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

     /*-
      * Copyright (c) 1991, 1993
      *      The Regents of the University of California.  All rights reserved.
      * Copyright (c) 1994 John S. Dyson
      * All rights reserved.
      * Copyright (c) 1994 David Greenman
      * All rights reserved.
      *
      *
     * This code is derived from software contributed to Berkeley by
     * The Mach Operating System project at Carnegie-Mellon University.
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. All advertising materials mentioning features or use of this software
     *    must display the following acknowledgement:
     *      This product includes software developed by the University of
     *      California, Berkeley and its contributors.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      from: @(#)vm_fault.c    8.4 (Berkeley) 1/12/94
     *
     *
     * 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.
     */
    
    /*
     *      Page fault handling module.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_ktrace.h"
    #include "opt_vm.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/lock.h>
    #include <sys/proc.h>
    #include <sys/resourcevar.h>
    #include <sys/rwlock.h>
    #include <sys/sysctl.h>
    #include <sys/vmmeter.h>
    #include <sys/vnode.h>
    #ifdef KTRACE
    #include <sys/ktrace.h>
    #endif
    
    #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_kern.h>
   #include <vm/vm_pager.h>
   #include <vm/vm_extern.h>
   #include <vm/vm_reserv.h>
   
   #define PFBAK 4
   #define PFFOR 4
   
   static int vm_fault_additional_pages(vm_page_t, int, int, vm_page_t *, int *);
   
   #define VM_FAULT_READ_BEHIND    8
   #define VM_FAULT_READ_MAX       (1 + VM_FAULT_READ_AHEAD_MAX)
   #define VM_FAULT_NINCR          (VM_FAULT_READ_MAX / VM_FAULT_READ_BEHIND)
   #define VM_FAULT_SUM            (VM_FAULT_NINCR * (VM_FAULT_NINCR + 1) / 2)
   #define VM_FAULT_CACHE_BEHIND   (VM_FAULT_READ_BEHIND * VM_FAULT_SUM)
   
   struct faultstate {
           vm_page_t m;
           vm_object_t object;
           vm_pindex_t pindex;
           vm_page_t first_m;
           vm_object_t     first_object;
           vm_pindex_t first_pindex;
           vm_map_t map;
           vm_map_entry_t entry;
           int lookup_still_valid;
           int map_generation;
           struct vnode *vp;
   };
   
   static void vm_fault_cache_behind(const struct faultstate *fs, int distance);
   static void vm_fault_prefault(const struct faultstate *fs, vm_offset_t addra,
               int faultcount, int reqpage);
   
   static inline void
   release_page(struct faultstate *fs)
   {
   
           vm_page_xunbusy(fs->m);
           vm_page_lock(fs->m);
           vm_page_deactivate(fs->m);
           vm_page_unlock(fs->m);
           fs->m = NULL;
   }
   
   static inline void
   unlock_map(struct faultstate *fs)
   {
   
           if (fs->lookup_still_valid) {
                   vm_map_lookup_done(fs->map, fs->entry);
                   fs->lookup_still_valid = FALSE;
           }
   }
   
   static void
   unlock_vp(struct faultstate *fs)
   {
   
           if (fs->vp != NULL) {
                   vput(fs->vp);
                   fs->vp = NULL;
           }
   }
   
   static void
   unlock_and_deallocate(struct faultstate *fs)
   {
   
           vm_object_pip_wakeup(fs->object);
           VM_OBJECT_WUNLOCK(fs->object);
           if (fs->object != fs->first_object) {
                   VM_OBJECT_WLOCK(fs->first_object);
                   vm_page_lock(fs->first_m);
                   vm_page_free(fs->first_m);
                   vm_page_unlock(fs->first_m);
                   vm_object_pip_wakeup(fs->first_object);
                   VM_OBJECT_WUNLOCK(fs->first_object);
                   fs->first_m = NULL;
           }
           vm_object_deallocate(fs->first_object);
           unlock_map(fs);
           unlock_vp(fs);
   }
   
   static void
   vm_fault_dirty(vm_map_entry_t entry, vm_page_t m, vm_prot_t prot,
       vm_prot_t fault_type, int fault_flags, bool set_wd)
   {
           bool need_dirty;
   
           if (((prot & VM_PROT_WRITE) == 0 &&
               (fault_flags & VM_FAULT_DIRTY) == 0) ||
               (m->oflags & VPO_UNMANAGED) != 0)
                   return;
   
           VM_OBJECT_ASSERT_LOCKED(m->object);
   
           need_dirty = ((fault_type & VM_PROT_WRITE) != 0 &&
               (fault_flags & VM_FAULT_WIRE) == 0) ||
               (fault_flags & VM_FAULT_DIRTY) != 0;
   
           if (set_wd)
                   vm_object_set_writeable_dirty(m->object);
           else
                   /*
                    * If two callers of vm_fault_dirty() with set_wd ==
                    * FALSE, one for the map entry with MAP_ENTRY_NOSYNC
                    * flag set, other with flag clear, race, it is
                    * possible for the no-NOSYNC thread to see m->dirty
                    * != 0 and not clear VPO_NOSYNC.  Take vm_page lock
                    * around manipulation of VPO_NOSYNC and
                    * vm_page_dirty() call, to avoid the race and keep
                    * m->oflags consistent.
                    */
                   vm_page_lock(m);
   
           /*
            * If this is a NOSYNC mmap we do not want to set VPO_NOSYNC
            * if the page is already dirty to prevent data written with
            * the expectation of being synced from not being synced.
            * Likewise if this entry does not request NOSYNC then make
            * sure the page isn't marked NOSYNC.  Applications sharing
            * data should use the same flags to avoid ping ponging.
            */
           if ((entry->eflags & MAP_ENTRY_NOSYNC) != 0) {
                   if (m->dirty == 0) {
                           m->oflags |= VPO_NOSYNC;
                   }
           } else {
                   m->oflags &= ~VPO_NOSYNC;
           }
   
           /*
            * If the fault is a write, we know that this page is being
            * written NOW so dirty it explicitly to save on
            * pmap_is_modified() calls later.
            *
            * Also tell the backing pager, if any, that it should remove
            * any swap backing since the page is now dirty.
            */
           if (need_dirty)
                   vm_page_dirty(m);
           if (!set_wd)
                   vm_page_unlock(m);
           if (need_dirty)
                   vm_pager_page_unswapped(m);
   }
   
   static void
   vm_fault_fill_hold(vm_page_t *m_hold, vm_page_t m)
   {
   
           if (m_hold != NULL) {
                   *m_hold = m;
                   vm_page_lock(m);
                   vm_page_hold(m);
                   vm_page_unlock(m);
           }
   }
   
   /*
    * Unlocks fs.first_object and fs.map on success.
    */
   static int
   vm_fault_soft_fast(struct faultstate *fs, vm_offset_t vaddr, vm_prot_t prot,
       int fault_type, int fault_flags, boolean_t wired, vm_page_t *m_hold)
   {
           vm_page_t m;
           int rv;
   
