xref: /dflybsd-src/sys/vm/vm_fault.c (revision 2e7bf158f373428dba2c765c927f14d9e94f00a4)

/*
 * (MPSAFE)
 *
 * 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 1.47 2008/07/01 02:02:56 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 <sys/vkernel.h>
#include <sys/lock.h>
#include <sys/sysctl.h>

#include <cpu/lwbuf.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_pageout.h>
#include <vm/vm_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>

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

static int vm_fast_fault = 1;
SYSCTL_INT(_vm, OID_AUTO, fast_fault, CTLFLAG_RW, &vm_fast_fault, 0, "");
static int debug_cluster = 0;
SYSCTL_INT(_vm, OID_AUTO, debug_cluster, CTLFLAG_RW, &debug_cluster, 0, "");

static int vm_fault_object(struct faultstate *, vm_pindex_t, vm_prot_t);
static int vm_fault_vpagetable(struct faultstate *, vm_pindex_t *, vpte_t, int);
#if 0
static int vm_fault_additional_pages (vm_page_t, int, int, vm_page_t *, int *);
#endif
static int vm_fault_ratelimit(struct vmspace *);
static void vm_prefault(pmap_t pmap, vm_offset_t addra, vm_map_entry_t entry,
			int prot);

/*
 * The caller must hold vm_token.
 */
static __inline void
release_page(struct faultstate *fs)
{
	vm_page_deactivate(fs->m);
	vm_page_wakeup(fs->m);
	fs->m = NULL;
}

/*
 * The caller must hold vm_token.
 */
static __inline void
unlock_map(struct faultstate *fs)
{
	if (fs->lookup_still_valid && fs->map) {
		vm_map_lookup_done(fs->map, fs->entry, 0);
		fs->lookup_still_valid = FALSE;
	}
}

/*
 * Clean up after a successful call to vm_fault_object() so another call
 * to vm_fault_object() can be made.
 *
 * The caller must hold vm_token.
 */
static void
_cleanup_successful_fault(struct faultstate *fs, int relock)
{
	if (fs->object != fs->first_object) {
		vm_page_free(fs->first_m);
		vm_object_pip_wakeup(fs->object);
		fs->first_m = NULL;
	}
	fs->object = fs->first_object;
	if (relock && fs->lookup_still_valid == FALSE) {
		if (fs->map)
			vm_map_lock_read(fs->map);
		fs->lookup_still_valid = TRUE;
	}
}

/*
 * The caller must hold vm_token.
 */
static void
_unlock_things(struct faultstate *fs, int dealloc)
{
	vm_object_pip_wakeup(fs->first_object);
	_cleanup_successful_fault(fs, 0);
	if (dealloc) {
		vm_object_deallocate(fs->first_object);
		fs->first_object = NULL;
	}
	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)
#define cleanup_successful_fault(fs) _cleanup_successful_fault(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.
 * No other requirements.
 */
int
vm_fault(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type, int fault_flags)
{
	int result;
	vm_pindex_t first_pindex;
	struct faultstate fs;
	int growstack;

	mycpu->gd_cnt.v_vm_faults++;

	fs.didlimit = 0;
	fs.hardfault = 0;
	fs.fault_flags = fault_flags;
	growstack = 1;

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 try to 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,
			       &first_pindex, &fs.first_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 ||
		    (fs.fault_flags & VM_FAULT_WIRE_MASK) != VM_FAULT_USER_WIRE)
		{
			if (result == KERN_INVALID_ADDRESS && growstack &&
			    map != &kernel_map && curproc != NULL) {
				result = vm_map_growstack(curproc, vaddr);
				if (result != KERN_SUCCESS)
					return (KERN_FAILURE);
				growstack = 0;
				goto RetryFault;
			}
			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,
				       &first_pindex, &fs.first_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.
	 *
	 * The vm_token is needed to manipulate the vm_object
	 */
	lwkt_gettoken(&vm_token);
	vm_object_reference(fs.first_object);
	fs.vp = vnode_pager_lock(fs.first_object);
	vm_object_pip_add(fs.first_object, 1);
	lwkt_reltoken(&vm_token);

	fs.lookup_still_valid = TRUE;
	fs.first_m = NULL;
	fs.object = fs.first_object;	/* so unlock_and_deallocate works */

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

	/*
	 * The page we want is at (first_object, first_pindex), but if the
	 * vm_map_entry is VM_MAPTYPE_VPAGETABLE we have to traverse the
	 * page table to figure out the actual pindex.
	 *
	 * NOTE!  DEVELOPMENT IN PROGRESS, THIS IS AN INITIAL IMPLEMENTATION
	 * ONLY
	 */
	if (fs.entry->maptype == VM_MAPTYPE_VPAGETABLE) {
		result = vm_fault_vpagetable(&fs, &first_pindex,
					     fs.entry->aux.master_pde,
					     fault_type);
		if (result == KERN_TRY_AGAIN)
			goto RetryFault;
		if (result != KERN_SUCCESS)
			return (result);
	}

