[PATCH v3] lib: xarray: free unused spare node in xas_create_range()

Shardul Bankar
2025-12-01T07:46:17Z

xas_create_range() is typically called in a retry loop that uses
xas_nomem() to handle -ENOMEM errors. xas_nomem() may allocate a spare
xa_node and store it in xas->xa_alloc for use in the retry.

If the lock is dropped after xas_nomem(), another thread can expand the
xarray tree in the meantime. On the next retry, xas_create_range() can
then succeed without consuming the spare node stored in xas->xa_alloc.
If the function returns without freeing this spare node, it leaks.

xas_create_range() calls xas_create() multiple times in a loop for
different index ranges. A spare node that isn't needed for one range
iteration might be needed for the next, so we cannot free it after each
xas_create() call. We can only safely free it after xas_create_range()
completes.

Fix this by calling xas_destroy() at the end of xas_create_range() to
free any unused spare node. This makes the API safer by default and
prevents callers from needing to remember cleanup.

This fixes a memory leak in mm/khugepaged.c and potentially other
callers that use xas_nomem() with xas_create_range().

Link: https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c
Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow")
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
v3:
- Move fix from collapse_file() to xas_create_range() as suggested by Matthew Wilcox
- Fix in library function makes API safer by default, preventing callers from needing
to remember cleanup
- Use shared cleanup label that both restore: and success: paths jump to
- Clean up unused spare node on both success and error exit paths
v2:
- Call xas_destroy() on both success and failure
- Explained retry semantics and xa_alloc / concurrency risk
- Dropped cleanup_empty_nodes from previous proposal
lib/xarray.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/xarray.c b/lib/xarray.c
index 9a8b4916540c..a924421c0c4c 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -744,11 +744,17 @@ void xas_create_range(struct xa_state *xas)
xas->xa_shift = shift;
xas->xa_sibs = sibs;
xas->xa_index = index;
- return;
+ goto cleanup;
+
success:
xas->xa_index = index;
if (xas->xa_node)
xas_set_offset(xas);
+
+cleanup:
+ /* Free any unused spare node from xas_nomem() */
+ if (xas->xa_alloc)
+ xas_destroy(xas);
}
EXPORT_SYMBOL_GPL(xas_create_range);

--
2.34.1