vm_fault: Shoot down shared mappings in vm_fault_copy_entry()

[FreeBSD/FreeBSD.git] / 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/mutex.h>
#include <sys/proc.h>
#include <sys/racct.h>
#include <sys/refcount.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_DONTNEED_MIN   1048576

struct faultstate {
        /* Fault parameters. */
        vm_offset_t     vaddr;
        vm_page_t       *m_hold;
        vm_prot_t       fault_type;
        vm_prot_t       prot;
        int             fault_flags;
        boolean_t       wired;

        /* Control state. */
        struct timeval  oom_start_time;
        bool            oom_started;
        int             nera;

        /* Page reference for cow. */
        vm_page_t m_cow;

        /* Current object. */
        vm_object_t     object;
        vm_pindex_t     pindex;
        vm_page_t       m;

        /* Top-level map object. */
        vm_object_t     first_object;
        vm_pindex_t     first_pindex;
        vm_page_t       first_m;

        /* Map state. */
        vm_map_t        map;
        vm_map_entry_t  entry;
        int             map_generation;
        bool            lookup_still_valid;

        /* Vnode if locked. */
        struct vnode    *vp;
};

/*
 * Return codes for internal fault routines.
 */
enum fault_status {
        FAULT_SUCCESS = 1,      /* Return success to user. */
        FAULT_FAILURE,          /* Return failure to user. */
        FAULT_CONTINUE,         /* Continue faulting. */
        FAULT_RESTART,          /* Restart fault. */
        FAULT_OUT_OF_BOUNDS,    /* Invalid address for pager. */
        FAULT_HARD,             /* Performed I/O. */
        FAULT_SOFT,             /* Found valid page. */
        FAULT_PROTECTION_FAILURE, /* Invalid access. */
};

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
fault_page_release(vm_page_t *mp)
{
        vm_page_t m;

        m = *mp;
        if (m != NULL) {
                /*
                 * We are likely to loop around again and attempt to busy
                 * this page.  Deactivating it leaves it available for
                 * pageout while optimizing fault restarts.
                 */
                vm_page_deactivate(m);
                vm_page_xunbusy(m);
                *mp = NULL;
        }
}

static inline void
fault_page_free(vm_page_t *mp)
{
        vm_page_t m;

        m = *mp;
        if (m != NULL) {
                VM_OBJECT_ASSERT_WLOCKED(m->object);
                if (!vm_page_wired(m))
                        vm_page_free(m);
                else
                        vm_page_xunbusy(m);
                *mp = 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
fault_deallocate(struct faultstate *fs)
{

        fault_page_release(&fs->m_cow);
        fault_page_release(&fs->m);
        vm_object_pip_wakeup(fs->object);
        if (fs->object != fs->first_object) {
                VM_OBJECT_WLOCK(fs->first_object);
                fault_page_free(&fs->first_m);
                VM_OBJECT_WUNLOCK(fs->first_object);
                vm_object_pip_wakeup(fs->first_object);
        }
        vm_object_deallocate(fs->first_object);
        unlock_map(fs);
        unlock_vp(fs);
}

static void
unlock_and_deallocate(struct faultstate *fs)
{

        VM_OBJECT_WUNLOCK(fs->object);
        fault_deallocate(fs);
}

static void
vm_fault_dirty(struct faultstate *fs, vm_page_t m)
{
        bool need_dirty;

        if (((fs->prot & VM_PROT_WRITE) == 0 &&
            (fs->fault_flags & VM_FAULT_DIRTY) == 0) ||
            (m->oflags & VPO_UNMANAGED) != 0)
                return;

        VM_PAGE_OBJECT_BUSY_ASSERT(m);

        need_dirty = ((fs->fault_type & VM_PROT_WRITE) != 0 &&
            (fs->fault_flags & VM_FAULT_WIRE) == 0) ||
            (fs->fault_flags & VM_FAULT_DIRTY) != 0;

        vm_object_set_writeable_dirty(m->object);

        /*
         * 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.
         */
        if (need_dirty && vm_page_set_dirty(m) == 0) {
                /*
                 * If this is a NOSYNC mmap we do not want to set PGA_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 ((fs->entry->eflags & MAP_ENTRY_NOSYNC) != 0)
                        vm_page_aflag_set(m, PGA_NOSYNC);
                else
                        vm_page_aflag_clear(m, PGA_NOSYNC);
        }