	/*
	 * Now we have the actual (object, pindex), fault in the page.  If
	 * vm_fault_object() fails it will unlock and deallocate the FS
	 * data.   If it succeeds everything remains locked and fs->object
	 * will have an additional PIP count if it is not equal to
	 * fs->first_object
	 *
	 * vm_fault_object will set fs->prot for the pmap operation.  It is
	 * allowed to set VM_PROT_WRITE if fault_type == VM_PROT_READ if the
	 * page can be safely written.  However, it will force a read-only
	 * mapping for a read fault if the memory is managed by a virtual
	 * page table.
	 */
	result = vm_fault_object(&fs, first_pindex, fault_type);

	if (result == KERN_TRY_AGAIN)
		goto RetryFault;
	if (result != KERN_SUCCESS)
		return (result);

	/*
	 * On success vm_fault_object() does not unlock or 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);

	/*
	 * Burst in a few more pages if possible.  The fs.map should still
	 * be locked.
	 */
	if (fault_flags & VM_FAULT_BURST) {
		if ((fs.fault_flags & VM_FAULT_WIRE_MASK) == 0 &&
		    fs.wired == 0) {
			vm_prefault(fs.map->pmap, vaddr, fs.entry, fs.prot);
		}
	}
	unlock_things(&fs);

	vm_page_flag_clear(fs.m, PG_ZERO);
	vm_page_flag_set(fs.m, PG_REFERENCED);

	/*
	 * If the page is not wired down, then put it where the pageout daemon
	 * can find it.
	 *
	 * We do not really need to get vm_token here but since all the
	 * vm_*() calls have to doing it here improves efficiency.
	 */
	lwkt_gettoken(&vm_token);
	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 (curthread->td_lwp) {
		if (fs.hardfault) {
			curthread->td_lwp->lwp_ru.ru_majflt++;
		} else {
			curthread->td_lwp->lwp_ru.ru_minflt++;
		}
	}

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

	return (KERN_SUCCESS);
}

/*
 * Fault in the specified virtual address in the current process map,
 * returning a held VM page or NULL.  See vm_fault_page() for more
 * information.
 *
 * No requirements.
 */
vm_page_t
vm_fault_page_quick(vm_offset_t va, vm_prot_t fault_type, int *errorp)
{
	struct lwp *lp = curthread->td_lwp;
	vm_page_t m;

	m = vm_fault_page(&lp->lwp_vmspace->vm_map, va,
			  fault_type, VM_FAULT_NORMAL, errorp);
	return(m);
}

/*
 * Fault in the specified virtual address in the specified map, doing all
 * necessary manipulation of the object store and all necessary I/O.  Return
 * a held VM page or NULL, and set *errorp.  The related pmap is not
 * updated.
 *
 * The returned page will be properly dirtied if VM_PROT_WRITE was specified,
 * and marked PG_REFERENCED as well.
 *
 * If the page cannot be faulted writable and VM_PROT_WRITE was specified, an
 * error will be returned.
 *
 * No requirements.
 */
vm_page_t
vm_fault_page(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
	      int fault_flags, int *errorp)
{
	vm_pindex_t first_pindex;
	struct faultstate fs;
	int result;
	vm_prot_t orig_fault_type = fault_type;

	mycpu->gd_cnt.v_vm_faults++;

	fs.didlimit = 0;
	fs.hardfault = 0;
	fs.fault_flags = fault_flags;
	KKASSERT((fault_flags & VM_FAULT_WIRE_MASK) == 0);

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,
			       &first_pindex, &fs.first_prot, &fs.wired);

	if (result != KERN_SUCCESS) {
		*errorp = result;
		return (NULL);
	}

	/*
	 * 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.
	 *
	 * The vm_token is needed to manipulate the vm_object
	 */
	lwkt_gettoken(&vm_token);
	vm_object_reference(fs.first_object);
	fs.vp = vnode_pager_lock(fs.first_object);
	vm_object_pip_add(fs.first_object, 1);
	lwkt_reltoken(&vm_token);

	fs.lookup_still_valid = TRUE;
	fs.first_m = NULL;
	fs.object = fs.first_object;	/* so unlock_and_deallocate works */

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

	/*
	 * The page we want is at (first_object, first_pindex), but if the
	 * vm_map_entry is VM_MAPTYPE_VPAGETABLE we have to traverse the
	 * page table to figure out the actual pindex.
	 *
	 * NOTE!  DEVELOPMENT IN PROGRESS, THIS IS AN INITIAL IMPLEMENTATION
	 * ONLY
	 */
	if (fs.entry->maptype == VM_MAPTYPE_VPAGETABLE) {
		result = vm_fault_vpagetable(&fs, &first_pindex,
					     fs.entry->aux.master_pde,
					     fault_type);
		if (result == KERN_TRY_AGAIN)
			goto RetryFault;
		if (result != KERN_SUCCESS) {
			*errorp = result;
			return (NULL);
		}
	}

