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

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2024/1/25 16:51, Yosry Ahmed 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>

LGTM, thanks!

Reviewed-by: Chengming Zhou <zhouchengming@bytedance.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);