}

/*
 * Unlocks fs.first_object and fs.map on success.
 */
static enum fault_status
vm_fault_soft_fast(struct faultstate *fs)
{
        vm_page_t m, m_map;
#if VM_NRESERVLEVEL > 0
        vm_page_t m_super;
        int flags;
#endif
        int psind;
        vm_offset_t vaddr;
        enum fault_status res;

        MPASS(fs->vp == NULL);

        res = FAULT_SUCCESS;
        vaddr = fs->vaddr;
        vm_object_busy(fs->first_object);
        m = vm_page_lookup(fs->first_object, fs->first_pindex);
        /* A busy page can be mapped for read|execute access. */
        if (m == NULL || ((fs->prot & VM_PROT_WRITE) != 0 &&
            vm_page_busied(m)) || !vm_page_all_valid(m)) {
                res = FAULT_FAILURE;
                goto out;
        }
        m_map = m;
        psind = 0;
#if 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)) && !fs->wired &&
            pmap_ps_enabled(fs->map->pmap)) {
                flags = PS_ALL_VALID;
                if ((fs->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)
                                fs->fault_type |= VM_PROT_WRITE;
                }
        }
#endif
        if (pmap_enter(fs->map->pmap, vaddr, m_map, fs->prot, fs->fault_type |
            PMAP_ENTER_NOSLEEP | (fs->wired ? PMAP_ENTER_WIRED : 0), psind) !=
            KERN_SUCCESS) {
                res = FAULT_FAILURE;
                goto out;
        }
        if (fs->m_hold != NULL) {
                (*fs->m_hold) = m;
                vm_page_wire(m);
        }
        if (psind == 0 && !fs->wired)
                vm_fault_prefault(fs, vaddr, PFBAK, PFFOR, true);
        VM_OBJECT_RUNLOCK(fs->first_object);
        vm_fault_dirty(fs, m);
        vm_map_lookup_done(fs->map, fs->entry);
        curthread->td_ru.ru_minflt++;

out:
        vm_object_unbusy(fs->first_object);
        return (res);
}

static void
vm_fault_restore_map_lock(struct faultstate *fs)
{

        VM_OBJECT_ASSERT_WLOCKED(fs->first_object);
        MPASS(blockcount_read(&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(vm_page_all_valid(m));
        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_deactivate(m);
                vm_page_xunbusy(m);
        }
}

static enum fault_status
vm_fault_populate(struct faultstate *fs)
{
        vm_offset_t vaddr;
        vm_page_t m;
        vm_pindex_t map_first, map_last, pager_first, pager_last, pidx;
        int bdry_idx, i, npages, psind, rv;
        enum fault_status res;

        MPASS(fs->object == fs->first_object);
        VM_OBJECT_ASSERT_WLOCKED(fs->first_object);
        MPASS(blockcount_read(&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);

        res = FAULT_SUCCESS;

        /*
         * 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,
            fs->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 (FAULT_RESTART);
                return (FAULT_CONTINUE);
        }
        if (rv != VM_PAGER_OK)
                return (FAULT_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);
        bdry_idx = (fs->entry->eflags & MAP_ENTRY_SPLIT_BOUNDARY_MASK) >>
            MAP_ENTRY_SPLIT_BOUNDARY_SHIFT;
        if (fs->map->timestamp != fs->map_generation) {
                if (bdry_idx == 0) {
                        vm_fault_populate_cleanup(fs->first_object, pager_first,
                            pager_last);
                } else {
                        m = vm_page_lookup(fs->first_object, pager_first);
                        if (m != fs->m)
                                vm_page_xunbusy(m);
                }
                return (FAULT_RESTART);
        }

