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Date:   Tue, 26 Jul 2022 14:04:55 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     mm-commits@vger.kernel.org,
        syzbot+2d2aeadc6ce1e1f11d45@syzkaller.appspotmail.com,
        shakeelb@google.com, roman.gushchin@linux.dev, hannes@cmpxchg.org,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: + mm-memcontrol-fix-potential-oom_lock-recursion-deadlock.patch
 added to mm-unstable branch
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 <ac3a7af1-08c6-e2de-b93c-327ce6839830@I-love.SAKURA.ne.jp>
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On Tue 26-07-22 20:31:17, Tetsuo Handa wrote:
> On 2022/07/26 17:14, Michal Hocko wrote:
> > As we have concluded there are two issues possible here which would be
> > great to have reflected in the changelog.
> > 
> > On Mon 25-07-22 15:00:32, Andrew Morton wrote:
> >> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> >> Subject: mm: memcontrol: fix potential oom_lock recursion deadlock
> >> Date: Fri, 22 Jul 2022 19:45:39 +0900
> >>
> >> syzbot is reporting GFP_KERNEL allocation with oom_lock held when
> >> reporting memcg OOM [1].  Such allocation request might deadlock the
> >> system, for __alloc_pages_may_oom() cannot invoke global OOM killer due to
> >> oom_lock being already held by the caller.
> > 
> > I would phrase it like this:
> 
> This report is difficult to explain correctly.
> 
> > syzbot is reporting GFP_KERNEL allocation with oom_lock held when
> > reporting memcg OOM [1].
> 
> Correct. But
> 
> >                          This is problematic because this creates a
> > dependency between GFP_NOFS and GFP_KERNEL over oom_lock which could
> > dead lock the system.
> 
> oom_lock is irrelevant when trying GFP_KERNEL allocation from GFP_NOFS
> context. Therefore, something like:

I meant to say there is a dependency chain
	potential_fs_lock
	GFP_NOFS
				oom_lock
				GFP_KERNEL
				potentiaL_lock
	oom_lock
 
> ----------
> syzbot is reporting GFP_KERNEL allocation with oom_lock held when
> reporting memcg OOM [1]. If this allocation triggers the global OOM
> situation then the system can livelock because the GFP_KERNEL allocation
> with oom_lock held cannot trigger the global OOM killer because
> __alloc_pages_may_oom() fails to hold oom_lock.
> 
> Fix this problem by removing the allocation from memory_stat_format()
> completely, and pass static buffer when calling from memcg OOM path.
> 
> Note that the caller holding filesystem lock was the trigger for syzbot
> to report this locking dependency. Doing GFP_KERNEL allocation with
> filesystem lock held can deadlock the system even without involving OOM
> situation.
> ----------

But this sounds good as well.

Thanks!

-- 
Michal Hocko
SUSE Labs