Re: [RFC]: userspace memory reaping

From: Suren Baghdasaryan <surenb@google.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>,
	linux-api@vger.kernel.org, linux-mm <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.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>,
	kernel-team <kernel-team@android.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [RFC]: userspace memory reaping
Date: Thu, 5 Nov 2020 08:50:58 -0800	[thread overview]
Message-ID: <CAJuCfpF5zAif97-uK8M+-fJhd0pab4fMPDMUNkAXYOB3MC7aXg@mail.gmail.com> (raw)
In-Reply-To: <20201105122012.GD21348@dhcp22.suse.cz>

On Thu, Nov 5, 2020 at 4:20 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 04-11-20 12:40:51, Minchan Kim wrote:
> > On Wed, Nov 04, 2020 at 07:58:44AM +0100, Michal Hocko wrote:
> > > On Tue 03-11-20 13:32:28, Minchan Kim wrote:
> > > > On Tue, Nov 03, 2020 at 10:35:50AM +0100, Michal Hocko wrote:
> > > > > On Mon 02-11-20 12:29:24, Suren Baghdasaryan wrote:
> > > > > [...]
> > > > > > To follow up on this. Should I post an RFC implementing SIGKILL_SYNC
> > > > > > which in addition to sending a kill signal would also reap the
> > > > > > victim's mm in the context of the caller? Maybe having some code will
> > > > > > get the discussion moving forward?
> > > > >
> > > > > Yeah, having a code, even preliminary, might help here. This definitely
> > > > > needs a good to go from process management people as that proper is land
> > > > > full of surprises...
> > > >
> > > > Just to remind a idea I suggested to reuse existing concept
> > > >
> > > >     fd = pidfd_open(victim process)
> > > >     fdatasync(fd);
> > > >     close(fd);
> > >
> > > I must have missed this proposal. Anyway, are you suggesting fdatasync
> > > to act as a destructive operation?
> >
> > write(fd) && fdatasync(fd) are already destructive operation if the file
> > is shared.
>
> I am likely missing something because fdatasync will not destroy any
> underlying data. It will sync
>
> > You don't need to reaping as destruptive operation. Rather than, just
> > commit on the asynchrnous status "write file into page cache and commit
> > with fsync" and "killing process and commit with fsync".
>
> I am sorry but I do not follow. The result of the memory reaping is a
> data loss. Any private mapping will simply lose it's content. The caller
> will get EFAULT when trying to access it but there is no way to
> reconstruct the data. This is everything but not resembling what I see
> f{data}sync is used for.

I think Minchan considers f{data}sync as a "commit" operation. So
write+f{data}sync would mean we write and commit written data,
kill+f{data}sync would mean we kill and commit that kill (reclaim the
resources).

>
> --
> Michal Hocko
> SUSE Labs