Date:	Tue, 21 Jun 2016 15:27:36 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	linux-mm@...ck.org, rientjes@...gle.com, oleg@...hat.com,
	vdavydov@...allels.com, mgorman@...hsingularity.net,
	hughd@...gle.com, riel@...hat.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: mm, oom_reaper: How to handle race with oom_killer_disable() ?

On Tue 21-06-16 13:46:43, Michal Hocko wrote:
> On Tue 21-06-16 20:03:17, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Mon 13-06-16 13:19:43, Michal Hocko wrote:
> > > [...]
> > > > I am trying to remember why we are disabling oom killer before kernel
> > > > threads are frozen but not really sure about that right away.
> > > 
> > > OK, I guess I remember now. Say that a task would depend on a freezable
> > > kernel thread to get to do_exit (stuck in wait_event etc...). We would
> > > simply get stuck in oom_killer_disable for ever. So we need to address
> > > it a different way.
> > > 
> > > One way would be what you are proposing but I guess it would be more
> > > systematic to never call exit_oom_victim on a remote task.  After [1] we
> > > have a solid foundation to rely only on MMF_REAPED even when TIF_MEMDIE
> > > is set. It is more code than your patch so I can see a reason to go with
> > > yours if the following one seems too large or ugly.
> > > 
> > > [1] http://lkml.kernel.org/r/1466426628-15074-1-git-send-email-mhocko@kernel.org
> > > 
> > > What do you think about the following?
> > 
> > I'm OK with not clearing TIF_MEMDIE from a remote task. But this patch is racy.
> > 
> > > @@ -567,40 +612,23 @@ static void oom_reap_task(struct task_struct *tsk)
> > >  	while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task(tsk))
> > >  		schedule_timeout_idle(HZ/10);
> > >  
> > > -	if (attempts > MAX_OOM_REAP_RETRIES) {
> > > -		struct task_struct *p;
> > > +	tsk->oom_reaper_list = NULL;
> > >  
> > > +	if (attempts > MAX_OOM_REAP_RETRIES) {
> > 
> > attempts > MAX_OOM_REAP_RETRIES would mean that down_read_trylock()
> > continuously failed. But it does not guarantee that the offending task
> > shall not call up_write(&mm->mmap_sem) and arrives at mmput() from exit_mm()
> > (as well as other threads which are blocked at down_read(&mm->mmap_sem) in
> > exit_mm() by the offending task arrive at mmput() from exit_mm()) when the
> > OOM reaper was preempted at this point.
> > 
> > Therefore, find_lock_task_mm() in requeue_oom_victim() could return NULL and
> > the OOM reaper could fail to set MMF_OOM_REAPED (and find_lock_task_mm() in
> > oom_scan_process_thread() could return NULL and the OOM killer could fail to
> > select next OOM victim as well) when __mmput() got stuck.
> 
> Fair enough. As this would break no-lockup requirement we cannot go that
> way. Let me think about it more.

Hmm, what about the following instead. It is rather a workaround than a
full flaged fix but it seems much more easier and shouldn't introduce
new issues.
---
>From 86bf010d2a6086491bb356494fab0e0fca80dee9 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Mon, 20 Jun 2016 17:28:18 +0200
Subject: [PATCH] oom, suspend: fix oom_reaper vs. oom_killer_disable race

Tetsuo has reported the following potential oom_killer_disable vs.
oom_reaper race:

(1) freeze_processes() starts freezing user space threads.
(2) Somebody (maybe a kenrel thread) calls out_of_memory().
(3) The OOM killer calls mark_oom_victim() on a user space thread
    P1 which is already in __refrigerator().
(4) oom_killer_disable() sets oom_killer_disabled = true.
(5) P1 leaves __refrigerator() and enters do_exit().
(6) The OOM reaper calls exit_oom_victim(P1) before P1 can call
    exit_oom_victim(P1).
(7) oom_killer_disable() returns while P1 not yet finished
(8) P1 perform IO/interfere with the freezer.

This situation is unfortunate. We cannot move oom_killer_disable after
all the freezable kernel threads are frozen because the oom victim might
depend on some of those kthreads to make a forward progress to exit so
we could deadlock. It is also far from trivial to teach the oom_reaper
to not call exit_oom_victim() because then we would lose a guarantee of
the OOM killer and oom_killer_disable forward progress because
exit_mm->mmput might block and never call exit_oom_victim.

It seems the easiest way forward is to workaround this race by calling
try_to_freeze_tasks again after oom_killer_disable. This will make sure
that all the tasks are frozen or it bails out.

Fixes: 449d777d7ad6 ("mm, oom_reaper: clear TIF_MEMDIE for all tasks queued for oom_reaper")
Reported-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 kernel/power/process.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/power/process.c b/kernel/power/process.c
index df058bed53ce..0c2ee9761d57 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -146,6 +146,18 @@ int freeze_processes(void)
 	if (!error && !oom_killer_disable())
 		error = -EBUSY;
 
+	/*
+	 * There is a hard to fix race between oom_reaper kernel thread
+	 * and oom_killer_disable. oom_reaper calls exit_oom_victim
+	 * before the victim reaches exit_mm so try to freeze all the tasks
+	 * again and catch such a left over task.
+	 */
+	if (!error) {
+		pr_info("Double checking all user space processes after OOM killer disable... ");
+		error = try_to_freeze_tasks(true);
+		pr_cont("\n");
+	}
+
 	if (error)
 		thaw_processes();
 	return error;
-- 
2.8.1


-- 
Michal Hocko
SUSE Labs

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