBJT_VNODE &&
                              (vp = (struct vnode *)object->handle) != NULL) {
                                  /*
                                   * Use new-style interface to clear VISDIRTY
                                   * because the vnode is not necessarily removed
                                   * from the syncer list(s) as often as it was
                                   * under the old interface, which can leave
                                   * the vnode on the syncer list after reclaim.
                                   */
                                  vclrobjdirty(vp);
                          }
                  }
          }
  
          /*
           * Do a pass to clean all the dirty pages we find.
           */
          do {
                  info.error = 0;
                  generation = object->generation;
                  vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                          vm_object_page_clean_pass2, &info);
          } while (info.error || generation != object->generation);
  
          vm_object_clear_flag(object, OBJ_CLEANING);
          vm_object_drop(object);
  }
  
  /*
   * The caller must hold the object.
   */
  static 
  int
  vm_object_page_clean_pass1(struct vm_page *p, void *data)
  {
          struct rb_vm_page_scan_info *info = data;
  
          vm_page_flag_set(p, PG_CLEANCHK);
          if ((info->limit & OBJPC_NOSYNC) && (p->flags & PG_NOSYNC)) {
                  info->error = 1;
          } else if (vm_page_busy_try(p, FALSE) == 0) {
                  vm_page_protect(p, VM_PROT_READ);       /* must not block */
                  vm_page_wakeup(p);
          } else {
                  info->error = 1;
          }
          lwkt_yield();
          return(0);
  }
  
  /*
   * The caller must hold the object
   */
  static 
  int
  vm_object_page_clean_pass2(struct vm_page *p, void *data)
  {
          struct rb_vm_page_scan_info *info = data;
          int generation;
  
          /*
           * Do not mess with pages that were inserted after we started
           * the cleaning pass.
           */
          if ((p->flags & PG_CLEANCHK) == 0)
                  goto done;
  
          generation = info->object->generation;
          vm_page_busy_wait(p, TRUE, "vpcwai");
          if (p->object != info->object ||
              info->object->generation != generation) {
                  info->error = 1;
                  vm_page_wakeup(p);
                  goto done;
          }
  
          /*
           * Before wasting time traversing the pmaps, check for trivial
           * cases where the page cannot be dirty.
           */
          if (p->valid == 0 || (p->queue - p->pc) == PQ_CACHE) {
                  KKASSERT((p->dirty & p->valid) == 0 &&
                           (p->flags & PG_NEED_COMMIT) == 0);
                  vm_page_wakeup(p);
                  goto done;
          }
  
          /*
           * Check whether the page is dirty or not.  The page has been set
           * to be read-only so the check will not race a user dirtying the
           * page.
           */
          vm_page_test_dirty(p);
          if ((p->dirty & p->valid) == 0 && (p->flags & PG_NEED_COMMIT) == 0) {
                  vm_page_flag_clear(p, PG_CLEANCHK);
                  vm_page_wakeup(p);
                  goto done;
          }
  
          /*
           * If we have been asked to skip nosync pages and this is a
           * nosync page, skip it.  Note that the object flags were
           * not cleared in this case (because pass1 will have returned an
           * error), so we do not have to set them.
           */
          if ((info->limit & OBJPC_NOSYNC) && (p->flags & PG_NOSYNC)) {
                  vm_page_flag_clear(p, PG_CLEANCHK);
                  vm_page_wakeup(p);
                  goto done;
          }
  
          /*
           * Flush as many pages as we can.  PG_CLEANCHK will be cleared on
           * the pages that get successfully flushed.  Set info->error if
           * we raced an object modification.
           */
          vm_object_page_collect_flush(info->object, p, info->pagerflags);
          vm_wait_nominal();
  done:
          lwkt_yield();
          return(0);
  }
  
  /*
   * Collect the specified page and nearby pages and flush them out.
   * The number of pages flushed is returned.  The passed page is busied
   * by the caller and we are responsible for its disposition.
   *
   * The caller must hold the object.
   */
  static void
  vm_object_page_collect_flush(vm_object_t object, vm_page_t p, int pagerflags)
  {
          int error;
          int is;
          int ib;
          int i;
          int page_base;
          vm_pindex_t pi;
          vm_page_t ma[BLIST_MAX_ALLOC];
  
          ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
  
          pi = p->pindex;
          page_base = pi % BLIST_MAX_ALLOC;
          ma[page_base] = p;
          ib = page_base - 1;
          is = page_base + 1;
  
          while (ib >= 0) {
                  vm_page_t tp;
  
                  tp = vm_page_lookup_busy_try(object, pi - page_base + ib,
                                               TRUE, &error);
                  if (error)
                          break;
                  if (tp == NULL)
                          break;
                  if ((pagerflags & VM_PAGER_IGNORE_CLEANCHK) == 0 &&
                      (tp->flags & PG_CLEANCHK) == 0) {
                          vm_page_wakeup(tp);
                          break;
                  }
                  if ((tp->queue - tp->pc) == PQ_CACHE) {
                          vm_page_flag_clear(tp, PG_CLEANCHK);
                          vm_page_wakeup(tp);
                          break;
                  }
                  vm_page_test_dirty(tp);
                  if ((tp->dirty & tp->valid) == 0 &&
                      (tp->flags & PG_NEED_COMMIT) == 0) {
                          vm_page_flag_clear(tp, PG_CLEANCHK);
                          vm_page_wakeup(tp);
                          break;
                  }
                  ma[ib] = tp;
                  --ib;
          }
          ++ib;   /* fixup */
  
          while (is < BLIST_MAX_ALLOC &&
                 pi - page_base + is < object->size) {
                  vm_page_t tp;
  
                  tp = vm_page_lookup_busy_try(object, pi - page_base + is,
                                               TRUE, &error);
                  if (error)
                          break;
                  if (tp == NULL)
                          break;
                  if ((pagerflags & VM_PAGER_IGNORE_CLEANCHK) == 0 &&
                      (tp->flags & PG_CLEANCHK) == 0) {
                          vm_page_wakeup(tp);
                          break;
                  }
                  if ((tp->queue - tp->pc) == PQ_CACHE) {
                          vm_page_flag_clear(tp, PG_CLEANCHK);
                          vm_page_wakeup(tp);
                          break;
                  }
                  vm_page_test_dirty(tp);
                  if ((tp->dirty & tp->valid) == 0 &&
                      (tp->flags & PG_NEED_COMMIT) == 0) {
                          vm_page_flag_clear(tp, PG_CLEANCHK);
                          vm_page_wakeup(tp);
                          break;
                  }
                  ma[is] = tp;
                  ++is;
          }
  
          /*
           * All pages in the ma[] array are busied now
           */
          for (i = ib; i < is; ++i) {
                  vm_page_flag_clear(ma[i], PG_CLEANCHK);
                  vm_page_hold(ma[i]);    /* XXX need this any more? */
          }
          vm_pageout_flush(&ma[ib], is - ib, pagerflags);
          for (i = ib; i < is; ++i)       /* XXX need this any more? */
                  vm_page_unhold(ma[i]);
  }
  
