MAP_VPAGETABLE support part 1/3.

[dragonfly.git] / sys / vm / vm_fault.c

/*
 * Copyright (c) 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 * Copyright (c) 1994 John S. Dyson
 * All rights reserved.
 * Copyright (c) 1994 David Greenman
 * All rights reserved.
 *
 *
 * This code is derived from software contributed to Berkeley by
 * The Mach Operating System project at Carnegie-Mellon University.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      from: @(#)vm_fault.c    8.4 (Berkeley) 1/12/94
 *
 *
 * Copyright (c) 1987, 1990 Carnegie-Mellon University.
 * All rights reserved.
 *
 * Authors: Avadis Tevanian, Jr., Michael Wayne Young
 *
 * Permission to use, copy, modify and distribute this software and
 * its documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 *
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 *
 * Carnegie Mellon requests users of this software to return to
 *
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 *
 * any improvements or extensions that they make and grant Carnegie the
 * rights to redistribute these changes.
 *
 * $FreeBSD: src/sys/vm/vm_fault.c,v 1.108.2.8 2002/02/26 05:49:27 silby Exp $
 * $DragonFly: src/sys/vm/vm_fault.c,v 1.26 2006/09/12 18:41:32 dillon Exp $
 */

/*
 *      Page fault handling module.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/vnode.h>
#include <sys/resourcevar.h>
#include <sys/vmmeter.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <sys/lock.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/vnode_pager.h>
#include <vm/vm_extern.h>

#include <sys/thread2.h>
#include <vm/vm_page2.h>

#define VM_FAULT_READ_AHEAD 8
#define VM_FAULT_READ_BEHIND 7
#define VM_FAULT_READ (VM_FAULT_READ_AHEAD+VM_FAULT_READ_BEHIND+1)

struct faultstate {
        vm_page_t m;
        vm_object_t object;
        vm_pindex_t pindex;
        vm_page_t first_m;
        vm_object_t first_object;
        vm_pindex_t first_pindex;
        vm_map_t map;
        vm_map_entry_t entry;
        int lookup_still_valid;
        int didlimit;
        int hardfault;
        vm_prot_t prot;
        int fault_flags;
        int map_generation;
        boolean_t wired;
        struct vnode *vp;
};

static int vm_fault_object(struct faultstate *fs, vm_prot_t);
static int vm_fault_additional_pages (vm_page_t, int, int, vm_page_t *, int *);
static int vm_fault_ratelimit(struct vmspace *vmspace);

static __inline void
release_page(struct faultstate *fs)
{
        vm_page_wakeup(fs->m);
        vm_page_deactivate(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, 0);
                fs->lookup_still_valid = FALSE;
        }
}

static void
_unlock_things(struct faultstate *fs, int dealloc)
{
        vm_object_pip_wakeup(fs->object);
        if (fs->object != fs->first_object) {
                vm_page_free(fs->first_m);
                vm_object_pip_wakeup(fs->first_object);
                fs->first_m = NULL;
        }
        if (dealloc) {
                vm_object_deallocate(fs->first_object);
        }
        unlock_map(fs); 
        if (fs->vp != NULL) { 
                vput(fs->vp);
                fs->vp = NULL;
        }
}

#define unlock_things(fs) _unlock_things(fs, 0)
#define unlock_and_deallocate(fs) _unlock_things(fs, 1)

/*
 * TRYPAGER 
 *
 * Determine if the pager for the current object *might* contain the page.
 *
 * We only need to try the pager if this is not a default object (default
 * objects are zero-fill and have no real pager), and if we are not taking
 * a wiring fault or if the FS entry is wired.
 */
#define TRYPAGER(fs)    \
                (fs->object->type != OBJT_DEFAULT && \
                (((fs->fault_flags & VM_FAULT_WIRE_MASK) == 0) || fs->wired))

/*
 * vm_fault:
 *
 * Handle a page fault occuring 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.
 * The caller may hold no locks.
 */
int
vm_fault(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type, int fault_flags)
{
        int result;
        struct faultstate fs;

        mycpu->gd_cnt.v_vm_faults++;

        fs.didlimit = 0;
        fs.hardfault = 0;
        fs.fault_flags = fault_flags;

RetryFault:
        /*
         * Find the vm_map_entry representing the backing store and resolve
         * the top level object and page index.  This may have the side
         * effect of executing a copy-on-write on the map entry and/or
         * creating a shadow object, but will not COW any actual VM pages.
         *
         * On success fs.map is left read-locked and various other fields 
         * are initialized but not otherwise referenced or locked.
         *
         * NOTE!  vm_map_lookup will upgrade the fault_type to VM_FAULT_WRITE
         * if the map entry is a virtual page table and also writable,
         * so we can set the 'A'accessed bit in the virtual page table entry.
         */
        fs.map = map;
        result = vm_map_lookup(&fs.map, vaddr, fault_type,
                               &fs.entry, &fs.first_object,
                               &fs.first_pindex, &fs.prot, &fs.wired);