	/*
	 * Now we have the actual (object, pindex), fault in the page.  If
	 * vm_fault_object() fails it will unlock and deallocate the FS
	 * data.   If it succeeds everything remains locked and fs->object
	 * will have an additinal PIP count if it is not equal to
	 * fs->first_object
	 */
	result = vm_fault_object(&fs, first_pindex, fault_type);

	if (result == KERN_TRY_AGAIN)
		goto RetryFault;
	if (result != KERN_SUCCESS) {
		*errorp = result;
		return(NULL);
	}

	if ((orig_fault_type & VM_PROT_WRITE) &&
	    (fs.prot & VM_PROT_WRITE) == 0) {
		*errorp = KERN_PROTECTION_FAILURE;
		unlock_and_deallocate(&fs);
		return(NULL);
	}

	/*
	 * On success vm_fault_object() does not unlock or deallocate, and fs.m
	 * will contain a busied page.
	 */
	unlock_things(&fs);

	/*
	 * Return a held page.  We are not doing any pmap manipulation so do
	 * not set PG_MAPPED.  However, adjust the page flags according to
	 * the fault type because the caller may not use a managed pmapping
	 * (so we don't want to lose the fact that the page will be dirtied
	 * if a write fault was specified).
	 */
	lwkt_gettoken(&vm_token);
	vm_page_hold(fs.m);
	vm_page_flag_clear(fs.m, PG_ZERO);
	if (fault_type & VM_PROT_WRITE)
		vm_page_dirty(fs.m);

	/*
	 * Update the pmap.  We really only have to do this if a COW
	 * occured to replace the read-only page with the new page.  For
	 * now just do it unconditionally. XXX
	 */
	pmap_enter(fs.map->pmap, vaddr, fs.m, fs.prot, fs.wired);
	vm_page_flag_set(fs.m, PG_REFERENCED);

	/*
	 * Unbusy the page by activating it.  It remains held and will not
	 * be reclaimed.
	 */
	vm_page_activate(fs.m);

	if (curthread->td_lwp) {
		if (fs.hardfault) {
			curthread->td_lwp->lwp_ru.ru_majflt++;
		} else {
			curthread->td_lwp->lwp_ru.ru_minflt++;
		}
	}

	/*
	 * Unlock everything, and return the held page.
	 */
	vm_page_wakeup(fs.m);
	vm_object_deallocate(fs.first_object);
	lwkt_reltoken(&vm_token);

	*errorp = 0;
	return(fs.m);
}

/*
 * Fault in the specified (object,offset), dirty the returned page as
 * needed.  If the requested fault_type cannot be done NULL and an
 * error is returned.
 *
 * A held (but not busied) page is returned.
 *
 * No requirements.
 */
vm_page_t
vm_fault_object_page(vm_object_t object, vm_ooffset_t offset,
		     vm_prot_t fault_type, int fault_flags, int *errorp)
{
	int result;
	vm_pindex_t first_pindex;
	struct faultstate fs;
	struct vm_map_entry entry;

	bzero(&entry, sizeof(entry));
	entry.object.vm_object = object;
	entry.maptype = VM_MAPTYPE_NORMAL;
	entry.protection = entry.max_protection = fault_type;

	fs.didlimit = 0;
	fs.hardfault = 0;
	fs.fault_flags = fault_flags;
	fs.map = NULL;
	KKASSERT((fault_flags & VM_FAULT_WIRE_MASK) == 0);

RetryFault:

	fs.first_object = object;
	first_pindex = OFF_TO_IDX(offset);
	fs.entry = &entry;
	fs.first_prot = fault_type;
	fs.wired = 0;
	/*fs.map_generation = 0; unused */

	/*
	 * 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.
	 */
	lwkt_gettoken(&vm_token);
	vm_object_reference(fs.first_object);
	fs.vp = vnode_pager_lock(fs.first_object);
	vm_object_pip_add(fs.first_object, 1);
	lwkt_reltoken(&vm_token);

	fs.lookup_still_valid = TRUE;
	fs.first_m = NULL;
	fs.object = fs.first_object;	/* so unlock_and_deallocate works */

#if 0
	/* XXX future - ability to operate on VM object using vpagetable */
	if (fs.entry->maptype == VM_MAPTYPE_VPAGETABLE) {
		result = vm_fault_vpagetable(&fs, &first_pindex,
					     fs.entry->aux.master_pde,
					     fault_type);
		if (result == KERN_TRY_AGAIN)
			goto RetryFault;
		if (result != KERN_SUCCESS) {
			*errorp = result;
			return (NULL);
		}
	}
#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.   If it succeeds everything remains locked and fs->object
	 * will have an additinal PIP count if it is not equal to
	 * fs->first_object
	 */
	result = vm_fault_object(&fs, first_pindex, fault_type);

	if (result == KERN_TRY_AGAIN)
		goto RetryFault;
	if (result != KERN_SUCCESS) {
		*errorp = result;
		return(NULL);
	}

	if ((fault_type & VM_PROT_WRITE) && (fs.prot & VM_PROT_WRITE) == 0) {
		*errorp = KERN_PROTECTION_FAILURE;
		unlock_and_deallocate(&fs);
		return(NULL);
	}

