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Subject: Re: [PATCH v13 0/7] cgroup-aware OOM killer
From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
References: <0d018c7e-a3de-a23a-3996-bed8b28b1e4a@i-love.sakura.ne.jp>
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 <201807170055.w6H0tHn5075670@www262.sakura.ne.jp>
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Date: Tue, 31 Jul 2018 23:14:01 +0900
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To: Roman Gushchin <guro@fb.com>, Andrew Morton <akpm@linux-foundation.org>
Cc: David Rientjes <rientjes@google.com>, Michal Hocko <mhocko@kernel.org>, linux-mm@vger.kernel.org, Vladimir Davydov <vdavydov.dev@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>, kernel-team@fb.com, cgroups@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org

On 2018/07/17 9:55, Tetsuo Handa wrote:
>> I don't get, why it's necessary to drop the cgroup oom killer to merge your fix?
>> I'm happy to help with rebasing and everything else.
> 
> Yes, I wish you rebase your series on top of OOM lockup (CVE-2016-10723) mitigation
> patch ( https://marc.info/?l=linux-mm&m=153112243424285&w=4 ). It is a trivial change
> and easy to cleanly backport (if applied before your series).
> 
> Also, I expect you to check whether my cleanup patch which removes "abort" path
> ( [PATCH 1/2] at https://marc.info/?l=linux-mm&m=153119509215026&w=4 ) helps
> simplifying your series. I don't know detailed behavior of your series, but I
> assume that your series do not kill threads which current thread should not wait
> for MMF_OOM_SKIP.

syzbot is hitting WARN(1) due to mem_cgroup_out_of_memory() == false.
https://syzkaller.appspot.com/bug?id=ea8c7912757d253537375e981b61749b2da69258

I can't tell what change is triggering this race. Maybe removal of oom_lock from
the oom reaper made more likely to hit. But anyway I suspect that

static bool oom_kill_memcg_victim(struct oom_control *oc)
{
        if (oc->chosen_memcg == NULL || oc->chosen_memcg == INFLIGHT_VICTIM)
                return oc->chosen_memcg; // <= This line is still broken

because

                /* We have one or more terminating processes at this point. */
                oc->chosen_task = INFLIGHT_VICTIM;

is not called.

Also, that patch is causing confusion by reviving schedule_timeout_killable(1)
with oom_lock held.

Can we temporarily drop cgroup-aware OOM killer from linux-next.git and
apply my cleanup patch? Since the merge window is approaching, I really want to
see how next -rc1 would look like...