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Date:   Sun, 27 Jan 2019 23:00:38 +0000
From:   Roman Gushchin <guro@...com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
CC:     Michal Hocko <mhocko@...nel.org>,
        Arkadiusz Miśkiewicz <a.miskiewicz@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Tejun Heo" <tj@...nel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Aleksa Sarai <asarai@...e.de>, Jay Kamat <jgkamat@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v3] oom, oom_reaper: do not enqueue same task twice

On Sun, Jan 27, 2019 at 11:57:38PM +0900, Tetsuo Handa wrote:
> On 2019/01/27 20:40, Michal Hocko wrote:
> > On Sun 27-01-19 19:56:06, Tetsuo Handa wrote:
> >> On 2019/01/27 17:37, Michal Hocko wrote:
> >>> Thanks for the analysis and the patch. This should work, I believe but
> >>> I am not really thrilled to overload the meaning of the MMF_UNSTABLE.
> >>> The flag is meant to signal accessing address space is not stable and it
> >>> is not aimed to synchronize oom reaper with the oom path.
> >>>
> >>> Can we make use mark_oom_victim directly? I didn't get to think that
> >>> through right now so I might be missing something but this should
> >>> prevent repeating queueing as well.
> >>
> >> Yes, TIF_MEMDIE would work. But you are planning to remove TIF_MEMDIE. Also,
> >> TIF_MEMDIE can't avoid enqueuing many threads sharing mm_struct to the OOM
> >> reaper. There is no need to enqueue many threads sharing mm_struct because
> >> the OOM reaper acts on mm_struct rather than task_struct. Thus, enqueuing
> >> based on per mm_struct flag sounds better, but MMF_OOM_VICTIM cannot be
> >> set from wake_oom_reaper(victim) because victim's mm might be already inside
> >> exit_mmap() when wake_oom_reaper(victim) is called after task_unlock(victim).
> >>
> >> We could reintroduce MMF_OOM_KILLED in commit 855b018325737f76
> >> ("oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task")
> >> if you don't like overloading the meaning of the MMF_UNSTABLE. But since
> >> MMF_UNSTABLE is available in Linux 4.9+ kernels (which covers all LTS stable
> >> versions with the OOM reaper support), we can temporarily use MMF_UNSTABLE
> >> for ease of backporting.
> > 
> > I agree that a per-mm state is more optimal but I would rather fix the
> > issue in a clear way first and only then think about an optimization on
> > top. Queueing based on mark_oom_victim (whatever that uses to guarantee
> > the victim is marked atomically and only once) makes sense from the
> > conceptual point of view and it makes a lot of sense to start from
> > there. MMF_UNSTABLE has a completely different purpose. So unless you
> > see a correctness issue with that then I would rather go that way.
> > 
> 
> Then, adding a per mm_struct flag is better. I don't see the difference
> between reusing MMF_UNSTABLE as a flag for whether wake_oom_reaper() for
> that victim's memory was already called (what you think as an overload)
> and reusing TIF_MEMDIE as a flag for whether wake_oom_reaper() for that
> victim thread can be called (what I think as an overload). We want to
> remove TIF_MEMDIE, and we can actually remove TIF_MEMDIE if you stop
> whack-a-mole "can you observe it in real workload/program?" game.
> I don't see a correctness issue with TIF_MEMDIE but I don't want to go
> TIF_MEMDIE way.
> 
> 
> 
> From 9c9e935fc038342c48461aabca666f1b544e32b1 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Date: Sun, 27 Jan 2019 23:51:37 +0900
> Subject: [PATCH v3] oom, oom_reaper: do not enqueue same task twice
> 
> Arkadiusz reported that enabling memcg's group oom killing causes
> strange memcg statistics where there is no task in a memcg despite
> the number of tasks in that memcg is not 0. It turned out that there
> is a bug in wake_oom_reaper() which allows enqueuing same task twice
> which makes impossible to decrease the number of tasks in that memcg
> due to a refcount leak.
> 
> This bug existed since the OOM reaper became invokable from
> task_will_free_mem(current) path in out_of_memory() in Linux 4.7,
> but memcg's group oom killing made it easier to trigger this bug by
> calling wake_oom_reaper() on the same task from one out_of_memory()
> request.
> 
> Fix this bug using an approach used by commit 855b018325737f76
> ("oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task").
> As a side effect of this patch, this patch also avoids enqueuing
> multiple threads sharing memory via task_will_free_mem(current) path.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Reported-by: Arkadiusz Miśkiewicz <arekm@...en.pl>
> Tested-by: Arkadiusz Miśkiewicz <arekm@...en.pl>
> Fixes: af8e15cc85a25315 ("oom, oom_reaper: do not enqueue task if it is on the oom_reaper_list head")

Thank you, Tetsuo!

Acked-by: Roman Gushchin <guro@...com>

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