linux-kernel - Re: Multiple oom_reaper BUGs: unmap_page_range racing with exit

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Message-Id: <201712080044.BID56711.FFVOLMStJOQHOF@I-love.SAKURA.ne.jp>
Date:   Fri, 8 Dec 2017 00:44:11 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org, rientjes@...gle.com
Cc:     akpm@...ux-foundation.org, aarcange@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Multiple oom_reaper BUGs: unmap_page_range racing with exit_mmap

Michal Hocko wrote:
> David, could you test with this patch please?

Even if this patch solved David's case, you need to update

	 * tsk_is_oom_victim() cannot be set from under us
	 * either because current->mm is already set to NULL
	 * under task_lock before calling mmput and oom_mm is
	 * set not NULL by the OOM killer only if current->mm
	 * is found not NULL while holding the task_lock.

part as well, for it is the explanation of why
tsk_is_oom_victim() test was expected to work.

Also, do we need to do

  set_bit(MMF_OOM_SKIP, &mm->flags);

if mm_is_oom_victim(mm) == false?

exit_mmap() is called means that nobody can reach this mm
except ->signal->oom_mm, and mm_is_oom_victim(mm) == false
means that this mm cannot be reached by ->signal->oom_mm .

Then, I think we do not need to set MMF_OOM_SKIP on this mm
at exit_mmap() if mm_is_oom_victim(mm) == false.