        /*
         * If the lookup failed or the map protections are incompatible,
         * the fault generally fails.  However, if the caller is trying
         * to do a user wiring we have more work to do.
         */
        if (result != KERN_SUCCESS) {
                if (result != KERN_PROTECTION_FAILURE)
                        return result;
                if ((fs.fault_flags & VM_FAULT_WIRE_MASK) != VM_FAULT_USER_WIRE)
                        return result;

                /*
                 * If we are user-wiring a r/w segment, and it is COW, then
                 * we need to do the COW operation.  Note that we don't
                 * currently COW RO sections now, because it is NOT desirable
                 * to COW .text.  We simply keep .text from ever being COW'ed
                 * and take the heat that one cannot debug wired .text sections.
                 */
                result = vm_map_lookup(&fs.map, vaddr,
                                       VM_PROT_READ|VM_PROT_WRITE|
                                        VM_PROT_OVERRIDE_WRITE,
                                       &fs.entry, &fs.first_object,
                                       &fs.first_pindex, &fs.prot, &fs.wired);
                if (result != KERN_SUCCESS)
                        return result;

                /*
                 * If we don't COW now, on a user wire, the user will never
                 * be able to write to the mapping.  If we don't make this
                 * restriction, the bookkeeping would be nearly impossible.
                 */
                if ((fs.entry->protection & VM_PROT_WRITE) == 0)
                        fs.entry->max_protection &= ~VM_PROT_WRITE;
        }

        /*
         * fs.map is read-locked
         *
         * Misc checks.  Save the map generation number to detect races.
         */
        fs.map_generation = fs.map->timestamp;

        if (fs.entry->eflags & MAP_ENTRY_NOFAULT) {
                panic("vm_fault: fault on nofault entry, addr: %lx",
                    (u_long)vaddr);
        }

        /*
         * A system map entry may return a NULL object.  No object means
         * no pager means an unrecoverable kernel fault.
         */
        if (fs.first_object == NULL) {
                panic("vm_fault: unrecoverable fault at %p in entry %p",
                        (void *)vaddr, fs.entry);
        }

        /*
         * Make a reference to this object to prevent its disposal while we
         * are messing with it.  Once we have the reference, the map is free
         * to be diddled.  Since objects reference their shadows (and copies),
         * they will stay around as well.
         *
         * Bump the paging-in-progress count to prevent size changes (e.g.
         * truncation operations) during I/O.  This must be done after
         * obtaining the vnode lock in order to avoid possible deadlocks.
         */
        vm_object_reference(fs.first_object);
        fs.vp = vnode_pager_lock(fs.first_object);
        vm_object_pip_add(fs.first_object, 1);

        fs.lookup_still_valid = TRUE;
        fs.first_m = NULL;

        /*
         * If the entry is wired we cannot change the page protection.
         */
        if (fs.wired)
                fault_type = fs.prot;

        /*
         * The page we want is at (object, pindex), but if the vm_map_entry
         * is VM_MAPTYPE_VPAGETABLE we have to traverse the page table to
         * figure out the actual pindex.
         */
        fs.object = fs.first_object;
        fs.pindex = fs.first_pindex;
#if 0
        /* COMING SOON */
        if (fs.entry->maptype == VM_MAPTYPE_VPAGETABLE) {
                fs.pindex = fs.entry->avail_ssize;      /* page directory */
                result = vm_fault_object(&fs, fault_type);
                if (result == KERN_TRY_AGAIN)
                        goto RetryFault;
                if (result != KERN_SUCCESS) {
                        /* unlock_and_deallocate(&fs); */
                        return (result);
                }

                /*
                 * fs.m is busy on return
                 */
                XXX
        }
#endif

        /*
         * Now we have the actual (object, pindex), fault in the page.  If
         * vm_fault_object() fails it will unlock and deallocate the FS
         * data.  
         */
        result = vm_fault_object(&fs, fault_type);
        if (result == KERN_TRY_AGAIN)
                goto RetryFault;
        if (result != KERN_SUCCESS)
                return (result);

        /*
         * On success vm_fault_object() unlocks but does not deallocate, and
         * fs.m will contain a busied page.
         *
         * Enter the page into the pmap and do pmap-related adjustments.
         */
        pmap_enter(fs.map->pmap, vaddr, fs.m, fs.prot, fs.wired);

        if (((fs.fault_flags & VM_FAULT_WIRE_MASK) == 0) && (fs.wired == 0)) {
                pmap_prefault(fs.map->pmap, vaddr, fs.entry);
        }

        vm_page_flag_clear(fs.m, PG_ZERO);
        vm_page_flag_set(fs.m, PG_MAPPED|PG_REFERENCED);
        if (fs.fault_flags & VM_FAULT_HOLD)
                vm_page_hold(fs.m);

