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/20250604041533.91198-1-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/lib/xarray.c b/lib/xarray.c
index 76dde3a1cacf5f17a2c4d9141e17bb82a2a9b2e8..ae3d80f4b4ee3804ef1bed668bacf69a57167986 100644 (file)
--- 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);