           MPASS(fs->vp == NULL);
           m = vm_page_lookup(fs->first_object, fs->first_pindex);
           /* A busy page can be mapped for read|execute access. */
           if (m == NULL || ((prot & VM_PROT_WRITE) != 0 &&
               vm_page_busied(m)) || m->valid != VM_PAGE_BITS_ALL)
                   return (KERN_FAILURE);
           rv = pmap_enter(fs->map->pmap, vaddr, m, prot, fault_type |
               PMAP_ENTER_NOSLEEP | (wired ? PMAP_ENTER_WIRED : 0), 0);
           if (rv != KERN_SUCCESS)
                   return (rv);
           vm_fault_fill_hold(m_hold, m);
           vm_fault_dirty(fs->entry, m, prot, fault_type, fault_flags, false);
           VM_OBJECT_RUNLOCK(fs->first_object);
           if (!wired)
                   vm_fault_prefault(fs, vaddr, 0, 0);
           vm_map_lookup_done(fs->map, fs->entry);
           curthread->td_ru.ru_minflt++;
           return (KERN_SUCCESS);
   }
   
   /*
    *      vm_fault:
    *
    *      Handle a page fault occurring at the given address,
    *      requiring the given permissions, in the map specified.
    *      If successful, the page is inserted into the
    *      associated physical map.
    *
    *      NOTE: the given address should be truncated to the
    *      proper page address.
    *
    *      KERN_SUCCESS is returned if the page fault is handled; otherwise,
    *      a standard error specifying why the fault is fatal is returned.
    *
    *      The map in question must be referenced, and remains so.
    *      Caller may hold no locks.
    */
   int
   vm_fault(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
       int fault_flags)
   {
           struct thread *td;
           int result;
   
           td = curthread;
           if ((td->td_pflags & TDP_NOFAULTING) != 0)
                   return (KERN_PROTECTION_FAILURE);
   #ifdef KTRACE
           if (map != kernel_map && KTRPOINT(td, KTR_FAULT))
                   ktrfault(vaddr, fault_type);
   #endif
           result = vm_fault_hold(map, trunc_page(vaddr), fault_type, fault_flags,
               NULL);
   #ifdef KTRACE
           if (map != kernel_map && KTRPOINT(td, KTR_FAULTEND))
                   ktrfaultend(result);
   #endif
           return (result);
   }
   
   int
   vm_fault_hold(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
       int fault_flags, vm_page_t *m_hold)
   {
           vm_prot_t prot;
           long ahead, behind;
           int alloc_req, era, faultcount, nera, reqpage, result;
           boolean_t dead, is_first_object_locked, wired;
           vm_object_t next_object;
           vm_page_t marray[VM_FAULT_READ_MAX];
           int hardfault;
           struct faultstate fs;
           struct vnode *vp;
           int locked, error;
   
           hardfault = 0;
           PCPU_INC(cnt.v_vm_faults);
           fs.vp = NULL;
           faultcount = reqpage = 0;
   
   RetryFault:;
   
           /*
            * Find the backing store object and offset into it to begin the
            * search.
            */
           fs.map = map;
           result = vm_map_lookup(&fs.map, vaddr, fault_type |
               VM_PROT_FAULT_LOOKUP, &fs.entry, &fs.first_object,
               &fs.first_pindex, &prot, &wired);
           if (result != KERN_SUCCESS) {
                   unlock_vp(&fs);
                   return (result);
           }
   
           fs.map_generation = fs.map->timestamp;
   
           if (fs.entry->eflags & MAP_ENTRY_NOFAULT) {
                   panic("vm_fault: fault on nofault entry, addr: %lx",
                       (u_long)vaddr);
           }
   
           if (fs.entry->eflags & MAP_ENTRY_IN_TRANSITION &&
               fs.entry->wiring_thread != curthread) {
                   vm_map_unlock_read(fs.map);
                   vm_map_lock(fs.map);
                   if (vm_map_lookup_entry(fs.map, vaddr, &fs.entry) &&
                       (fs.entry->eflags & MAP_ENTRY_IN_TRANSITION)) {
                           unlock_vp(&fs);
                           fs.entry->eflags |= MAP_ENTRY_NEEDS_WAKEUP;
                           vm_map_unlock_and_wait(fs.map, 0);
                   } else
                           vm_map_unlock(fs.map);
                   goto RetryFault;
           }
   
           MPASS((fs.entry->eflags & MAP_ENTRY_GUARD) == 0);
   
           if (wired)
                   fault_type = prot | (fault_type & VM_PROT_COPY);
           else
                   KASSERT((fault_flags & VM_FAULT_WIRE) == 0,
                       ("!wired && VM_FAULT_WIRE"));
   
           /*
            * Try to avoid lock contention on the top-level object through
            * special-case handling of some types of page faults, specifically,
            * those that are both (1) mapping an existing page from the top-
            * level object and (2) not having to mark that object as containing
            * dirty pages.  Under these conditions, a read lock on the top-level
            * object suffices, allowing multiple page faults of a similar type to
            * run in parallel on the same top-level object.
            */
           if (fs.vp == NULL /* avoid locked vnode leak */ &&
               (fault_flags & (VM_FAULT_WIRE | VM_FAULT_DIRTY)) == 0 &&
               /* avoid calling vm_object_set_writeable_dirty() */
               ((prot & VM_PROT_WRITE) == 0 ||
               (fs.first_object->type != OBJT_VNODE &&
               (fs.first_object->flags & OBJ_TMPFS_NODE) == 0) ||
               (fs.first_object->flags & OBJ_MIGHTBEDIRTY) != 0)) {
                   VM_OBJECT_RLOCK(fs.first_object);
                   if ((prot & VM_PROT_WRITE) == 0 ||
                       (fs.first_object->type != OBJT_VNODE &&
                       (fs.first_object->flags & OBJ_TMPFS_NODE) == 0) ||
                       (fs.first_object->flags & OBJ_MIGHTBEDIRTY) != 0) {
                           result = vm_fault_soft_fast(&fs, vaddr, prot,
                               fault_type, fault_flags, wired, m_hold);
                           if (result == KERN_SUCCESS)
                                   return (result);
                   }
                   if (!VM_OBJECT_TRYUPGRADE(fs.first_object)) {
                           VM_OBJECT_RUNLOCK(fs.first_object);
                           VM_OBJECT_WLOCK(fs.first_object);
                   }
           } else {
                   VM_OBJECT_WLOCK(fs.first_object);
           }
   
           /*
            * Make a reference to this object to prevent its disposal while we
            * are messing with it.  Once we have the reference, the map is free
            * to be diddled.  Since objects reference their shadows (and copies),
            * they will stay around as well.
            *
            * Bump the paging-in-progress count to prevent size changes (e.g. 
            * truncation operations) during I/O.  This must be done after
            * obtaining the vnode lock in order to avoid possible deadlocks.
            */
           vm_object_reference_locked(fs.first_object);
           vm_object_pip_add(fs.first_object, 1);
   
           fs.lookup_still_valid = TRUE;
   
           fs.first_m = NULL;
   
