Xarray: move forward index correctly in xas_pause()

After xas_load(), xas->index could point to mid of found multi-index entry
and xas->index's bits under node->shift maybe non-zero.  The afterward
xas_pause() will move forward xas->index with xa->node->shift with bits
under node->shift un-masked and thus skip some index unexpectedly.

Consider following case:
Assume XA_CHUNK_SHIFT is 4.
xa_store_range(xa, 16, 31, ...)
xa_store(xa, 32, ...)
XA_STATE(xas, xa, 17);
xas_for_each(&xas,...)
xas_load(&xas)
/* xas->index = 17, xas->xa_offset = 1, xas->xa_node->xa_shift = 4 */
xas_pause()
/* xas->index = 33, xas->xa_offset = 2, xas->xa_node->xa_shift = 4 */
As we can see, index of 32 is skipped unexpectedly.

Fix this by mask bit under node->xa_shift when move forward index in
xas_pause().

For now, this will not cause serious problems.  Only minor problem like
cachestat return less number of page status could happen.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kemeng Shi <[email protected]>
Cc: Mattew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Kemeng Shi

lib/test_xarray.c

@@ -1511,6 +1511,41 @@ static noinline void check_pause(struct kunit *test)
 	XA_BUG_ON(xa, count != order_limit);
 
 	xa_destroy(xa);
+
+	index = 0;
+	for (order = XA_CHUNK_SHIFT; order > 0; order--) {
+		XA_BUG_ON(xa, xa_store_order(xa, index, order,
+					xa_mk_index(index), GFP_KERNEL));
+		index += 1UL << order;
+	}
+
+	index = 0;
+	count = 0;
+	xas_set(&xas, 0);
+	rcu_read_lock();
+	xas_for_each(&xas, entry, ULONG_MAX) {
+		XA_BUG_ON(xa, entry != xa_mk_index(index));
+		index += 1UL << (XA_CHUNK_SHIFT - count);
+		count++;
+	}
+	rcu_read_unlock();
+	XA_BUG_ON(xa, count != XA_CHUNK_SHIFT);
+
+	index = 0;
+	count = 0;
+	xas_set(&xas, XA_CHUNK_SIZE / 2 + 1);
+	rcu_read_lock();
+	xas_for_each(&xas, entry, ULONG_MAX) {
+		XA_BUG_ON(xa, entry != xa_mk_index(index));
+		index += 1UL << (XA_CHUNK_SHIFT - count);
+		count++;
+		xas_pause(&xas);
+	}
+	rcu_read_unlock();
+	XA_BUG_ON(xa, count != XA_CHUNK_SHIFT);
+
+	xa_destroy(xa);
+
 }
 
 static noinline void check_move_tiny(struct kunit *test)

lib/xarray.c

@@ -1152,6 +1152,7 @@ void xas_pause(struct xa_state *xas)
 			if (!xa_is_sibling(xa_entry(xas->xa, node, offset)))
 				break;
 		}
+		xas->xa_index &= ~0UL << node->shift;
 		xas->xa_index += (offset - xas->xa_offset) << node->shift;
 		if (xas->xa_index == 0)
 			xas->xa_node = XAS_BOUNDS;