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

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Message-Id: <201804182328.HIC57360.QHMFJtOLVFOSFO@I-love.SAKURA.ne.jp>
Date:   Wed, 18 Apr 2018 23:28:01 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org
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

Michal Hocko wrote:
> > > > Then, I'm tempted to call __oom_reap_task_mm() before holding mmap_sem for write.
> > > > It would be OK to call __oom_reap_task_mm() at the beginning of __mmput()...
> > > 
> > > I am not sure I understand.
> > 
> > To reduce possibility of __oom_reap_task_mm() giving up reclaim and
> > setting MMF_OOM_SKIP.
> 
> Still do not understand. Do you want to call __oom_reap_task_mm from
> __mmput?

Yes.

>          If yes why would you do so when exit_mmap does a stronger
> version of it?

Because memory which can be reclaimed by the OOM reaper is guaranteed
to be reclaimed before setting MMF_OOM_SKIP when the OOM reaper and
exit_mmap() contended, because the OOM reaper (weak reclaim) sets
MMF_OOM_SKIP after one second for safety in case of exit_mmap()
(strong reclaim) failing to make forward progress.