        /*
         * The map is unchanged after our last unlock.  Process the fault.
         *
         * First, the special case of largepage mappings, where
         * populate only busies the first page in superpage run.
         */
        if (bdry_idx != 0) {
                KASSERT(PMAP_HAS_LARGEPAGES,
                    ("missing pmap support for large pages"));
                m = vm_page_lookup(fs->first_object, pager_first);
                vm_fault_populate_check_page(m);
                VM_OBJECT_WUNLOCK(fs->first_object);
                vaddr = fs->entry->start + IDX_TO_OFF(pager_first) -
                    fs->entry->offset;
                /* assert alignment for entry */
                KASSERT((vaddr & (pagesizes[bdry_idx] - 1)) == 0,
    ("unaligned superpage start %#jx pager_first %#jx offset %#jx vaddr %#jx",
                    (uintmax_t)fs->entry->start, (uintmax_t)pager_first,
                    (uintmax_t)fs->entry->offset, (uintmax_t)vaddr));
                KASSERT((VM_PAGE_TO_PHYS(m) & (pagesizes[bdry_idx] - 1)) == 0,
                    ("unaligned superpage m %p %#jx", m,
                    (uintmax_t)VM_PAGE_TO_PHYS(m)));
                rv = pmap_enter(fs->map->pmap, vaddr, m, fs->prot,
                    fs->fault_type | (fs->wired ? PMAP_ENTER_WIRED : 0) |
                    PMAP_ENTER_LARGEPAGE, bdry_idx);
                VM_OBJECT_WLOCK(fs->first_object);
                vm_page_xunbusy(m);
                if (rv != KERN_SUCCESS) {
                        res = FAULT_FAILURE;
                        goto out;
                }
                if ((fs->fault_flags & VM_FAULT_WIRE) != 0) {
                        for (i = 0; i < atop(pagesizes[bdry_idx]); i++)
                                vm_page_wire(m + i);
                }
                if (fs->m_hold != NULL) {
                        *fs->m_hold = m + (fs->first_pindex - pager_first);
                        vm_page_wire(*fs->m_hold);
                }
                goto out;
        }

        /*
         * 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;

                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) || fs->wired))
                        psind = 0;

                npages = atop(pagesizes[psind]);
                for (i = 0; i < npages; i++) {
                        vm_fault_populate_check_page(&m[i]);
                        vm_fault_dirty(fs, &m[i]);
                }
                VM_OBJECT_WUNLOCK(fs->first_object);
                rv = pmap_enter(fs->map->pmap, vaddr, m, fs->prot, fs->fault_type |
                    (fs->wired ? PMAP_ENTER_WIRED : 0), psind);

                /*
                 * pmap_enter() may fail for a superpage mapping if additional
                 * protection policies prevent the full mapping.
                 * For example, this will happen on amd64 if the entire
                 * address range does not share the same userspace protection
                 * key.  Revert to single-page mappings if this happens.
                 */
                MPASS(rv == KERN_SUCCESS ||
                    (psind > 0 && rv == KERN_PROTECTION_FAILURE));
                if (__predict_false(psind > 0 &&
                    rv == KERN_PROTECTION_FAILURE)) {
                        MPASS(!fs->wired);
                        for (i = 0; i < npages; i++) {
                                rv = pmap_enter(fs->map->pmap, vaddr + ptoa(i),
                                    &m[i], fs->prot, fs->fault_type, 0);
                                MPASS(rv == KERN_SUCCESS);
                        }
                }

                VM_OBJECT_WLOCK(fs->first_object);
                for (i = 0; i < npages; i++) {
                        if ((fs->fault_flags & VM_FAULT_WIRE) != 0 &&
                            m[i].pindex == fs->first_pindex)
                                vm_page_wire(&m[i]);
                        else
                                vm_page_activate(&m[i]);
                        if (fs->m_hold != NULL &&
                            m[i].pindex == fs->first_pindex) {
                                (*fs->m_hold) = &m[i];
                                vm_page_wire(&m[i]);
                        }
                        vm_page_xunbusy(&m[i]);
                }
        }
out:
        curthread->td_ru.ru_majflt++;
        return (res);
}

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 enum fault_status
vm_fault_lock_vnode(struct faultstate *fs, bool objlocked)
{
        struct vnode *vp;
        int error, locked;

        if (fs->object->type != OBJT_VNODE)
                return (FAULT_CONTINUE);
        vp = fs->object->handle;
        if (vp == fs->vp) {
                ASSERT_VOP_LOCKED(vp, "saved vnode is not locked");
                return (FAULT_CONTINUE);
        }

