linux-kernel - Re: [PATCH 07/10] mm, oom: fortify task_will_free

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Message-Id: <201606162154.CGE05294.HJQOSMFFVFtOOL@I-love.SAKURA.ne.jp>
Date:	Thu, 16 Jun 2016 21:54:27 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org
Cc:	linux-mm@...ck.org, rientjes@...gle.com, oleg@...hat.com,
	vdavydov@...allels.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/10] mm, oom: fortify task_will_free_mem

Michal Hocko wrote:
> On Sat 11-06-16 17:10:03, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > > Also, I think setting TIF_MEMDIE on p when find_lock_task_mm(p) != p is
> > > > wrong. While oom_reap_task() will anyway clear TIF_MEMDIE even if we set
> > > > TIF_MEMDIE on p when p->mm == NULL, it is not true for CONFIG_MMU=n case.
> > > 
> > > Yes this would be racy for !CONFIG_MMU but does it actually matter?
> > 
> > I don't know because I've never used CONFIG_MMU=n kernels. But I think it
> > actually matters. You fixed this race by commit 83363b917a2982dd ("oom:
> > make sure that TIF_MEMDIE is set under task_lock").
> 
> Yes and that commit was trying to address a highly theoretical issue
> reported by you. Let me quote:
> :oom_kill_process is currently prone to a race condition when the OOM
> :victim is already exiting and TIF_MEMDIE is set after the task releases
> :its address space.  This might theoretically lead to OOM livelock if the
> :OOM victim blocks on an allocation later during exiting because it
> :wouldn't kill any other process and the exiting one won't be able to exit.
> :The situation is highly unlikely because the OOM victim is expected to
> :release some memory which should help to sort out OOM situation.
> 
> Even if such a race is possible it wouldn't be with the oom
> reaper. Regarding CONFIG_MMU=n I am even less sure it is possible and
> I would rather focus on CONFIG_MMU=y where we know that problems exist
> rather than speculating about something as special as nommu which even
> might not care at all.

I still don't like it. current->mm == NULL in

-	if (current->mm &&
-	    (fatal_signal_pending(current) || task_will_free_mem(current))) {
+	if (task_will_free_mem(current)) {

is not highly unlikely. You obviously break commit d7a94e7e11badf84
("oom: don't count on mm-less current process") on CONFIG_MMU=n kernels.

Also, since commit f44666b04605d1c7 ("mm,oom: speed up select_bad_process() loop")
changed to iterate using thread group leaders, it is no longer highly unlikely
that p is a thread group leader which already released mm. What you call "a highly
theoretical issue" (which is true as of commit 83363b917a2982dd ("oom: make sure
that TIF_MEMDIE is set under task_lock") was proposed) may not be true any more.
Regarding CONFIG_MMU=n kernels, making sure that TIF_MEMDIE is set on a thread
with non-NULL mm does matter.