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From:   David Stevens <stevensd@chromium.org>
Date:   Tue, 18 Apr 2023 17:13:02 +0900
Message-ID: <CAD=HUj5fhD3kcav5mbJ8Zegin-Bpb1cYChNzTUDLzUE5kbv9Ew@mail.gmail.com>
Subject: Re: [PATCH] mm/shmem: Fix race in shmem_undo_range w/THP
To:     linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Suleiman Souhlal <suleiman@google.com>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
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On Tue, Apr 18, 2023 at 3:22=E2=80=AFPM David Stevens <stevensd@chromium.or=
g> 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 f=
olios")
> 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, l=
off_t lstart, loff_t lend,
>                                 }
>                                 VM_BUG_ON_FOLIO(folio_test_writeback(foli=
o),
>                                                 folio);
> -                               truncate_inode_folio(mapping, folio);
> +                               truncate_inode_partial_folio(folio, lstar=
t, 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
>