        /*
         * If the page is not wired down, then put it where the pageout daemon
         * can find it.
         */
        if (fs.fault_flags & VM_FAULT_WIRE_MASK) {
                if (fs.wired)
                        vm_page_wire(fs.m);
                else
                        vm_page_unwire(fs.m, 1);
        } else {
                vm_page_activate(fs.m);
        }

        if (curproc && (curproc->p_flag & P_SWAPPEDOUT) == 0 &&
            curproc->p_stats) {
                if (fs.hardfault) {
                        curproc->p_stats->p_ru.ru_majflt++;
                } else {
                        curproc->p_stats->p_ru.ru_minflt++;
                }
        }

        /*
         * Unlock everything, and return
         */
        vm_page_wakeup(fs.m);
        vm_object_deallocate(fs.first_object);

        return (KERN_SUCCESS);
}

/*
 * Do all operations required to fault in (fs.object, fs.pindex).  Run
 * through the shadow chain as necessary and do required COW or virtual
 * copy operations.  The caller has already fully resolved the vm_map_entry
 * and, if appropriate, has created a copy-on-write layer.  All we need to
 * do is iterate the object chain.
 *
 * On failure (fs) is unlocked and deallocated and the caller may return or
 * retry depending on the failure code.  On success (fs) is unlocked but not
 * deallocated, and fs.m will contain a resolved busied page.
 */
static
int
vm_fault_object(struct faultstate *fs, vm_prot_t fault_type)
{
        vm_object_t next_object;
        vm_page_t marray[VM_FAULT_READ];
        int faultcount;

        for (;;) {
                /*
                 * If the object is dead, we stop here
                 */
                if (fs->object->flags & OBJ_DEAD) {
                        unlock_and_deallocate(fs);
                        return (KERN_PROTECTION_FAILURE);
                }

                /*
                 * See if page is resident.  spl protection is required
                 * to avoid an interrupt unbusy/free race against our
                 * lookup.  We must hold the protection through a page
                 * allocation or busy.
                 */
                crit_enter();
                fs->m = vm_page_lookup(fs->object, fs->pindex);
                if (fs->m != NULL) {
                        int queue;
                        /*
                         * Wait/Retry if the page is busy.  We have to do this
                         * if the page is busy via either PG_BUSY or 
                         * vm_page_t->busy because the vm_pager may be using
                         * vm_page_t->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 vm_page_t->busy page except, perhaps,
                         * to pmap it.
                         */
                        if ((fs->m->flags & PG_BUSY) || fs->m->busy) {
                                unlock_things(fs);
                                vm_page_sleep_busy(fs->m, TRUE, "vmpfw");
                                mycpu->gd_cnt.v_intrans++;
                                vm_object_deallocate(fs->first_object);
                                crit_exit();
                                return (KERN_TRY_AGAIN);
                        }

                        /*
                         * If reactivating a page from PQ_CACHE we may have
                         * to rate-limit.
                         */
                        queue = fs->m->queue;
                        vm_page_unqueue_nowakeup(fs->m);

                        if ((queue - fs->m->pc) == PQ_CACHE && 
                            vm_page_count_severe()) {
                                vm_page_activate(fs->m);
                                unlock_and_deallocate(fs);
                                vm_waitpfault();
                                crit_exit();
                                return (KERN_TRY_AGAIN);
                        }

                        /*
                         * Mark page busy for other processes, and the 
                         * pagedaemon.  If it still isn't completely valid
                         * (readable), jump to readrest, else we found the
                         * page and can return.
                         *
                         * We can release the spl once we have marked the
                         * page busy.
                         */
                        vm_page_busy(fs->m);
                        crit_exit();

                        if (((fs->m->valid & VM_PAGE_BITS_ALL) != VM_PAGE_BITS_ALL) &&
                            fs->m->object != kernel_object &&
                            fs->m->object != kmem_object) {
                                goto readrest;
                        }
                        break; /* break to PAGE HAS BEEN FOUND */
                }

                /*
                 * Page is not resident, If this is the search termination
                 * or the pager might contain the page, allocate a new page.
                 *
                 * NOTE: We are still in a critical section.
                 */
                if (TRYPAGER(fs) || fs->object == fs->first_object) {
                        /*
                         * If the page is beyond the object size we fail
                         */
                        if (fs->pindex >= fs->object->size) {
                                crit_exit();
                                unlock_and_deallocate(fs);
                                return (KERN_PROTECTION_FAILURE);
                        }

                        /*
                         * Ratelimit.
                         */
                        if (fs->didlimit == 0 && curproc != NULL) {
                                int limticks;

                                limticks = vm_fault_ratelimit(curproc->p_vmspace);
                                if (limticks) {
                                        crit_exit();
                                        unlock_and_deallocate(fs);
                                        tsleep(curproc, 0, "vmrate", limticks);
                                        fs->didlimit = 1;
                                        return (KERN_TRY_AGAIN);
                                }
                        }

