linux-kernel - Re: [RFC]: userspace memory reaping

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Date:   Thu, 5 Nov 2020 18:16:11 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Minchan Kim <minchan@...nel.org>, linux-api@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Christian Brauner <christian@...uner.io>,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Murray <timmurray@...gle.com>,
        kernel-team <kernel-team@...roid.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [RFC]: userspace memory reaping

On Thu 05-11-20 08:50:58, Suren Baghdasaryan wrote:
> On Thu, Nov 5, 2020 at 4:20 AM Michal Hocko <mhocko@...e.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.

But there is nothing like commit in that operation. It is simply a
destroy operation. ftruncate as Minchan mentions in another reply would
be a closer fit but how do you interpret the length argument? What about
memory regions which cannot be reaped?

I do understand that reusing an existing mechanism is usually preferable
but the semantic should be reasonable and easy to reason about.

-- 
Michal Hocko
SUSE Labs