Date: Wed, 22 Jul 2026 22:13:29 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Hao Jia <jiahao.kernel@gmail.com>
Cc: akpm@linux-foundation.org, tj@kernel.org, shakeel.butt@linux.dev,
	mhocko@kernel.org, yosry@kernel.org, mkoutny@suse.com,
	nphamcs@gmail.com, chengming.zhou@linux.dev, muchun.song@linux.dev,
	roman.gushchin@linux.dev, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	Hao Jia <jiahao1@lixiang.com>, stable@vger.kernel.org
Subject: Re: [PATCH v2 1/2] mm/zswap: Fix global shrinker when memory cgroup
 is disabled
Message-ID: <amF4yRobGT9x67y3@cmpxchg.org>
References: <20260717085151.22822-1-jiahao.kernel@gmail.com>
 <20260717085151.22822-2-jiahao.kernel@gmail.com>
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In-Reply-To: <20260717085151.22822-2-jiahao.kernel@gmail.com>

On Fri, Jul 17, 2026 at 04:51:50PM +0800, Hao Jia wrote:
> From: Hao Jia <jiahao1@lixiang.com>
> 
> Zswap writeback on hitting the pool limit is broken when memory cgroup
> is disabled, because mem_cgroup_iter() always returns NULL. Therefore,
> the global shrinker shrink_worker() always takes the !memcg branch.
> After MAX_RECLAIM_RETRIES empty walks, the worker simply gives up, so it
> fails to write back anything.
> 
> Therefore, when memory cgroup is disabled, fall through with the !memcg
> branch and shrink the root memcg directly.
> 
> With memcg disabled, shrink_memcg() only returns -ENOENT when the root
> LRU is empty, which means the total pages are already below thr. In the
> absence of heavy concurrent zswap stores, the loop then safely bails out
> via the zswap_total_pages() <= thr check; otherwise, it will resume
> shrinking the memcg after processing the reschedule check. For any other
> return value from shrink_memcg(), the loop is guaranteed to terminate,
> either after MAX_RECLAIM_RETRIES failures or once the threshold is met.
> 
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Cc: stable@vger.kernel.org
> Suggested-by: Nhat Pham <nphamcs@gmail.com>
> Acked-by: Nhat Pham <nphamcs@gmail.com>
> Acked-by: Yosry Ahmed <yosry@kernel.org>
> Reported-by: Yosry Ahmed <yosry@kernel.org>
> Closes: https://lore.kernel.org/all/CAO9r8zPVzMKFbCixxD-qgtRrkFxWVrHiZZeLc=eyTPKPVQgX4g@mail.gmail.com
> Signed-off-by: Hao Jia <jiahao1@lixiang.com>

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