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Subject: Re: [PATCH v2] oom, oom_reaper: do not enqueue same task twice
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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <ec0d0580-a2dd-f329-9707-0cb91205a216@i-love.sakura.ne.jp>
Date: Sun, 27 Jan 2019 19:56:06 +0900
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To: Michal Hocko <mhocko@kernel.org>
Cc: =?UTF-8?Q?Arkadiusz_Mi=c5=9bkiewicz?= <a.miskiewicz@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Tejun Heo <tj@kernel.org>, cgroups@vger.kernel.org, Aleksa Sarai <asarai@suse.de>, Jay Kamat <jgkamat@fb.com>, Roman Gushchin <guro@fb.com>, Johannes Weiner <hannes@cmpxchg.org>, linux-kernel@vger.kernel.org, Linus Torvalds <torvalds@linux-foundation.org>, linux-mm <linux-mm@kvack.org>

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.