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

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Message-ID: <20201105122012.GD21348@dhcp22.suse.cz>
Date:   Thu, 5 Nov 2020 13:20:12 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, 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 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.

-- 
Michal Hocko
SUSE Labs