                        /*
                         * Allocate a new page for this object/offset pair.
                         */
                        fs->m = NULL;
                        if (!vm_page_count_severe()) {
                                fs->m = vm_page_alloc(fs->object, fs->pindex,
                                    (fs->vp || fs->object->backing_object) ? VM_ALLOC_NORMAL : VM_ALLOC_NORMAL | VM_ALLOC_ZERO);
                        }
                        if (fs->m == NULL) {
                                crit_exit();
                                unlock_and_deallocate(fs);
                                vm_waitpfault();
                                return (KERN_TRY_AGAIN);
                        }
                }
                crit_exit();

readrest:
                /*
                 * We have found a valid page or we have allocated a new page.
                 * The page thus may not be valid or may not be entirely 
                 * valid.
                 *
                 * Attempt to fault-in the page if there is a chance that the
                 * pager has it, and potentially fault in additional pages
                 * at the same time.
                 *
                 * We are NOT in splvm here and if TRYPAGER is true then
                 * fs.m will be non-NULL and will be PG_BUSY for us.
                 */

                if (TRYPAGER(fs)) {
                        int rv;
                        int reqpage;
                        int ahead, behind;
                        u_char behavior = vm_map_entry_behavior(fs->entry);

                        if (behavior == MAP_ENTRY_BEHAV_RANDOM) {
                                ahead = 0;
                                behind = 0;
                        } else {
                                behind = fs->pindex;
                                if (behind > VM_FAULT_READ_BEHIND)
                                        behind = VM_FAULT_READ_BEHIND;

                                ahead = fs->object->size - fs->pindex;
                                if (ahead < 1)
                                        ahead = 1;
                                if (ahead > VM_FAULT_READ_AHEAD)
                                        ahead = VM_FAULT_READ_AHEAD;
                        }

                        if ((fs->first_object->type != OBJT_DEVICE) &&
                            (behavior == MAP_ENTRY_BEHAV_SEQUENTIAL ||
                                (behavior != MAP_ENTRY_BEHAV_RANDOM &&
                                fs->pindex >= fs->entry->lastr &&
                                fs->pindex < fs->entry->lastr + VM_FAULT_READ))
                        ) {
                                vm_pindex_t firstpindex, tmppindex;

                                if (fs->first_pindex < 2 * VM_FAULT_READ)
                                        firstpindex = 0;
                                else
                                        firstpindex = fs->first_pindex - 2 * VM_FAULT_READ;

                                /*
                                 * note: partially valid pages cannot be 
                                 * included in the lookahead - NFS piecemeal
                                 * writes will barf on it badly.
                                 *
                                 * spl protection is required to avoid races
                                 * between the lookup and an interrupt
                                 * unbusy/free sequence occuring prior to
                                 * our busy check.
                                 */
                                crit_enter();
                                for (tmppindex = fs->first_pindex - 1;
                                    tmppindex >= firstpindex;
                                    --tmppindex
                                ) {
                                        vm_page_t mt;

                                        mt = vm_page_lookup(fs->first_object, tmppindex);
                                        if (mt == NULL || (mt->valid != VM_PAGE_BITS_ALL))
                                                break;
                                        if (mt->busy ||
                                                (mt->flags & (PG_BUSY | PG_FICTITIOUS | PG_UNMANAGED)) ||
                                                mt->hold_count ||
                                                mt->wire_count) 
                                                continue;
                                        if (mt->dirty == 0)
                                                vm_page_test_dirty(mt);
                                        if (mt->dirty) {
                                                vm_page_protect(mt, VM_PROT_NONE);
                                                vm_page_deactivate(mt);
                                        } else {
                                                vm_page_cache(mt);
                                        }
                                }
                                crit_exit();

                                ahead += behind;
                                behind = 0;
                        }

                        /*
                         * now we find out if any other pages should be paged
                         * in at this time this routine checks to see if the
                         * pages surrounding this fault reside in the same
                         * object as the page for this fault.  If they do,
                         * then they are faulted in also into the object.  The
                         * array "marray" returned contains an array of
                         * vm_page_t structs where one of them is the
                         * vm_page_t passed to the routine.  The reqpage
                         * return value is the index into the marray for the
                         * vm_page_t passed to the routine.
                         *
                         * fs.m plus the additional pages are PG_BUSY'd.
                         */
                        faultcount = vm_fault_additional_pages(
                            fs->m, behind, ahead, marray, &reqpage);

                        /*
                         * update lastr imperfectly (we do not know how much
                         * getpages will actually read), but good enough.
                         */
                        fs->entry->lastr = fs->pindex + faultcount - behind;

                        /*
                         * Call the pager to retrieve the data, if any, after
                         * releasing the lock on the map.  We hold a ref on
                         * fs.object and the pages are PG_BUSY'd.
                         */
                        unlock_map(fs);

                        if (faultcount) {
                                rv = vm_pager_get_pages(fs->object, marray, 
                                                        faultcount, reqpage);
                        } else {
                                rv = VM_PAGER_FAIL;
                        }

                        if (rv == VM_PAGER_OK) {
                                /*
                                 * Found the page. Leave it busy while we play
                                 * with it.
                                 */

