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

     /*-
      * SPDX-License-Identifier: (BSD-4-Clause AND MIT-CMU)
      *
      * 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/mman.h>
    #include <sys/proc.h>
    #include <sys/racct.h>
    #include <sys/resourcevar.h>
    #include <sys/rwlock.h>
    #include <sys/signalvar.h>
    #include <sys/sysctl.h>
    #include <sys/sysent.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
   
   #define VM_FAULT_READ_DEFAULT   (1 + VM_FAULT_READ_AHEAD_INIT)
   #define VM_FAULT_READ_MAX       (1 + VM_FAULT_READ_AHEAD_MAX)
   
   #define VM_FAULT_DONTNEED_MIN   1048576
   
   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 map_generation;
           bool lookup_still_valid;
           struct vnode *vp;
   };
   
   static void vm_fault_dontneed(const struct faultstate *fs, vm_offset_t vaddr,
               int ahead);
   static void vm_fault_prefault(const struct faultstate *fs, vm_offset_t addra,
               int backward, int forward, bool obj_locked);
   
   static int vm_pfault_oom_attempts = 3;
   SYSCTL_INT(_vm, OID_AUTO, pfault_oom_attempts, CTLFLAG_RWTUN,
       &vm_pfault_oom_attempts, 0,
       "Number of page allocation attempts in page fault handler before it "
       "triggers OOM handling");
   
   static int vm_pfault_oom_wait = 10;
   SYSCTL_INT(_vm, OID_AUTO, pfault_oom_wait, CTLFLAG_RWTUN,
       &vm_pfault_oom_wait, 0,
       "Number of seconds to wait for free pages before retrying "
       "the page fault handler");
   
   static inline void
   release_page(struct faultstate *fs)
   {
   
           if (fs->m != NULL) {
                   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, since the page is now dirty, we can possibly tell
            * the pager to release any swap backing the page.  Calling
            * the pager requires a write lock on the object.
            */
           if (need_dirty)
                   vm_page_dirty(m);
           if (!set_wd)
                   vm_page_unlock(m);
           else 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, m_map;
   #if (defined(__aarch64__) || defined(__amd64__) || (defined(__arm__) && \
       __ARM_ARCH >= 6) || defined(__i386__) || defined(__riscv)) && \
       VM_NRESERVLEVEL > 0
           vm_page_t m_super;
           int flags;
   #endif
           int psind, 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);
           m_map = m;
           psind = 0;
   #if (defined(__aarch64__) || defined(__amd64__) || (defined(__arm__) && \
       __ARM_ARCH >= 6) || defined(__i386__) || defined(__riscv)) && \
       VM_NRESERVLEVEL > 0
           if ((m->flags & PG_FICTITIOUS) == 0 &&
               (m_super = vm_reserv_to_superpage(m)) != NULL &&
               rounddown2(vaddr, pagesizes[m_super->psind]) >= fs->entry->start &&
               roundup2(vaddr + 1, pagesizes[m_super->psind]) <= fs->entry->end &&
               (vaddr & (pagesizes[m_super->psind] - 1)) == (VM_PAGE_TO_PHYS(m) &
               (pagesizes[m_super->psind] - 1)) &&
               pmap_ps_enabled(fs->map->pmap)) {
                   flags = PS_ALL_VALID;
                   if ((prot & VM_PROT_WRITE) != 0) {
                           /*
                            * Create a superpage mapping allowing write access
                            * only if none of the constituent pages are busy and
                            * all of them are already dirty (except possibly for
                            * the page that was faulted on).
                            */
                           flags |= PS_NONE_BUSY;
                           if ((fs->first_object->flags & OBJ_UNMANAGED) == 0)
                                   flags |= PS_ALL_DIRTY;
                   }
                   if (vm_page_ps_test(m_super, flags, m)) {
                           m_map = m_super;
                           psind = m_super->psind;
                           vaddr = rounddown2(vaddr, pagesizes[psind]);
                           /* Preset the modified bit for dirty superpages. */
                           if ((flags & PS_ALL_DIRTY) != 0)
                                   fault_type |= VM_PROT_WRITE;
                   }
           }
   #endif
           rv = pmap_enter(fs->map->pmap, vaddr, m_map, prot, fault_type |
               PMAP_ENTER_NOSLEEP | (wired ? PMAP_ENTER_WIRED : 0), psind);
           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);
           if (psind == 0 && !wired)
                   vm_fault_prefault(fs, vaddr, PFBAK, PFFOR, true);
           VM_OBJECT_RUNLOCK(fs->first_object);
           vm_map_lookup_done(fs->map, fs->entry);
           curthread->td_ru.ru_minflt++;
           return (KERN_SUCCESS);
   }
   