           /*
            * Search for the page at object/offset.
            */
           fs.object = fs.first_object;
           fs.pindex = fs.first_pindex;
           while (TRUE) {
                   /*
                    * If the object is marked for imminent termination,
                    * we retry here, since the collapse pass has raced
                    * with us.  Otherwise, if we see terminally dead
                    * object, return fail.
                    */
                   if ((fs.object->flags & OBJ_DEAD) != 0) {
                           dead = fs.object->type == OBJT_DEAD;
                           unlock_and_deallocate(&fs);
                           if (dead)
                                   return (KERN_PROTECTION_FAILURE);
                           pause("vmf_de", 1);
                           goto RetryFault;
                   }
   
                   /*
                    * See if page is resident
                    */
                   fs.m = vm_page_lookup(fs.object, fs.pindex);
                   if (fs.m != NULL) {
                           /*
                            * Wait/Retry if the page is busy.  We have to do this
                            * if the page is either exclusive or shared busy
                            * because the vm_pager may be using read busy for
                            * pageouts (and even pageins if it is the vnode
                            * pager), and we could end up trying to pagein and
                            * pageout the same page simultaneously.
                            *
                            * We can theoretically allow the busy case on a read
                            * fault if the page is marked valid, but since such
                            * pages are typically already pmap'd, putting that
                            * special case in might be more effort then it is 
                            * worth.  We cannot under any circumstances mess
                            * around with a shared busied page except, perhaps,
                            * to pmap it.
                            */
                           if (vm_page_busied(fs.m)) {
                                   /*
                                    * Reference the page before unlocking and
                                    * sleeping so that the page daemon is less
                                    * likely to reclaim it. 
                                    */
                                   vm_page_aflag_set(fs.m, PGA_REFERENCED);
                                   if (fs.object != fs.first_object) {
                                           if (!VM_OBJECT_TRYWLOCK(
                                               fs.first_object)) {
                                                   VM_OBJECT_WUNLOCK(fs.object);
                                                   VM_OBJECT_WLOCK(fs.first_object);
                                                   VM_OBJECT_WLOCK(fs.object);
                                           }
                                           vm_page_lock(fs.first_m);
                                           vm_page_free(fs.first_m);
                                           vm_page_unlock(fs.first_m);
                                           vm_object_pip_wakeup(fs.first_object);
                                           VM_OBJECT_WUNLOCK(fs.first_object);
                                           fs.first_m = NULL;
                                   }
                                   unlock_map(&fs);
                                   if (fs.m == vm_page_lookup(fs.object,
                                       fs.pindex)) {
                                           vm_page_sleep_if_busy(fs.m, "vmpfw");
                                   }
                                   vm_object_pip_wakeup(fs.object);
                                   VM_OBJECT_WUNLOCK(fs.object);
                                   PCPU_INC(cnt.v_intrans);
                                   vm_object_deallocate(fs.first_object);
                                   goto RetryFault;
                           }
                           vm_page_lock(fs.m);
                           vm_page_remque(fs.m);
                           vm_page_unlock(fs.m);
   
                           /*
                            * Mark page busy for other processes, and the 
                            * pagedaemon.  If it still isn't completely valid
                            * (readable), jump to readrest, else break-out ( we
                            * found the page ).
                            */
                           vm_page_xbusy(fs.m);
                           if (fs.m->valid != VM_PAGE_BITS_ALL)
                                   goto readrest;
                           break;
                   }
   
                   /*
                    * Page is not resident.  If this is the search termination
                    * or the pager might contain the page, allocate a new page.
                    * Default objects are zero-fill, there is no real pager.
                    */
                   if (fs.object->type != OBJT_DEFAULT ||
                       fs.object == fs.first_object) {
                           if (fs.pindex >= fs.object->size) {
                                   unlock_and_deallocate(&fs);
                                   return (KERN_PROTECTION_FAILURE);
                           }
   
                           /*
                            * Allocate a new page for this object/offset pair.
                            *
                            * Unlocked read of the p_flag is harmless. At
                            * worst, the P_KILLED might be not observed
                            * there, and allocation can fail, causing
                            * restart and new reading of the p_flag.
                            */
                           fs.m = NULL;
                           if (!vm_page_count_severe() || P_KILLED(curproc)) {
   #if VM_NRESERVLEVEL > 0
                                   if ((fs.object->flags & OBJ_COLORED) == 0) {
                                           fs.object->flags |= OBJ_COLORED;
                                           fs.object->pg_color = atop(vaddr) -
                                               fs.pindex;
                                   }
   #endif
                                   alloc_req = P_KILLED(curproc) ?
                                       VM_ALLOC_SYSTEM : VM_ALLOC_NORMAL;
                                   if (fs.object->type != OBJT_VNODE &&
                                       fs.object->backing_object == NULL)
                                           alloc_req |= VM_ALLOC_ZERO;
                                   fs.m = vm_page_alloc(fs.object, fs.pindex,
                                       alloc_req);
                           }
                           if (fs.m == NULL) {
                                   unlock_and_deallocate(&fs);
                                   VM_WAITPFAULT;
                                   goto RetryFault;
                           } else if (fs.m->valid == VM_PAGE_BITS_ALL)
                                   break;
                   }
   
   readrest:
                   /*
                    * We have found a valid page or we have allocated a new page.
                    * The page thus may not be valid or may not be entirely 
                    * valid.
                    *
                    * Attempt to fault-in the page if there is a chance that the
                    * pager has it, and potentially fault in additional pages
                    * at the same time.  For default objects simply provide
                    * zero-filled pages.
                    */
                   if (fs.object->type != OBJT_DEFAULT) {
                           int rv;
                           u_char behavior = vm_map_entry_behavior(fs.entry);
   
                           if (behavior == MAP_ENTRY_BEHAV_RANDOM ||
                               P_KILLED(curproc)) {
                                   behind = 0;
                                   ahead = 0;
                           } else if (behavior == MAP_ENTRY_BEHAV_SEQUENTIAL) {
                                   behind = 0;
                                   ahead = atop(fs.entry->end - vaddr) - 1;
                                   if (ahead > VM_FAULT_READ_AHEAD_MAX)
                                           ahead = VM_FAULT_READ_AHEAD_MAX;
                                   if (fs.pindex == fs.entry->next_read)
                                           vm_fault_cache_behind(&fs,
                                               VM_FAULT_READ_MAX);
                           } else {
                                   /*
                                    * If this is a sequential page fault, then
                                    * arithmetically increase the number of pages
                                    * in the read-ahead window.  Otherwise, reset
                                    * the read-ahead window to its smallest size.
                                    */
                                   behind = atop(vaddr - fs.entry->start);
                                   if (behind > VM_FAULT_READ_BEHIND)
                                           behind = VM_FAULT_READ_BEHIND;
                                   ahead = atop(fs.entry->end - vaddr) - 1;
                                   era = fs.entry->read_ahead;
                                   if (fs.pindex == fs.entry->next_read) {
                                           nera = era + behind;
                                           if (nera > VM_FAULT_READ_AHEAD_MAX)
                                                   nera = VM_FAULT_READ_AHEAD_MAX;
                                           behind = 0;
                                           if (ahead > nera)
                                                   ahead = nera;
                                           if (era == VM_FAULT_READ_AHEAD_MAX)
                                                   vm_fault_cache_behind(&fs,
                                                       VM_FAULT_CACHE_BEHIND);
                                   } else if (ahead > VM_FAULT_READ_AHEAD_MIN)
                                           ahead = VM_FAULT_READ_AHEAD_MIN;
                                   if (era != ahead)
                                           fs.entry->read_ahead = ahead;
                           }
   
