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.
