Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race path

[Nhat Pham <nphamcs@gmail.com>]

On Thu, Jan 25, 2024 at 12:51 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> In zswap_writeback_entry(), after we get a folio from
> __read_swap_cache_async(), we grab the tree lock again to check that the
> swap entry was not invalidated and recycled. If it was, we delete the
> folio we just added to the swap cache and exit.
>
> However, __read_swap_cache_async() returns the folio locked when it is
> newly allocated, which is always true for this path, and the folio is
> ref'd. Make sure to unlock and put the folio before returning.
>
> This was discovered by code inspection, probably because this path
> handles a race condition that should not happen often, and the bug would
> not crash the system, it will only strand the folio indefinitely.
>
> Fixes: 04fc7816089c ("mm: fix zswap writeback race condition")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  mm/zswap.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 8f4a7efc2bdae..00e90b9b5417d 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1448,6 +1448,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>         if (zswap_rb_search(&tree->rbroot, swp_offset(entry->swpentry)) != entry) {
>                 spin_unlock(&tree->lock);
>                 delete_from_swap_cache(folio);
> +               folio_unlock(folio);
> +               folio_put(folio);
>                 return -ENOMEM;
>         }
>         spin_unlock(&tree->lock);
> --
> 2.43.0.429.g432eaa2c6b-goog
>

Oof. Yeah this is probably rare IRL (that looks like a very specific
race condition), and the symptoms are rather subtle (no kernel crash).
LGTM.
Reviewed-by: Nhat Pham <nphamcs@gmail.com>