                           /*
                            * Call the pager to retrieve the data, if any, after
                            * releasing the lock on the map.  We hold a ref on
                            * fs.object and the pages are exclusive busied.
                            */
                           unlock_map(&fs);
   
                           if (fs.object->type == OBJT_VNODE &&
                               (vp = fs.object->handle) != fs.vp) {
                                   unlock_vp(&fs);
                                   locked = VOP_ISLOCKED(vp);
   
                                   if (locked != LK_EXCLUSIVE)
                                           locked = LK_SHARED;
                                   /* Do not sleep for vnode lock while fs.m is busy */
                                   error = vget(vp, locked | LK_CANRECURSE |
                                       LK_NOWAIT, curthread);
                                   if (error != 0) {
                                           vhold(vp);
                                           release_page(&fs);
                                           unlock_and_deallocate(&fs);
                                           error = vget(vp, locked | LK_RETRY |
                                               LK_CANRECURSE, curthread);
                                           vdrop(vp);
                                           fs.vp = vp;
                                           KASSERT(error == 0,
                                               ("vm_fault: vget failed"));
                                           goto RetryFault;
                                   }
                                   fs.vp = vp;
                           }
                           KASSERT(fs.vp == NULL || !fs.map->system_map,
                               ("vm_fault: vnode-backed object mapped by system map"));
   
                           /*
                            * now we find out if any other pages should be paged
                            * in at this time this routine checks to see if the
                            * pages surrounding this fault reside in the same
                            * object as the page for this fault.  If they do,
                            * then they are faulted in also into the object.  The
                            * array "marray" returned contains an array of
                            * vm_page_t structs where one of them is the
                            * vm_page_t passed to the routine.  The reqpage
                            * return value is the index into the marray for the
                            * vm_page_t passed to the routine.
                            *
                            * fs.m plus the additional pages are exclusive busied.
                            */
                           faultcount = vm_fault_additional_pages(
                               fs.m, behind, ahead, marray, &reqpage);
   
                           rv = faultcount ?
                               vm_pager_get_pages(fs.object, marray, faultcount,
                                   reqpage) : VM_PAGER_FAIL;
   
                           if (rv == VM_PAGER_OK) {
                                   /*
                                    * Found the page. Leave it busy while we play
                                    * with it.
                                    */
   
                                   /*
                                    * Relookup in case pager changed page. Pager
                                    * is responsible for disposition of old page
                                    * if moved.
                                    */
                                   fs.m = vm_page_lookup(fs.object, fs.pindex);
                                   if (!fs.m) {
                                           unlock_and_deallocate(&fs);
                                           goto RetryFault;
                                   }
   
                                   hardfault++;
                                   break; /* break to PAGE HAS BEEN FOUND */
                           }
                           /*
                            * Remove the bogus page (which does not exist at this
                            * object/offset); before doing so, we must get back
                            * our object lock to preserve our invariant.
                            *
                            * Also wake up any other process that may want to bring
                            * in this page.
                            *
                            * If this is the top-level object, we must leave the
                            * busy page to prevent another process from rushing
                            * past us, and inserting the page in that object at
                            * the same time that we are.
                            */
                           if (rv == VM_PAGER_ERROR)
                                   printf("vm_fault: pager read error, pid %d (%s)\n",
                                       curproc->p_pid, curproc->p_comm);
                           /*
                            * Data outside the range of the pager or an I/O error
                            */
                           /*
                            * XXX - the check for kernel_map is a kludge to work
                            * around having the machine panic on a kernel space
                            * fault w/ I/O error.
                            */
                           if (((fs.map != kernel_map) && (rv == VM_PAGER_ERROR)) ||
                                   (rv == VM_PAGER_BAD)) {
                                   vm_page_lock(fs.m);
                                   vm_page_free(fs.m);
                                   vm_page_unlock(fs.m);
                                   fs.m = NULL;
                                   unlock_and_deallocate(&fs);
                                   return ((rv == VM_PAGER_ERROR) ? KERN_FAILURE : KERN_PROTECTION_FAILURE);
                           }
                           if (fs.object != fs.first_object) {
                                   vm_page_lock(fs.m);
                                   vm_page_free(fs.m);
                                   vm_page_unlock(fs.m);
                                   fs.m = NULL;
                                   /*
                                    * XXX - we cannot just fall out at this
                                    * point, m has been freed and is invalid!
                                    */
                           }
                   }
   
                   /*
                    * We get here if the object has default pager (or unwiring) 
                    * or the pager doesn't have the page.
                    */
                   if (fs.object == fs.first_object)
                           fs.first_m = fs.m;
   
                   /*
                    * Move on to the next object.  Lock the next object before
                    * unlocking the current one.
                    */
                   fs.pindex += OFF_TO_IDX(fs.object->backing_object_offset);
                   next_object = fs.object->backing_object;
                   if (next_object == NULL) {
                           /*
                            * If there's no object left, fill the page in the top
                            * object with zeros.
                            */
                           if (fs.object != fs.first_object) {
                                   vm_object_pip_wakeup(fs.object);
                                   VM_OBJECT_WUNLOCK(fs.object);
   
                                   fs.object = fs.first_object;
                                   fs.pindex = fs.first_pindex;
                                   fs.m = fs.first_m;
                                   VM_OBJECT_WLOCK(fs.object);
                           }
                           fs.first_m = NULL;
   
                           /*
                            * Zero the page if necessary and mark it valid.
                            */
                           if ((fs.m->flags & PG_ZERO) == 0) {
                                   pmap_zero_page(fs.m);
                           } else {
                                   PCPU_INC(cnt.v_ozfod);
                           }
                           PCPU_INC(cnt.v_zfod);
                           fs.m->valid = VM_PAGE_BITS_ALL;
                           /* Don't try to prefault neighboring pages. */
                           faultcount = 1;
                           break;  /* break to PAGE HAS BEEN FOUND */
                   } else {
                           KASSERT(fs.object != next_object,
                               ("object loop %p", next_object));
                           VM_OBJECT_WLOCK(next_object);
                           vm_object_pip_add(next_object, 1);
                           if (fs.object != fs.first_object)
                                   vm_object_pip_wakeup(fs.object);
                           VM_OBJECT_WUNLOCK(fs.object);
                           fs.object = next_object;
                   }
           }
   
           vm_page_assert_xbusied(fs.m);
   
