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

From: Shakeel Butt
Date: Thu Jan 03 2019 - 12:04:15 EST

On Thu, Jan 3, 2019 at 8:57 AM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
> On 1/2/19 1:45 PM, Shakeel Butt wrote:
> > On Wed, Jan 2, 2019 at 12:06 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> 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?