Re: [RFC PATCH] mm/khugepaged: free empty xa_nodes when rollbacks in collapse_file

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jan 21, 2026 at 02:22:43PM +0800, Jinjiang Tu wrote:
> collapse_file() calls xas_create_range() to pre-create all slots needed.
> If collapse_file() finally fails, these pre-created slots are empty nodes
> and aren't destroyed.

try this instead

diff --git a/fs/inode.c b/fs/inode.c
index cff1d3af0d57..85886af1e7ab 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -744,22 +744,18 @@ void dump_mapping(const struct address_space *mapping)
 
 void clear_inode(struct inode *inode)
 {
-	/*
-	 * We have to cycle the i_pages lock here because reclaim can be in the
-	 * process of removing the last page (in __filemap_remove_folio())
-	 * and we must not free the mapping under it.
-	 */
-	xa_lock_irq(&inode->i_data.i_pages);
-	BUG_ON(inode->i_data.nrpages);
 	/*
 	 * Almost always, mapping_empty(&inode->i_data) here; but there are
 	 * two known and long-standing ways in which nodes may get left behind
 	 * (when deep radix-tree node allocation failed partway; or when THP
-	 * collapse_file() failed). Until those two known cases are cleaned up,
-	 * or a cleanup function is called here, do not BUG_ON(!mapping_empty),
-	 * nor even WARN_ON(!mapping_empty).
+	 * collapse_file() failed).
+	 *
+	 * xa_destroy() also cycles the lock for us, which is needed because
+	 * reclaim can be in the process of removing the last folio (in
+	 * __filemap_remove_folio()) and we must not free the mapping under it.
 	 */
-	xa_unlock_irq(&inode->i_data.i_pages);
+	xa_destroy(&inode->i_data.i_pages);
+	BUG_ON(inode->i_data.nrpages);
 	BUG_ON(!list_empty(&inode->i_data.i_private_list));
 	BUG_ON(!(inode->i_state & I_FREEING));
 	BUG_ON(inode->i_state & I_CLEAR);