  /*
   * Same as vm_object_pmap_copy, except range checking really
   * works, and is meant for small sections of an object.
   *
   * This code protects resident pages by making them read-only
   * and is typically called on a fork or split when a page
   * is converted to copy-on-write.  
   *
   * NOTE: If the page is already at VM_PROT_NONE, calling
   * vm_page_protect will have no effect.
   */
  void
  vm_object_pmap_copy_1(vm_object_t object, vm_pindex_t start, vm_pindex_t end)
  {
          vm_pindex_t idx;
          vm_page_t p;
  
          if (object == NULL || (object->flags & OBJ_WRITEABLE) == 0)
                  return;
  
          vm_object_hold(object);
          for (idx = start; idx < end; idx++) {
                  p = vm_page_lookup(object, idx);
                  if (p == NULL)
                          continue;
                  vm_page_protect(p, VM_PROT_READ);
          }
          vm_object_drop(object);
  }
  
  /*
   * Removes all physical pages in the specified object range from all
   * physical maps.
   *
   * The object must *not* be locked.
   */
  
  static int vm_object_pmap_remove_callback(vm_page_t p, void *data);
  
  void
  vm_object_pmap_remove(vm_object_t object, vm_pindex_t start, vm_pindex_t end)
  {
          struct rb_vm_page_scan_info info;
  
          if (object == NULL)
                  return;
          info.start_pindex = start;
          info.end_pindex = end - 1;
  
          vm_object_hold(object);
          vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                  vm_object_pmap_remove_callback, &info);
          if (start == 0 && end == object->size)
                  vm_object_clear_flag(object, OBJ_WRITEABLE);
          vm_object_drop(object);
  }
  
  /*
   * The caller must hold the object
   */
  static int
  vm_object_pmap_remove_callback(vm_page_t p, void *data __unused)
  {
          vm_page_protect(p, VM_PROT_NONE);
          return(0);
  }
  
  /*
   * Implements the madvise function at the object/page level.
   *
   * MADV_WILLNEED        (any object)
   *
   *      Activate the specified pages if they are resident.
   *
   * MADV_DONTNEED        (any object)
   *
   *      Deactivate the specified pages if they are resident.
   *
   * MADV_FREE    (OBJT_DEFAULT/OBJT_SWAP objects, OBJ_ONEMAPPING only)
   *
   *      Deactivate and clean the specified pages if they are
   *      resident.  This permits the process to reuse the pages
   *      without faulting or the kernel to reclaim the pages
   *      without I/O.
   *
   * No requirements.
   */
  void
  vm_object_madvise(vm_object_t object, vm_pindex_t pindex, int count, int advise)
  {
          vm_pindex_t end, tpindex;
          vm_object_t tobject;
          vm_object_t xobj;
          vm_page_t m;
          int error;
  
          if (object == NULL)
                  return;
  
          end = pindex + count;
  
          vm_object_hold(object);
          tobject = object;
  
          /*
           * Locate and adjust resident pages
           */
          for (; pindex < end; pindex += 1) {
  relookup:
                  if (tobject != object)
                          vm_object_drop(tobject);
                  tobject = object;
                  tpindex = pindex;
  shadowlookup:
                  /*
                   * MADV_FREE only operates on OBJT_DEFAULT or OBJT_SWAP pages
                   * and those pages must be OBJ_ONEMAPPING.
                   */
                  if (advise == MADV_FREE) {
                          if ((tobject->type != OBJT_DEFAULT &&
                               tobject->type != OBJT_SWAP) ||
                              (tobject->flags & OBJ_ONEMAPPING) == 0) {
                                  continue;
                          }
                  }
  
                  m = vm_page_lookup_busy_try(tobject, tpindex, TRUE, &error);
  
                  if (error) {
                          vm_page_sleep_busy(m, TRUE, "madvpo");
                          goto relookup;
                  }
                  if (m == NULL) {
                          /*
                           * There may be swap even if there is no backing page
                           */
                          if (advise == MADV_FREE && tobject->type == OBJT_SWAP)
                                  swap_pager_freespace(tobject, tpindex, 1);
  
                          /*
                           * next object
                           */
                          while ((xobj = tobject->backing_object) != NULL) {
                                  KKASSERT(xobj != object);
                                  vm_object_hold(xobj);
                                  if (xobj == tobject->backing_object)
                                          break;
                                  vm_object_drop(xobj);
                          }
                          if (xobj == NULL)
                                  continue;
                          tpindex += OFF_TO_IDX(tobject->backing_object_offset);
                          if (tobject != object) {
                                  vm_object_lock_swap();
                                  vm_object_drop(tobject);
                          }
                          tobject = xobj;
                          goto shadowlookup;
                  }
  
                  /*
                   * If the page is not in a normal active state, we skip it.
                   * If the page is not managed there are no page queues to
                   * mess with.  Things can break if we mess with pages in
                   * any of the below states.
                   */
                  if (m->wire_count ||
                      (m->flags & (PG_UNMANAGED | PG_NEED_COMMIT)) ||
                      m->valid != VM_PAGE_BITS_ALL
                  ) {
                          vm_page_wakeup(m);
                          continue;
                  }
  
                  /*
                   * Theoretically once a page is known not to be busy, an
                   * interrupt cannot come along and rip it out from under us.
                   */
  
                  if (advise == MADV_WILLNEED) {
                          vm_page_activate(m);
                  } else if (advise == MADV_DONTNEED) {
                          vm_page_dontneed(m);
                  } else if (advise == MADV_FREE) {
                          /*
                           * Mark the page clean.  This will allow the page
                           * to be freed up by the system.  However, such pages
                           * are often reused quickly by malloc()/free()
                           * so we do not do anything that would cause
                           * a page fault if we can help it.
                           *
                           * Specifically, we do not try to actually free
                           * the page now nor do we try to put it in the
                           * cache (which would cause a page fault on reuse).
                           *
                           * But we do make the page is freeable as we
                           * can without actually taking the step of unmapping
                           * it.
                           */
                          pmap_clear_modify(m);
                          m->dirty = 0;
                          m->act_count = 0;
                          vm_page_dontneed(m);
                          if (tobject->type == OBJT_SWAP)
                                  swap_pager_freespace(tobject, tpindex, 1);
                  }
                  vm_page_wakeup(m);
          }       
          if (tobject != object)
                  vm_object_drop(tobject);
          vm_object_drop(object);
  }
  
  /*
   * Create a new object which is backed by the specified existing object
   * range.  Replace the pointer and offset that was pointing at the existing
   * object with the pointer/offset for the new object.
   *
   * No other requirements.
   */
  void
  vm_object_shadow(vm_object_t *objectp, vm_ooffset_t *offset, vm_size_t length,
                   int addref)
  {
          vm_object_t source;
          vm_object_t result;
          int useshadowlist;
  
          source = *objectp;
  