           /*
            * PAGE HAS BEEN FOUND. [Loop invariant still holds -- the object lock
            * is held.]
            */
   
           /*
            * If the page is being written, but isn't already owned by the
            * top-level object, we have to copy it into a new page owned by the
            * top-level object.
            */
           if (fs.object != fs.first_object) {
                   /*
                    * We only really need to copy if we want to write it.
                    */
                   if ((fault_type & (VM_PROT_COPY | VM_PROT_WRITE)) != 0) {
                           /*
                            * This allows pages to be virtually copied from a 
                            * backing_object into the first_object, where the 
                            * backing object has no other refs to it, and cannot
                            * gain any more refs.  Instead of a bcopy, we just 
                            * move the page from the backing object to the 
                            * first object.  Note that we must mark the page 
                            * dirty in the first object so that it will go out 
                            * to swap when needed.
                            */
                           is_first_object_locked = FALSE;
                           if (
                                   /*
                                    * Only one shadow object
                                    */
                                   (fs.object->shadow_count == 1) &&
                                   /*
                                    * No COW refs, except us
                                    */
                                   (fs.object->ref_count == 1) &&
                                   /*
                                    * No one else can look this object up
                                    */
                                   (fs.object->handle == NULL) &&
                                   /*
                                    * No other ways to look the object up
                                    */
                                   ((fs.object->type == OBJT_DEFAULT) ||
                                    (fs.object->type == OBJT_SWAP)) &&
                               (is_first_object_locked = VM_OBJECT_TRYWLOCK(fs.first_object)) &&
                                   /*
                                    * We don't chase down the shadow chain
                                    */
                               fs.object == fs.first_object->backing_object) {
                                   /*
                                    * get rid of the unnecessary page
                                    */
                                   vm_page_lock(fs.first_m);
                                   vm_page_free(fs.first_m);
                                   vm_page_unlock(fs.first_m);
                                   /*
                                    * grab the page and put it into the 
                                    * process'es object.  The page is 
                                    * automatically made dirty.
                                    */
                                   if (vm_page_rename(fs.m, fs.first_object,
                                       fs.first_pindex)) {
                                           unlock_and_deallocate(&fs);
                                           goto RetryFault;
                                   }
   #if VM_NRESERVLEVEL > 0
                                   /*
                                    * Rename the reservation.
                                    */
                                   vm_reserv_rename(fs.m, fs.first_object,
                                       fs.object, OFF_TO_IDX(
                                       fs.first_object->backing_object_offset));
   #endif
                                   vm_page_xbusy(fs.m);
                                   fs.first_m = fs.m;
                                   fs.m = NULL;
                                   PCPU_INC(cnt.v_cow_optim);
                           } else {
                                   /*
                                    * Oh, well, lets copy it.
                                    */
                                   pmap_copy_page(fs.m, fs.first_m);
                                   fs.first_m->valid = VM_PAGE_BITS_ALL;
                                   if ((fault_flags & VM_FAULT_WIRE) == 0) {
                                           prot &= ~VM_PROT_WRITE;
                                           fault_type &= ~VM_PROT_WRITE;
                                   }
                                   if (wired && (fault_flags &
                                       VM_FAULT_WIRE) == 0) {
                                           vm_page_lock(fs.first_m);
                                           vm_page_wire(fs.first_m);
                                           vm_page_unlock(fs.first_m);
                                           
                                           vm_page_lock(fs.m);
                                           vm_page_unwire(fs.m, FALSE);
                                           vm_page_unlock(fs.m);
                                   }
                                   /*
                                    * We no longer need the old page or object.
                                    */
                                   release_page(&fs);
                           }
                           /*
                            * fs.object != fs.first_object due to above 
                            * conditional
                            */
                           vm_object_pip_wakeup(fs.object);
                           VM_OBJECT_WUNLOCK(fs.object);
                           /*
                            * Only use the new page below...
                            */
                           fs.object = fs.first_object;
                           fs.pindex = fs.first_pindex;
                           fs.m = fs.first_m;
                           if (!is_first_object_locked)
                                   VM_OBJECT_WLOCK(fs.object);
                           PCPU_INC(cnt.v_cow_faults);
                           curthread->td_cow++;
                   } else {
                           prot &= ~VM_PROT_WRITE;
                   }
           }
   
           /*
            * We must verify that the maps have not changed since our last
            * lookup.
            */
           if (!fs.lookup_still_valid) {
                   vm_object_t retry_object;
                   vm_pindex_t retry_pindex;
                   vm_prot_t retry_prot;
   
                   if (!vm_map_trylock_read(fs.map)) {
                           release_page(&fs);
                           unlock_and_deallocate(&fs);
                           goto RetryFault;
                   }
                   fs.lookup_still_valid = TRUE;
                   if (fs.map->timestamp != fs.map_generation) {
                           result = vm_map_lookup_locked(&fs.map, vaddr, fault_type,
                               &fs.entry, &retry_object, &retry_pindex, &retry_prot, &wired);
   
                           /*
                            * If we don't need the page any longer, put it on the inactive
                            * list (the easiest thing to do here).  If no one needs it,
                            * pageout will grab it eventually.
                            */
                           if (result != KERN_SUCCESS) {
                                   release_page(&fs);
                                   unlock_and_deallocate(&fs);
   
                                   /*
                                    * If retry of map lookup would have blocked then
                                    * retry fault from start.
                                    */
                                   if (result == KERN_FAILURE)
                                           goto RetryFault;
                                   return (result);
                           }
                           if ((retry_object != fs.first_object) ||
                               (retry_pindex != fs.first_pindex)) {
                                   release_page(&fs);
                                   unlock_and_deallocate(&fs);
                                   goto RetryFault;
                           }
   
                           /*
                            * Check whether the protection has changed or the object has
                            * been copied while we left the map unlocked. Changing from
                            * read to write permission is OK - we leave the page
                            * write-protected, and catch the write fault. Changing from
                            * write to read permission means that we can't mark the page
                            * write-enabled after all.
                            */
                           prot &= retry_prot;
                   }
           }
           /*
            * If the page was filled by a pager, update the map entry's
            * last read offset.  Since the pager does not return the
            * actual set of pages that it read, this update is based on
            * the requested set.  Typically, the requested and actual
            * sets are the same.
            *
            * XXX The following assignment modifies the map
            * without holding a write lock on it.
            */
          if (hardfault)
                  fs.entry->next_read = fs.pindex + faultcount - reqpage;
  
          vm_fault_dirty(fs.entry, fs.m, prot, fault_type, fault_flags, true);
          vm_page_assert_xbusied(fs.m);
  
          /*
           * Page must be completely valid or it is not fit to
           * map into user space.  vm_pager_get_pages() ensures this.
           */
          KASSERT(fs.m->valid == VM_PAGE_BITS_ALL,
              ("vm_fault: page %p partially invalid", fs.m));
          VM_OBJECT_WUNLOCK(fs.object);
  
