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

From: Andrew Morton <akpm@linux-foundation.org>
To: Suren Baghdasaryan <surenb@google.com>
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 17:18:10 -0800	[thread overview]
Message-ID: <20201113171810.bebf66608b145cced85bf54c@linux-foundation.org> (raw)
In-Reply-To: <CAJuCfpHS3hZi-E=JCp257u0AG+RoMAG4kLa3NQydONGfp9oXQQ@mail.gmail.com>

On Fri, 13 Nov 2020 17:09:37 -0800 Suren Baghdasaryan <surenb@google.com> wrote:

> > > > 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.

Why is the `vector=NULL' needed?  Can't `vector' point at a single iovec
which spans the whole address range?

> If that's the
> preferred method then I can post it quite quickly.

I assume you've tested that prototype.  How did its usefulness compare
with this SIGKILL-based approach?