XArray: Fix xas_pause at ULONG_MAX

If we were unlucky enough to call xas_pause() when the index was at
ULONG_MAX (or a multi-slot entry which ends at ULONG_MAX), we would
wrap the index back around to 0 and restart the iteration from the
beginning.  Use the XAS_BOUNDS state to indicate that we should just
stop the iteration.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

lib/test_xarray.c

@@ -1132,6 +1132,27 @@ static noinline void check_move_tiny(struct xarray *xa)
 	XA_BUG_ON(xa, !xa_empty(xa));
 }
 
+static noinline void check_move_max(struct xarray *xa)
+{
+	XA_STATE(xas, xa, 0);
+
+	xa_store_index(xa, ULONG_MAX, GFP_KERNEL);
+	rcu_read_lock();
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != xa_mk_index(ULONG_MAX));
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != NULL);
+	rcu_read_unlock();
+
+	xas_set(&xas, 0);
+	rcu_read_lock();
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != xa_mk_index(ULONG_MAX));
+	xas_pause(&xas);
+	XA_BUG_ON(xa, xas_find(&xas, ULONG_MAX) != NULL);
+	rcu_read_unlock();
+
+	xa_erase_index(xa, ULONG_MAX);
+	XA_BUG_ON(xa, !xa_empty(xa));
+}
+
 static noinline void check_move_small(struct xarray *xa, unsigned long idx)
 {
 	XA_STATE(xas, xa, 0);
@@ -1240,6 +1261,7 @@ static noinline void check_move(struct xarray *xa)
 	xa_destroy(xa);
 
 	check_move_tiny(xa);
+	check_move_max(xa);
 
 	for (i = 0; i < 16; i++)
 		check_move_small(xa, 1UL << i);

lib/xarray.c

@@ -967,6 +967,7 @@ void xas_pause(struct xa_state *xas)
 	if (xas_invalid(xas))
 		return;
 
+	xas->xa_node = XAS_RESTART;
 	if (node) {
 		unsigned int offset = xas->xa_offset;
 		while (++offset < XA_CHUNK_SIZE) {
@@ -974,10 +975,11 @@ void xas_pause(struct xa_state *xas)
 				break;
 		}
 		xas->xa_index += (offset - xas->xa_offset) << node->shift;
+		if (xas->xa_index == 0)
+			xas->xa_node = XAS_BOUNDS;
 	} else {
 		xas->xa_index++;
 	}
-	xas->xa_node = XAS_RESTART;
 }
 EXPORT_SYMBOL_GPL(xas_pause);
 
@@ -1079,13 +1081,13 @@ void *xas_find(struct xa_state *xas, unsigned long max)
 {
 	void *entry;
 
-	if (xas_error(xas))
+	if (xas_error(xas) || xas->xa_node == XAS_BOUNDS)
 		return NULL;
 
 	if (!xas->xa_node) {
 		xas->xa_index = 1;
 		return set_bounds(xas);
-	} else if (xas_top(xas->xa_node)) {
+	} else if (xas->xa_node == XAS_RESTART) {
 		entry = xas_load(xas);
 		if (entry || xas_not_node(xas->xa_node))
 			return entry;