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 07:45:06 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
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

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.

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:

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