diff --git a/sys/vm/vm_map.c b/sys/vm/vm_map.c
index 33e819d1aa6d..6e96db37b5ba 100644
--- a/sys/vm/vm_map.c
+++ b/sys/vm/vm_map.c
@@ -346,6 +346,8 @@ vmspace_exitfree(struct proc *p)
  * vmspace_swap_count() - count the approximate swap useage in pages for a
  *			  vmspace.
  *
+ *	The map must be locked.
+ *
  *	Swap useage is determined by taking the proportional swap used by
  *	VM objects backing the VM map.  To make up for fractional losses,
  *	if the VM object has any swap use at all the associated map entries
@@ -358,7 +360,6 @@ vmspace_swap_count(struct vmspace *vmspace)
 	vm_map_entry_t cur;
 	int count = 0;
 
-	vm_map_lock_read(map);
 	for (cur = map->header.next; cur != &map->header; cur = cur->next) {
 		vm_object_t object;
 
@@ -374,7 +375,6 @@ vmspace_swap_count(struct vmspace *vmspace)
 			}
 		}
 	}
-	vm_map_unlock_read(map);
 	return (count);
 }
 
@@ -438,6 +438,17 @@ _vm_map_trylock(vm_map_t map, const char *file, int line)
 	return (error == 0);
 }
 
+int
+_vm_map_trylock_read(vm_map_t map, const char *file, int line)
+{
+	int error;
+
+	error = map->system_map ?
+	    !_mtx_trylock(&map->system_mtx, 0, file, line) :
+	    lockmgr(&map->lock, LK_EXCLUSIVE | LK_NOWAIT, NULL, curthread);
+	return (error == 0);
+}
+
 int
 _vm_map_lock_upgrade(vm_map_t map, const char *file, int line)
 {
diff --git a/sys/vm/vm_map.h b/sys/vm/vm_map.h
index eef9a481b399..b685a18b01e2 100644
--- a/sys/vm/vm_map.h
+++ b/sys/vm/vm_map.h
@@ -248,6 +248,7 @@ void _vm_map_unlock(vm_map_t map, const char *file, int line);
 void _vm_map_lock_read(vm_map_t map, const char *file, int line);
 void _vm_map_unlock_read(vm_map_t map, const char *file, int line);
 int _vm_map_trylock(vm_map_t map, const char *file, int line);
+int _vm_map_trylock_read(vm_map_t map, const char *file, int line);
 int _vm_map_lock_upgrade(vm_map_t map, const char *file, int line);
 void _vm_map_lock_downgrade(vm_map_t map, const char *file, int line);
 int vm_map_unlock_and_wait(vm_map_t map, boolean_t user_wait);
@@ -258,6 +259,8 @@ void vm_map_wakeup(vm_map_t map);
 #define	vm_map_lock_read(map)	_vm_map_lock_read(map, LOCK_FILE, LOCK_LINE)
 #define	vm_map_unlock_read(map)	_vm_map_unlock_read(map, LOCK_FILE, LOCK_LINE)
 #define	vm_map_trylock(map)	_vm_map_trylock(map, LOCK_FILE, LOCK_LINE)
+#define	vm_map_trylock_read(map)	\
+			_vm_map_trylock_read(map, LOCK_FILE, LOCK_LINE)
 #define	vm_map_lock_upgrade(map)	\
 			_vm_map_lock_upgrade(map, LOCK_FILE, LOCK_LINE)
 #define	vm_map_lock_downgrade(map)	\
diff --git a/sys/vm/vm_pageout.c b/sys/vm/vm_pageout.c
index 8e3c39f8b90d..dab4ae2bacad 100644
--- a/sys/vm/vm_pageout.c
+++ b/sys/vm/vm_pageout.c
@@ -1214,8 +1214,13 @@ vm_pageout_scan(int pass)
 			/*
 			 * get the process size
 			 */
-			size = vmspace_resident_count(p->p_vmspace) +
-				vmspace_swap_count(p->p_vmspace);
+			if (!vm_map_trylock_read(&p->p_vmspace->vm_map)) {
+				PROC_UNLOCK(p);
+				continue;
+			}
+			size = vmspace_swap_count(p->p_vmspace);
+			vm_map_unlock_read(&p->p_vmspace->vm_map);
+			size += vmspace_resident_count(p->p_vmspace);
 			/*
 			 * if the this process is bigger than the biggest one
 			 * remember it.