linux-kernel - Re: [PATCH v2] mm,oom: Exclude TIF_MEMDIE processes from candidates.

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Date:	Wed, 13 Jan 2016 19:52:08 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	rientjes@...gle.com
Cc:	mhocko@...nel.org, hannes@...xchg.org, akpm@...ux-foundation.org,
	mgorman@...e.de, torvalds@...ux-foundation.org, oleg@...hat.com,
	hughd@...gle.com, andrea@...nel.org, riel@...hat.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm,oom: Exclude TIF_MEMDIE processes from candidates.

David Rientjes wrote:
> > @@ -171,7 +195,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
> >  	if (oom_unkillable_task(p, memcg, nodemask))
> >  		return 0;
> > 
> > -	p = find_lock_task_mm(p);
> > +	p = find_lock_non_victim_task_mm(p);
> >  	if (!p)
> >  		return 0;
> > 
> 
> I understand how this may make your test case pass, but I simply don't 
> understand how this could possibly be the correct thing to do.  This would 
> cause oom_badness() to return 0 for any process where a thread has 
> TIF_MEMDIE set.  If the oom killer is called from the page allocator, 
> kills a thread, and it is recalled before that thread may exit, then this 
> will panic the system if there are no other eligible processes to kill.
> 
Why? oom_badness() is called after oom_scan_process_thread() returned OOM_SCAN_OK.
oom_scan_process_thread() returns OOM_SCAN_ABORT if a thread has TIF_MEMDIE set.

If the TIF_MEMDIE thread already exited, find_lock_non_victim_task_mm() acts like
find_lock_task_mm(). Otherwise, oom_scan_process_thread() acts like a blocker.