Re: [PATCH] mm/shmem: Fix race in shmem_undo_range w/THP

[David Stevens]

On Tue, Apr 18, 2023 at 3:22 PM David Stevens <stevensd@xxxxxxxxxxxx> wrote:
>
> From: David Stevens <stevensd@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
> ---
>  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
>