Re: [PATCH 08/16] mm/shmem: stop retrying folios that cannot be split

[Zi Yan]

On Mon Oct 26, 2026 at 5:40 AM EDT, David Hildenbrand (Arm) wrote:
> From: Chris Mason <mason@xxxxxxxxxx>
>
> A fault or collapse can insert a large folio crossing EOF after
> shmem_undo_range() has handled the boundary folios. If splitting that
> folio fails, truncate_inode_partial_folio() retains it when it is dirty.
> The final scan then repeatedly finds it because it restarts from an
> unchanged start.
>
> Truncation keeps the inode lock and ignores fatal signals while looping.
> A concurrent unlink() can then block other directory operations while
> holding the directory lock and waiting for the file's lock.
>
> Advance start past the retained folio, whose truncated part has already
> been zeroed. Do not move start backwards if the failed split has shrunk
> the folio.
>
> Fix generated by an LLM and sent by Chris Mason in private.
>
> Fixes: 55ac8bbe358b ("mm/shmem: fix race in shmem_undo_range w/THP")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chris Mason <mason@xxxxxxxxxx>
> [ david: split fixes, simplify descriptions, adjust comments ]
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---
>  mm/shmem.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>

I rebased your patch on mm-stable, which has Zhang Yi's fix, and
1. tweaked the comment,
2. skipped the dropped folio (this is to avoid a narrow case below).

Since after Zhang Yi's patchset, truncate_inode_partial_folio() is
changed to return false when it drops the entire folio. Restetting
start for a dropped folio in the scenario below would leave zeroed
folios still in the memory, causing a waste, although it is very narrow:

folio A=[0, 512), B=[512, 1024), C=[1024, 1536],

1. A, B and, C are inserted after the same_folio block check, so only the
   code at whole_folios label sees all of them,
2. A is split partially and the split failed, A becomes [0, 32) and
   [32, 512) zeroed,
   truncate_inode_partial_folio() returns false, start is set to 32,
3. B is dropped entirely, false is returned, start is set to 1024,
4. C is split successfully

After 4, index is start=1024. The [32, 512) folio is not dropped. That
is why I added the folio->mapping check to avoid changing start in 3.



diff --git a/mm/shmem.c b/mm/shmem.c
index fadf23f9e21ed..efd54a624974c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1480,6 +1480,16 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 						index = start;
 						break;
 					}
+				} else {
+					/*
+					 * Splitting failed, but the part in the
+					 * range is already zeroed. If the folio
+					 * was not dropped, skip it when
+					 * restarting the scan to avoid retrying
+					 * it forever.
+					 */
+					if (folio->mapping)
+						start = max(start, folio_next_index(folio));
 				}
 			}
 			folio_unlock(folio);


-- 
Best Regards,
Yan, Zi