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

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Date:   Tue, 24 Apr 2018 14:07:52 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrea Arcangeli <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 reaperunmap

On Tue, 24 Apr 2018, Michal Hocko wrote:

> > > > My patch has passed intensive testing on both x86 and powerpc, so I'll ask 
> > > > that it's pushed for 4.17-rc3.  Many thanks to Tetsuo for the suggestion 
> > > > on calling __oom_reap_task_mm() from exit_mmap().
> > > 
> > > Yeah, but your patch does have a problem with blockable mmu notifiers
> > > IIUC.
> > 
> > What on earth are you talking about?  exit_mmap() does 
> > mmu_notifier_release().  There are no blockable mmu notifiers.
> 
> MMF_OOM_SKIP - remember? The thing that guarantees a forward progress.
> So we cannot really depend on setting MMF_OOM_SKIP if a
> mmu_notifier_release blocks for an excessive/unbounded amount of time.
> 

If the thread is blocked in exit_mmap() because of mmu_notifier_release() 
then the oom reaper will eventually grab mm->mmap_sem (nothing holding it 
in exit_mmap()), return true, and oom_reap_task() will set MMF_OOM_SKIP.  
This is unchanged with the patch and is a completely separate issue.

> Look I am not really interested in disussing this to death but it would
> be really _nice_ if you could calm down a bit, stop fighting for the solution
> you have proposed and ignore the feedback you are getting.
> 

I assume we should spend more time considering the two untested patches 
you have sent, one of which killed 17 processes while a 8GB memory hog was 
exiting because the oom reaper couldn't grab mm->mmap_sem and set 
MMF_OOM_SKIP.

> There are two things to care about here. Stop the race that can blow up
> and do not regress MMF_OOM_SKIP guarantee. Can we please do that.

My patch does both.

Thanks.