linux-kernel - Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space

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Date:   Tue, 21 Nov 2023 14:00:21 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Liu Shixin <liushixin2@...wei.com>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Huang Ying <ying.huang@...el.com>,
        Sachin Sant <sachinp@...ux.ibm.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no
 swap space

On Tue 21-11-23 17:06:24, Liu Shixin wrote:
> When spaces of swap devices are exhausted, only file pages can be
> reclaimed.  But there are still some swapcache pages in anon lru list.
> This can lead to a premature out-of-memory.
> 
> The problem is found with such step:
> 
>  Firstly, set a 9MB disk swap space, then create a cgroup with 10MB
>  memory limit, then runs an program to allocates about 15MB memory.
> 
> The problem occurs occasionally, which may need about 100 times [1].
> 
> Fix it by checking number of swapcache pages in can_reclaim_anon_pages().
> If the number is not zero, return true and set swapcache_only to 1.
> When scan anon lru list in swapcache_only mode, non-swapcache pages will
> be skipped to isolate in order to accelerate reclaim efficiency.
> 
> However, in swapcache_only mode, the scan count still increased when scan
> non-swapcache pages because there are large number of non-swapcache pages
> and rare swapcache pages in swapcache_only mode, and if the non-swapcache
> is skipped and do not count, the scan of pages in isolate_lru_folios() can
> eventually lead to hung task, just as Sachin reported [2].

I find this paragraph really confusing! I guess what you meant to say is
that a real swapcache_only is problematic because it can end up not
making any progress, correct? 

AFAIU you have addressed that problem by making swapcache_only anon LRU
specific, right? That would be certainly more robust as you can still
reclaim from file LRUs. I cannot say I like that because swapcache_only
is a bit confusing and I do not think we want to grow more special
purpose reclaim types. Would it be possible/reasonable to instead put
swapcache pages on the file LRU instead?
-- 
Michal Hocko
SUSE Labs