Re: [PATCH 10/16] mm/shmem: discard whole new large folios on fallocate rollback

[Zi Yan]

On Mon Oct 26, 2026 at 5:40 AM EDT, David Hildenbrand (Arm) wrote:
> When undoing a failed fallocate(), shmem_undo_range() must discard each
> newly allocated folio. Its second scan instead passes large
> folios to truncate_inode_partial_folio(), which does not necessarily
> discard the entire large folio, but instead tries splitting it to
> discard only the requested range.
>
> For example, with huge=always, a failed allocation starting at offset
> 4 KiB can leave page 0 allocated in an otherwise empty file.
>
> Discard the whole folio when unfalloc is set. Note that we enter this
> path only for folios that are not uptodate, and can consequently be
> dropped without losing any data.
>
> Fixes: 55ac8bbe358b ("mm/shmem: fix race in shmem_undo_range w/THP")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---
>  mm/shmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Rebase it on top of mm-stable:

diff --git a/mm/shmem.c b/mm/shmem.c
index e9f98d526fc52..7cf5a4d9d1ace 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1472,7 +1472,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 				if (!unfalloc && folio_test_large(folio))
 					folio_mark_dirty(folio);
 
-				if (!folio_test_large(folio)) {
+				if (unfalloc || !folio_test_large(folio)) {
 					truncate_inode_folio(mapping, folio);
 				} else if (truncate_inode_partial_folio(folio,
 						lstart, lend, NULL, NULL)) {



-- 
Best Regards,
Yan, Zi