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Date: Thu, 25 Jan 2024 09:49:52 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Nhat Pham <nphamcs@gmail.com>,
	Chengming Zhou <zhouchengming@bytedance.com>,
	Domenico Cerasuolo <cerasuolodomenico@gmail.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race
 path
Message-ID: <20240125144952.GE1567330@cmpxchg.org>
References: <20240125085127.1327013-1-yosryahmed@google.com>
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In-Reply-To: <20240125085127.1327013-1-yosryahmed@google.com>

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>