linux-kernel - Re: [PATCH 1/1] RFC: add pidfd_send_signal flag to reclaim mm while killing a process

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Date:   Wed, 18 Nov 2020 11:32:40 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, mhocko@...nel.org,
        rientjes@...gle.com, willy@...radead.org, hannes@...xchg.org,
        guro@...com, riel@...riel.com, minchan@...nel.org,
        christian@...uner.io, oleg@...hat.com, timmurray@...gle.com,
        linux-api@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/1] RFC: add pidfd_send_signal flag to reclaim mm while
 killing a process

On Fri, Nov 13, 2020 at 03:55:39PM -0800, Andrew Morton wrote:
> On Fri, 13 Nov 2020 09:34:48 -0800 Suren Baghdasaryan <surenb@...gle.com> wrote:
> 
> > When a process is being killed it might be in an uninterruptible sleep
> > which leads to an unpredictable delay in its memory reclaim. In low memory
> > situations, when it's important to free up memory quickly, such delay is
> > problematic. Kernel solves this problem with oom-reaper thread which
> > performs memory reclaim even when the victim process is not runnable.
> > Userspace currently lacks such mechanisms and the need and potential
> > solutions were discussed before (see links below).
> > This patch provides a mechanism to perform memory reclaim in the context
> > of the process that sends SIGKILL signal. New SYNC_REAP_MM flag for
> > pidfd_send_signal syscall can be used only when sending SIGKILL signal
> > and will lead to the caller synchronously reclaiming the memory that
> > belongs to the victim and can be easily reclaimed.
> 
> hm.
> 
> Seems to me that the ability to reap another process's memory is a
> generally useful one, and that it should not be tied to delivering a
> signal in this fashion.

I agree and I see you've already had some good ideas how to tie this to
process_madvise(). If that's workable for your use-case then I'd prefer
that approach. Signals are almost always not a great choice.

Christian