                                /*
                                 * Relookup in case pager changed page. Pager
                                 * is responsible for disposition of old page
                                 * if moved.
                                 *
                                 * XXX other code segments do relookups too.
                                 * It's a bad abstraction that needs to be
                                 * fixed/removed.
                                 */
                                fs->m = vm_page_lookup(fs->object, fs->pindex);
                                if (fs->m == NULL) {
                                        unlock_and_deallocate(fs);
                                        return (KERN_TRY_AGAIN);
                                }

                                ++fs->hardfault;
                                break; /* break to PAGE HAS BEEN FOUND */
                        }

                        /*
                         * Remove the bogus page (which does not exist at this
                         * object/offset); before doing so, we must get back
                         * our object lock to preserve our invariant.
                         *
                         * Also wake up any other process that may want to bring
                         * in this page.
                         *
                         * If this is the top-level object, we must leave the
                         * busy page to prevent another process from rushing
                         * past us, and inserting the page in that object at
                         * the same time that we are.
                         */
                        if (rv == VM_PAGER_ERROR) {
                                if (curproc)
                                        printf("vm_fault: pager read error, pid %d (%s)\n", curproc->p_pid, curproc->p_comm);
                                else
                                        printf("vm_fault: pager read error, thread %p (%s)\n", curthread, curproc->p_comm);
                        }
                        /*
                         * Data outside the range of the pager or an I/O error
                         */
                        /*
                         * XXX - the check for kernel_map is a kludge to work
                         * around having the machine panic on a kernel space
                         * fault w/ I/O error.
                         */
                        if (((fs->map != kernel_map) && (rv == VM_PAGER_ERROR)) ||
                                (rv == VM_PAGER_BAD)) {
                                vm_page_free(fs->m);
                                fs->m = NULL;
                                unlock_and_deallocate(fs);
                                if (rv == VM_PAGER_ERROR)
                                        return (KERN_FAILURE);
                                else
                                        return (KERN_PROTECTION_FAILURE);
                                /* NOT REACHED */
                        }
                        if (fs->object != fs->first_object) {
                                vm_page_free(fs->m);
                                fs->m = NULL;
                                /*
                                 * XXX - we cannot just fall out at this
                                 * point, m has been freed and is invalid!
                                 */
                        }
                }

                /*
                 * We get here if the object has a default pager (or unwiring) 
                 * or the pager doesn't have the page.
                 */
                if (fs->object == fs->first_object)
                        fs->first_m = fs->m;

                /*
                 * Move on to the next object.  Lock the next object before
                 * unlocking the current one.
                 */
                fs->pindex += OFF_TO_IDX(fs->object->backing_object_offset);
                next_object = fs->object->backing_object;
                if (next_object == NULL) {
                        /*
                         * If there's no object left, fill the page in the top
                         * object with zeros.
                         */
                        if (fs->object != fs->first_object) {
                                vm_object_pip_wakeup(fs->object);

                                fs->object = fs->first_object;
                                fs->pindex = fs->first_pindex;
                                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) {
                                vm_page_zero_fill(fs->m);
                        } else {
                                mycpu->gd_cnt.v_ozfod++;
                        }
                        mycpu->gd_cnt.v_zfod++;
                        fs->m->valid = VM_PAGE_BITS_ALL;
                        break;  /* break to PAGE HAS BEEN FOUND */
                } else {
                        if (fs->object != fs->first_object) {
                                vm_object_pip_wakeup(fs->object);
                        }
                        KASSERT(fs->object != next_object, ("object loop %p", next_object));
                        fs->object = next_object;
                        vm_object_pip_add(fs->object, 1);
                }
        }

        KASSERT((fs->m->flags & PG_BUSY) != 0,
                ("vm_fault: not busy after main loop"));

        /*
         * 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_WRITE) {
                        /*
                         * 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.
                         */
                        if (fs->map_generation == fs->map->timestamp &&
                                /*
                                 * 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)) &&
                                /*
                                 * We don't chase down the shadow chain
                                 */
                                (fs->object == fs->first_object->backing_object) &&

                                /*
                                 * grab the lock if we need to
                                 */
                                (fs->lookup_still_valid ||
                                 lockmgr(&fs->map->lock, LK_EXCLUSIVE|LK_NOWAIT) == 0)
                            ) {
                                
                                fs->lookup_still_valid = 1;
                                /*
                                 * get rid of the unnecessary page
                                 */
                                vm_page_protect(fs->first_m, VM_PROT_NONE);
                                vm_page_free(fs->first_m);
                                fs->first_m = NULL;

                                /*
                                 * grab the page and put it into the 
                                 * process'es object.  The page is 
                                 * automatically made dirty.
                                 */
                                vm_page_rename(fs->m, fs->first_object, fs->first_pindex);
                                fs->first_m = fs->m;
                                vm_page_busy(fs->first_m);
                                fs->m = NULL;
                                mycpu->gd_cnt.v_cow_optim++;
                        } else {
                                /*
                                 * Oh, well, lets copy it.
                                 */
                                vm_page_copy(fs->m, fs->first_m);
                        }