        /*
         * 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);
        if (error == 0) {
                fs->vp = vp;
                return (FAULT_CONTINUE);
        }

        vhold(vp);
        if (objlocked)
                unlock_and_deallocate(fs);
        else
                fault_deallocate(fs);
        error = vget(vp, locked | LK_RETRY | LK_CANRECURSE);
        vdrop(vp);
        fs->vp = vp;
        KASSERT(error == 0, ("vm_fault: vget failed %d", error));
        return (FAULT_RESTART);
}

/*
 * Calculate the desired readahead.  Handle drop-behind.
 *
 * Returns the number of readahead blocks to pass to the pager.
 */
static int
vm_fault_readahead(struct faultstate *fs)
{
        int era, nera;
        u_char behavior;

        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 (fs->vaddr == fs->entry->next_read)
                        vm_fault_dontneed(fs, fs->vaddr, nera);
        } else if (fs->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, 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;
        }

        return (nera);
}

static int
vm_fault_lookup(struct faultstate *fs)
{
        int result;

        KASSERT(!fs->lookup_still_valid,
           ("vm_fault_lookup: Map already locked."));
        result = vm_map_lookup(&fs->map, fs->vaddr, fs->fault_type |
            VM_PROT_FAULT_LOOKUP, &fs->entry, &fs->first_object,
            &fs->first_pindex, &fs->prot, &fs->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)fs->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, fs->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);
                return (KERN_RESOURCE_SHORTAGE);
        }

        MPASS((fs->entry->eflags & MAP_ENTRY_GUARD) == 0);

        if (fs->wired)
                fs->fault_type = fs->prot | (fs->fault_type & VM_PROT_COPY);
        else
                KASSERT((fs->fault_flags & VM_FAULT_WIRE) == 0,
                    ("!fs->wired && VM_FAULT_WIRE"));
        fs->lookup_still_valid = true;

        return (KERN_SUCCESS);
}

static int
vm_fault_relookup(struct faultstate *fs)
{
        vm_object_t retry_object;
        vm_pindex_t retry_pindex;
        vm_prot_t retry_prot;
        int result;

        if (!vm_map_trylock_read(fs->map))
                return (KERN_RESTART);

        fs->lookup_still_valid = true;
        if (fs->map->timestamp == fs->map_generation)
                return (KERN_SUCCESS);

        result = vm_map_lookup_locked(&fs->map, fs->vaddr, fs->fault_type,
            &fs->entry, &retry_object, &retry_pindex, &retry_prot,
            &fs->wired);
        if (result != KERN_SUCCESS) {
                /*
                 * If retry of map lookup would have blocked then
                 * retry fault from start.
                 */
                if (result == KERN_FAILURE)
                        return (KERN_RESTART);
                return (result);
        }
        if (retry_object != fs->first_object ||
            retry_pindex != fs->first_pindex)
                return (KERN_RESTART);

        /*
         * 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.
         */
        fs->prot &= retry_prot;
        fs->fault_type &= retry_prot;
        if (fs->prot == 0)
                return (KERN_RESTART);

        /* Reassert because wired may have changed. */
        KASSERT(fs->wired || (fs->fault_flags & VM_FAULT_WIRE) == 0,
            ("!wired && VM_FAULT_WIRE"));

        return (KERN_SUCCESS);
}

static void
vm_fault_cow(struct faultstate *fs)
{
        bool is_first_object_locked;

        KASSERT(fs->object != fs->first_object,
            ("source and target COW objects are identical"));

        /*
         * 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 and no other refs.
             */
            fs->object->shadow_count == 1 && fs->object->ref_count == 1 &&
            /*
             * No other ways to look the object up
             */
            fs->object->handle == NULL && (fs->object->flags & OBJ_ANON) != 0 &&
            /*
             * We don't chase down the shadow chain and we can acquire locks.
             */
            (is_first_object_locked = VM_OBJECT_TRYWLOCK(fs->first_object)) &&
            fs->object == fs->first_object->backing_object &&
            VM_OBJECT_TRYWLOCK(fs->object)) {
                /*
                 * Remove but keep xbusy for replace.  fs->m is moved into
                 * fs->first_object and left busy while fs->first_m is
                 * conditionally freed.
                 */
                vm_page_remove_xbusy(fs->m);
                vm_page_replace(fs->m, fs->first_object, fs->first_pindex,
                    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
                VM_OBJECT_WUNLOCK(fs->object);
                VM_OBJECT_WUNLOCK(fs->first_object);
                fs->first_m = fs->m;
                fs->m = NULL;
                VM_CNT_INC(v_cow_optim);
        } else {
                if (is_first_object_locked)
                        VM_OBJECT_WUNLOCK(fs->first_object);
                /*
                 * Oh, well, lets copy it.
                 */
                pmap_copy_page(fs->m, fs->first_m);
                vm_page_valid(fs->first_m);
                if (fs->wired && (fs->fault_flags & VM_FAULT_WIRE) == 0) {
                        vm_page_wire(fs->first_m);
                        vm_page_unwire(fs->m, PQ_INACTIVE);
                }
                /*
                 * Save the cow page to be released after
                 * pmap_enter is complete.
                 */
                fs->m_cow = fs->m;
                fs->m = NULL;

