Re: [PATCH v6] mm: vmscan: try to reclaim swapcache pages if no swap space

[Liu Shixin]

On 2023/10/5 0:18, Andrew Morton wrote:
> On Fri, 15 Sep 2023 16:34:17 +0800 Liu Shixin <liushixin2@xxxxxxxxxx> 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].
>>
>> By the way, since there are enough times of memory reclaim before OOM, it
>> is not need to isolate too much swapcache pages in one times.
>>
> mhocko earlier suspected this might impact global reclaim.  Have you
> looked into that further?
> .
Alreadly test the case of global reclaim using ltp testcases. In previous version,
the reclaim can stall in isolate_lru_folios() because I didn't count non-swapcache
pages into scan so it will waste too much times scanning non-swapcache pages.
In this version, I count non-swapcache pages into scan too and terminate when
scan reaches the threshold. So it will not stall in reclaim now.