                        if (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);

                        /*
                         * Only use the new page below...
                         */

                        mycpu->gd_cnt.v_cow_faults++;
                        fs->m = fs->first_m;
                        fs->object = fs->first_object;
                        fs->pindex = fs->first_pindex;
                } else {
                        /*
                         * If it wasn't a write fault avoid having to copy
                         * the page by mapping it read-only.
                         */
                        fs->prot &= ~VM_PROT_WRITE;
                }
        }

        /*
         * We may have had to unlock a map to do I/O.  If we did then
         * lookup_still_valid will be FALSE.  If the map generation count
         * also changed then all sorts of things could have happened while
         * we were doing the I/O and we need to retry.
         */

        if (!fs->lookup_still_valid &&
            (fs->map->timestamp != fs->map_generation)) {
                release_page(fs);
                unlock_and_deallocate(fs);
                return (KERN_TRY_AGAIN);
        }


#if 0
        /*
         * THIS IS THE PREVIOUS CODE, WHICH TRIED TO OPTIMIZE THE RETRY PATH.
         * SINCE WE HAD TO DO I/O ANYWAY, DON'T BOTHER
         */
        if (!fs->lookup_still_valid &&
            (fs->map->timestamp != fs->map_generation)) {
                vm_object_t retry_object;
                vm_pindex_t retry_pindex;
                vm_prot_t retry_prot;

                /*
                 * Since map entries may be pageable, make sure we can take a
                 * page fault on them.
                 */

                /*
                 * Unlock vnode before the lookup to avoid deadlock.   E.G.
                 * avoid a deadlock between the inode and exec_map that can
                 * occur due to locks being obtained in different orders.
                 */

                if (fs->vp != NULL) {
                        vput(fs->vp);
                        fs->vp = NULL;
                }
                
                if (fs->map->infork) {
                        release_page(fs);
                        unlock_and_deallocate(fs);
                        return (KERN_TRY_AGAIN);
                }

                /*
                 * To avoid trying to write_lock the map while another process
                 * has it read_locked (in vm_map_wire), we do not try for
                 * write permission.  If the page is still writable, we will
                 * get write permission.  If it is not, or has been marked
                 * needs_copy, we enter the mapping without write permission,
                 * and will merely take another fault.
                 */
                result = vm_map_lookup(fs->map, vaddr,
                                       fault_type & ~VM_PROT_WRITE, &fs->entry,
                                       &retry_object, &retry_pindex,
                                       &retry_prot, &fs->wired);
                fs->map_generation = fs->map->timestamp;

                /*
                 * If we don't need the page any longer, put it on the active
                 * 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);
                        return (result);
                }
                fs->lookup_still_valid = TRUE;

                if ((retry_object != fs->first_object) ||
                    (retry_pindex != fs->first_pindex)) {
                        release_page(fs);
                        unlock_and_deallocate(fs);
                        return (KERN_TRY_AGAIN);
                }
                /*
                 * 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;
        }
#endif

        /*
         * Put this page into the physical map. We had to do the unlock above
         * because pmap_enter may cause other faults.   We don't put the page
         * back on the active queue until later so that the page-out daemon
         * won't find us (yet).
         */
        if (fs->prot & VM_PROT_WRITE) {
                vm_page_flag_set(fs->m, PG_WRITEABLE);
                vm_object_set_writeable_dirty(fs->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.
                 *
                 * If this is a NOSYNC mmap we do not want to set PG_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.
                 *
                 * Also tell the backing pager, if any, that it should remove
                 * any swap backing since the page is now dirty.
                 */
                if (fs->entry->eflags & MAP_ENTRY_NOSYNC) {
                        if (fs->m->dirty == 0)
                                vm_page_flag_set(fs->m, PG_NOSYNC);
                } else {
                        vm_page_flag_clear(fs->m, PG_NOSYNC);
                }
                if (fs->fault_flags & VM_FAULT_DIRTY) {
                        crit_enter();
                        vm_page_dirty(fs->m);
                        vm_pager_page_unswapped(fs->m);
                        crit_exit();
                }
        }

        /*
         * Page had better still be busy
         */

        KASSERT(fs->m->flags & PG_BUSY,
                ("vm_fault: page %p not busy!", fs->m));

        unlock_things(fs);

        /*
         * Sanity check: page must be completely valid or it is not fit to
         * map into user space.  vm_pager_get_pages() ensures this.
         */
        if (fs->m->valid != VM_PAGE_BITS_ALL) {
                vm_page_zero_invalid(fs->m, TRUE);
                printf("Warning: page %p partially invalid on fault\n", fs->m);
        }

        return (KERN_SUCCESS);
}

/*
 * quick version of vm_fault
 */
int
vm_fault_quick(caddr_t v, int prot)
{
        int r;

        if (prot & VM_PROT_WRITE)
                r = subyte(v, fubyte(v));
        else
                r = fubyte(v);
        return(r);
}