                /*
                 * 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 m_cow 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_cow);
                if ((fs->first_object->flags & OBJ_ONEMAPPING) == 0)
                        pmap_remove_all(fs->m_cow);
        }

        vm_object_pip_wakeup(fs->object);

        /*
         * Only use the new page below...
         */
        fs->object = fs->first_object;
        fs->pindex = fs->first_pindex;
        fs->m = fs->first_m;
        VM_CNT_INC(v_cow_faults);
        curthread->td_cow++;
}

static bool
vm_fault_next(struct faultstate *fs)
{
        vm_object_t next_object;

        /*
         * 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) {
                fs->first_m = fs->m;
                fs->m = NULL;
        } else
                fault_page_free(&fs->m);

        /*
         * Move on to the next object.  Lock the next object before
         * unlocking the current one.
         */
        VM_OBJECT_ASSERT_WLOCKED(fs->object);
        next_object = fs->object->backing_object;
        if (next_object == NULL)
                return (false);
        MPASS(fs->first_m != NULL);
        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;

        return (true);
}

static void
vm_fault_zerofill(struct faultstate *fs)
{

        /*
         * 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);
                fs->object = fs->first_object;
                fs->pindex = fs->first_pindex;
        }
        MPASS(fs->first_m != NULL);
        MPASS(fs->m == NULL);
        fs->m = fs->first_m;
        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);
        vm_page_valid(fs->m);
}

/*
 * Initiate page fault after timeout.  Returns true if caller should
 * do vm_waitpfault() after the call.
 */
static bool
vm_fault_allocate_oom(struct faultstate *fs)
{
        struct timeval now;

        unlock_and_deallocate(fs);
        if (vm_pfault_oom_attempts < 0)
                return (true);
        if (!fs->oom_started) {
                fs->oom_started = true;
                getmicrotime(&fs->oom_start_time);
                return (true);
        }

        getmicrotime(&now);
        timevalsub(&now, &fs->oom_start_time);
        if (now.tv_sec < vm_pfault_oom_attempts * vm_pfault_oom_wait)
                return (true);

        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);
        fs->oom_started = false;
        return (false);
}

/*
 * Allocate a page directly or via the object populate method.
 */
static enum fault_status
vm_fault_allocate(struct faultstate *fs)
{
        struct domainset *dset;
        int alloc_req;
        enum fault_status res;

        if ((fs->object->flags & OBJ_SIZEVNLOCK) != 0) {
                res = vm_fault_lock_vnode(fs, true);
                MPASS(res == FAULT_CONTINUE || res == FAULT_RESTART);
                if (res == FAULT_RESTART)
                        return (res);
        }

        if (fs->pindex >= fs->object->size) {
                unlock_and_deallocate(fs);
                return (FAULT_OUT_OF_BOUNDS);
        }

        if (fs->object == fs->first_object &&
            (fs->first_object->flags & OBJ_POPULATE) != 0 &&
            fs->first_object->shadow_count == 0) {
                res = vm_fault_populate(fs);
                switch (res) {
                case FAULT_SUCCESS:
                case FAULT_FAILURE:
                case FAULT_RESTART:
                        unlock_and_deallocate(fs);
                        return (res);
                case FAULT_CONTINUE:
                        /*
                         * 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(fs->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) {
                if (vm_fault_allocate_oom(fs))
                        vm_waitpfault(dset, vm_pfault_oom_wait * hz);
                return (FAULT_RESTART);
        }
        fs->oom_started = false;

        return (FAULT_CONTINUE);
}

/*
 * 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.
 */
static enum fault_status
vm_fault_getpages(struct faultstate *fs, int *behindp, int *aheadp)
{
        vm_offset_t e_end, e_start;
        int ahead, behind, cluster_offset, rv;
        enum fault_status status;
        u_char behavior;

