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

From: Suren Baghdasaryan <surenb@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
	Christian Brauner <christian@brauner.io>,
	Oleg Nesterov <oleg@redhat.com>,
	Tim Murray <timmurray@google.com>,
	linux-api@vger.kernel.org, linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>,
	Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH 1/1] RFC: add pidfd_send_signal flag to reclaim mm while killing a process
Date: Fri, 13 Nov 2020 17:09:37 -0800	[thread overview]
Message-ID: <CAJuCfpHS3hZi-E=JCp257u0AG+RoMAG4kLa3NQydONGfp9oXQQ@mail.gmail.com> (raw)
In-Reply-To: <20201113170032.7aa56ea273c900f97e6ccbdc@linux-foundation.org>

On Fri, Nov 13, 2020 at 5:00 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 13 Nov 2020 16:06:25 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
>
> > On Fri, Nov 13, 2020 at 3:55 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Fri, 13 Nov 2020 09:34:48 -0800 Suren Baghdasaryan <surenb@google.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.
> > >
> > > And we do have the new process_madvise(MADV_PAGEOUT).  It may need a
> > > few changes and tweaks, but can't that be used to solve this problem?
> >
> > Thank you for the feedback, Andrew. process_madvise(MADV_DONTNEED) was
> > one of the options recently discussed in
> > https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@mail.gmail.com
> > . The thread describes some of the issues with that approach but if we
> > limit it to processes with pending SIGKILL only then I think that
> > would be doable.
>
> Why would it be necessary to read /proc/pid/maps?  I'd have thought
> that a starting effort would be
>
>         madvise((void *)0, (void *)-1, MADV_PAGEOUT)
>
> (after translation into process_madvise() speak).  Which is equivalent
> to the proposed process_madvise(MADV_DONTNEED_MM)?

Yep, this is very similar to option #3 in
https://lore.kernel.org/linux-api/CAJuCfpGz1kPM3G1gZH+09Z7aoWKg05QSAMMisJ7H5MdmRrRhNQ@mail.gmail.com
and I actually have a tested prototype for that. If that's the
preferred method then I can post it quite quickly.

>
> There may be things which trip this up, such as mlocked regions or
> whatever, but we could add another madvise `advice' mode to handle
> this?