On Tue, Apr 18, 2023 at 3:22 PM David Stevens stevensd@chromium.org wrote:

From: David Stevens stevensd@chromium.org

Split folios during the second loop of shmem_undo_range. It's not sufficient to only split folios when dealing with partial pages, since it's possible for a THP to be faulted in after that point. Calling truncate_inode_folio in that situation can result in throwing away data outside of the range being targeted.

Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios") Cc: stable@vger.kernel.org Signed-off-by: David Stevens stevensd@chromium.org

---

mm/shmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c index 9218c955f482..317cbeb0fb6b 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1033,7 +1033,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend, } VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);

•                           truncate_inode_folio(mapping, folio);
  

•                           truncate_inode_partial_folio(folio, lstart, lend);
  

It was pointed out to me that truncate_inode_partial_folio only sometimes frees the target pages. So this patch does fix the data loss, but it ends up making partial hole punches on a THP not actually free memory. I'll send out a v2 that properly calls truncate_inode_folio after splitting a THP.

-David

                    }
                    folio_unlock(folio);
            }

-- 2.40.0.634.g4ca3ef3211-goog