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Subject: Re: [PATCH] mm/oom: Suppress unnecessary "sharing same memory"message.
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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Message-Id: <201505292140.JHE18273.SFFMJFHOtQLOVO@I-love.SAKURA.ne.jp>
Date: Fri, 29 May 2015 21:40:47 +0900
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To: mhocko@suse.cz
Cc: linux-mm@kvack.org

Michal Hocko wrote:
> On Thu 28-05-15 06:59:32, Tetsuo Handa wrote:
> > I just imagined a case where p is blocked at down_read() in acct_collect() from
> > do_exit() when p is sharing mm with other processes, and other process is doing
> > blocking operation with mm->mmap_sem held for writing. Is such case impossible?
> 
> It is very much possible and I have missed this case when proposing
> my alternative. The other process could be doing an address space
> operation e.g. mmap which requires an allocation.

Are there locations that do memory allocations with mm->mmap_sem held for
writing? Is it possible that thread1 is doing memory allocation between
down_write(&current->mm->mmap_sem) and up_write(&current->mm->mmap_sem),
thread2 sharing the same mm is waiting at down_read(&current->mm->mmap_sem),
and the OOM killer invoked by thread3 chooses thread2 as the OOM victim and
sets TIF_MEMDIE to thread2?

If yes, I think setting TIF_MEMDIE to only one thread can cause deadlock
problem when mm is shared by multiple threads, for thread2 cannot be terminated
because thread1 will not call up_write(&current->mm->mmap_sem) until thread1's
memory allocation completes, resulting in hang up. If thread2->mm &&
task_will_free_mem(thread2) was true when the OOM killer chooses thread2 as
the OOM victim, it will result in annoying silent hang up.

If there are locations that do memory allocations with mm->mmap_sem held for
writing, don't we need to send SIGKILL and set TIF_MEMDIE to all threads which
could block the OOM victim?

Maybe we can use "struct mm_struct"->"bool chosen_by_oom_killer" and checking
for (current->mm && current->mm->chosen_by_oom_killer) than
test_thread_flag(TIF_MEMDIE) inside the memory allocator?

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