linux-kernel - Re: [PATCH] oom_reaper: close race without using oom

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Message-Id: <201707190551.GJE30718.OFHOQMFJtVSFOL@I-love.SAKURA.ne.jp>
Date:   Wed, 19 Jul 2017 05:51:03 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org
Cc:     linux-mm@...ck.org, hannes@...xchg.org, rientjes@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] oom_reaper: close race without using oom_lock

Michal Hocko wrote:
> On Tue 18-07-17 23:06:50, Tetsuo Handa wrote:
> > Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task")
> > guarded whole OOM reaping operations using oom_lock. But there was no
> > need to guard whole operations. We needed to guard only setting of
> > MMF_OOM_REAPED flag because get_page_from_freelist() in
> > __alloc_pages_may_oom() is called with oom_lock held.
> > 
> > If we change to guard only setting of MMF_OOM_SKIP flag, the OOM reaper
> > can start reaping operations as soon as wake_oom_reaper() is called.
> > But since setting of MMF_OOM_SKIP flag at __mmput() is not guarded with
> > oom_lock, guarding only the OOM reaper side is not sufficient.
> > 
> > If we change the OOM killer side to ignore MMF_OOM_SKIP flag once,
> > there is no need to guard setting of MMF_OOM_SKIP flag, and we can
> > guarantee a chance to call get_page_from_freelist() in
> > __alloc_pages_may_oom() without depending on oom_lock serialization.
> > 
> > This patch makes MMF_OOM_SKIP act as if MMF_OOM_REAPED, and adds a new
> > flag which acts as if MMF_OOM_SKIP, in order to close both race window
> > (the OOM reaper side and __mmput() side) without using oom_lock.
> 
> Why do we need this patch when
> http://lkml.kernel.org/r/20170626130346.26314-1-mhocko@kernel.org
> already removes the lock and solves another problem at once?

We haven't got an answer from Hugh and/or Andrea whether that patch is safe.
Even if that patch is safe, this patch still helps with CONFIG_MMU=n case.