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From:   Shakeel Butt <shakeelb@google.com>
Date:   Mon, 21 Sep 2020 08:55:15 -0700
Message-ID: <CALvZod5JpEsmNvbaCMEfRMt83GbeLJk2oM0=siCj+aXEnxYh4Q@mail.gmail.com>
Subject: Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2
To:     zangchunxin@bytedance.com
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, kafai@fb.com,
        Song Liu <songliubraving@fb.com>, yhs@fb.com, andriin@fb.com,
        john.fastabend@gmail.com, kpsingh@chromium.org,
        Cgroups <cgroups@vger.kernel.org>, linux-doc@vger.kernel.org,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>, bpf@vger.kernel.org
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On Mon, Sep 21, 2020 at 1:05 AM <zangchunxin@bytedance.com> wrote:
>
> From: Chunxin Zang <zangchunxin@bytedance.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@bytedance.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