          /*
           * Put this page into the physical map.  We had to do the unlock above
           * because pmap_enter() may sleep.  We don't put the page
           * back on the active queue until later so that the pageout daemon
           * won't find it (yet).
           */
          pmap_enter(fs.map->pmap, vaddr, fs.m, prot,
              fault_type | (wired ? PMAP_ENTER_WIRED : 0), 0);
          if (faultcount != 1 && (fault_flags & VM_FAULT_WIRE) == 0 &&
              wired == 0)
                  vm_fault_prefault(&fs, vaddr, faultcount, reqpage);
          VM_OBJECT_WLOCK(fs.object);
          vm_page_lock(fs.m);
  
          /*
           * If the page is not wired down, then put it where the pageout daemon
           * can find it.
           */
          if ((fault_flags & VM_FAULT_WIRE) != 0) {
                  KASSERT(wired, ("VM_FAULT_WIRE && !wired"));
                  vm_page_wire(fs.m);
          } else
                  vm_page_activate(fs.m);
          if (m_hold != NULL) {
                  *m_hold = fs.m;
                  vm_page_hold(fs.m);
          }
          vm_page_unlock(fs.m);
          vm_page_xunbusy(fs.m);
  
          /*
           * Unlock everything, and return
           */
          unlock_and_deallocate(&fs);
          if (hardfault) {
                  PCPU_INC(cnt.v_io_faults);
                  curthread->td_ru.ru_majflt++;
          } else 
                  curthread->td_ru.ru_minflt++;
  
          return (KERN_SUCCESS);
  }
  
  /*
   * Speed up the reclamation of up to "distance" pages that precede the
   * faulting pindex within the first object of the shadow chain.
   */
  static void
  vm_fault_cache_behind(const struct faultstate *fs, int distance)
  {
          vm_object_t first_object, object;
          vm_page_t m, m_prev;
          vm_pindex_t pindex;
  
          object = fs->object;
          VM_OBJECT_ASSERT_WLOCKED(object);
          first_object = fs->first_object;
          if (first_object != object) {
                  if (!VM_OBJECT_TRYWLOCK(first_object)) {
                          VM_OBJECT_WUNLOCK(object);
                          VM_OBJECT_WLOCK(first_object);
                          VM_OBJECT_WLOCK(object);
                  }
          }
          /* Neither fictitious nor unmanaged pages can be cached. */
          if ((first_object->flags & (OBJ_FICTITIOUS | OBJ_UNMANAGED)) == 0) {
                  if (fs->first_pindex < distance)
                          pindex = 0;
                  else
                          pindex = fs->first_pindex - distance;
                  if (pindex < OFF_TO_IDX(fs->entry->offset))
                          pindex = OFF_TO_IDX(fs->entry->offset);
                  m = first_object != object ? fs->first_m : fs->m;
                  vm_page_assert_xbusied(m);
                  m_prev = vm_page_prev(m);
                  while ((m = m_prev) != NULL && m->pindex >= pindex &&
                      m->valid == VM_PAGE_BITS_ALL) {
                          m_prev = vm_page_prev(m);
                          if (vm_page_busied(m))
                                  continue;
                          vm_page_lock(m);
                          if (m->hold_count == 0 && m->wire_count == 0) {
                                  pmap_remove_all(m);
                                  vm_page_aflag_clear(m, PGA_REFERENCED);
                                  if (m->dirty != 0)
                                          vm_page_deactivate(m);
                                  else
                                          vm_page_cache(m);
                          }
                          vm_page_unlock(m);
                  }
          }
          if (first_object != object)
                  VM_OBJECT_WUNLOCK(first_object);
  }
  
  /*
   * vm_fault_prefault provides a quick way of clustering
   * pagefaults into a processes address space.  It is a "cousin"
   * of vm_map_pmap_enter, except it runs at page fault time instead
   * of mmap time.
   */
  static void
  vm_fault_prefault(const struct faultstate *fs, vm_offset_t addra,
      int faultcount, int reqpage)
  {
          pmap_t pmap;
          vm_map_entry_t entry;
          vm_object_t backing_object, lobject;
          vm_offset_t addr, starta;
          vm_pindex_t pindex;
          vm_page_t m;
          int backward, forward, i;
  
          pmap = fs->map->pmap;
          if (pmap != vmspace_pmap(curthread->td_proc->p_vmspace))
                  return;
  
          if (faultcount > 0) {
                  backward = reqpage;
                  forward = faultcount - reqpage - 1;
          } else {
                  backward = PFBAK;
                  forward = PFFOR;
          }
          entry = fs->entry;
  
          if (addra < backward * PAGE_SIZE) {
                  starta = entry->start;
          } else {
                  starta = addra - backward * PAGE_SIZE;
                  if (starta < entry->start)
                          starta = entry->start;
          }
  
          /*
           * Generate the sequence of virtual addresses that are candidates for
           * prefaulting in an outward spiral from the faulting virtual address,
           * "addra".  Specifically, the sequence is "addra - PAGE_SIZE", "addra
           * + PAGE_SIZE", "addra - 2 * PAGE_SIZE", "addra + 2 * PAGE_SIZE", ...
           * If the candidate address doesn't have a backing physical page, then
           * the loop immediately terminates.
           */
          for (i = 0; i < 2 * imax(backward, forward); i++) {
                  addr = addra + ((i >> 1) + 1) * ((i & 1) == 0 ? -PAGE_SIZE :
                      PAGE_SIZE);
                  if (addr > addra + forward * PAGE_SIZE)
                          addr = 0;
  
                  if (addr < starta || addr >= entry->end)
                          continue;
  
                  if (!pmap_is_prefaultable(pmap, addr))
                          continue;
  
                  pindex = ((addr - entry->start) + entry->offset) >> PAGE_SHIFT;
                  lobject = entry->object.vm_object;
                  VM_OBJECT_RLOCK(lobject);
                  while ((m = vm_page_lookup(lobject, pindex)) == NULL &&
                      lobject->type == OBJT_DEFAULT &&
                      (backing_object = lobject->backing_object) != NULL) {
                          KASSERT((lobject->backing_object_offset & PAGE_MASK) ==
                              0, ("vm_fault_prefault: unaligned object offset"));
                          pindex += lobject->backing_object_offset >> PAGE_SHIFT;
                          VM_OBJECT_RLOCK(backing_object);
                          VM_OBJECT_RUNLOCK(lobject);
                          lobject = backing_object;
                  }
                  if (m == NULL) {
                          VM_OBJECT_RUNLOCK(lobject);
                          break;
                  }
                  if (m->valid == VM_PAGE_BITS_ALL &&
                      (m->flags & PG_FICTITIOUS) == 0)
                          pmap_enter_quick(pmap, addr, m, entry->protection);
                  VM_OBJECT_RUNLOCK(lobject);
          }
  }
  
