Re: [RFC PATCH] mm, oom: allow oom reaper to race with exit_mmap

From: Hugh Dickins <hughd@google.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <andrea@kernel.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	David Rientjes <rientjes@google.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org
Subject: Re: [RFC PATCH] mm, oom: allow oom reaper to race with exit_mmap
Date: Sun, 23 Jul 2017 23:39:15 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LSU.2.11.1707232331250.2154@eggly.anvils> (raw)
In-Reply-To: <20170720130541.GH9058@dhcp22.suse.cz>

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

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