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Date:   Mon, 21 Sep 2020 08:55:15 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     zangchunxin@...edance.com
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, kafai@...com,
        Song Liu <songliubraving@...com>, yhs@...com, andriin@...com,
        john.fastabend@...il.com, kpsingh@...omium.org,
        Cgroups <cgroups@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org
Subject: Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2

On Mon, Sep 21, 2020 at 1:05 AM <zangchunxin@...edance.com> wrote:
>
> From: Chunxin Zang <zangchunxin@...edance.com>
>
> In the cgroup v1, we have 'force_mepty' interface. This is very
> useful for userspace to actively release memory. But the cgroup
> v2 does not.
>
> This patch reuse cgroup v1's function, but have a new name for
> the interface. Because I think 'drop_cache' may be is easier to
> understand :)
>
> Signed-off-by: Chunxin Zang <zangchunxin@...edance.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 11 +++++++++++
>  mm/memcontrol.c                         |  5 +++++
>  2 files changed, 16 insertions(+)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index ce3e05e41724..fbff959c8116 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1181,6 +1181,17 @@ PAGE_SIZE multiple when read back.
>         high limit is used and monitored properly, this limit's
>         utility is limited to providing the final safety net.
>
> +  memory.drop_cache
> +    A write-only single value file which exists on non-root
> +    cgroups.
> +
> +    Provide a mechanism for users to actively trigger memory
> +    reclaim. The cgroup will be reclaimed and as many pages
> +    reclaimed as possible.
> +
> +    It will broke low boundary. Because it tries to reclaim the
> +    memory many times, until the memory drops to a certain level.
> +

drop_cache is not really force_empty(). What is your use-case? Maybe
you can use memory.reclaim [1] for your use-case. It is already in
Andrew's tree.

[1] https://lkml.kernel.org/r/20200909215752.1725525-1-shakeelb@google.com

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