   static void
   vm_fault_restore_map_lock(struct faultstate *fs)
   {
   
           VM_OBJECT_ASSERT_WLOCKED(fs->first_object);
           MPASS(fs->first_object->paging_in_progress > 0);
   
           if (!vm_map_trylock_read(fs->map)) {
                   VM_OBJECT_WUNLOCK(fs->first_object);
                   vm_map_lock_read(fs->map);
                   VM_OBJECT_WLOCK(fs->first_object);
           }
           fs->lookup_still_valid = true;
   }
   
   static void
   vm_fault_populate_check_page(vm_page_t m)
   {
   
           /*
            * Check each page to ensure that the pager is obeying the
            * interface: the page must be installed in the object, fully
            * valid, and exclusively busied.
            */
           MPASS(m != NULL);
           MPASS(m->valid == VM_PAGE_BITS_ALL);
           MPASS(vm_page_xbusied(m));
   }
   
   static void
   vm_fault_populate_cleanup(vm_object_t object, vm_pindex_t first,
       vm_pindex_t last)
   {
           vm_page_t m;
           vm_pindex_t pidx;
   
           VM_OBJECT_ASSERT_WLOCKED(object);
           MPASS(first <= last);
           for (pidx = first, m = vm_page_lookup(object, pidx);
               pidx <= last; pidx++, m = vm_page_next(m)) {
                   vm_fault_populate_check_page(m);
                   vm_page_lock(m);
                   vm_page_deactivate(m);
                   vm_page_unlock(m);
                   vm_page_xunbusy(m);
           }
   }
   
   static int
   vm_fault_populate(struct faultstate *fs, vm_prot_t prot, int fault_type,
       int fault_flags, boolean_t wired, vm_page_t *m_hold)
   {
           struct mtx *m_mtx;
           vm_offset_t vaddr;
           vm_page_t m;
           vm_pindex_t map_first, map_last, pager_first, pager_last, pidx;
           int i, npages, psind, rv;
   
           MPASS(fs->object == fs->first_object);
           VM_OBJECT_ASSERT_WLOCKED(fs->first_object);
           MPASS(fs->first_object->paging_in_progress > 0);
           MPASS(fs->first_object->backing_object == NULL);
           MPASS(fs->lookup_still_valid);
   
           pager_first = OFF_TO_IDX(fs->entry->offset);
           pager_last = pager_first + atop(fs->entry->end - fs->entry->start) - 1;
           unlock_map(fs);
           unlock_vp(fs);
   
           /*
            * Call the pager (driver) populate() method.
            *
            * There is no guarantee that the method will be called again
            * if the current fault is for read, and a future fault is
            * for write.  Report the entry's maximum allowed protection
            * to the driver.
            */
           rv = vm_pager_populate(fs->first_object, fs->first_pindex,
               fault_type, fs->entry->max_protection, &pager_first, &pager_last);
   
           VM_OBJECT_ASSERT_WLOCKED(fs->first_object);
           if (rv == VM_PAGER_BAD) {
                   /*
                    * VM_PAGER_BAD is the backdoor for a pager to request
                    * normal fault handling.
                    */
                   vm_fault_restore_map_lock(fs);
                   if (fs->map->timestamp != fs->map_generation)
                           return (KERN_RESOURCE_SHORTAGE); /* RetryFault */
                   return (KERN_NOT_RECEIVER);
           }
           if (rv != VM_PAGER_OK)
                   return (KERN_FAILURE); /* AKA SIGSEGV */
   
           /* Ensure that the driver is obeying the interface. */
           MPASS(pager_first <= pager_last);
           MPASS(fs->first_pindex <= pager_last);
           MPASS(fs->first_pindex >= pager_first);
           MPASS(pager_last < fs->first_object->size);
   
           vm_fault_restore_map_lock(fs);
           if (fs->map->timestamp != fs->map_generation) {
                   vm_fault_populate_cleanup(fs->first_object, pager_first,
                       pager_last);
                   return (KERN_RESOURCE_SHORTAGE); /* RetryFault */
           }
   
