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

[David Rientjes]

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.