Re: [patch] oom: give current access to memory reserves if it hasbeen killed

[David Rientjes]

On Tue, 30 Mar 2010, Oleg Nesterov wrote:

> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -681,6 +681,16 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
> >  	}
> >
> >  	/*
> > +	 * If current has a pending SIGKILL, then automatically select it.  The
> > +	 * goal is to allow it to allocate so that it may quickly exit and free
> > +	 * its memory.
> > +	 */
> > +	if (fatal_signal_pending(current)) {
> > +		__oom_kill_task(current);
> 
> I am worried...
> 
> Note that __oom_kill_task() does force_sig(SIGKILL) which assumes that
> ->sighand != NULL. This is not true if out_of_memory() is called after
> current has already passed exit_notify().
> 

We have an even bigger problem if current is in the oom killer at 
exit_notify() since it has already detached its ->mm in exit_mm() :)

> Hmm. looking at oom_kill.c... Afaics there are more problems with mt
> apllications. select_bad_process() does for_each_process() which can
> only see the group leaders. This is fine, but what if ->group_leader
> has already exited? In this case its ->mm == NULL, and we ignore the
> whole thread group.
> 
> IOW, unless I missed something, it is very easy to hide the process
> from oom-kill:
> 
> 	int main()
> 	{
> 		pthread_create(memory_hog_func);
> 		syscall(__NR_exit);
> 	}
> 

The check for !p->mm was moved in the -mm tree (and the oom killer was 
entirely rewritten in that tree, so I encourage you to work off of it 
instead) with 
oom-avoid-race-for-oom-killed-tasks-detaching-mm-prior-to-exit.patch to 
even after the check for PF_EXITING.  This is set in the exit path before 
the ->mm is detached so if the oom killer finds an already exiting task, 
it will become a no-op since it should eventually free memory and avoids a 
needless oom kill.
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