           /*
            * The map is unchanged after our last unlock.  Process the fault.
            *
            * The range [pager_first, pager_last] that is given to the
            * pager is only a hint.  The pager may populate any range
            * within the object that includes the requested page index.
            * In case the pager expanded the range, clip it to fit into
            * the map entry.
            */
           map_first = OFF_TO_IDX(fs->entry->offset);
           if (map_first > pager_first) {
                   vm_fault_populate_cleanup(fs->first_object, pager_first,
                       map_first - 1);
                   pager_first = map_first;
           }
           map_last = map_first + atop(fs->entry->end - fs->entry->start) - 1;
           if (map_last < pager_last) {
                   vm_fault_populate_cleanup(fs->first_object, map_last + 1,
                       pager_last);
                   pager_last = map_last;
           }
           for (pidx = pager_first, m = vm_page_lookup(fs->first_object, pidx);
               pidx <= pager_last;
               pidx += npages, m = vm_page_next(&m[npages - 1])) {
                   vaddr = fs->entry->start + IDX_TO_OFF(pidx) - fs->entry->offset;
   #if defined(__aarch64__) || defined(__amd64__) || (defined(__arm__) && \
       __ARM_ARCH >= 6) || defined(__i386__) || defined(__riscv)
                   psind = m->psind;
                   if (psind > 0 && ((vaddr & (pagesizes[psind] - 1)) != 0 ||
                       pidx + OFF_TO_IDX(pagesizes[psind]) - 1 > pager_last ||
                       !pmap_ps_enabled(fs->map->pmap)))
                           psind = 0;
   #else
                   psind = 0;
   #endif          
                   npages = atop(pagesizes[psind]);
                   for (i = 0; i < npages; i++) {
                           vm_fault_populate_check_page(&m[i]);
                           vm_fault_dirty(fs->entry, &m[i], prot, fault_type,
                               fault_flags, true);
                   }
                   VM_OBJECT_WUNLOCK(fs->first_object);
                   rv = pmap_enter(fs->map->pmap, vaddr, m, prot, fault_type |
                       (wired ? PMAP_ENTER_WIRED : 0), psind);
   #if defined(__amd64__)
                   if (psind > 0 && rv == KERN_FAILURE) {
                           for (i = 0; i < npages; i++) {
                                   rv = pmap_enter(fs->map->pmap, vaddr + ptoa(i),
                                       &m[i], prot, fault_type |
                                       (wired ? PMAP_ENTER_WIRED : 0), 0);
                                   MPASS(rv == KERN_SUCCESS);
                           }
                   }
   #else
                   MPASS(rv == KERN_SUCCESS);
   #endif
                   VM_OBJECT_WLOCK(fs->first_object);
                   m_mtx = NULL;
                   for (i = 0; i < npages; i++) {
                           vm_page_change_lock(&m[i], &m_mtx);
                           if ((fault_flags & VM_FAULT_WIRE) != 0)
                                   vm_page_wire(&m[i]);
                           else
                                   vm_page_activate(&m[i]);
                           if (m_hold != NULL && m[i].pindex == fs->first_pindex) {
                                   *m_hold = &m[i];
                                   vm_page_hold(&m[i]);
                           }
                           vm_page_xunbusy_maybelocked(&m[i]);
                   }
                   if (m_mtx != NULL)
                           mtx_unlock(m_mtx);
           }
           curthread->td_ru.ru_majflt++;
           return (KERN_SUCCESS);
   }
   
   static int prot_fault_translation;
   SYSCTL_INT(_machdep, OID_AUTO, prot_fault_translation, CTLFLAG_RWTUN,
       &prot_fault_translation, 0,
       "Control signal to deliver on protection fault");
   
   /* compat definition to keep common code for signal translation */
   #define UCODE_PAGEFLT   12
   #ifdef T_PAGEFLT
   _Static_assert(UCODE_PAGEFLT == T_PAGEFLT, "T_PAGEFLT");
   #endif
   
   /*
    *      vm_fault_trap:
    *
    *      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_trap(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
       int fault_flags, int *signo, int *ucode)
   {
           int result;
   
           MPASS(signo == NULL || ucode != NULL);
   #ifdef KTRACE
           if (map != kernel_map && KTRPOINT(curthread, KTR_FAULT))
                   ktrfault(vaddr, fault_type);
   #endif
           result = vm_fault(map, trunc_page(vaddr), fault_type, fault_flags,
               NULL);
           KASSERT(result == KERN_SUCCESS || result == KERN_FAILURE ||
               result == KERN_INVALID_ADDRESS ||
               result == KERN_RESOURCE_SHORTAGE ||
               result == KERN_PROTECTION_FAILURE ||
               result == KERN_OUT_OF_BOUNDS,
               ("Unexpected Mach error %d from vm_fault()", result));
   #ifdef KTRACE
           if (map != kernel_map && KTRPOINT(curthread, KTR_FAULTEND))
                   ktrfaultend(result);
   #endif
           if (result != KERN_SUCCESS && signo != NULL) {
                   switch (result) {
                   case KERN_FAILURE:
                   case KERN_INVALID_ADDRESS:
                           *signo = SIGSEGV;
                           *ucode = SEGV_MAPERR;
                           break;
                   case KERN_RESOURCE_SHORTAGE:
                           *signo = SIGBUS;
                           *ucode = BUS_OOMERR;
                           break;
                   case KERN_OUT_OF_BOUNDS:
                           *signo = SIGBUS;
                           *ucode = BUS_OBJERR;
                           break;
                   case KERN_PROTECTION_FAILURE:
                           if (prot_fault_translation == 0) {
                                   /*
                                    * Autodetect.  This check also covers
                                    * the images without the ABI-tag ELF
                                    * note.
                                    */
                                   if (SV_CURPROC_ABI() == SV_ABI_FREEBSD &&
                                       curproc->p_osrel >= P_OSREL_SIGSEGV) {
                                           *signo = SIGSEGV;
                                           *ucode = SEGV_ACCERR;
                                   } else {
                                           *signo = SIGBUS;
                                           *ucode = UCODE_PAGEFLT;
                                   }
                           } else if (prot_fault_translation == 1) {
                                   /* Always compat mode. */
                                   *signo = SIGBUS;
                                   *ucode = UCODE_PAGEFLT;
                           } else {
                                   /* Always SIGSEGV mode. */
                                   *signo = SIGSEGV;
                                   *ucode = SEGV_ACCERR;
                           }
                           break;
                   default:
                           KASSERT(0, ("Unexpected Mach error %d from vm_fault()",
                               result));
                           break;
                   }
           }
           return (result);
   }
   
