linux-kernel - Re: [RFC PATCH] mm, oom: allow oom reaper to race with exit

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Date:   Sun, 23 Jul 2017 23:39:15 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <andrea@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        David Rientjes <rientjes@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [RFC PATCH] mm, oom: allow oom reaper to race with exit_mmap

On Thu, 20 Jul 2017, Michal Hocko wrote:
> On Wed 19-07-17 18:18:27, Hugh Dickins wrote:
> > 
> > But I haven't looked at the oom_kill or oom_reaper end of it at all,
> > perhaps you have an overriding argument on the placement from that end.
> 
> Well, the main problem here is that the oom_reaper tries to
> MADV_DONTNEED the oom victim and then hide it from the oom killer (by
> setting MMF_OOM_SKIP) to guarantee a forward progress. In order to do
> that it needs mmap_sem for read. Currently we try to avoid races with
> the eixt path by checking mm->mm_users and that can lead to premature
> MMF_OOM_SKIP and that in turn to additional oom victim(s) selection
> while the current one is still tearing the address space down.
> 
> One way around that is to allow final unmap race with the oom_reaper
> tear down.
> 
> I hope this clarify the motivation

Thanks, yes, if you have a good reason of that kind, then I agree that
it's appropriate to leave the down_write(mmap_sem) until reaching the
free_pgtables() stage.

Hugh