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().

I encountered another memory leak issue in xas_create_range().

collapse_file() calls xas_create_range() to pre-create all slots needed.
If collapse_file() finally fails, these pre-created slots are empty nodes.
When the file is deleted, shmem_evict_inode()->shmem_truncate_range()->shmem_undo_range()
calls xas_store(&xas, NULL) for each entries to delete nodes, but leaving those pre-created
empty nodes leaked.

I can reproduce it with following steps.
1) create file /tmp/test_madvise_collapse and ftruncate to 4MB size, and then mmap the file
2) memset for the first 2MB
3) madvise(MADV_COLLAPSE) for the second 2MB.
4) unlink the file

in 3), collapse_file() calls xas_create_range() to expand xarray depth, and fails to collapse
due to the whole 2M region is empty, leading to the new created empty nodes leaked.

To fix it, maybe we should add a new function xas_delete_range() to revert what xas_create_range()
does when xas_create_range() runs into rollback path?

Link: https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c
Link: https://lore.kernel.org/all/20251201074540.3576327-1-shardul.b@mpiricsoftware.com/ ("v3")
Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow")
Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
 v4:
 - Drop redundant `if (xa_alloc)` around xas_destroy(), as xas_destroy()
   already checks xa_alloc internally.
 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 | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/lib/xarray.c b/lib/xarray.c
index 9a8b4916540c..f49ccfa5f57d 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -744,11 +744,16 @@ 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() */
+	xas_destroy(xas);
 }
 EXPORT_SYMBOL_GPL(xas_create_range);