/*
 * Wire down a range of virtual addresses in a map.  The entry in question
 * should be marked in-transition and the map must be locked.  We must
 * release the map temporarily while faulting-in the page to avoid a
 * deadlock.  Note that the entry may be clipped while we are blocked but
 * will never be freed.
 */
int
vm_fault_wire(vm_map_t map, vm_map_entry_t entry, boolean_t user_wire)
{
        boolean_t fictitious;
        vm_offset_t start;
        vm_offset_t end;
        vm_offset_t va;
        vm_paddr_t pa;
        pmap_t pmap;
        int rv;

        pmap = vm_map_pmap(map);
        start = entry->start;
        end = entry->end;
        fictitious = entry->object.vm_object &&
                        (entry->object.vm_object->type == OBJT_DEVICE);

        vm_map_unlock(map);
        map->timestamp++;

        /*
         * We simulate a fault to get the page and enter it in the physical
         * map.
         */
        for (va = start; va < end; va += PAGE_SIZE) {
                if (user_wire) {
                        rv = vm_fault(map, va, VM_PROT_READ, 
                                        VM_FAULT_USER_WIRE);
                } else {
                        rv = vm_fault(map, va, VM_PROT_READ|VM_PROT_WRITE,
                                        VM_FAULT_CHANGE_WIRING);
                }
                if (rv) {
                        while (va > start) {
                                va -= PAGE_SIZE;
                                if ((pa = pmap_extract(pmap, va)) == 0)
                                        continue;
                                pmap_change_wiring(pmap, va, FALSE);
                                if (!fictitious)
                                        vm_page_unwire(PHYS_TO_VM_PAGE(pa), 1);
                        }
                        vm_map_lock(map);
                        return (rv);
                }
        }
        vm_map_lock(map);
        return (KERN_SUCCESS);
}

/*
 * Unwire a range of virtual addresses in a map.  The map should be
 * locked.
 */
void
vm_fault_unwire(vm_map_t map, vm_map_entry_t entry)
{
        boolean_t fictitious;
        vm_offset_t start;
        vm_offset_t end;
        vm_offset_t va;
        vm_paddr_t pa;
        pmap_t pmap;

        pmap = vm_map_pmap(map);
        start = entry->start;
        end = entry->end;
        fictitious = entry->object.vm_object &&
                        (entry->object.vm_object->type == OBJT_DEVICE);

        /*
         * Since the pages are wired down, we must be able to get their
         * mappings from the physical map system.
         */
        for (va = start; va < end; va += PAGE_SIZE) {
                pa = pmap_extract(pmap, va);
                if (pa != 0) {
                        pmap_change_wiring(pmap, va, FALSE);
                        if (!fictitious)
                                vm_page_unwire(PHYS_TO_VM_PAGE(pa), 1);
                }
        }
}

/*
 * Reduce the rate at which memory is allocated to a process based
 * on the perceived load on the VM system. As the load increases
 * the allocation burst rate goes down and the delay increases. 
 *
 * Rate limiting does not apply when faulting active or inactive
 * pages.  When faulting 'cache' pages, rate limiting only applies
 * if the system currently has a severe page deficit.
 *
 * XXX vm_pagesupply should be increased when a page is freed.
 *
 * We sleep up to 1/10 of a second.
 */
static int
vm_fault_ratelimit(struct vmspace *vmspace)
{
        if (vm_load_enable == 0)
                return(0);
        if (vmspace->vm_pagesupply > 0) {
                --vmspace->vm_pagesupply;
                return(0);
        }
#ifdef INVARIANTS
        if (vm_load_debug) {
                printf("load %-4d give %d pgs, wait %d, pid %-5d (%s)\n",
                        vm_load, 
                        (1000 - vm_load ) / 10, vm_load * hz / 10000,
                        curproc->p_pid, curproc->p_comm);
        }
#endif
        vmspace->vm_pagesupply = (1000 - vm_load) / 10;
        return(vm_load * hz / 10000);
}

/*
 *      Routine:
 *              vm_fault_copy_entry
 *      Function:
 *              Copy all of the pages from a wired-down map entry to another.
 *
 *      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_object_t dst_object;
        vm_object_t src_object;
        vm_ooffset_t dst_offset;
        vm_ooffset_t src_offset;
        vm_prot_t prot;
        vm_offset_t vaddr;
        vm_page_t dst_m;
        vm_page_t src_m;

#ifdef  lint
        src_map++;
#endif  /* lint */

        src_object = src_entry->object.vm_object;
        src_offset = src_entry->offset;

        /*
         * 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,
            (vm_size_t) OFF_TO_IDX(dst_entry->end - dst_entry->start));

        dst_entry->object.vm_object = dst_object;
        dst_entry->offset = 0;

        prot = dst_entry->max_protection;

