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.

1 files changed, 41 insertions, 1 deletions

diff --git a/sys/vm/vm_map.c b/sys/vm/vm_map.c
index 35552a6..1a51af8 100644
--- a/sys/vm/vm_map.c
+++ b/sys/vm/vm_map.c
@@ -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, vm_offset_t min, vm_offset_t max)
 	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 start, vm_offset_t end,
 			    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_ooffset_t *fork_charge)
 	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;