	/*
	 * On success vm_fault_object() does not unlock or deallocate, and fs.m
	 * will contain a busied page.
	 */
	unlock_things(&fs);

	/*
	 * Return a held page.  We are not doing any pmap manipulation so do
	 * not set PG_MAPPED.  However, adjust the page flags according to
	 * the fault type because the caller may not use a managed pmapping
	 * (so we don't want to lose the fact that the page will be dirtied
	 * if a write fault was specified).
	 */
	lwkt_gettoken(&vm_token);
	vm_page_hold(fs.m);
	vm_page_flag_clear(fs.m, PG_ZERO);
	if (fault_type & VM_PROT_WRITE)
		vm_page_dirty(fs.m);

	/*
	 * Indicate that the page was accessed.
	 */
	vm_page_flag_set(fs.m, PG_REFERENCED);

	/*
	 * Unbusy the page by activating it.  It remains held and will not
	 * be reclaimed.
	 */
	vm_page_activate(fs.m);

	if (curthread->td_lwp) {
		if (fs.hardfault) {
			mycpu->gd_cnt.v_vm_faults++;
			curthread->td_lwp->lwp_ru.ru_majflt++;
		} else {
			curthread->td_lwp->lwp_ru.ru_minflt++;
		}
	}

	/*
	 * Unlock everything, and return the held page.
	 */
	vm_page_wakeup(fs.m);
	vm_object_deallocate(fs.first_object);
	lwkt_reltoken(&vm_token);

	*errorp = 0;
	return(fs.m);
}

/*
 * Translate the virtual page number (first_pindex) that is relative
 * to the address space into a logical page number that is relative to the
 * backing object.  Use the virtual page table pointed to by (vpte).
 *
 * This implements an N-level page table.  Any level can terminate the
 * scan by setting VPTE_PS.   A linear mapping is accomplished by setting
 * VPTE_PS in the master page directory entry set via mcontrol(MADV_SETMAP).
 *
 * No requirements (vm_token need not be held).
 */
static
int
vm_fault_vpagetable(struct faultstate *fs, vm_pindex_t *pindex,
		    vpte_t vpte, int fault_type)
{
	struct lwbuf *lwb;
	int vshift = VPTE_FRAME_END - PAGE_SHIFT; /* index bits remaining */
	int result = KERN_SUCCESS;
	vpte_t *ptep;

	for (;;) {
		/*
		 * We cannot proceed if the vpte is not valid, not readable
		 * for a read fault, or not writable for a write fault.
		 */
		if ((vpte & VPTE_V) == 0) {
			unlock_and_deallocate(fs);
			return (KERN_FAILURE);
		}
		if ((fault_type & VM_PROT_READ) && (vpte & VPTE_R) == 0) {
			unlock_and_deallocate(fs);
			return (KERN_FAILURE);
		}
		if ((fault_type & VM_PROT_WRITE) && (vpte & VPTE_W) == 0) {
			unlock_and_deallocate(fs);
			return (KERN_FAILURE);
		}
		if ((vpte & VPTE_PS) || vshift == 0)
			break;
		KKASSERT(vshift >= VPTE_PAGE_BITS);

		/*
		 * Get the page table page.  Nominally we only read the page
		 * table, but since we are actively setting VPTE_M and VPTE_A,
		 * tell vm_fault_object() that we are writing it.
		 *
		 * There is currently no real need to optimize this.
		 */
		result = vm_fault_object(fs, (vpte & VPTE_FRAME) >> PAGE_SHIFT,
					 VM_PROT_READ|VM_PROT_WRITE);
		if (result != KERN_SUCCESS)
			return (result);

		/*
		 * Process the returned fs.m and look up the page table
		 * entry in the page table page.
		 */
		vshift -= VPTE_PAGE_BITS;
		lwb = lwbuf_alloc(fs->m);
		ptep = ((vpte_t *)lwbuf_kva(lwb) +
		        ((*pindex >> vshift) & VPTE_PAGE_MASK));
		vpte = *ptep;