          /*
           * Don't create the new object if the old object isn't shared.
           * We have to chain wait before adding the reference to avoid
           * racing a collapse or deallocation.
           *
           * Add the additional ref to source here to avoid racing a later
           * collapse or deallocation. Clear the ONEMAPPING flag whether
           * addref is TRUE or not in this case because the original object
           * will be shadowed.
           */
          useshadowlist = 0;
          if (source) {
                  if (source->type != OBJT_VNODE) {
                          useshadowlist = 1;
                          vm_object_hold(source);
                          vm_object_chain_wait(source, 0);
                          if (source->ref_count == 1 &&
                              source->handle == NULL &&
                              (source->type == OBJT_DEFAULT ||
                               source->type == OBJT_SWAP)) {
                                  if (addref) {
                                          vm_object_reference_locked(source);
                                          vm_object_clear_flag(source, OBJ_ONEMAPPING);
                                  }
                                  vm_object_drop(source);
                                  return;
                          }
                          vm_object_reference_locked(source);
                          vm_object_clear_flag(source, OBJ_ONEMAPPING);
                  } else {
                          vm_object_reference_quick(source);
                          vm_object_clear_flag(source, OBJ_ONEMAPPING);
                  }
          }
  
          /*
           * Allocate a new object with the given length.  The new object
           * is returned referenced but we may have to add another one.
           * If we are adding a second reference we must clear OBJ_ONEMAPPING.
           * (typically because the caller is about to clone a vm_map_entry).
           *
           * The source object currently has an extra reference to prevent
           * collapses into it while we mess with its shadow list, which
           * we will remove later in this routine.
           */
          if ((result = vm_object_allocate(OBJT_DEFAULT, length)) == NULL)
                  panic("vm_object_shadow: no object for shadowing");
          vm_object_hold(result);
          if (addref) {
                  vm_object_reference_locked(result);
                  vm_object_clear_flag(result, OBJ_ONEMAPPING);
          }
  
          /*
           * The new object shadows the source object.  Chain wait before
           * adjusting shadow_count or the shadow list to avoid races.
           *
           * Try to optimize the result object's page color when shadowing
           * in order to maintain page coloring consistency in the combined 
           * shadowed object.
           *
           * SHADOWING IS NOT APPLICABLE TO OBJT_VNODE OBJECTS
           */
          KKASSERT(result->backing_object == NULL);
          result->backing_object = source;
          if (source) {
                  if (useshadowlist) {
                          vm_object_chain_wait(source, 0);
                          LIST_INSERT_HEAD(&source->shadow_head,
                                           result, shadow_list);
                          source->shadow_count++;
                          source->generation++;
                          vm_object_set_flag(result, OBJ_ONSHADOW);
                  }
                  /* cpu localization twist */
                  result->pg_color = (int)(intptr_t)curthread;
          }
  
          /*
           * Adjust the return storage.  Drop the ref on source before
           * returning.
           */
          result->backing_object_offset = *offset;
          vm_object_drop(result);
          *offset = 0;
          if (source) {
                  if (useshadowlist) {
                          vm_object_deallocate_locked(source);
                          vm_object_drop(source);
                  } else {
                          vm_object_deallocate(source);
                  }
          }
  
          /*
           * Return the new things
           */
          *objectp = result;
  }
  
  #define OBSC_TEST_ALL_SHADOWED  0x0001
  #define OBSC_COLLAPSE_NOWAIT    0x0002
  #define OBSC_COLLAPSE_WAIT      0x0004
  
  static int vm_object_backing_scan_callback(vm_page_t p, void *data);
  
  /*
   * The caller must hold the object.
   */
  static __inline int
  vm_object_backing_scan(vm_object_t object, vm_object_t backing_object, int op)
  {
          struct rb_vm_page_scan_info info;
          int n;
  
          vm_object_assert_held(object);
          vm_object_assert_held(backing_object);
  
          KKASSERT(backing_object == object->backing_object);
          info.backing_offset_index = OFF_TO_IDX(object->backing_object_offset);
  
          /*
           * Initial conditions
           */
          if (op & OBSC_TEST_ALL_SHADOWED) {
                  /*
                   * We do not want to have to test for the existence of
                   * swap pages in the backing object.  XXX but with the
                   * new swapper this would be pretty easy to do.
                   *
                   * XXX what about anonymous MAP_SHARED memory that hasn't
                   * been ZFOD faulted yet?  If we do not test for this, the
                   * shadow test may succeed! XXX
                   */
                  if (backing_object->type != OBJT_DEFAULT)
                          return(0);
          }
          if (op & OBSC_COLLAPSE_WAIT) {
                  KKASSERT((backing_object->flags & OBJ_DEAD) == 0);
                  vm_object_set_flag(backing_object, OBJ_DEAD);
  
                  n = VMOBJ_HASH(backing_object);
                  lwkt_gettoken(&vmobj_tokens[n]);
                  TAILQ_REMOVE(&vm_object_lists[n], backing_object, object_list);
                  lwkt_reltoken(&vmobj_tokens[n]);
                  atomic_add_long(&vm_object_count, -1);
          }
  
          /*
           * Our scan.   We have to retry if a negative error code is returned,
           * otherwise 0 or 1 will be returned in info.error.  0 Indicates that
           * the scan had to be stopped because the parent does not completely
           * shadow the child.
           */
          info.object = object;
          info.backing_object = backing_object;
          info.limit = op;
          do {
                  info.error = 1;
                  vm_page_rb_tree_RB_SCAN(&backing_object->rb_memq, NULL,
                                          vm_object_backing_scan_callback,
                                          &info);
          } while (info.error < 0);
  
          return(info.error);
  }
  
  /*
   * The caller must hold the object.
   */
  static int
  vm_object_backing_scan_callback(vm_page_t p, void *data)
  {
          struct rb_vm_page_scan_info *info = data;
          vm_object_t backing_object;
          vm_object_t object;
          vm_pindex_t pindex;
          vm_pindex_t new_pindex;
          vm_pindex_t backing_offset_index;
          int op;
  
          pindex = p->pindex;
          new_pindex = pindex - info->backing_offset_index;
          op = info->limit;
          object = info->object;
          backing_object = info->backing_object;
          backing_offset_index = info->backing_offset_index;
  
          if (op & OBSC_TEST_ALL_SHADOWED) {
                  vm_page_t pp;
  
                  /*
                   * Ignore pages outside the parent object's range
                   * and outside the parent object's mapping of the 
                   * backing object.
                   *
                   * note that we do not busy the backing object's
                   * page.
                   */
                  if (pindex < backing_offset_index ||
                      new_pindex >= object->size
                  ) {
                          return(0);
                  }
  
                  /*
                   * See if the parent has the page or if the parent's
                   * object pager has the page.  If the parent has the
                   * page but the page is not valid, the parent's
                   * object pager must have the page.
                   *
                   * If this fails, the parent does not completely shadow
                   * the object and we might as well give up now.
                   */
                  pp = vm_page_lookup(object, new_pindex);
                  if ((pp == NULL || pp->valid == 0) &&
                      !vm_pager_has_page(object, new_pindex)
                  ) {
                          info->error = 0;        /* problemo */
                          return(-1);             /* stop the scan */
                  }
          }
  