   static int
   vm_fault_lock_vnode(struct faultstate *fs)
   {
           struct vnode *vp;
           int error, locked;
   
           if (fs->object->type != OBJT_VNODE)
                   return (KERN_SUCCESS);
           vp = fs->object->handle;
           if (vp == fs->vp) {
                   ASSERT_VOP_LOCKED(vp, "saved vnode is not locked");
                   return (KERN_SUCCESS);
           }
   
           /*
            * Perform an unlock in case the desired vnode changed while
            * the map was unlocked during a retry.
            */
           unlock_vp(fs);
   
           locked = VOP_ISLOCKED(vp);
           if (locked != LK_EXCLUSIVE)
                   locked = LK_SHARED;
   
           /*
            * We must not sleep acquiring the vnode lock while we have
            * the page exclusive busied or the object's
            * paging-in-progress count incremented.  Otherwise, we could
            * deadlock.
            */
           error = vget(vp, locked | LK_CANRECURSE | LK_NOWAIT, curthread);
           if (error == 0) {
                   fs->vp = vp;
                   return (KERN_SUCCESS);
           }
   
           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 %d", error));
           return (KERN_RESOURCE_SHORTAGE);
   }
   
   int
   vm_fault(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
       int fault_flags, vm_page_t *m_hold)
   {
           struct faultstate fs;
           struct domainset *dset;
           vm_object_t next_object, retry_object;
           vm_offset_t e_end, e_start;
           vm_pindex_t retry_pindex;
           vm_prot_t prot, retry_prot;
           int ahead, alloc_req, behind, cluster_offset, era, faultcount;
           int nera, oom, result, rv;
           u_char behavior;
           boolean_t wired;        /* Passed by reference. */
           bool dead, hardfault, is_first_object_locked;
   
           VM_CNT_INC(v_vm_faults);
   
           if ((curthread->td_pflags & TDP_NOFAULTING) != 0)
                   return (KERN_PROTECTION_FAILURE);
   
           fs.vp = NULL;
           faultcount = 0;
           nera = -1;
           hardfault = false;
   
   RetryFault:
           oom = 0;
   RetryFault_oom:
   
           /*
            * 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("%s: fault on nofault entry, addr: %#lx",
                       __func__, (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) {
                           rv = vm_fault_soft_fast(&fs, vaddr, prot, fault_type,
                               fault_flags, wired, m_hold);
                           if (rv == KERN_SUCCESS)
                                   return (rv);
                   }
                   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.
            */
           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);
                                   VM_CNT_INC(v_intrans);
                                   vm_object_deallocate(fs.first_object);
                                   goto RetryFault;
                           }
   
                           /*
                            * 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; /* break to PAGE HAS BEEN FOUND */
                   }
                   KASSERT(fs.m == NULL, ("fs.m should be NULL, not %p", fs.m));
   
                   /*
                    * Page is not resident.  If the pager might contain the page
                    * or this is the beginning of the search, allocate a new
                    * page.  (Default objects are zero-fill, so there is no real
                    * pager for them.)
                    */
                   if (fs.object->type != OBJT_DEFAULT ||
                       fs.object == fs.first_object) {
                           if ((fs.object->flags & OBJ_SIZEVNLOCK) != 0) {
                                   rv = vm_fault_lock_vnode(&fs);
                                   MPASS(rv == KERN_SUCCESS ||
                                       rv == KERN_RESOURCE_SHORTAGE);
                                   if (rv == KERN_RESOURCE_SHORTAGE)
                                           goto RetryFault;
                           }
                           if (fs.pindex >= fs.object->size) {
                                   unlock_and_deallocate(&fs);
                                   return (KERN_OUT_OF_BOUNDS);
                           }
   