  /*
   * Hold each of the physical pages that are mapped by the specified range of
   * virtual addresses, ["addr", "addr" + "len"), if those mappings are valid
   * and allow the specified types of access, "prot".  If all of the implied
   * pages are successfully held, then the number of held pages is returned
   * together with pointers to those pages in the array "ma".  However, if any
   * of the pages cannot be held, -1 is returned.
   */
  int
  vm_fault_quick_hold_pages(vm_map_t map, vm_offset_t addr, vm_size_t len,
      vm_prot_t prot, vm_page_t *ma, int max_count)
  {
          vm_offset_t end, va;
          vm_page_t *mp;
          int count;
          boolean_t pmap_failed;
  
          if (len == 0)
                  return (0);
          end = round_page(addr + len);
          addr = trunc_page(addr);
  
          /*
           * Check for illegal addresses.
           */
          if (addr < vm_map_min(map) || addr > end || end > vm_map_max(map))
                  return (-1);
  
          if (atop(end - addr) > max_count)
                  panic("vm_fault_quick_hold_pages: count > max_count");
          count = atop(end - addr);
  
          /*
           * Most likely, the physical pages are resident in the pmap, so it is
           * faster to try pmap_extract_and_hold() first.
           */
          pmap_failed = FALSE;
          for (mp = ma, va = addr; va < end; mp++, va += PAGE_SIZE) {
                  *mp = pmap_extract_and_hold(map->pmap, va, prot);
                  if (*mp == NULL)
                          pmap_failed = TRUE;
                  else if ((prot & VM_PROT_WRITE) != 0 &&
                      (*mp)->dirty != VM_PAGE_BITS_ALL) {
                          /*
                           * Explicitly dirty the physical page.  Otherwise, the
                           * caller's changes may go unnoticed because they are
                           * performed through an unmanaged mapping or by a DMA
                           * operation.
                           *
                           * The object lock is not held here.
                           * See vm_page_clear_dirty_mask().
                           */
                          vm_page_dirty(*mp);
                  }
          }
          if (pmap_failed) {
                  /*
                   * One or more pages could not be held by the pmap.  Either no
                   * page was mapped at the specified virtual address or that
                   * mapping had insufficient permissions.  Attempt to fault in
                   * and hold these pages.
                   */
                  for (mp = ma, va = addr; va < end; mp++, va += PAGE_SIZE)
                          if (*mp == NULL && vm_fault_hold(map, va, prot,
                              VM_FAULT_NORMAL, mp) != KERN_SUCCESS)
                                  goto error;
          }
          return (count);
  error:  
          for (mp = ma; mp < ma + count; mp++)
                  if (*mp != NULL) {
                          vm_page_lock(*mp);
                          vm_page_unhold(*mp);
                          vm_page_unlock(*mp);
                  }
          return (-1);
  }
  
  /*
   *      Routine:
   *              vm_fault_copy_entry
   *      Function:
   *              Create new shadow object backing dst_entry with private copy of
   *              all underlying pages. When src_entry is equal to dst_entry,
   *              function implements COW for wired-down map entry. Otherwise,
   *              it forks wired entry into dst_map.
   *
   *      In/out conditions:
   *              The source and destination maps must be locked for write.
   *              The source map entry must be wired down (or be a sharing map
   *              entry corresponding to a main map entry that is wired down).
   */
  void
  vm_fault_copy_entry(vm_map_t dst_map, vm_map_t src_map,
      vm_map_entry_t dst_entry, vm_map_entry_t src_entry,
      vm_ooffset_t *fork_charge)
  {
          vm_object_t backing_object, dst_object, object, src_object;
          vm_pindex_t dst_pindex, pindex, src_pindex;
          vm_prot_t access, prot;
          vm_offset_t vaddr;
          vm_page_t dst_m;
          vm_page_t src_m;
          boolean_t upgrade;
  
  #ifdef  lint
          src_map++;
  #endif  /* lint */
  
          upgrade = src_entry == dst_entry;
          access = prot = dst_entry->protection;
  
          src_object = src_entry->object.vm_object;
          src_pindex = OFF_TO_IDX(src_entry->offset);
  
          if (upgrade && (dst_entry->eflags & MAP_ENTRY_NEEDS_COPY) == 0) {
                  dst_object = src_object;
                  vm_object_reference(dst_object);
          } else {
                  /*
                   * Create the top-level object for the destination entry. (Doesn't
                   * actually shadow anything - we copy the pages directly.)
                   */
                  dst_object = vm_object_allocate(OBJT_DEFAULT,
                      OFF_TO_IDX(dst_entry->end - dst_entry->start));
  #if VM_NRESERVLEVEL > 0
                  dst_object->flags |= OBJ_COLORED;
                  dst_object->pg_color = atop(dst_entry->start);
  #endif
          }
  
          VM_OBJECT_WLOCK(dst_object);
          KASSERT(upgrade || dst_entry->object.vm_object == NULL,
              ("vm_fault_copy_entry: vm_object not NULL"));
          if (src_object != dst_object) {
                  dst_entry->object.vm_object = dst_object;
                  dst_entry->offset = 0;
                  dst_object->charge = dst_entry->end - dst_entry->start;
          }
          if (fork_charge != NULL) {
                  KASSERT(dst_entry->cred == NULL,
                      ("vm_fault_copy_entry: leaked swp charge"));
                  dst_object->cred = curthread->td_ucred;
                  crhold(dst_object->cred);
                  *fork_charge += dst_object->charge;
          } else if (dst_object->cred == NULL) {
                  KASSERT(dst_entry->cred != NULL, ("no cred for entry %p",
                      dst_entry));
                  dst_object->cred = dst_entry->cred;
                  dst_entry->cred = NULL;
          }
  
          /*
           * If not an upgrade, then enter the mappings in the pmap as
           * read and/or execute accesses.  Otherwise, enter them as
           * write accesses.
           *
           * A writeable large page mapping is only created if all of
           * the constituent small page mappings are modified. Marking
           * PTEs as modified on inception allows promotion to happen
           * without taking potentially large number of soft faults.
           */
          if (!upgrade)
                  access &= ~VM_PROT_WRITE;
  
          /*
           * Loop through all of the virtual pages within the entry's
           * range, copying each page from the source object to the
           * destination object.  Since the source is wired, those pages
           * must exist.  In contrast, the destination is pageable.
           * Since the destination object does share any backing storage
           * with the source object, all of its pages must be dirtied,
           * regardless of whether they can be written.
           */
          for (vaddr = dst_entry->start, dst_pindex = 0;
              vaddr < dst_entry->end;
              vaddr += PAGE_SIZE, dst_pindex++) {
  again:
                  /*
                   * Find the page in the source object, and copy it in.
                   * Because the source is wired down, the page will be
                   * in memory.
                   */
                  if (src_object != dst_object)
                          VM_OBJECT_RLOCK(src_object);
                  object = src_object;
                  pindex = src_pindex + dst_pindex;
                  while ((src_m = vm_page_lookup(object, pindex)) == NULL &&
                      (backing_object = object->backing_object) != NULL) {
                          /*
                           * Unless the source mapping is read-only or
                           * it is presently being upgraded from
                           * read-only, the first object in the shadow
                           * chain should provide all of the pages.  In
                           * other words, this loop body should never be
                           * executed when the source mapping is already
                           * read/write.
                           */
                          KASSERT((src_entry->protection & VM_PROT_WRITE) == 0 ||
                              upgrade,
                              ("vm_fault_copy_entry: main object missing page"));
  