        /*
         * 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;
        behavior = vm_map_entry_behavior(fs->entry);

        /*
         * 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->nera == -1 && !P_KILLED(curproc))
                fs->nera = vm_fault_readahead(fs);

        /*
         * 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);

        status = vm_fault_lock_vnode(fs, false);
        MPASS(status == FAULT_CONTINUE || status == FAULT_RESTART);
        if (status == FAULT_RESTART)
                return (status);
        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 (fs->nera == -1 || behavior == MAP_ENTRY_BEHAV_RANDOM ||
            P_KILLED(curproc)) {
                behind = 0;
                ahead = 0;
        } else {
                /* Is this a sequential fault? */
                if (fs->nera > 0) {
                        behind = 0;
                        ahead = fs->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(fs->vaddr - e_start));
                        ahead = VM_FAULT_READ_DEFAULT - 1 - cluster_offset;
                }
                ahead = ulmin(ahead, atop(e_end - fs->vaddr) - 1);
        }
        *behindp = behind;
        *aheadp = ahead;
        rv = vm_pager_get_pages(fs->object, &fs->m, 1, behindp, aheadp);
        if (rv == VM_PAGER_OK)
                return (FAULT_HARD);
        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_OBJECT_WLOCK(fs->object);
                fault_page_free(&fs->m);
                unlock_and_deallocate(fs);
                return (FAULT_OUT_OF_BOUNDS);
        }
        KASSERT(rv == VM_PAGER_FAIL,
            ("%s: unepxected pager error %d", __func__, rv));
        return (FAULT_CONTINUE);
}

/*
 * 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.
 */
static void
vm_fault_busy_sleep(struct faultstate *fs)
{
        /*
         * 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) {
                fault_page_release(&fs->first_m);
                vm_object_pip_wakeup(fs->first_object);
        }
        vm_object_pip_wakeup(fs->object);
        unlock_map(fs);
        if (fs->m != vm_page_lookup(fs->object, fs->pindex) ||
            !vm_page_busy_sleep(fs->m, "vmpfw", 0))
                VM_OBJECT_WUNLOCK(fs->object);
        VM_CNT_INC(v_intrans);
        vm_object_deallocate(fs->first_object);
}

/*
 * Handle page lookup, populate, allocate, page-in for the current
 * object.
 *
 * The object is locked on entry and will remain locked with a return
 * code of FAULT_CONTINUE so that fault may follow the shadow chain.
 * Otherwise, the object will be unlocked upon return.
 */
static enum fault_status
vm_fault_object(struct faultstate *fs, int *behindp, int *aheadp)
{
        enum fault_status res;
        bool dead;

        /*
         * 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 (FAULT_PROTECTION_FAILURE);
                pause("vmf_de", 1);
                return (FAULT_RESTART);
        }

        /*
         * See if the page is resident.
         */
        fs->m = vm_page_lookup(fs->object, fs->pindex);
        if (fs->m != NULL) {
                if (!vm_page_tryxbusy(fs->m)) {
                        vm_fault_busy_sleep(fs);
                        return (FAULT_RESTART);
                }

                /*
                 * The page is marked busy for other processes and the
                 * pagedaemon.  If it is still completely valid we are
                 * done.
                 */
                if (vm_page_all_valid(fs->m)) {
                        VM_OBJECT_WUNLOCK(fs->object);
                        return (FAULT_SOFT);
                }
        }
        VM_OBJECT_ASSERT_WLOCKED(fs->object);

        /*
         * 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->m == NULL && (fs->object->type != OBJT_DEFAULT ||
            fs->object == fs->first_object)) {
                res = vm_fault_allocate(fs);
                if (res != FAULT_CONTINUE)
                        return (res);
        }