                           if (fs.object == fs.first_object &&
                               (fs.first_object->flags & OBJ_POPULATE) != 0 &&
                               fs.first_object->shadow_count == 0) {
                                   rv = vm_fault_populate(&fs, prot, fault_type,
                                       fault_flags, wired, m_hold);
                                   switch (rv) {
                                   case KERN_SUCCESS:
                                   case KERN_FAILURE:
                                           unlock_and_deallocate(&fs);
                                           return (rv);
                                   case KERN_RESOURCE_SHORTAGE:
                                           unlock_and_deallocate(&fs);
                                           goto RetryFault;
                                   case KERN_NOT_RECEIVER:
                                           /*
                                            * Pager's populate() method
                                            * returned VM_PAGER_BAD.
                                            */
                                           break;
                                   default:
                                           panic("inconsistent return codes");
                                   }
                           }
   
                           /*
                            * 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.
                            */
                           dset = fs.object->domain.dr_policy;
                           if (dset == NULL)
                                   dset = curthread->td_domain.dr_policy;
                           if (!vm_page_count_severe_set(&dset->ds_mask) ||
                               P_KILLED(curproc)) {
   #if VM_NRESERVLEVEL > 0
                                   vm_object_color(fs.object, 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);
                                   if (vm_pfault_oom_attempts < 0 ||
                                       oom < vm_pfault_oom_attempts) {
                                           oom++;
                                           vm_waitpfault(dset,
                                               vm_pfault_oom_wait * hz);
                                           goto RetryFault_oom;
                                   }
                                   if (bootverbose)
                                           printf(
           "proc %d (%s) failed to alloc page on fault, starting OOM\n",
                                               curproc->p_pid, curproc->p_comm);
                                   vm_pageout_oom(VM_OOM_MEM_PF);
                                   goto RetryFault;
                           }
                   }
   
   readrest:
                   /*
                    * At this point, we have either allocated a new page or found
                    * an existing page that is only partially valid.
                    *
                    * We hold a reference on the current object and the page is
                    * exclusive busied.
                    */
   
                   /*
                    * If the pager for the current object might have the page,
                    * then determine the number of additional pages to read and
                    * potentially reprioritize previously read pages for earlier
                    * reclamation.  These operations should only be performed
                    * once per page fault.  Even if the current pager doesn't
                    * have the page, the number of additional pages to read will
                    * apply to subsequent objects in the shadow chain.
                    */
                   if (fs.object->type != OBJT_DEFAULT && nera == -1 &&
                       !P_KILLED(curproc)) {
                           KASSERT(fs.lookup_still_valid, ("map unlocked"));
                           era = fs.entry->read_ahead;
                          behavior = vm_map_entry_behavior(fs.entry);
                          if (behavior == MAP_ENTRY_BEHAV_RANDOM) {
                                  nera = 0;
                          } else if (behavior == MAP_ENTRY_BEHAV_SEQUENTIAL) {
                                  nera = VM_FAULT_READ_AHEAD_MAX;
                                  if (vaddr == fs.entry->next_read)
                                          vm_fault_dontneed(&fs, vaddr, nera);
                          } else if (vaddr == fs.entry->next_read) {
                                  /*
                                   * This is a sequential fault.  Arithmetically
                                   * increase the requested number of pages in
                                   * the read-ahead window.  The requested
                                   * number of pages is "# of sequential faults
                                   * x (read ahead min + 1) + read ahead min"
                                   */
                                  nera = VM_FAULT_READ_AHEAD_MIN;
                                  if (era > 0) {
                                          nera += era + 1;
                                          if (nera > VM_FAULT_READ_AHEAD_MAX)
                                                  nera = VM_FAULT_READ_AHEAD_MAX;
                                  }
                                  if (era == VM_FAULT_READ_AHEAD_MAX)
                                          vm_fault_dontneed(&fs, vaddr, nera);
                          } else {
                                  /*
                                   * This is a non-sequential fault.
                                   */
                                  nera = 0;
                          }
                          if (era != nera) {
                                  /*
                                   * A read lock on the map suffices to update
                                   * the read ahead count safely.
                                   */
                                  fs.entry->read_ahead = nera;
                          }
  
                          /*
                           * Prepare for unlocking the map.  Save the map
                           * entry's start and end addresses, which are used to
                           * optimize the size of the pager operation below.
                           * Even if the map entry's addresses change after
                           * unlocking the map, using the saved addresses is
                           * safe.
                           */
                          e_start = fs.entry->start;
                          e_end = fs.entry->end;
                  }
  
