[PATCH] mm: khugepaged: fix memory leak in collapse_file rollback path

[Shardul Bankar <shardul.b@mpiricsoftware.com>]

When collapse_file() fails after xas_create_range() succeeds, the
rollback path does not clean up pre-allocated XArray nodes stored in
xas->xa_alloc. These nodes are allocated by xas_nomem() when
xas_create() fails with GFP_NOWAIT and need to be freed.

The leak occurs because:
1. xas_create_range() may call xas_nomem() which allocates a node
   and stores it in xas->xa_alloc
2. If the collapse operation fails later, the rollback path jumps
   to the 'rollback:' label
3. The rollback path cleans up folios but does not call xas_destroy()
   to free the pre-allocated nodes in xas->xa_alloc

Fix this by calling xas_destroy(&xas) at the beginning of the rollback
path to free any pre-allocated nodes. This is safe because xas_destroy()
only frees nodes in xas->xa_alloc that were never inserted into the
XArray tree.

Note: This fixes the leak of pre-allocated nodes. A separate fix will
be needed to clean up empty nodes that were inserted into the tree by
xas_create_range() but never populated.

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>
---
 mm/khugepaged.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index abe54f0043c7..f2fe7924afa0 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2230,6 +2230,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	goto out;
 
 rollback:
+	xas_destroy(&xas);
 	/* Something went wrong: roll back page cache changes */
 	if (nr_none) {
 		xas_lock_irq(&xas);
-- 
2.34.1