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

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Date: Thu, 25 Jan 2024 09:49:52 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Nhat Pham <nphamcs@...il.com>,
	Chengming Zhou <zhouchengming@...edance.com>,
	Domenico Cerasuolo <cerasuolodomenico@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race
 path

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@...r.kernel.org
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

Ouch, good catch.

Acked-by: Johannes Weiner <hannes@...xchg.org>