* Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race path
2024-01-25 8:51 [PATCH] mm: zswap: fix missing folio cleanup in writeback race path Yosry Ahmed
@ 2024-01-25 8:54 ` Chengming Zhou
2024-01-25 14:49 ` Johannes Weiner
2024-01-25 18:29 ` Nhat Pham
2 siblings, 0 replies; 4+ messages in thread
From: Chengming Zhou @ 2024-01-25 8:54 UTC (permalink / raw)
To: Yosry Ahmed, Andrew Morton
Cc: Johannes Weiner, Nhat Pham, Domenico Cerasuolo, linux-mm,
linux-kernel, stable
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);
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race path
2024-01-25 8:51 [PATCH] mm: zswap: fix missing folio cleanup in writeback race path Yosry Ahmed
2024-01-25 8:54 ` Chengming Zhou
@ 2024-01-25 14:49 ` Johannes Weiner
2024-01-25 18:29 ` Nhat Pham
2 siblings, 0 replies; 4+ messages in thread
From: Johannes Weiner @ 2024-01-25 14:49 UTC (permalink / raw)
To: Yosry Ahmed
Cc: Andrew Morton, Nhat Pham, Chengming Zhou, Domenico Cerasuolo,
linux-mm, linux-kernel, stable
On Thu, Jan 25, 2024 at 08:51:27AM +0000, 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>
Ouch, good catch.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race path
2024-01-25 8:51 [PATCH] mm: zswap: fix missing folio cleanup in writeback race path Yosry Ahmed
2024-01-25 8:54 ` Chengming Zhou
2024-01-25 14:49 ` Johannes Weiner
@ 2024-01-25 18:29 ` Nhat Pham
2 siblings, 0 replies; 4+ messages in thread
From: Nhat Pham @ 2024-01-25 18:29 UTC (permalink / raw)
To: Yosry Ahmed
Cc: Andrew Morton, Johannes Weiner, Chengming Zhou,
Domenico Cerasuolo, linux-mm, linux-kernel, stable
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>
^ permalink raw reply [flat|nested] 4+ messages in thread