		/*
		 * Page table write-back.  If the vpte is valid for the
		 * requested operation, do a write-back to the page table.
		 *
		 * XXX VPTE_M is not set properly for page directory pages.
		 * It doesn't get set in the page directory if the page table
		 * is modified during a read access.
		 */
		if ((fault_type & VM_PROT_WRITE) && (vpte & VPTE_V) &&
		    (vpte & VPTE_W)) {
			if ((vpte & (VPTE_M|VPTE_A)) != (VPTE_M|VPTE_A)) {
				atomic_set_long(ptep, VPTE_M | VPTE_A);
				vm_page_dirty(fs->m);
			}
		}
		if ((fault_type & VM_PROT_READ) && (vpte & VPTE_V) &&
		    (vpte & VPTE_R)) {
			if ((vpte & VPTE_A) == 0) {
				atomic_set_long(ptep, VPTE_A);
				vm_page_dirty(fs->m);
			}
		}
		lwbuf_free(lwb);
		vm_page_flag_set(fs->m, PG_REFERENCED);
		vm_page_activate(fs->m);
		vm_page_wakeup(fs->m);
		cleanup_successful_fault(fs);
	}
	/*
	 * Combine remaining address bits with the vpte.
	 */
	/* JG how many bits from each? */
	*pindex = ((vpte & VPTE_FRAME) >> PAGE_SHIFT) +
		  (*pindex & ((1L << vshift) - 1));
	return (KERN_SUCCESS);
}


/*
 * This is the core of the vm_fault code.
 *
 * Do all operations required to fault-in (fs.first_object, 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 NOT unlocked or
 * deallocated, fs.m will contained a resolved, busied page, and fs.object
 * will have an additional PIP count if it is not equal to fs.first_object.
 *
 * No requirements.
 */
static
int
vm_fault_object(struct faultstate *fs,
		vm_pindex_t first_pindex, vm_prot_t fault_type)
{
	vm_object_t next_object;
	vm_pindex_t pindex;

	fs->prot = fs->first_prot;
	fs->object = fs->first_object;
	pindex = first_pindex;

	/*
	 * If a read fault occurs we try to make the page writable if
	 * possible.  There are three cases where we cannot make the
	 * page mapping writable:
	 *
	 * (1) The mapping is read-only or the VM object is read-only,
	 *     fs->prot above will simply not have VM_PROT_WRITE set.
	 *
	 * (2) If the mapping is a virtual page table we need to be able
	 *     to detect writes so we can set VPTE_M in the virtual page
	 *     table.
	 *
	 * (3) If the VM page is read-only or copy-on-write, upgrading would
	 *     just result in an unnecessary COW fault.
	 *
	 * VM_PROT_VPAGED is set if faulting via a virtual page table and
	 * causes adjustments to the 'M'odify bit to also turn off write
	 * access to force a re-fault.
	 */
	if (fs->entry->maptype == VM_MAPTYPE_VPAGETABLE) {
		if ((fault_type & VM_PROT_WRITE) == 0)
			fs->prot &= ~VM_PROT_WRITE;
	}

	lwkt_gettoken(&vm_token);

	for (;;) {
		/*
		 * If the object is dead, we stop here
		 */
		if (fs->object->flags & OBJ_DEAD) {
			unlock_and_deallocate(fs);
			lwkt_reltoken(&vm_token);
			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, 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);
				fs->first_object = NULL;
				lwkt_reltoken(&vm_token);
				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();
				lwkt_reltoken(&vm_token);
				crit_exit();
				return (KERN_TRY_AGAIN);
			}

			/*
			 * Mark page busy for other processes, and the
			 * pagedaemon.  If it still isn't completely valid
			 * (readable), or if a read-ahead-mark is set on
			 * the VM page, 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->object != &kernel_object) {
				if ((fs->m->valid & VM_PAGE_BITS_ALL) !=
				    VM_PAGE_BITS_ALL) {
					goto readrest;
				}
				if (fs->m->flags & PG_RAM) {
					if (debug_cluster)
						kprintf("R");
					vm_page_flag_clear(fs->m, PG_RAM);
					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 (pindex >= fs->object->size) {
				lwkt_reltoken(&vm_token);
				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) {
					lwkt_reltoken(&vm_token);
					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, pindex,
				    (fs->vp || fs->object->backing_object) ? VM_ALLOC_NORMAL : VM_ALLOC_NORMAL | VM_ALLOC_ZERO);
			}
			if (fs->m == NULL) {
				lwkt_reltoken(&vm_token);
				crit_exit();
				unlock_and_deallocate(fs);
				vm_waitpfault();
				return (KERN_TRY_AGAIN);
			}
		}
		crit_exit();

readrest:
		/*
		 * We have found an invalid or partially valid page, a
		 * page with a read-ahead mark which might be partially or
		 * fully valid (and maybe dirty too), or we have allocated
		 * a new page.
		 *
		 * 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 seqaccess;
			u_char behavior = vm_map_entry_behavior(fs->entry);

			if (behavior == MAP_ENTRY_BEHAV_RANDOM)
				seqaccess = 0;
			else
				seqaccess = -1;

