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

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Date:   Thu, 3 Jan 2019 09:03:58 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] mm: memcontrol: do not try to do swap when force empty

On Thu, Jan 3, 2019 at 8:57 AM Yang Shi <yang.shi@...ux.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@...ux.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