Re: [PATCH v2] mm: vmscan: reclaim anon pages if there are swapcache pages

[Yosry Ahmed]

On Wed, Aug 23, 2023 at 6:12 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 23-08-23 10:00:58, Liu Shixin wrote:
> >
> >
> > On 2023/8/23 0:35, Yosry Ahmed wrote:
> > > On Mon, Aug 21, 2023 at 6:54 PM 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.
> > >>
> > >> This problem can be fixed by checking number of swapcache pages in
> > >> can_reclaim_anon_pages(). For memcg v2, there are swapcache stat that can
> > >> be used directly. For memcg v1, use total_swapcache_pages() instead, which
> > >> may not accurate but can solve the problem.
> > > Interesting find. I wonder if we really don't have any handling of
> > > this situation.
> > I have alreadly test this problem and can confirm that it is a real problem.
> > With 9MB swap space and 10MB mem_cgroup limit，when allocate 15MB memory,
> > there is a probability that OOM occurs.
>
> Could you be more specific about the test and the oom report?

I actually couldn't reproduce it using 9MB of zram and a cgroup with a
10MB limit trying to allocate 15MB of tmpfs, no matter how many
repetitions I do.

>
> --
> Michal Hocko
> SUSE Labs