			/*
			 * If sequential access is detected then attempt
			 * to deactivate/cache pages behind the scan to
			 * prevent resource hogging.
			 *
			 * Use of PG_RAM to detect sequential access
			 * also simulates multi-zone sequential access
			 * detection for free.
			 *
			 * NOTE: Partially valid dirty pages cannot be
			 *	 deactivated without causing NFS picemeal
			 *	 writes to barf.
			 */
			if ((fs->first_object->type != OBJT_DEVICE) &&
			    (behavior == MAP_ENTRY_BEHAV_SEQUENTIAL ||
                                (behavior != MAP_ENTRY_BEHAV_RANDOM &&
				 (fs->m->flags & PG_RAM)))
			) {
				vm_pindex_t scan_pindex;
				int scan_count = 16;

				if (first_pindex < 16) {
					scan_pindex = 0;
					scan_count = 0;
				} else {
					scan_pindex = first_pindex - 16;
					if (scan_pindex < 16)
						scan_count = scan_pindex;
					else
						scan_count = 16;
				}

				crit_enter();
				while (scan_count) {
					vm_page_t mt;

					mt = vm_page_lookup(fs->first_object,
							    scan_pindex);
					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)  {
						goto skip;
					}
					if (mt->dirty == 0)
						vm_page_test_dirty(mt);
					if (mt->dirty) {
						vm_page_busy(mt);
						vm_page_protect(mt,
								VM_PROT_NONE);
						vm_page_deactivate(mt);
						vm_page_wakeup(mt);
					} else {
						vm_page_cache(mt);
					}
skip:
					--scan_count;
					--scan_pindex;
				}
				crit_exit();

				seqaccess = 1;
			}

			/*
			 * Avoid deadlocking against the map when doing I/O.
			 * fs.object and the page is PG_BUSY'd.
			 */
			unlock_map(fs);

			/*
			 * Acquire the page data.  We still hold a ref on
			 * fs.object and the page has been PG_BUSY's.
			 *
			 * The pager may replace the page (for example, in
			 * order to enter a fictitious page into the
			 * object).  If it does so it is responsible for
			 * cleaning up the passed page and properly setting
			 * the new page PG_BUSY.
			 *
			 * If we got here through a PG_RAM read-ahead
			 * mark the page may be partially dirty and thus
			 * not freeable.  Don't bother checking to see
			 * if the pager has the page because we can't free
			 * it anyway.  We have to depend on the get_page
			 * operation filling in any gaps whether there is
			 * backing store or not.
			 */
			rv = vm_pager_get_page(fs->object, &fs->m, seqaccess);

			if (rv == VM_PAGER_OK) {
				/*
				 * 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, pindex);
				if (fs->m == NULL) {
					lwkt_reltoken(&vm_token);
					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)
					kprintf("vm_fault: pager read error, pid %d (%s)\n", curproc->p_pid, curproc->p_comm);
				else
					kprintf("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
			 *
			 * The page may have been wired during the pagein,
			 * e.g. by the buffer cache, and cannot simply be
			 * freed.  Call vnode_pager_freepage() to deal with it.
			 */
			/*
			 * 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)) {
				vnode_pager_freepage(fs->m);
				lwkt_reltoken(&vm_token);
				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) {
				vnode_pager_freepage(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.
		 */
		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;
				pindex = 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 */
		}
		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);
	}

	/*
	 * PAGE HAS BEEN FOUND. [Loop invariant still holds -- the object lock
	 * is held.]
	 *
	 * vm_token is still 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.
	 */
	KASSERT((fs->m->flags & PG_BUSY) != 0,
		("vm_fault: not busy after main loop"));

	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 (
				/*
				 * Map, if present, has not changed
				 */
				(fs->map == NULL ||
				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 ||
				 fs->map == NULL ||
				 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, 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);
				vm_page_event(fs->m, VMEVENT_COW);
			}

			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;
			pindex = 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 != NULL &&
	    (fs->map->timestamp != fs->map_generation)) {
		release_page(fs);
		lwkt_reltoken(&vm_token);
		unlock_and_deallocate(fs);
		return (KERN_TRY_AGAIN);
	}

	/*
	 * 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->prot & VM_PROT_WRITE) {
		vm_object_set_writeable_dirty(fs->m->object);
		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);
			swap_pager_unswapped(fs->m);
			crit_exit();
		}
	}

	lwkt_reltoken(&vm_token);

	/*
	 * Page had better still be busy.  We are still locked up and
	 * fs->object will have another PIP reference if it is not equal
	 * to fs->first_object.
	 */
	KASSERT(fs->m->flags & PG_BUSY,
		("vm_fault: page %p not busy!", fs->m));

	/*
	 * 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);
		kprintf("Warning: page %p partially invalid on fault\n", fs->m);
	}

	return (KERN_SUCCESS);
}

/*
 * 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.
 *
 * No requirements.
 */
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);

	lwkt_gettoken(&vm_token);
	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);
			lwkt_reltoken(&vm_token);
			return (rv);
		}
	}
	vm_map_lock(map);
	lwkt_reltoken(&vm_token);
	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.
	 */
	lwkt_gettoken(&vm_token);
	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);
		}
	}
	lwkt_reltoken(&vm_token);
}

