linux-kernel - Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

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Date:   Thu, 19 Apr 2018 14:48:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     rientjes@...gle.com, akpm@...ux-foundation.org,
        aarcange@...hat.com, guro@...com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

On Thu 19-04-18 20:51:45, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > > We need to teach the OOM reaper stop reaping as soon as entering exit_mmap().
> > > Maybe let the OOM reaper poll for progress (e.g. none of get_mm_counter(mm, *)
> > > decreased for last 1 second) ?
> > 
> > Can we start simple and build a more elaborate heuristics on top _please_?
> > In other words holding the mmap_sem for write for oom victims in
> > exit_mmap should handle the problem. We can then enhance this to probe
> > for progress or any other clever tricks if we find out that the race
> > happens too often and we kill more than necessary.
> > 
> > Let's not repeat the error of trying to be too clever from the beginning
> > as we did previously. This are is just too subtle and obviously error
> > prone.
> > 
> Something like this?

Not really. This is still building on the tricky locking protocol we
have and proven to be error prone.  Can we simply take the mmap_sem for
write for oom victims before munlocking and release it after munmapping?
 
I am OK with building on the current protocol if taking the mmap_sem for
the whole section has some serious down sides but I haven't heard any
yet, to be honest.
-- 
Michal Hocko
SUSE Labs