        /*
         * Default objects have no pager so no exclusive busy exists
         * to protect this page in the chain.  Skip to the next
         * object without dropping the lock to preserve atomicity of
         * shadow faults.
         */
        if (fs->object->type != OBJT_DEFAULT) {
                /*
                 * 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.  The exclusive busy
                 * prevents simultaneous faults and collapses while
                 * the object lock is dropped.
                 */
                VM_OBJECT_WUNLOCK(fs->object);
                res = vm_fault_getpages(fs, behindp, aheadp);
                if (res == FAULT_CONTINUE)
                        VM_OBJECT_WLOCK(fs->object);
        } else {
                res = FAULT_CONTINUE;
        }
        return (res);
}

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;
        int ahead, behind, faultcount, rv;
        enum fault_status res;
        bool hardfault;

        VM_CNT_INC(v_vm_faults);

        if ((curthread->td_pflags & TDP_NOFAULTING) != 0)
                return (KERN_PROTECTION_FAILURE);

        fs.vp = NULL;
        fs.vaddr = vaddr;
        fs.m_hold = m_hold;
        fs.fault_flags = fault_flags;
        fs.map = map;
        fs.lookup_still_valid = false;
        fs.oom_started = false;
        fs.nera = -1;
        faultcount = 0;
        hardfault = false;

RetryFault:
        fs.fault_type = fault_type;

        /*
         * Find the backing store object and offset into it to begin the
         * search.
         */
        rv = vm_fault_lookup(&fs);
        if (rv != KERN_SUCCESS) {
                if (rv == KERN_RESOURCE_SHORTAGE)
                        goto RetryFault;
                return (rv);
        }

        /*
         * Try to avoid lock contention on the top-level object through
         * special-case handling of some types of page faults, specifically,
         * those that are mapping an existing page from the top-level object.
         * Under this condition, a read lock on the object suffices, allowing
         * multiple page faults of a similar type to run in parallel.
         */
        if (fs.vp == NULL /* avoid locked vnode leak */ &&
            (fs.entry->eflags & MAP_ENTRY_SPLIT_BOUNDARY_MASK) == 0 &&
            (fs.fault_flags & (VM_FAULT_WIRE | VM_FAULT_DIRTY)) == 0) {
                VM_OBJECT_RLOCK(fs.first_object);
                res = vm_fault_soft_fast(&fs);
                if (res == FAULT_SUCCESS)
                        return (KERN_SUCCESS);
                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.m_cow = fs.m = fs.first_m = NULL;

        /*
         * Search for the page at object/offset.
         */
        fs.object = fs.first_object;
        fs.pindex = fs.first_pindex;

        if ((fs.entry->eflags & MAP_ENTRY_SPLIT_BOUNDARY_MASK) != 0) {
                res = vm_fault_allocate(&fs);
                switch (res) {
                case FAULT_RESTART:
                        goto RetryFault;
                case FAULT_SUCCESS:
                        return (KERN_SUCCESS);
                case FAULT_FAILURE:
                        return (KERN_FAILURE);
                case FAULT_OUT_OF_BOUNDS:
                        return (KERN_OUT_OF_BOUNDS);
                case FAULT_CONTINUE:
                        break;
                default:
                        panic("vm_fault: Unhandled status %d", res);
                }
        }

        while (TRUE) {
                KASSERT(fs.m == NULL,
                    ("page still set %p at loop start", fs.m));

                res = vm_fault_object(&fs, &behind, &ahead);
                switch (res) {
                case FAULT_SOFT:
                        goto found;
                case FAULT_HARD:
                        faultcount = behind + 1 + ahead;
                        hardfault = true;
                        goto found;
                case FAULT_RESTART:
                        goto RetryFault;
                case FAULT_SUCCESS:
                        return (KERN_SUCCESS);
                case FAULT_FAILURE:
                        return (KERN_FAILURE);
                case FAULT_OUT_OF_BOUNDS:
                        return (KERN_OUT_OF_BOUNDS);
                case FAULT_PROTECTION_FAILURE:
                        return (KERN_PROTECTION_FAILURE);
                case FAULT_CONTINUE:
                        break;
                default:
                        panic("vm_fault: Unhandled status %d", res);
                }