          /*
           * Check for busy page.  Note that we may have lost (p) when we
           * possibly blocked above.
           */
          if (op & (OBSC_COLLAPSE_WAIT | OBSC_COLLAPSE_NOWAIT)) {
                  vm_page_t pp;
  
                  if (vm_page_busy_try(p, TRUE)) {
                          if (op & OBSC_COLLAPSE_NOWAIT) {
                                  return(0);
                          } else {
                                  /*
                                   * If we slept, anything could have
                                   * happened.   Ask that the scan be restarted.
                                   *
                                   * Since the object is marked dead, the
                                   * backing offset should not have changed.  
                                   */
                                  vm_page_sleep_busy(p, TRUE, "vmocol");
                                  info->error = -1;
                                  return(-1);
                          }
                  }
  
                  /*
                   * If (p) is no longer valid restart the scan.
                   */
                  if (p->object != backing_object || p->pindex != pindex) {
                          kprintf("vm_object_backing_scan: Warning: page "
                                  "%p ripped out from under us\n", p);
                          vm_page_wakeup(p);
                          info->error = -1;
                          return(-1);
                  }
  
                  if (op & OBSC_COLLAPSE_NOWAIT) {
                          if (p->valid == 0 ||
                              p->wire_count ||
                              (p->flags & PG_NEED_COMMIT)) {
                                  vm_page_wakeup(p);
                                  return(0);
                          }
                  } else {
                          /* XXX what if p->valid == 0 , hold_count, etc? */
                  }
  
                  KASSERT(
                      p->object == backing_object,
                      ("vm_object_qcollapse(): object mismatch")
                  );
  
                  /*
                   * Destroy any associated swap
                   */
                  if (backing_object->type == OBJT_SWAP)
                          swap_pager_freespace(backing_object, p->pindex, 1);
  
                  if (
                      p->pindex < backing_offset_index ||
                      new_pindex >= object->size
                  ) {
                          /*
                           * Page is out of the parent object's range, we 
                           * can simply destroy it. 
                           */
                          vm_page_protect(p, VM_PROT_NONE);
                          vm_page_free(p);
                          return(0);
                  }
  
                  pp = vm_page_lookup(object, new_pindex);
                  if (pp != NULL || vm_pager_has_page(object, new_pindex)) {
                          /*
                           * page already exists in parent OR swap exists
                           * for this location in the parent.  Destroy 
                           * the original page from the backing object.
                           *
                           * Leave the parent's page alone
                           */
                          vm_page_protect(p, VM_PROT_NONE);
                          vm_page_free(p);
                          return(0);
                  }
  
                  /*
                   * Page does not exist in parent, rename the
                   * page from the backing object to the main object. 
                   *
                   * If the page was mapped to a process, it can remain 
                   * mapped through the rename.
                   */
                  if ((p->queue - p->pc) == PQ_CACHE)
                          vm_page_deactivate(p);
  
                  vm_page_rename(p, object, new_pindex);
                  vm_page_wakeup(p);
                  /* page automatically made dirty by rename */
          }
          return(0);
  }
  
  /*
   * This version of collapse allows the operation to occur earlier and
   * when paging_in_progress is true for an object...  This is not a complete
   * operation, but should plug 99.9% of the rest of the leaks.
   *
   * The caller must hold the object and backing_object and both must be
   * chainlocked.
   *
   * (only called from vm_object_collapse)
   */
  static void
  vm_object_qcollapse(vm_object_t object, vm_object_t backing_object)
  {
          if (backing_object->ref_count == 1) {
                  atomic_add_int(&backing_object->ref_count, 2);
                  vm_object_backing_scan(object, backing_object,
                                         OBSC_COLLAPSE_NOWAIT);
                  atomic_add_int(&backing_object->ref_count, -2);
          }
  }
  
  /*
   * Collapse an object with the object backing it.  Pages in the backing
   * object are moved into the parent, and the backing object is deallocated.
   * Any conflict is resolved in favor of the parent's existing pages.
   *
   * object must be held and chain-locked on call.
   *
   * The caller must have an extra ref on object to prevent a race from
   * destroying it during the collapse.
   */
  void
  vm_object_collapse(vm_object_t object, struct vm_object_dealloc_list **dlistp)
  {
          struct vm_object_dealloc_list *dlist = NULL;
          vm_object_t backing_object;
  
          /*
           * Only one thread is attempting a collapse at any given moment.
           * There are few restrictions for (object) that callers of this
           * function check so reentrancy is likely.
           */
          KKASSERT(object != NULL);
          vm_object_assert_held(object);
          KKASSERT(object->chainlk & (CHAINLK_MASK | CHAINLK_EXCL));
  
          for (;;) {
                  vm_object_t bbobj;
                  int dodealloc;
  
                  /*
                   * We can only collapse a DEFAULT/SWAP object with a
                   * DEFAULT/SWAP object.
                   */
                  if (object->type != OBJT_DEFAULT && object->type != OBJT_SWAP) {
                          backing_object = NULL;
                          break;
                  }
  
                  backing_object = object->backing_object;
                  if (backing_object == NULL)
                          break;
                  if (backing_object->type != OBJT_DEFAULT &&
                      backing_object->type != OBJT_SWAP) {
                          backing_object = NULL;
                          break;
                  }
  
                  /*
                   * Hold the backing_object and check for races
                   */
                  vm_object_hold(backing_object);
                  if (backing_object != object->backing_object ||
                      (backing_object->type != OBJT_DEFAULT &&
                       backing_object->type != OBJT_SWAP)) {
                          vm_object_drop(backing_object);
                          continue;
                  }
  
                  /*
                   * Chain-lock the backing object too because if we
                   * successfully merge its pages into the top object we
                   * will collapse backing_object->backing_object as the
                   * new backing_object.  Re-check that it is still our
                   * backing object.
                   */
                  vm_object_chain_acquire(backing_object, 0);
                  if (backing_object != object->backing_object) {
                          vm_object_chain_release(backing_object);
                          vm_object_drop(backing_object);
                          continue;
                  }
  
                  /*
                   * we check the backing object first, because it is most likely
                   * not collapsable.
                   */
                  if (backing_object->handle != NULL ||
                      (backing_object->type != OBJT_DEFAULT &&
                       backing_object->type != OBJT_SWAP) ||
                      (backing_object->flags & OBJ_DEAD) ||
                      object->handle != NULL ||
                      (object->type != OBJT_DEFAULT &&
                       object->type != OBJT_SWAP) ||
                      (object->flags & OBJ_DEAD)) {
                          break;
                  }
  
                  /*
                   * If paging is in progress we can't do a normal collapse.
                   */
                  if (
                      object->paging_in_progress != 0 ||
                      backing_object->paging_in_progress != 0
                  ) {
                          vm_object_qcollapse(object, backing_object);
                          break;
                  }
  