                  /*
                   * Call the pager to retrieve the page if there is a chance
                   * that the pager has it, and potentially retrieve additional
                   * pages at the same time.
                   */
                  if (fs.object->type != OBJT_DEFAULT) {
                          /*
                           * Release the map lock before locking the vnode or
                           * sleeping in the pager.  (If the current object has
                           * a shadow, then an earlier iteration of this loop
                           * may have already unlocked the map.)
                           */
                          unlock_map(&fs);
  
                          rv = vm_fault_lock_vnode(&fs);
                          MPASS(rv == KERN_SUCCESS ||
                              rv == KERN_RESOURCE_SHORTAGE);
                          if (rv == KERN_RESOURCE_SHORTAGE)
                                  goto RetryFault;
                          KASSERT(fs.vp == NULL || !fs.map->system_map,
                              ("vm_fault: vnode-backed object mapped by system map"));
  
                          /*
                           * Page in the requested page and hint the pager,
                           * that it may bring up surrounding pages.
                           */
                          if (nera == -1 || behavior == MAP_ENTRY_BEHAV_RANDOM ||
                              P_KILLED(curproc)) {
                                  behind = 0;
                                  ahead = 0;
                          } else {
                                  /* Is this a sequential fault? */
                                  if (nera > 0) {
                                          behind = 0;
                                          ahead = nera;
                                  } else {
                                          /*
                                           * Request a cluster of pages that is
                                           * aligned to a VM_FAULT_READ_DEFAULT
                                           * page offset boundary within the
                                           * object.  Alignment to a page offset
                                           * boundary is more likely to coincide
                                           * with the underlying file system
                                           * block than alignment to a virtual
                                           * address boundary.
                                           */
                                          cluster_offset = fs.pindex %
                                              VM_FAULT_READ_DEFAULT;
                                          behind = ulmin(cluster_offset,
                                              atop(vaddr - e_start));
                                          ahead = VM_FAULT_READ_DEFAULT - 1 -
                                              cluster_offset;
                                  }
                                  ahead = ulmin(ahead, atop(e_end - vaddr) - 1);
                          }
                          rv = vm_pager_get_pages(fs.object, &fs.m, 1,
                              &behind, &ahead);
                          if (rv == VM_PAGER_OK) {
                                  faultcount = behind + 1 + ahead;
                                  hardfault = true;
                                  break; /* break to PAGE HAS BEEN FOUND */
                          }
                          if (rv == VM_PAGER_ERROR)
                                  printf("vm_fault: pager read error, pid %d (%s)\n",
                                      curproc->p_pid, curproc->p_comm);
  
                          /*
                           * If an I/O error occurred or the requested page was
                           * outside the range of the pager, clean up and return
                           * an error.
                           */
                          if (rv == VM_PAGER_ERROR || rv == VM_PAGER_BAD) {
                                  vm_page_lock(fs.m);
                                  if (!vm_page_wired(fs.m))
                                          vm_page_free(fs.m);
                                  else
                                          vm_page_xunbusy_maybelocked(fs.m);
                                  vm_page_unlock(fs.m);
                                  fs.m = NULL;
                                  unlock_and_deallocate(&fs);
                                  return (KERN_OUT_OF_BOUNDS);
                          }
  
                          /*
                           * The requested page does not exist at this object/
                           * offset.  Remove the invalid page from the object,
                           * waking up anyone waiting for it, and continue on to
                           * the next object.  However, if this is the top-level
                           * object, we must leave the busy page in place to
                           * prevent another process from rushing past us, and
                           * inserting the page in that object at the same time
                           * that we are.
                           */
                          if (fs.object != fs.first_object) {
                                  vm_page_lock(fs.m);
                                  if (!vm_page_wired(fs.m))
                                          vm_page_free(fs.m);
                                  else
                                          vm_page_xunbusy_maybelocked(fs.m);
                                  vm_page_unlock(fs.m);
                                  fs.m = NULL;
                          }
                  }
  
                  /*
                   * 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.
                   */
                  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 {
                                  VM_CNT_INC(v_ozfod);
                          }
                          VM_CNT_INC(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);
                          fs.pindex +=
                              OFF_TO_IDX(fs.object->backing_object_offset);
                          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) {
                                  vm_page_lock(fs.m);
                                  vm_page_dequeue(fs.m);
                                  (void)vm_page_remove(fs.m);
                                  vm_page_unlock(fs.m);
                                  vm_page_lock(fs.first_m);
                                  vm_page_replace_checked(fs.m, fs.first_object,
                                      fs.first_pindex, fs.first_m);
                                  vm_page_free(fs.first_m);
                                  vm_page_unlock(fs.first_m);
                                  vm_page_dirty(fs.m);
  #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
                                  /*
                                   * Removing the page from the backing object
                                   * unbusied it.
                                   */
                                  vm_page_xbusy(fs.m);
                                  fs.first_m = fs.m;
                                  fs.m = NULL;
                                  VM_CNT_INC(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 (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, PQ_INACTIVE);
                                          vm_page_unlock(fs.m);
                                  }
  
