Re: [PATCH v3 1/2] mm/zswap: Fix global shrinker when memory cgroup is disabled

[Andrew Morton]

On Wed, 29 Jul 2026 16:42:05 +0800 Hao Jia <jiahao.kernel@xxxxxxxxx> wrote:

> Zswap writeback when the global pool limit is hit fails when memory
> cgroup is disabled. The pool remains full until it is organically
> drained by swapins or memory freeing, leading to zswap store failures
> and pages bypassing getting written directly to the backing swap device,
> causing LRU inversion (hotter pages with higher fault latency).
> 
> This happens because mem_cgroup_iter() always returns NULL when
> memory cgroups are disabled. As a result, the global shrinker
> shrink_worker() repeatedly takes empty walks. After MAX_RECLAIM_RETRIES
> failed attempts, the worker gives up without writing back any pages.
> 
> 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@xxxxxxxxxxxxxxx

How does this affect users?  What behavior do they observe when it
occurs?

> Closes: https://lore.kernel.org/all/CAO9r8zPVzMKFbCixxD-qgtRrkFxWVrHiZZeLc=eyTPKPVQgX4g@xxxxxxxxxxxxxx

hm, that isn't really a bug report and doesn't answer the above
question.


AI review asked a couple of questions:
	https://sashiko.dev/#/patchset/20260729084206.77793-1-jiahao.kernel@xxxxxxxxx

Thanks.