linux-kernel - Re: [PATCH] mm/oom_kill.c: don't kill TASK

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Date:	Thu, 17 Sep 2015 21:22:04 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kyle Walker <kwalker@...hat.com>
Cc:	akpm@...ux-foundation.org, mhocko@...e.cz, rientjes@...gle.com,
	hannes@...xchg.org, vdavydov@...allels.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Stanislav Kozina <skozina@...hat.com>
Subject: Re: [PATCH] mm/oom_kill.c: don't kill TASK_UNINTERRUPTIBLE tasks

Add cc's.

On 09/17, Kyle Walker wrote:
>
> Currently, the oom killer will attempt to kill a process that is in
> TASK_UNINTERRUPTIBLE state. For tasks in this state for an exceptional
> period of time, such as processes writing to a frozen filesystem during
> a lengthy backup operation, this can result in a deadlock condition as
> related processes memory access will stall within the page fault
> handler.
>
> Within oom_unkillable_task(), check for processes in
> TASK_UNINTERRUPTIBLE (TASK_KILLABLE omitted). The oom killer will
> move on to another task.
>
> Signed-off-by: Kyle Walker <kwalker@...hat.com>
> ---
>  mm/oom_kill.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 1ecc0bc..66f03f8 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -131,6 +131,10 @@ static bool oom_unkillable_task(struct task_struct *p,
>  	if (memcg && !task_in_mem_cgroup(p, memcg))
>  		return true;
>
> +	/* Uninterruptible tasks should not be killed unless in TASK_WAKEKILL */
> +	if (p->state == TASK_UNINTERRUPTIBLE)
> +		return true;
> +

So we can skip a memory hog which, say, does mutex_lock(). And this can't
help if this task is multithreaded, unless all its sub-threads are in "D"
state too oom killer will pick another thread with the same ->mm. Plus
other problems.

But yes, such a deadlock is possible. I would really like to see the comments
from maintainers. In particular, I seem to recall that someone suggested to
try to kill another !TIF_MEMDIE process after timeout, perhaps this is what
we should actually do...

Oleg.

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