                  /*
                   * We know that we can either collapse the backing object (if
                   * the parent is the only reference to it) or (perhaps) have
                   * the parent bypass the object if the parent happens to shadow
                   * all the resident pages in the entire backing object.
                   *
                   * This is ignoring pager-backed pages such as swap pages.
                   * vm_object_backing_scan fails the shadowing test in this
                   * case.
                   */
                  if (backing_object->ref_count == 1) {
                          /*
                           * If there is exactly one reference to the backing
                           * object, we can collapse it into the parent.  
                           */
                          KKASSERT(object->backing_object == backing_object);
                          vm_object_backing_scan(object, backing_object,
                                                 OBSC_COLLAPSE_WAIT);
  
                          /*
                           * Move the pager from backing_object to object.
                           */
                          if (backing_object->type == OBJT_SWAP) {
                                  vm_object_pip_add(backing_object, 1);
  
                                  /*
                                   * scrap the paging_offset junk and do a 
                                   * discrete copy.  This also removes major 
                                   * assumptions about how the swap-pager 
                                   * works from where it doesn't belong.  The
                                   * new swapper is able to optimize the
                                   * destroy-source case.
                                   */
                                  vm_object_pip_add(object, 1);
                                  swap_pager_copy(backing_object, object,
                                      OFF_TO_IDX(object->backing_object_offset),
                                      TRUE);
                                  vm_object_pip_wakeup(object);
                                  vm_object_pip_wakeup(backing_object);
                          }
  
                          /*
                           * Object now shadows whatever backing_object did.
                           * Remove object from backing_object's shadow_list.
                           */
                          KKASSERT(object->backing_object == backing_object);
                          if (object->flags & OBJ_ONSHADOW) {
                                  LIST_REMOVE(object, shadow_list);
                                  backing_object->shadow_count--;
                                  backing_object->generation++;
                                  vm_object_clear_flag(object, OBJ_ONSHADOW);
                          }
  
                          /*
                           * backing_object->backing_object moves from within
                           * backing_object to within object.
                           *
                           * OBJT_VNODE bbobj's should have empty shadow lists.
                           */
                          while ((bbobj = backing_object->backing_object) != NULL) {
                                  if (bbobj->type == OBJT_VNODE)
                                          vm_object_hold_shared(bbobj);
                                  else
                                          vm_object_hold(bbobj);
                                  if (bbobj == backing_object->backing_object)
                                          break;
                                  vm_object_drop(bbobj);
                          }
                          if (bbobj) {
                                  if (backing_object->flags & OBJ_ONSHADOW) {
                                          /* not locked exclusively if vnode */
                                          KKASSERT(bbobj->type != OBJT_VNODE);
                                          LIST_REMOVE(backing_object,
                                                      shadow_list);
                                          bbobj->shadow_count--;
                                          bbobj->generation++;
                                          vm_object_clear_flag(backing_object,
                                                               OBJ_ONSHADOW);
                                  }
                                  backing_object->backing_object = NULL;
                          }
                          object->backing_object = bbobj;
                          if (bbobj) {
                                  if (bbobj->type != OBJT_VNODE) {
                                          LIST_INSERT_HEAD(&bbobj->shadow_head,
                                                           object, shadow_list);
                                          bbobj->shadow_count++;
                                          bbobj->generation++;
                                          vm_object_set_flag(object,
                                                             OBJ_ONSHADOW);
                                  }
                          }
  
                          object->backing_object_offset +=
                                  backing_object->backing_object_offset;
  
                          vm_object_drop(bbobj);
  
                          /*
                           * Discard the old backing_object.  Nothing should be
                           * able to ref it, other than a vm_map_split(),
                           * and vm_map_split() will stall on our chain lock.
                           * And we control the parent so it shouldn't be
                           * possible for it to go away either.
                           *
                           * Since the backing object has no pages, no pager
                           * left, and no object references within it, all
                           * that is necessary is to dispose of it.
                           */
                          KASSERT(backing_object->ref_count == 1,
                                  ("backing_object %p was somehow "
                                   "re-referenced during collapse!",
                                   backing_object));
                          KASSERT(RB_EMPTY(&backing_object->rb_memq),
                                  ("backing_object %p somehow has left "
                                   "over pages during collapse!",
                                   backing_object));
  
                          /*
                           * The object can be destroyed.
                           *
                           * XXX just fall through and dodealloc instead
                           *     of forcing destruction?
                           */
                          atomic_add_int(&backing_object->ref_count, -1);
                          if ((backing_object->flags & OBJ_DEAD) == 0)
                                  vm_object_terminate(backing_object);
                          object_collapses++;
                          dodealloc = 0;
                  } else {
                          /*
                           * If we do not entirely shadow the backing object,
                           * there is nothing we can do so we give up.
                           */
                          if (vm_object_backing_scan(object, backing_object,
                                                  OBSC_TEST_ALL_SHADOWED) == 0) {
                                  break;
                          }
  
                          /*
                           * bbobj is backing_object->backing_object.  Since
                           * object completely shadows backing_object we can
                           * bypass it and become backed by bbobj instead.
                           *
                           * The shadow list for vnode backing objects is not
                           * used and a shared hold is allowed.
                           */
                          while ((bbobj = backing_object->backing_object) != NULL) {
                                  if (bbobj->type == OBJT_VNODE)
                                          vm_object_hold_shared(bbobj);
                                  else
                                          vm_object_hold(bbobj);
                                  if (bbobj == backing_object->backing_object)
                                          break;
                                  vm_object_drop(bbobj);
                          }
  
                          /*
                           * Make object shadow bbobj instead of backing_object.
                           * Remove object from backing_object's shadow list.
                           *
                           * Deallocating backing_object will not remove
                           * it, since its reference count is at least 2.
                           */
                          KKASSERT(object->backing_object == backing_object);
                          if (object->flags & OBJ_ONSHADOW) {
                                  LIST_REMOVE(object, shadow_list);
                                  backing_object->shadow_count--;
                                  backing_object->generation++;
                                  vm_object_clear_flag(object, OBJ_ONSHADOW);
                          }
  
                          /*
                           * Add a ref to bbobj, bbobj now shadows object.
                           *
                           * NOTE: backing_object->backing_object still points
                           *       to bbobj.  That relationship remains intact
                           *       because backing_object has > 1 ref, so
                           *       someone else is pointing to it (hence why
                           *       we can't collapse it into object and can
                           *       only handle the all-shadowed bypass case).
                           */
                          if (bbobj) {
                                  if (bbobj->type != OBJT_VNODE) {
                                          vm_object_chain_wait(bbobj, 0);
                                          vm_object_reference_locked(bbobj);
                                          LIST_INSERT_HEAD(&bbobj->shadow_head,
                                                           object, shadow_list);
                                          bbobj->shadow_count++;
                                          bbobj->generation++;
                                          vm_object_set_flag(object,
                                                             OBJ_ONSHADOW);
                                  } else {
                                          vm_object_reference_quick(bbobj);
                                  }
                                  object->backing_object_offset +=
                                          backing_object->backing_object_offset;
                                  object->backing_object = bbobj;
                                  vm_object_drop(bbobj);
                          } else {
                                  object->backing_object = NULL;
                          }
  