/*
 * 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;	/* SMP race ok */
		return(0);
	}
#ifdef INVARIANTS
	if (vm_load_debug) {
		kprintf("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);
}

/*
 * Copy all of the pages from a wired-down map entry to another.
 *
 * 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).
 *
 * No other requirements.
 */
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.)
	 */
	vm_map_entry_allocate_object(dst_entry);
	dst_object = dst_entry->object.vm_object;

	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(0);
			}
		} 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);
		vm_page_event(src_m, VMEVENT_COW);

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

		vm_page_flag_clear(dst_m, PG_ZERO);
		pmap_enter(dst_map->pmap, vaddr, dst_m, prot, FALSE);

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

#if 0

/*
 * 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)) {
		*reqpage = 0;	/* not used by caller, fix compiler warn */
		return 0;
	}

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

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

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

	/*
	 * Do not do any readahead if we have insufficient free memory.
	 *
	 * XXX code was broken disabled before and has instability
	 * with this conditonal fixed, so shortcut for now.
	 */
	if (burst_fault == 0 || vm_page_count_severe()) {
		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();
		lwkt_gettoken(&vm_token);
		for (tpindex = pindex; tpindex > startpindex; --tpindex) {
			if (vm_page_lookup(object, tpindex - 1))
				break;
		}

		i = 0;
		while (tpindex < pindex) {
			rtm = vm_page_alloc(object, tpindex, VM_ALLOC_SYSTEM);
			if (rtm == NULL) {
				lwkt_reltoken(&vm_token);
				crit_exit();
				for (j = 0; j < i; j++) {
					vm_page_free(marray[j]);
				}
				marray[0] = m;
				*reqpage = 0;
				return 1;
			}
			marray[i] = rtm;
			++i;
			++tpindex;
		}
		lwkt_reltoken(&vm_token);
		crit_exit();
	} else {
		i = 0;
	}

	/*
	 * Assign requested page
	 */
	marray[i] = m;
	*reqpage = i;
	++i;

	/*
	 * Scan forwards for read-ahead pages
	 */
	tpindex = pindex + 1;
	endpindex = tpindex + rahead;
	if (endpindex > object->size)
		endpindex = object->size;

	crit_enter();
	lwkt_gettoken(&vm_token);
	while (tpindex < endpindex) {
		if (vm_page_lookup(object, tpindex))
			break;
		rtm = vm_page_alloc(object, tpindex, VM_ALLOC_SYSTEM);
		if (rtm == NULL)
			break;
		marray[i] = rtm;
		++i;
		++tpindex;
	}
	lwkt_reltoken(&vm_token);
	crit_exit();

	return (i);
}

#endif

/*
 * vm_prefault() provides a quick way of clustering pagefaults into a
 * processes address space.  It is a "cousin" of pmap_object_init_pt,
 * except it runs at page fault time instead of mmap time.
 *
 * This code used to be per-platform pmap_prefault().  It is now
 * machine-independent and enhanced to also pre-fault zero-fill pages
 * (see vm.fast_fault) as well as make them writable, which greatly
 * reduces the number of page faults programs incur.
 *
 * Application performance when pre-faulting zero-fill pages is heavily
 * dependent on the application.  Very tiny applications like /bin/echo
 * lose a little performance while applications of any appreciable size
 * gain performance.  Prefaulting multiple pages also reduces SMP
 * congestion and can improve SMP performance significantly.
 *
 * NOTE!  prot may allow writing but this only applies to the top level
 *	  object.  If we wind up mapping a page extracted from a backing
 *	  object we have to make sure it is read-only.
 *
 * NOTE!  The caller has already handled any COW operations on the
 *	  vm_map_entry via the normal fault code.  Do NOT call this
 *	  shortcut unless the normal fault code has run on this entry.
 *
 * No other requirements.
 */
#define PFBAK 4
#define PFFOR 4
#define PAGEORDER_SIZE (PFBAK+PFFOR)

static int vm_prefault_pageorder[] = {
	-PAGE_SIZE, PAGE_SIZE,
	-2 * PAGE_SIZE, 2 * PAGE_SIZE,
	-3 * PAGE_SIZE, 3 * PAGE_SIZE,
	-4 * PAGE_SIZE, 4 * PAGE_SIZE
};

static void
vm_prefault(pmap_t pmap, vm_offset_t addra, vm_map_entry_t entry, int prot)
{
	struct lwp *lp;
	vm_page_t m;
	vm_offset_t starta;
	vm_offset_t addr;
	vm_pindex_t index;
	vm_pindex_t pindex;
	vm_object_t object;
	int pprot;
	int i;

