linux-kernel - Re: [PATCH 3/5] oom: clear TIF_MEMDIE after oom_reaper managed to unmap the address space

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Date:	Sat, 6 Feb 2016 23:33:24 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com, mgorman@...e.de,
	oleg@...hat.com, torvalds@...ux-foundation.org, hughd@...gle.com,
	andrea@...nel.org, riel@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] oom: clear TIF_MEMDIE after oom_reaper managed to unmap the address space

Michal Hocko wrote:
> On Thu 04-02-16 15:43:19, Michal Hocko wrote:
> > On Thu 04-02-16 23:22:18, Tetsuo Handa wrote:
> > > Michal Hocko wrote:
> > > > From: Michal Hocko <mhocko@...e.com>
> > > > 
> > > > When oom_reaper manages to unmap all the eligible vmas there shouldn't
> > > > be much of the freable memory held by the oom victim left anymore so it
> > > > makes sense to clear the TIF_MEMDIE flag for the victim and allow the
> > > > OOM killer to select another task.
> > > 
> > > Just a confirmation. Is it safe to clear TIF_MEMDIE without reaching do_exit()
> > > with regard to freezing_slow_path()? Since clearing TIF_MEMDIE from the OOM
> > > reaper confuses
> > > 
> > >     wait_event(oom_victims_wait, !atomic_read(&oom_victims));
> > > 
> > > in oom_killer_disable(), I'm worrying that the freezing operation continues
> > > before the OOM victim which escaped the __refrigerator() actually releases
> > > memory. Does this cause consistency problem?
> > 
> > This is a good question! At first sight it seems this is not safe and we
> > might need to make the oom_reaper freezable so that it doesn't wake up
> > during suspend and interfere. Let me think about that.
> 
> OK, I was thinking about it some more and it seems you are right here.
> oom_reaper as a kernel thread is not freezable automatically and so it
> might interfere after all the processes/kernel threads are considered
> frozen. Then it really might shut down TIF_MEMDIE too early and wake out
> oom_killer_disable. wait_event_freezable is not sufficient because the
> oom_reaper might running while the PM freezer is freezing tasks and it
> will miss it because it doesn't see it.

I'm not using PM freezer, but your answer is opposite to my guess.
I thought try_to_freeze_tasks(false) is called by freeze_kernel_threads()
after oom_killer_disable() succeeded, and try_to_freeze_tasks(false) will
freeze both userspace tasks (including OOM victims which got TIF_MEMDIE
cleared by the OOM reaper) and kernel threads (including the OOM reaper).
Thus, I was guessing that clearing TIF_MEMDIE without reaching do_exit() is
safe.

> 
> So I think we might need this. I am heading to vacation today and will
> be offline for the next week so I will prepare the full patch with the
> proper changelog after I get back:
> 
I can't judge whether we need this set_freezable().

> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index ca61e6cfae52..7e9953a64489 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -521,6 +521,8 @@ static void oom_reap_task(struct task_struct *tsk)
>  
>  static int oom_reaper(void *unused)
>  {
> +	set_freezable();
> +
>  	while (true) {
>  		struct task_struct *tsk = NULL;
>  
> -- 
> Michal Hocko
> SUSE Labs
>