                          /*
                           * Drop the reference count on backing_object.  To
                           * handle ref_count races properly we can't assume
                           * that the ref_count is still at least 2 so we
                           * have to actually call vm_object_deallocate()
                           * (after clearing the chainlock).
                           */
                          object_bypasses++;
                          dodealloc = 1;
                  }
  
                  /*
                   * Ok, we want to loop on the new object->bbobj association,
                   * possibly collapsing it further.  However if dodealloc is
                   * non-zero we have to deallocate the backing_object which
                   * itself can potentially undergo a collapse, creating a
                   * recursion depth issue with the LWKT token subsystem.
                   *
                   * In the case where we must deallocate the backing_object
                   * it is possible now that the backing_object has a single
                   * shadow count on some other object (not represented here
                   * as yet), since it no longer shadows us.  Thus when we
                   * call vm_object_deallocate() it may attempt to collapse
                   * itself into its remaining parent.
                   */
                  if (dodealloc) {
                          struct vm_object_dealloc_list *dtmp;
  
                          vm_object_chain_release(backing_object);
                          vm_object_unlock(backing_object);
                          /* backing_object remains held */
  
                          /*
                           * Auto-deallocation list for caller convenience.
                           */
                          if (dlistp == NULL)
                                  dlistp = &dlist;
  
                          dtmp = kmalloc(sizeof(*dtmp), M_TEMP, M_WAITOK);
                          dtmp->object = backing_object;
                          dtmp->next = *dlistp;
                          *dlistp = dtmp;
                  } else {
                          vm_object_chain_release(backing_object);
                          vm_object_drop(backing_object);
                  }
                  /* backing_object = NULL; not needed */
                  /* loop */
          }
  
          /*
           * Clean up any left over backing_object
           */
          if (backing_object) {
                  vm_object_chain_release(backing_object);
                  vm_object_drop(backing_object);
          }
  
          /*
           * Clean up any auto-deallocation list.  This is a convenience
           * for top-level callers so they don't have to pass &dlist.
           * Do not clean up any caller-passed dlistp, the caller will
           * do that.
           */
          if (dlist)
                  vm_object_deallocate_list(&dlist);
  
  }
  
  /*
   * vm_object_collapse() may collect additional objects in need of
   * deallocation.  This routine deallocates these objects.  The
   * deallocation itself can trigger additional collapses (which the
   * deallocate function takes care of).  This procedure is used to
   * reduce procedural recursion since these vm_object shadow chains
   * can become quite long.
   */
  void
  vm_object_deallocate_list(struct vm_object_dealloc_list **dlistp)
  {
          struct vm_object_dealloc_list *dlist;
  
          while ((dlist = *dlistp) != NULL) {
                  *dlistp = dlist->next;
                  vm_object_lock(dlist->object);
                  vm_object_deallocate_locked(dlist->object);
                  vm_object_drop(dlist->object);
                  kfree(dlist, M_TEMP);
          }
  }
  
  /*
   * Removes all physical pages in the specified object range from the
   * object's list of pages.
   *
   * No requirements.
   */
  static int vm_object_page_remove_callback(vm_page_t p, void *data);
  
  void
  vm_object_page_remove(vm_object_t object, vm_pindex_t start, vm_pindex_t end,
                        boolean_t clean_only)
  {
          struct rb_vm_page_scan_info info;
          int all;
  
          /*
           * Degenerate cases and assertions
           */
          vm_object_hold(object);
          if (object == NULL ||
              (object->resident_page_count == 0 && object->swblock_count == 0)) {
                  vm_object_drop(object);
                  return;
          }
          KASSERT(object->type != OBJT_PHYS, 
                  ("attempt to remove pages from a physical object"));
  
          /*
           * Indicate that paging is occuring on the object
           */
          vm_object_pip_add(object, 1);
  
          /*
           * Figure out the actual removal range and whether we are removing
           * the entire contents of the object or not.  If removing the entire
           * contents, be sure to get all pages, even those that might be 
           * beyond the end of the object.
           */
          info.start_pindex = start;
          if (end == 0)
                  info.end_pindex = (vm_pindex_t)-1;
          else
                  info.end_pindex = end - 1;
          info.limit = clean_only;
          all = (start == 0 && info.end_pindex >= object->size - 1);
  
          /*
           * Loop until we are sure we have gotten them all.
           */
          do {
                  info.error = 0;
                  vm_page_rb_tree_RB_SCAN(&object->rb_memq, rb_vm_page_scancmp,
                                          vm_object_page_remove_callback, &info);
          } while (info.error);
  
          /*
           * Remove any related swap if throwing away pages, or for
           * non-swap objects (the swap is a clean copy in that case).
           */
          if (object->type != OBJT_SWAP || clean_only == FALSE) {
                  if (all)
                          swap_pager_freespace_all(object);
                  else
                          swap_pager_freespace(object, info.start_pindex,
                               info.end_pindex - info.start_pindex + 1);
          }
  
          /*
           * Cleanup
           */
          vm_object_pip_wakeup(object);
          vm_object_drop(object);
  }
  
  /*
   * The caller must hold the object
   */
  static int
  vm_object_page_remove_callback(vm_page_t p, void *data)
  {
          struct rb_vm_page_scan_info *info = data;
  
          if (vm_page_busy_try(p, TRUE)) {
                  vm_page_sleep_busy(p, TRUE, "vmopar");
                  info->error = 1;
                  return(0);
          }
  
          /*
           * Wired pages cannot be destroyed, but they can be invalidated
           * and we do so if clean_only (limit) is not set.
           *
           * WARNING!  The page may be wired due to being part of a buffer
           *           cache buffer, and the buffer might be marked B_CACHE.
           *           This is fine as part of a truncation but VFSs must be
           *           sure to fix the buffer up when re-extending the file.
           *
           * NOTE!     PG_NEED_COMMIT is ignored.
           */
          if (p->wire_count != 0) {
                  vm_page_protect(p, VM_PROT_NONE);
                  if (info->limit == 0)
                          p->valid = 0;
                  vm_page_wakeup(p);
                  return(0);
          }
  
          /*
           * limit is our clean_only flag.  If set and the page is dirty or
           * requires a commit, do not free it.  If set and the page is being
           * held by someone, do not free it.
           */
          if (info->limit && p->valid) {
                  vm_page_test_dirty(p);
                  if ((p->valid & p->dirty) || (p->flags & PG_NEED_COMMIT)) {
                          vm_page_wakeup(p);
                          return(0);
                  }
          }
  
          /*
           * Destroy the page
           */
          vm_page_protect(p, VM_PROT_NONE);
          vm_page_free(p);
          return(0);
  }
  
