Re: [PATCH 2/3] mm: memcontrol: do not try to do swap when force empty

[Shakeel Butt <shakeelb@google.com>]

On Thu, Jan 3, 2019 at 8:57 AM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>
>
> On 1/2/19 1:45 PM, Shakeel Butt wrote:
> > On Wed, Jan 2, 2019 at 12:06 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
> >> The typical usecase of force empty is to try to reclaim as much as
> >> possible memory before offlining a memcg.  Since there should be no
> >> attached tasks to offlining memcg, the tasks anonymous pages would have
> >> already been freed or uncharged.
> > Anon pages can come from tmpfs files as well.
>
> Yes, but they are charged to swap space as regular anon pages.
>

The point was the lifetime of tmpfs anon pages are not tied to any
task. Even though there aren't any task attached to a memcg, the tmpfs
anon pages will remain charged. Other than that, the old anon pages of
a task which have migrated away might still be charged to the old
memcg (if move_charge_at_immigrate is not set).

> >
> >> Even though anonymous pages get
> >> swapped out, but they still get charged to swap space.  So, it sounds
> >> pointless to do swap for force empty.
> >>
> > I understand that force_empty is typically used before rmdir'ing a
> > memcg but it might be used differently by some users. We use this
> > interface to test memory reclaim behavior (anon and file).
>
> Thanks for sharing your usecase. So, you uses this for test only?
>

Yes.

Shakeel