        /*
         * Loop through all of the pages in the entry's range, copying each
         * one from the source object (it should be there) to the destination
         * object.
         */
        for (vaddr = dst_entry->start, dst_offset = 0;
            vaddr < dst_entry->end;
            vaddr += PAGE_SIZE, dst_offset += PAGE_SIZE) {

                /*
                 * Allocate a page in the destination object
                 */
                do {
                        dst_m = vm_page_alloc(dst_object,
                                OFF_TO_IDX(dst_offset), VM_ALLOC_NORMAL);
                        if (dst_m == NULL) {
                                vm_wait();
                        }
                } while (dst_m == NULL);

                /*
                 * Find the page in the source object, and copy it in.
                 * (Because the source is wired down, the page will be in
                 * memory.)
                 */
                src_m = vm_page_lookup(src_object,
                        OFF_TO_IDX(dst_offset + src_offset));
                if (src_m == NULL)
                        panic("vm_fault_copy_wired: page missing");

                vm_page_copy(src_m, dst_m);

                /*
                 * Enter it in the pmap...
                 */

                vm_page_flag_clear(dst_m, PG_ZERO);
                pmap_enter(dst_map->pmap, vaddr, dst_m, prot, FALSE);
                vm_page_flag_set(dst_m, PG_WRITEABLE|PG_MAPPED);

                /*
                 * Mark it no longer busy, and put it on the active list.
                 */
                vm_page_activate(dst_m);
                vm_page_wakeup(dst_m);
        }
}


/*
 * This routine checks around the requested page for other pages that
 * might be able to be faulted in.  This routine brackets the viable
 * pages for the pages to be paged in.
 *
 * Inputs:
 *      m, rbehind, rahead
 *
 * Outputs:
 *  marray (array of vm_page_t), reqpage (index of requested page)
 *
 * Return value:
 *  number of pages in marray
 */
static int
vm_fault_additional_pages(vm_page_t m, int rbehind, int rahead,
    vm_page_t *marray, int *reqpage)
{
        int i,j;
        vm_object_t object;
        vm_pindex_t pindex, startpindex, endpindex, tpindex;
        vm_page_t rtm;
        int cbehind, cahead;

        object = m->object;
        pindex = m->pindex;

        /*
         * we don't fault-ahead for device pager
         */
        if (object->type == OBJT_DEVICE) {
                *reqpage = 0;
                marray[0] = m;
                return 1;
        }

        /*
         * if the requested page is not available, then give up now
         */

        if (!vm_pager_has_page(object, pindex, &cbehind, &cahead)) {
                return 0;
        }

        if ((cbehind == 0) && (cahead == 0)) {
                *reqpage = 0;
                marray[0] = m;
                return 1;
        }

        if (rahead > cahead) {
                rahead = cahead;
        }

        if (rbehind > cbehind) {
                rbehind = cbehind;
        }

        /*
         * try to do any readahead that we might have free pages for.
         */
        if ((rahead + rbehind) >
                ((vmstats.v_free_count + vmstats.v_cache_count) - vmstats.v_free_reserved)) {
                pagedaemon_wakeup();
                marray[0] = m;
                *reqpage = 0;
                return 1;
        }

        /*
         * scan backward for the read behind pages -- in memory 
         *
         * Assume that if the page is not found an interrupt will not
         * create it.  Theoretically interrupts can only remove (busy)
         * pages, not create new associations.
         */
        if (pindex > 0) {
                if (rbehind > pindex) {
                        rbehind = pindex;
                        startpindex = 0;
                } else {
                        startpindex = pindex - rbehind;
                }

                crit_enter();
                for ( tpindex = pindex - 1; tpindex >= startpindex; tpindex -= 1) {
                        if (vm_page_lookup( object, tpindex)) {
                                startpindex = tpindex + 1;
                                break;
                        }
                        if (tpindex == 0)
                                break;
                }

                for(i = 0, tpindex = startpindex; tpindex < pindex; i++, tpindex++) {

                        rtm = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL);
                        if (rtm == NULL) {
                                crit_exit();
                                for (j = 0; j < i; j++) {
                                        vm_page_free(marray[j]);
                                }
                                marray[0] = m;
                                *reqpage = 0;
                                return 1;
                        }

                        marray[i] = rtm;
                }
                crit_exit();
        } else {
                startpindex = 0;
                i = 0;
        }

        marray[i] = m;
        /* page offset of the required page */
        *reqpage = i;

        tpindex = pindex + 1;
        i++;

        /*
         * scan forward for the read ahead pages
         */
        endpindex = tpindex + rahead;
        if (endpindex > object->size)
                endpindex = object->size;

        crit_enter();
        for( ; tpindex < endpindex; i++, tpindex++) {

                if (vm_page_lookup(object, tpindex)) {
                        break;
                }

                rtm = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL);
                if (rtm == NULL) {
                        break;
                }

                marray[i] = rtm;
        }
        crit_exit();

        /* return number of bytes of pages */
        return i;
}