  /*
   * Coalesces two objects backing up adjoining regions of memory into a
   * single object.
   *
   * returns TRUE if objects were combined.
   *
   * NOTE: Only works at the moment if the second object is NULL -
   *       if it's not, which object do we lock first?
   *
   * Parameters:
   *      prev_object     First object to coalesce
   *      prev_offset     Offset into prev_object
   *      next_object     Second object into coalesce
   *      next_offset     Offset into next_object
   *
   *      prev_size       Size of reference to prev_object
   *      next_size       Size of reference to next_object
   *
   * The caller does not need to hold (prev_object) but must have a stable
   * pointer to it (typically by holding the vm_map locked).
   */
  boolean_t
  vm_object_coalesce(vm_object_t prev_object, vm_pindex_t prev_pindex,
                     vm_size_t prev_size, vm_size_t next_size)
  {
          vm_pindex_t next_pindex;
  
          if (prev_object == NULL)
                  return (TRUE);
  
          vm_object_hold(prev_object);
  
          if (prev_object->type != OBJT_DEFAULT &&
              prev_object->type != OBJT_SWAP) {
                  vm_object_drop(prev_object);
                  return (FALSE);
          }
  
          /*
           * Try to collapse the object first
           */
          vm_object_chain_acquire(prev_object, 0);
          vm_object_collapse(prev_object, NULL);
  
          /*
           * Can't coalesce if: . more than one reference . paged out . shadows
           * another object . has a copy elsewhere (any of which mean that the
           * pages not mapped to prev_entry may be in use anyway)
           */
  
          if (prev_object->backing_object != NULL) {
                  vm_object_chain_release(prev_object);
                  vm_object_drop(prev_object);
                  return (FALSE);
          }
  
          prev_size >>= PAGE_SHIFT;
          next_size >>= PAGE_SHIFT;
          next_pindex = prev_pindex + prev_size;
  
          if ((prev_object->ref_count > 1) &&
              (prev_object->size != next_pindex)) {
                  vm_object_chain_release(prev_object);
                  vm_object_drop(prev_object);
                  return (FALSE);
          }
  
          /*
           * Remove any pages that may still be in the object from a previous
           * deallocation.
           */
          if (next_pindex < prev_object->size) {
                  vm_object_page_remove(prev_object,
                                        next_pindex,
                                        next_pindex + next_size, FALSE);
                  if (prev_object->type == OBJT_SWAP)
                          swap_pager_freespace(prev_object,
                                               next_pindex, next_size);
          }
  
          /*
           * Extend the object if necessary.
           */
          if (next_pindex + next_size > prev_object->size)
                  prev_object->size = next_pindex + next_size;
  
          vm_object_chain_release(prev_object);
          vm_object_drop(prev_object);
          return (TRUE);
  }
  
  /*
   * Make the object writable and flag is being possibly dirty.
   *
   * The object might not be held (or might be held but held shared),
   * the related vnode is probably not held either.  Object and vnode are
   * stable by virtue of the vm_page busied by the caller preventing
   * destruction.
   *
   * If the related mount is flagged MNTK_THR_SYNC we need to call
   * vsetobjdirty().  Filesystems using this option usually shortcut
   * synchronization by only scanning the syncer list.
   */
  void
  vm_object_set_writeable_dirty(vm_object_t object)
  {
          struct vnode *vp;
  
          /*vm_object_assert_held(object);*/
          /*
           * Avoid contention in vm fault path by checking the state before
           * issuing an atomic op on it.
           */
          if ((object->flags & (OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY)) !=
              (OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY)) {
                  vm_object_set_flag(object, OBJ_WRITEABLE|OBJ_MIGHTBEDIRTY);
          }
          if (object->type == OBJT_VNODE &&
              (vp = (struct vnode *)object->handle) != NULL) {
                  if ((vp->v_flag & VOBJDIRTY) == 0) {
                          if (vp->v_mount &&
                              (vp->v_mount->mnt_kern_flag & MNTK_THR_SYNC)) {
                                  /*
                                   * New style THR_SYNC places vnodes on the
                                   * syncer list more deterministically.
                                   */
                                  vsetobjdirty(vp);
                          } else {
                                  /*
                                   * Old style scan would not necessarily place
                                   * a vnode on the syncer list when possibly
                                   * modified via mmap.
                                   */
                                  vsetflags(vp, VOBJDIRTY);
                          }
                  }
          }
  }
  
  #include "opt_ddb.h"
  #ifdef DDB
  #include <sys/kernel.h>
  
  #include <sys/cons.h>
  
  #include <ddb/ddb.h>
  
  static int      _vm_object_in_map (vm_map_t map, vm_object_t object,
                                         vm_map_entry_t entry);
  static int      vm_object_in_map (vm_object_t object);
  
  /*
   * The caller must hold the object.
   */
  static int
  _vm_object_in_map(vm_map_t map, vm_object_t object, vm_map_entry_t entry)
  {
          vm_map_t tmpm;
          vm_map_entry_t tmpe;
          vm_object_t obj, nobj;
          int entcount;
  
          if (map == 0)
                  return 0;
          if (entry == 0) {
                  tmpe = map->header.next;
                  entcount = map->nentries;
                  while (entcount-- && (tmpe != &map->header)) {
                          if( _vm_object_in_map(map, object, tmpe)) {
                                  return 1;
                          }
                          tmpe = tmpe->next;
                  }
                  return (0);
          }
          switch(entry->maptype) {
          case VM_MAPTYPE_SUBMAP:
                  tmpm = entry->object.sub_map;
                  tmpe = tmpm->header.next;
                  entcount = tmpm->nentries;
                  while (entcount-- && tmpe != &tmpm->header) {
                          if( _vm_object_in_map(tmpm, object, tmpe)) {
                                  return 1;
                          }
                          tmpe = tmpe->next;
                  }
                  break;
          case VM_MAPTYPE_NORMAL:
          case VM_MAPTYPE_VPAGETABLE:
                  obj = entry->object.vm_object;
                  while (obj) {
                          if (obj == object) {
                                  if (obj != entry->object.vm_object)
                                          vm_object_drop(obj);
                                  return 1;
                          }
                          while ((nobj = obj->backing_object) != NULL) {
                                  vm_object_hold(nobj);
                                  if (nobj == obj->backing_object)
                                          break;
                                  vm_object_drop(nobj);
                          }
                          if (obj != entry->object.vm_object) {
                                  if (nobj)
                                          vm_object_lock_swap();
                                  vm_object_drop(obj);
                          }
                          obj = nobj;
                  }
                  break;
          default:
                  break;
          }
          return 0;
  }
  
  static int vm_object_in_map_callback(struct proc *p, void *data);
  
  struct vm_object_in_map_info {
          vm_object_t object;
          int rv;
  };
  
