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Date:      Thu, 9 Dec 2010 21:02:22 +0000 (UTC)
From:      Max Laier <mlaier@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r216335 - head/sys/vm
Message-ID:  <201012092102.oB9L2MMG050677@svn.freebsd.org>
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Author: mlaier
Date: Thu Dec  9 21:02:22 2010
New Revision: 216335
URL: http://svn.freebsd.org/changeset/base/216335

Log:
  Fix a long standing (from the original 4.4BSD lite sources) race between
  vmspace_fork and vm_map_wire that would lead to "vm_fault_copy_wired: page
  missing" panics.  While faulting in pages for a map entry that is being
  wired down, mark the containing map as busy.  In vmspace_fork wait until the
  map is unbusy, before we try to copy the entries.
  
  Reviewed by:	kib
  MFC after:	5 days
  Sponsored by:	Isilon Systems, Inc.

Modified:
  head/sys/vm/vm_map.c
  head/sys/vm/vm_map.h

Modified: head/sys/vm/vm_map.c
==============================================================================
--- head/sys/vm/vm_map.c	Thu Dec  9 20:28:30 2010	(r216334)
+++ head/sys/vm/vm_map.c	Thu Dec  9 21:02:22 2010	(r216335)
@@ -671,6 +671,41 @@ vm_map_wakeup(vm_map_t map)
 	wakeup(&map->root);
 }
 
+void
+vm_map_busy(vm_map_t map)
+{
+
+	VM_MAP_ASSERT_LOCKED(map);
+	map->busy++;
+}
+
+void
+vm_map_unbusy(vm_map_t map)
+{
+
+	VM_MAP_ASSERT_LOCKED(map);
+	KASSERT(map->busy, ("vm_map_unbusy: not busy"));
+	if (--map->busy == 0 && (map->flags & MAP_BUSY_WAKEUP)) {
+		vm_map_modflags(map, 0, MAP_BUSY_WAKEUP);
+		wakeup(&map->busy);
+	}
+}
+
+void 
+vm_map_wait_busy(vm_map_t map)
+{
+
+	VM_MAP_ASSERT_LOCKED(map);
+	while (map->busy) {
+		vm_map_modflags(map, MAP_BUSY_WAKEUP, 0);
+		if (map->system_map)
+			msleep(&map->busy, &map->system_mtx, 0, "mbusy", 0);
+		else
+			sx_sleep(&map->busy, &map->lock, 0, "mbusy", 0);
+	}
+	map->timestamp++;
+}
+
 long
 vmspace_resident_count(struct vmspace *vmspace)
 {
@@ -718,6 +753,7 @@ _vm_map_init(vm_map_t map, pmap_t pmap, 
 	map->flags = 0;
 	map->root = NULL;
 	map->timestamp = 0;
+	map->busy = 0;
 }
 
 void
@@ -2382,12 +2418,14 @@ vm_map_wire(vm_map_t map, vm_offset_t st
 			    entry->object.vm_object->type == OBJT_SG);
 			/*
 			 * Release the map lock, relying on the in-transition
-			 * mark.
+			 * mark.  Mark the map busy for fork.
 			 */
+			vm_map_busy(map);
 			vm_map_unlock(map);
 			rv = vm_fault_wire(map, saved_start, saved_end,
 			    fictitious);
 			vm_map_lock(map);
+			vm_map_unbusy(map);
 			if (last_timestamp + 1 != map->timestamp) {
 				/*
 				 * Look again for the entry because the map was
@@ -2995,6 +3033,8 @@ vmspace_fork(struct vmspace *vm1, vm_oof
 	int locked;
 
 	vm_map_lock(old_map);
+	if (old_map->busy)
+		vm_map_wait_busy(old_map);
 	vm2 = vmspace_alloc(old_map->min_offset, old_map->max_offset);
 	if (vm2 == NULL)
 		goto unlock_and_return;

Modified: head/sys/vm/vm_map.h
==============================================================================
--- head/sys/vm/vm_map.h	Thu Dec  9 20:28:30 2010	(r216334)
+++ head/sys/vm/vm_map.h	Thu Dec  9 21:02:22 2010	(r216335)
@@ -187,12 +187,14 @@ struct vm_map {
 	pmap_t pmap;			/* (c) Physical map */
 #define	min_offset	header.start	/* (c) */
 #define	max_offset	header.end	/* (c) */
+	int busy;
 };
 
 /*
  * vm_flags_t values
  */
 #define MAP_WIREFUTURE		0x01	/* wire all future pages */
+#define	MAP_BUSY_WAKEUP		0x02
 
 #ifdef	_KERNEL
 static __inline vm_offset_t
@@ -275,6 +277,9 @@ int _vm_map_lock_upgrade(vm_map_t map, c
 void _vm_map_lock_downgrade(vm_map_t map, const char *file, int line);
 int vm_map_locked(vm_map_t map);
 void vm_map_wakeup(vm_map_t map);
+void vm_map_busy(vm_map_t map);
+void vm_map_unbusy(vm_map_t map);
+void vm_map_wait_busy(vm_map_t map);
 
 #define	vm_map_lock(map)	_vm_map_lock(map, LOCK_FILE, LOCK_LINE)
 #define	vm_map_unlock(map)	_vm_map_unlock(map, LOCK_FILE, LOCK_LINE)

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