                                  /*
                                   * Typically, the shadow object is either
                                   * private to this address space
                                   * (OBJ_ONEMAPPING) or its pages are read only.
                                   * In the highly unusual case where the pages of
                                   * a shadow object are read/write shared between
                                   * this and other address spaces, we need to
                                   * ensure that any pmap-level mappings to the
                                   * original, copy-on-write page from the backing
                                   * object are removed from those other address
                                   * spaces.
                                   *
                                   * The flag check is racy, but this is
                                   * tolerable: if OBJ_ONEMAPPING is cleared after
                                   * the check, the busy state ensures that new
                                   * mappings of fs.m can't be created.
                                   * pmap_enter() will replace an existing mapping
                                   * in the current address space.  If
                                   * OBJ_ONEMAPPING is set after the check,
                                   * removing mappings will at worse trigger some
                                   * unnecessary page faults.
                                   */
                                  vm_page_assert_xbusied(fs.m);
                                  if ((fs.first_object->flags & OBJ_ONEMAPPING) == 0)
                                          pmap_remove_all(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);
                          VM_CNT_INC(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) {
                  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;
                          fault_type &= retry_prot;
                          if (prot == 0) {
                                  release_page(&fs);
                                  unlock_and_deallocate(&fs);
                                  goto RetryFault;
                          }
  
                          /* Reassert because wired may have changed. */
                          KASSERT(wired || (fault_flags & VM_FAULT_WIRE) == 0,
                              ("!wired && VM_FAULT_WIRE"));
                  }
          }
  
          /*
           * If the page was filled by a pager, save the virtual address that
           * should be faulted on next under a sequential access pattern to the
           * map entry.  A read lock on the map suffices to update this address
           * safely.
           */
          if (hardfault)
                  fs.entry->next_read = vaddr + ptoa(ahead) + PAGE_SIZE;
  
          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 > 0 ? behind : PFBAK,
                      faultcount > 0 ? ahead : PFFOR, false);
          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)
                  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) {
                  VM_CNT_INC(v_io_faults);
                  curthread->td_ru.ru_majflt++;
  #ifdef RACCT
                  if (racct_enable && fs.object->type == OBJT_VNODE) {
                          PROC_LOCK(curproc);
                          if ((fault_type & (VM_PROT_COPY | VM_PROT_WRITE)) != 0) {
                                  racct_add_force(curproc, RACCT_WRITEBPS,
                                      PAGE_SIZE + behind * PAGE_SIZE);
                                  racct_add_force(curproc, RACCT_WRITEIOPS, 1);
                          } else {
                                  racct_add_force(curproc, RACCT_READBPS,
                                      PAGE_SIZE + ahead * PAGE_SIZE);
                                  racct_add_force(curproc, RACCT_READIOPS, 1);
                          }
                          PROC_UNLOCK(curproc);
                  }
  #endif
          } else 
                  curthread->td_ru.ru_minflt++;
  
          return (KERN_SUCCESS);
  }
  
  /*
   * Speed up the reclamation of pages that precede the faulting pindex within
   * the first object of the shadow chain.  Essentially, perform the equivalent
   * to madvise(..., MADV_DONTNEED) on a large cluster of pages that precedes
   * the faulting pindex by the cluster size when the pages read by vm_fault()
   * cross a cluster-size boundary.  The cluster size is the greater of the
   * smallest superpage size and VM_FAULT_DONTNEED_MIN.
   *
   * When "fs->first_object" is a shadow object, the pages in the backing object
   * that precede the faulting pindex are deactivated by vm_fault().  So, this
   * function must only be concerned with pages in the first object.
   */
  static void
  vm_fault_dontneed(const struct faultstate *fs, vm_offset_t vaddr, int ahead)
  {
          vm_map_entry_t entry;
          vm_object_t first_object, object;
          vm_offset_t end, start;
          vm_page_t m, m_next;
          vm_pindex_t pend, pstart;
          vm_size_t size;
  