                          VM_OBJECT_RLOCK(backing_object);
                          pindex += OFF_TO_IDX(object->backing_object_offset);
                          if (object != dst_object)
                                  VM_OBJECT_RUNLOCK(object);
                          object = backing_object;
                  }
                  KASSERT(src_m != NULL, ("vm_fault_copy_entry: page missing"));
  
                  if (object != dst_object) {
                          /*
                           * Allocate a page in the destination object.
                           */
                          dst_m = vm_page_alloc(dst_object, (src_object ==
                              dst_object ? src_pindex : 0) + dst_pindex,
                              VM_ALLOC_NORMAL);
                          if (dst_m == NULL) {
                                  VM_OBJECT_WUNLOCK(dst_object);
                                  VM_OBJECT_RUNLOCK(object);
                                  VM_WAIT;
                                  VM_OBJECT_WLOCK(dst_object);
                                  goto again;
                          }
                          pmap_copy_page(src_m, dst_m);
                          VM_OBJECT_RUNLOCK(object);
                          dst_m->valid = VM_PAGE_BITS_ALL;
                          dst_m->dirty = VM_PAGE_BITS_ALL;
                  } else {
                          dst_m = src_m;
                          if (vm_page_sleep_if_busy(dst_m, "fltupg"))
                                  goto again;
                          vm_page_xbusy(dst_m);
                          KASSERT(dst_m->valid == VM_PAGE_BITS_ALL,
                              ("invalid dst page %p", dst_m));
                  }
                  VM_OBJECT_WUNLOCK(dst_object);
  
                  /*
                   * Enter it in the pmap. If a wired, copy-on-write
                   * mapping is being replaced by a write-enabled
                   * mapping, then wire that new mapping.
                   */
                  pmap_enter(dst_map->pmap, vaddr, dst_m, prot,
                      access | (upgrade ? PMAP_ENTER_WIRED : 0), 0);
  
                  /*
                   * Mark it no longer busy, and put it on the active list.
                   */
                  VM_OBJECT_WLOCK(dst_object);
                  
                  if (upgrade) {
                          if (src_m != dst_m) {
                                  vm_page_lock(src_m);
                                  vm_page_unwire(src_m, 0);
                                  vm_page_unlock(src_m);
                                  vm_page_lock(dst_m);
                                  vm_page_wire(dst_m);
                                  vm_page_unlock(dst_m);
                          } else {
                                  KASSERT(dst_m->wire_count > 0,
                                      ("dst_m %p is not wired", dst_m));
                          }
                  } else {
                          vm_page_lock(dst_m);
                          vm_page_activate(dst_m);
                          vm_page_unlock(dst_m);
                  }
                  vm_page_xunbusy(dst_m);
          }
          VM_OBJECT_WUNLOCK(dst_object);
          if (upgrade) {
                  dst_entry->eflags &= ~(MAP_ENTRY_COW | MAP_ENTRY_NEEDS_COPY);
                  vm_object_deallocate(src_object);
          }
  }
  
  
  /*
   * This routine checks around the requested page for other pages that
   * might be able to be faulted in.  This routine brackets the viable
   * pages for the pages to be paged in.
   *
   * Inputs:
   *      m, rbehind, rahead
   *
   * Outputs:
   *  marray (array of vm_page_t), reqpage (index of requested page)
   *
   * Return value:
   *  number of pages in marray
   */
  static int
  vm_fault_additional_pages(m, rbehind, rahead, marray, reqpage)
          vm_page_t m;
          int rbehind;
          int rahead;
          vm_page_t *marray;
          int *reqpage;
  {
          int i,j;
          vm_object_t object;
          vm_pindex_t pindex, startpindex, endpindex, tpindex;
          vm_page_t rtm;
          int cbehind, cahead;
  
          VM_OBJECT_ASSERT_WLOCKED(m->object);
  
          object = m->object;
          pindex = m->pindex;
          cbehind = cahead = 0;
  
          /*
           * if the requested page is not available, then give up now
           */
          if (!vm_pager_has_page(object, pindex, &cbehind, &cahead)) {
                  return 0;
          }
  
          if ((cbehind == 0) && (cahead == 0)) {
                  *reqpage = 0;
                  marray[0] = m;
                  return 1;
          }
  
          if (rahead > cahead) {
                  rahead = cahead;
          }
  
          if (rbehind > cbehind) {
                  rbehind = cbehind;
          }
  
          /*
           * scan backward for the read behind pages -- in memory 
           */
          if (pindex > 0) {
                  if (rbehind > pindex) {
                          rbehind = pindex;
                          startpindex = 0;
                  } else {
                          startpindex = pindex - rbehind;
                  }
  
                  if ((rtm = TAILQ_PREV(m, pglist, listq)) != NULL &&
                      rtm->pindex >= startpindex)
                          startpindex = rtm->pindex + 1;
  
                  /* tpindex is unsigned; beware of numeric underflow. */
                  for (i = 0, tpindex = pindex - 1; tpindex >= startpindex &&
                      tpindex < pindex; i++, tpindex--) {
  
                          rtm = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL |
                              VM_ALLOC_IFNOTCACHED);
                          if (rtm == NULL) {
                                  /*
                                   * Shift the allocated pages to the
                                   * beginning of the array.
                                   */
                                  for (j = 0; j < i; j++) {
                                          marray[j] = marray[j + tpindex + 1 -
                                              startpindex];
                                  }
                                  break;
                          }
  
                          marray[tpindex - startpindex] = rtm;
                  }
          } else {
                  startpindex = 0;
                  i = 0;
          }
  
          marray[i] = m;
          /* page offset of the required page */
          *reqpage = i;
  
          tpindex = pindex + 1;
          i++;
  
          /*
           * scan forward for the read ahead pages
           */
          endpindex = tpindex + rahead;
          if ((rtm = TAILQ_NEXT(m, listq)) != NULL && rtm->pindex < endpindex)
                  endpindex = rtm->pindex;
          if (endpindex > object->size)
                  endpindex = object->size;
  
          for (; tpindex < endpindex; i++, tpindex++) {
  
                  rtm = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL |
                      VM_ALLOC_IFNOTCACHED);
                  if (rtm == NULL) {
                          break;
                  }
  
                  marray[i] = rtm;
          }
  
          /* return number of pages */
          return i;
  }
  
  /*
   * Block entry into the machine-independent layer's page fault handler by
   * the calling thread.  Subsequent calls to vm_fault() by that thread will
   * return KERN_PROTECTION_FAILURE.  Enable machine-dependent handling of
   * spurious page faults. 
   */
  int
  vm_fault_disable_pagefaults(void)
  {
  
          return (curthread_pflags_set(TDP_NOFAULTING | TDP_RESETSPUR));
  }
  
  void
  vm_fault_enable_pagefaults(int save)
  {
  
          curthread_pflags_restore(save);
  }

Cache object: cd215021c62f9bc550a00f5a7ef7321a