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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@vger.kernel.org > Signed-off-by: Chris Mason > [ david: split fixes, simplify descriptions, adjust comments ] > Signed-off-by: David Hildenbrand (Arm) > --- > 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=3D[0, 512), B=3D[512, 1024), C=3D[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=3D1024. 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, lo= ff_t lstart, uoff_t lend, index =3D 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 =3D max(start, folio_next_index(folio)); } } folio_unlock(folio); --=20 Best Regards, Yan, Zi