  /*
   * Debugging only
   */
  static int
  vm_object_in_map(vm_object_t object)
  {
          struct vm_object_in_map_info info;
  
          info.rv = 0;
          info.object = object;
  
          allproc_scan(vm_object_in_map_callback, &info);
          if (info.rv)
                  return 1;
          if( _vm_object_in_map(&kernel_map, object, 0))
                  return 1;
          if( _vm_object_in_map(&pager_map, object, 0))
                  return 1;
          if( _vm_object_in_map(&buffer_map, object, 0))
                  return 1;
          return 0;
  }
  
  /*
   * Debugging only
   */
  static int
  vm_object_in_map_callback(struct proc *p, void *data)
  {
          struct vm_object_in_map_info *info = data;
  
          if (p->p_vmspace) {
                  if (_vm_object_in_map(&p->p_vmspace->vm_map, info->object, 0)) {
                          info->rv = 1;
                          return -1;
                  }
          }
          return (0);
  }
  
  DB_SHOW_COMMAND(vmochk, vm_object_check)
  {
          vm_object_t object;
          int n;
  
          /*
           * make sure that internal objs are in a map somewhere
           * and none have zero ref counts.
           */
          for (n = 0; n < VMOBJ_HSIZE; ++n) {
                  for (object = TAILQ_FIRST(&vm_object_lists[n]);
                                  object != NULL;
                                  object = TAILQ_NEXT(object, object_list)) {
                          if (object->type == OBJT_MARKER)
                                  continue;
                          if (object->handle != NULL ||
                              (object->type != OBJT_DEFAULT &&
                               object->type != OBJT_SWAP)) {
                                  continue;
                          }
                          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))
                                  continue;
                          db_printf("vmochk: internal obj is not in a map: "
                                    "ref: %d, size: %lu: 0x%lx, "
                                    "backing_object: %p\n",
                                    object->ref_count, (u_long)object->size,
                                    (u_long)object->size,
                                    (void *)object->backing_object);
                  }
          }
  }
  
  /*
   * Debugging only
   */
  DB_SHOW_COMMAND(object, vm_object_print_static)
  {
          /* XXX convert args. */
          vm_object_t object = (vm_object_t)addr;
          boolean_t full = have_addr;
  
          vm_page_t p;
  
          /* XXX count is an (unused) arg.  Avoid shadowing it. */
  #define count   was_count
  
          int count;
  
          if (object == NULL)
                  return;
  
          db_iprintf(
              "Object %p: type=%d, size=0x%lx, res=%d, ref=%d, flags=0x%x\n",
              object, (int)object->type, (u_long)object->size,
              object->resident_page_count, object->ref_count, object->flags);
          /*
           * XXX no %qd in kernel.  Truncate object->backing_object_offset.
           */
          db_iprintf(" sref=%d, backing_object(%d)=(%p)+0x%lx\n",
              object->shadow_count, 
              object->backing_object ? object->backing_object->ref_count : 0,
              object->backing_object, (long)object->backing_object_offset);
  
          if (!full)
                  return;
  
          db_indent += 2;
          count = 0;
          RB_FOREACH(p, vm_page_rb_tree, &object->rb_memq) {
                  if (count == 0)
                          db_iprintf("memory:=");
                  else if (count == 6) {
                          db_printf("\n");
                          db_iprintf(" ...");
                          count = 0;
                  } else
                          db_printf(",");
                  count++;
  
                  db_printf("(off=0x%lx,page=0x%lx)",
                      (u_long) p->pindex, (u_long) VM_PAGE_TO_PHYS(p));
          }
          if (count != 0)
                  db_printf("\n");
          db_indent -= 2;
  }
  
  /* XXX. */
  #undef count
  
  /*
   * XXX need this non-static entry for calling from vm_map_print.
   *
   * Debugging only
   */
  void
  vm_object_print(/* db_expr_t */ long addr,
                  boolean_t have_addr,
                  /* db_expr_t */ long count,
                  char *modif)
  {
          vm_object_print_static(addr, have_addr, count, modif);
  }
  
  /*
   * Debugging only
   */
  DB_SHOW_COMMAND(vmopag, vm_object_print_pages)
  {
          vm_object_t object;
          int nl = 0;
          int c;
          int n;
  
          for (n = 0; n < VMOBJ_HSIZE; ++n) {
                  for (object = TAILQ_FIRST(&vm_object_lists[n]);
                                  object != NULL;
                                  object = TAILQ_NEXT(object, object_list)) {
                          vm_pindex_t idx, fidx;
                          vm_pindex_t osize;
                          vm_paddr_t pa = -1, padiff;
                          int rcount;
                          vm_page_t m;
  
                          if (object->type == OBJT_MARKER)
                                  continue;
                          db_printf("new object: %p\n", (void *)object);
                          if ( nl > 18) {
                                  c = cngetc();
                                  if (c != ' ')
                                          return;
                                  nl = 0;
                          }
                          nl++;
                          rcount = 0;
                          fidx = 0;
                          osize = object->size;
                          if (osize > 128)
                                  osize = 128;
                          for (idx = 0; idx < osize; idx++) {
                                  m = vm_page_lookup(object, idx);
                                  if (m == NULL) {
                                          if (rcount) {
                                                  db_printf(" index(%ld)run(%d)pa(0x%lx)\n",
                                                          (long)fidx, rcount, (long)pa);
                                                  if ( nl > 18) {
                                                          c = cngetc();
                                                          if (c != ' ')
                                                                  return;
                                                          nl = 0;
                                                  }
                                                  nl++;
                                                  rcount = 0;
                                          }
                                          continue;
                                  }
  
                                  if (rcount &&
                                          (VM_PAGE_TO_PHYS(m) == pa + rcount * PAGE_SIZE)) {
                                          ++rcount;
                                          continue;
                                  }
                                  if (rcount) {
                                          padiff = pa + rcount * PAGE_SIZE - VM_PAGE_TO_PHYS(m);
                                          padiff >>= PAGE_SHIFT;
                                          padiff &= PQ_L2_MASK;
                                          if (padiff == 0) {
                                                  pa = VM_PAGE_TO_PHYS(m) - rcount * PAGE_SIZE;
                                                  ++rcount;
                                                  continue;
                                          }
                                          db_printf(" index(%ld)run(%d)pa(0x%lx)",
                                                  (long)fidx, rcount, (long)pa);
                                          db_printf("pd(%ld)\n", (long)padiff);
                                          if ( nl > 18) {
                                                  c = cngetc();
                                                  if (c != ' ')
                                                          return;
                                                  nl = 0;
                                          }
                                          nl++;
                                  }
                                  fidx = idx;
                                  pa = VM_PAGE_TO_PHYS(m);
                                  rcount = 1;
                          }
                          if (rcount) {
                                  db_printf(" index(%ld)run(%d)pa(0x%lx)\n",
                                          (long)fidx, rcount, (long)pa);
                                  if ( nl > 18) {
                                          c = cngetc();
                                          if (c != ' ')
                                                  return;
                                          nl = 0;
                                  }
                                  nl++;
                          }
                  }
          }
  }
  #endif /* DDB */

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