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

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Message-ID: <alpine.DEB.2.10.1601131653420.3847@chino.kir.corp.google.com>
Date:	Wed, 13 Jan 2016 16:57:08 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
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.

On Wed, 13 Jan 2016, Tetsuo Handa wrote:

> 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.
> 

oom_scan_process_thread() checks for TIF_MEMDIE on p, not on p's threads.  
If one of p's threads has TIF_MEMDIE set and p does not, we actually want 
to set TIF_MEMDIE for p.  That's the current behavior since it will lead 
to p->mm memory freeing.  Your patch is excluding such processes entirely 
and selecting another process to kill unnecessarily.