                /*
                 * The page was not found in the current object.  Try to
                 * traverse into a backing object or zero fill if none is
                 * found.
                 */
                if (vm_fault_next(&fs))
                        continue;
                if ((fs.fault_flags & VM_FAULT_NOFILL) != 0) {
                        if (fs.first_object == fs.object)
                                fault_page_free(&fs.first_m);
                        unlock_and_deallocate(&fs);
                        return (KERN_OUT_OF_BOUNDS);
                }
                VM_OBJECT_WUNLOCK(fs.object);
                vm_fault_zerofill(&fs);
                /* Don't try to prefault neighboring pages. */
                faultcount = 1;
                break;
        }

found:
        /*
         * A valid page has been found and exclusively busied.  The
         * object lock must no longer be held.
         */
        vm_page_assert_xbusied(fs.m);
        VM_OBJECT_ASSERT_UNLOCKED(fs.object);

        /*
         * 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 ((fs.fault_type & (VM_PROT_COPY | VM_PROT_WRITE)) != 0) {
                        vm_fault_cow(&fs);
                        /*
                         * We only try to prefault read-only mappings to the
                         * neighboring pages when this copy-on-write fault is
                         * a hard fault.  In other cases, trying to prefault
                         * is typically wasted effort.
                         */
                        if (faultcount == 0)
                                faultcount = 1;

                } else {
                        fs.prot &= ~VM_PROT_WRITE;
                }
        }

        /*
         * We must verify that the maps have not changed since our last
         * lookup.
         */
        if (!fs.lookup_still_valid) {
                rv = vm_fault_relookup(&fs);
                if (rv != KERN_SUCCESS) {
                        fault_deallocate(&fs);
                        if (rv == KERN_RESTART)
                                goto RetryFault;
                        return (rv);
                }
        }
        VM_OBJECT_ASSERT_UNLOCKED(fs.object);

        /*
         * 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;

        /*
         * Page must be completely valid or it is not fit to
         * map into user space.  vm_pager_get_pages() ensures this.
         */
        vm_page_assert_xbusied(fs.m);
        KASSERT(vm_page_all_valid(fs.m),
            ("vm_fault: page %p partially invalid", fs.m));

        vm_fault_dirty(&fs, fs.m);

        /*
         * 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, fs.prot,
            fs.fault_type | (fs.wired ? PMAP_ENTER_WIRED : 0), 0);
        if (faultcount != 1 && (fs.fault_flags & VM_FAULT_WIRE) == 0 &&
            fs.wired == 0)
                vm_fault_prefault(&fs, vaddr,
                    faultcount > 0 ? behind : PFBAK,
                    faultcount > 0 ? ahead : PFFOR, false);

        /*
         * If the page is not wired down, then put it where the pageout daemon
         * can find it.
         */
        if ((fs.fault_flags & VM_FAULT_WIRE) != 0)
                vm_page_wire(fs.m);
        else
                vm_page_activate(fs.m);
        if (fs.m_hold != NULL) {
                (*fs.m_hold) = fs.m;
                vm_page_wire(fs.m);
        }
        vm_page_xunbusy(fs.m);
        fs.m = NULL;

        /*
         * Unlock everything, and return
         */
        fault_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 ((fs.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_UNLOCKED(object);
        first_object = fs->first_object;
        /* Neither fictitious nor unmanaged pages can be reclaimed. */
        if ((first_object->flags & (OBJ_FICTITIOUS | OBJ_UNMANAGED)) == 0) {
                VM_OBJECT_RLOCK(first_object);
                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 (!vm_page_all_valid(m) ||
                                    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.  The test for whether the page
                                 * is in the inactive queue is racy; in the
                                 * worst case we will requeue the page
                                 * unnecessarily.
                                 */
                                if (!vm_page_inactive(m))
                                        vm_page_deactivate(m);
                        }
                }
                VM_OBJECT_RUNLOCK(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 (vm_page_all_valid(m) &&
                    (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_unwire(*mp, PQ_INACTIVE);
        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_anon(atop(dst_entry->end -
                    dst_entry->start), NULL, NULL, 0);
#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->flags & OBJ_SWAP) != 0) &&
            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_busy_acquire(dst_m, VM_ALLOC_WAITFAIL) == 0)
                                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.
                                 */
                                vm_page_xunbusy(dst_m);
                                break;
                        }
                }
                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 (vm_page_all_valid(dst_m)) {
                        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_unwire(src_m, PQ_INACTIVE);
                                vm_page_wire(dst_m);
                        } else {
                                KASSERT(vm_page_wired(dst_m),
                                    ("dst_m %p is not wired", dst_m));
                        }
                } else {
                        vm_page_activate(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);
}