XArray: Fix xas_next() with a single entry at 0

If there is only a single entry at 0, the first time we call xas_next(),
we return the entry.  Unfortunately, all subsequent times we call
xas_next(), we also return the entry at 0 instead of noticing that the
xa_index is now greater than zero.  This broke find_get_pages_contig().

Fixes: 64d3e9a ("xarray: Step through an XArray")
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

lib/test_xarray.c

@@ -1110,6 +1110,28 @@ static noinline void check_find_entry(struct xarray *xa)
 	XA_BUG_ON(xa, !xa_empty(xa));
 }
 
+static noinline void check_move_tiny(struct xarray *xa)
+{
+	XA_STATE(xas, xa, 0);
+
+	XA_BUG_ON(xa, !xa_empty(xa));
+	rcu_read_lock();
+	XA_BUG_ON(xa, xas_next(&xas) != NULL);
+	XA_BUG_ON(xa, xas_next(&xas) != NULL);
+	rcu_read_unlock();
+	xa_store_index(xa, 0, GFP_KERNEL);
+	rcu_read_lock();
+	xas_set(&xas, 0);
+	XA_BUG_ON(xa, xas_next(&xas) != xa_mk_index(0));
+	XA_BUG_ON(xa, xas_next(&xas) != NULL);
+	xas_set(&xas, 0);
+	XA_BUG_ON(xa, xas_prev(&xas) != xa_mk_index(0));
+	XA_BUG_ON(xa, xas_prev(&xas) != NULL);
+	rcu_read_unlock();
+	xa_erase_index(xa, 0);
+	XA_BUG_ON(xa, !xa_empty(xa));
+}
+
 static noinline void check_move_small(struct xarray *xa, unsigned long idx)
 {
 	XA_STATE(xas, xa, 0);
@@ -1217,6 +1239,8 @@ static noinline void check_move(struct xarray *xa)
 
 	xa_destroy(xa);
 
+	check_move_tiny(xa);
+
 	for (i = 0; i < 16; i++)
 		check_move_small(xa, 1UL << i);
 

lib/xarray.c

@@ -994,6 +994,8 @@ void *__xas_prev(struct xa_state *xas)
 
 	if (!xas_frozen(xas->xa_node))
 		xas->xa_index--;
+	if (!xas->xa_node)
+		return set_bounds(xas);
 	if (xas_not_node(xas->xa_node))
 		return xas_load(xas);
 
@@ -1031,6 +1033,8 @@ void *__xas_next(struct xa_state *xas)
 
 	if (!xas_frozen(xas->xa_node))
 		xas->xa_index++;
+	if (!xas->xa_node)
+		return set_bounds(xas);
 	if (xas_not_node(xas->xa_node))
 		return xas_load(xas);