	/*
	 * We do not currently prefault mappings that use virtual page
	 * tables.  We do not prefault foreign pmaps.
	 */
	if (entry->maptype == VM_MAPTYPE_VPAGETABLE)
		return;
	lp = curthread->td_lwp;
	if (lp == NULL || (pmap != vmspace_pmap(lp->lwp_vmspace)))
		return;

	object = entry->object.vm_object;

	starta = addra - PFBAK * PAGE_SIZE;
	if (starta < entry->start)
		starta = entry->start;
	else if (starta > addra)
		starta = 0;

	/*
	 * critical section protection is required to maintain the
	 * page/object association, interrupts can free pages and remove
	 * them from their objects.
	 */
	crit_enter();
	lwkt_gettoken(&vm_token);
	for (i = 0; i < PAGEORDER_SIZE; i++) {
		vm_object_t lobject;
		int allocated = 0;

		addr = addra + vm_prefault_pageorder[i];
		if (addr > addra + (PFFOR * PAGE_SIZE))
			addr = 0;

		if (addr < starta || addr >= entry->end)
			continue;

		if (pmap_prefault_ok(pmap, addr) == 0)
			continue;

		/*
		 * Follow the VM object chain to obtain the page to be mapped
		 * into the pmap.
		 *
		 * If we reach the terminal object without finding a page
		 * and we determine it would be advantageous, then allocate
		 * a zero-fill page for the base object.  The base object
		 * is guaranteed to be OBJT_DEFAULT for this case.
		 *
		 * In order to not have to check the pager via *haspage*()
		 * we stop if any non-default object is encountered.  e.g.
		 * a vnode or swap object would stop the loop.
		 */
		index = ((addr - entry->start) + entry->offset) >> PAGE_SHIFT;
		lobject = object;
		pindex = index;
		pprot = prot;

		while ((m = vm_page_lookup(lobject, pindex)) == NULL) {
			if (lobject->type != OBJT_DEFAULT)
				break;
			if (lobject->backing_object == NULL) {
				if (vm_fast_fault == 0)
					break;
				if (vm_prefault_pageorder[i] < 0 ||
				    (prot & VM_PROT_WRITE) == 0 ||
				    vm_page_count_min(0)) {
					break;
				}
				/* note: allocate from base object */
				m = vm_page_alloc(object, index,
					      VM_ALLOC_NORMAL | VM_ALLOC_ZERO);

				if ((m->flags & PG_ZERO) == 0) {
					vm_page_zero_fill(m);
				} else {
					vm_page_flag_clear(m, PG_ZERO);
					mycpu->gd_cnt.v_ozfod++;
				}
				mycpu->gd_cnt.v_zfod++;
				m->valid = VM_PAGE_BITS_ALL;
				allocated = 1;
				pprot = prot;
				/* lobject = object .. not needed */
				break;
			}
			if (lobject->backing_object_offset & PAGE_MASK)
				break;
			pindex += lobject->backing_object_offset >> PAGE_SHIFT;
			lobject = lobject->backing_object;
			pprot &= ~VM_PROT_WRITE;
		}
		/*
		 * NOTE: lobject now invalid (if we did a zero-fill we didn't
		 *	 bother assigning lobject = object).
		 *
		 * Give-up if the page is not available.
		 */
		if (m == NULL)
			break;

		/*
		 * Do not conditionalize on PG_RAM.  If pages are present in
		 * the VM system we assume optimal caching.  If caching is
		 * not optimal the I/O gravy train will be restarted when we
		 * hit an unavailable page.  We do not want to try to restart
		 * the gravy train now because we really don't know how much
		 * of the object has been cached.  The cost for restarting
		 * the gravy train should be low (since accesses will likely
		 * be I/O bound anyway).
		 *
		 * The object must be marked dirty if we are mapping a
		 * writable page.
		 */
		if (pprot & VM_PROT_WRITE)
			vm_object_set_writeable_dirty(m->object);

		/*
		 * Enter the page into the pmap if appropriate.  If we had
		 * allocated the page we have to place it on a queue.  If not
		 * we just have to make sure it isn't on the cache queue
		 * (pages on the cache queue are not allowed to be mapped).
		 */
		if (allocated) {
			pmap_enter(pmap, addr, m, pprot, 0);
			vm_page_deactivate(m);
			vm_page_wakeup(m);
		} else if (((m->valid & VM_PAGE_BITS_ALL) == VM_PAGE_BITS_ALL) &&
		    (m->busy == 0) &&
		    (m->flags & (PG_BUSY | PG_FICTITIOUS)) == 0) {

			if ((m->queue - m->pc) == PQ_CACHE) {
				vm_page_deactivate(m);
			}
			vm_page_busy(m);
			pmap_enter(pmap, addr, m, pprot, 0);
			vm_page_wakeup(m);
		}
	}
	lwkt_reltoken(&vm_token);
	crit_exit();
}