          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 reclaimed. */
          if ((first_object->flags & (OBJ_FICTITIOUS | OBJ_UNMANAGED)) == 0) {
                  size = VM_FAULT_DONTNEED_MIN;
                  if (MAXPAGESIZES > 1 && size < pagesizes[1])
                          size = pagesizes[1];
                  end = rounddown2(vaddr, size);
                  if (vaddr - end >= size - PAGE_SIZE - ptoa(ahead) &&
                      (entry = fs->entry)->start < end) {
                          if (end - entry->start < size)
                                  start = entry->start;
                          else
                                  start = end - size;
                          pmap_advise(fs->map->pmap, start, end, MADV_DONTNEED);
                          pstart = OFF_TO_IDX(entry->offset) + atop(start -
                              entry->start);
                          m_next = vm_page_find_least(first_object, pstart);
                          pend = OFF_TO_IDX(entry->offset) + atop(end -
                              entry->start);
                          while ((m = m_next) != NULL && m->pindex < pend) {
                                  m_next = TAILQ_NEXT(m, listq);
                                  if (m->valid != VM_PAGE_BITS_ALL ||
                                      vm_page_busied(m))
                                          continue;
  
                                  /*
                                   * Don't clear PGA_REFERENCED, since it would
                                   * likely represent a reference by a different
                                   * process.
                                   *
                                   * Typically, at this point, prefetched pages
                                   * are still in the inactive queue.  Only
                                   * pages that triggered page faults are in the
                                   * active queue.
                                   */
                                  vm_page_lock(m);
                                  if (!vm_page_inactive(m))
                                          vm_page_deactivate(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 backward, int forward, bool obj_locked)
  {
          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 i;
  
          pmap = fs->map->pmap;
          if (pmap != vmspace_pmap(curthread->td_proc->p_vmspace))
                  return;
  
          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;
                  if (!obj_locked)
                          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);
                          if (!obj_locked || lobject != entry->object.vm_object)
                                  VM_OBJECT_RUNLOCK(lobject);
                          lobject = backing_object;
                  }
                  if (m == NULL) {
                          if (!obj_locked || lobject != entry->object.vm_object)
                                  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);
                  if (!obj_locked || lobject != entry->object.vm_object)
                          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);
  
          if (!vm_map_range_valid(map, addr, end))
                  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.
                   *
                   * If vm_fault_disable_pagefaults() was called,
                   * i.e., TDP_NOFAULTING is set, we must not sleep nor
                   * acquire MD VM locks, which means we must not call
                   * vm_fault().  Some (out of tree) callers mark
                   * too wide a code area with vm_fault_disable_pagefaults()
                   * already, use the VM_PROT_QUICK_NOFAULT flag to request
                   * the proper behaviour explicitly.
                   */
                  if ((prot & VM_PROT_QUICK_NOFAULT) != 0 &&
                      (curthread->td_pflags & TDP_NOFAULTING) != 0)
                          goto error;
                  for (mp = ma, va = addr; va < end; mp++, va += PAGE_SIZE)
                          if (*mp == NULL && vm_fault(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,
                      atop(dst_entry->end - dst_entry->start));
  #if VM_NRESERVLEVEL > 0
                  dst_object->flags |= OBJ_COLORED;
                  dst_object->pg_color = atop(dst_entry->start);
  #endif
                  dst_object->domain = src_object->domain;
                  dst_object->charge = dst_entry->end - dst_entry->start;
          }
  
          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_entry->eflags &= ~MAP_ENTRY_VN_EXEC;
          }
          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->type == OBJT_DEFAULT ||
              dst_object->type == OBJT_SWAP) &&
              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 doesn't 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(dst_object);
                                  VM_OBJECT_WLOCK(dst_object);
                                  goto again;
                          }
  
                          /*
                           * See the comment in vm_fault_cow().
                           */
                          if (src_object == dst_object &&
                              (object->flags & OBJ_ONEMAPPING) == 0)
                                  pmap_remove_all(src_m);
                          pmap_copy_page(src_m, dst_m);
                          VM_OBJECT_RUNLOCK(object);
                          dst_m->dirty = dst_m->valid = src_m->valid;
                  } else {
                          dst_m = src_m;
                          if (vm_page_sleep_if_busy(dst_m, "fltupg"))
                                  goto again;
                          if (dst_m->pindex >= dst_object->size)
                                  /*
                                   * We are upgrading.  Index can occur
                                   * out of bounds if the object type is
                                   * vnode and the file was truncated.
                                   */
                                  break;
                          vm_page_xbusy(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.
                   *
                   * The page can be invalid if the user called
                   * msync(MS_INVALIDATE) or truncated the backing vnode
                   * or shared memory object.  In this case, do not
                   * insert it into pmap, but still do the copy so that
                   * all copies of the wired map entry have similar
                   * backing pages.
                   */
                  if (dst_m->valid == VM_PAGE_BITS_ALL) {
                          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, PQ_INACTIVE);
                                  vm_page_unlock(src_m);
                                  vm_page_lock(dst_m);
                                  vm_page_wire(dst_m);
                                  vm_page_unlock(dst_m);
                          } else {
                                  KASSERT(vm_page_wired(dst_m),
                                      ("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);
          }
  }
  
  /*
   * 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: 960b37528bd4b214993e4a7be0db3ab7