xarray: add a BUG_ON() to ensure caller is not sibling

Suppose xas is pointing somewhere near the end of the multi-entry batch. 
Then it may happen that the computed slot already falls beyond the batch,
thus breaking the loop due to !xa_is_sibling(), and computing the wrong
order.

For example, suppose we have a shift-6 node having an order-9 entry => 8 -
1 = 7 siblings, so assume the slots are at offset 0 till 7 in this node. 
If xas->xa_offset is 6, then the code will compute order as 1 +
xas->xa_node->shift = 7.  Therefore, the order computation must start from
the beginning of the multi-slot entries, that is, the non-sibling entry.

Thus ensure that the caller is aware of this by triggering a BUG when the
entry is a sibling entry.  Note that this BUG_ON() is only active while
running selftests, so there is no overhead in a running kernel.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dev Jain <[email protected]>
Acked-by: Zi Yan <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Ryan Roberts <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 3 insertions, 0 deletions

diff --git a/lib/xarray.c b/lib/xarray.c
index 76dde3a1cacf..ae3d80f4b4ee 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1910,6 +1910,7 @@ EXPORT_SYMBOL(xa_store_range);
  * @xas: XArray operation state.
  *
  * Called after xas_load, the xas should not be in an error state.
+ * The xas should not be pointing to a sibling entry.
  *
  * Return: A number between 0 and 63 indicating the order of the entry.
  */
@@ -1920,6 +1921,8 @@ int xas_get_order(struct xa_state *xas)
 	if (!xas->xa_node)
 		return 0;
 
+	XA_NODE_BUG_ON(xas->xa_node, xa_is_sibling(xa_entry(xas->xa,
+		       xas->xa_node, xas->xa_offset)));
 	for (;;) {
 		unsigned int slot = xas->xa_offset + (1 << order);