Re: [PATCH 1/2] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves

[Michal Hocko <mhocko@kernel.org>]

On Mon 30-11-15 14:17:03, David Rientjes wrote:
> On Thu, 26 Nov 2015, Michal Hocko wrote:
> 
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > index 8034909faad2..94b04c1e894a 100644
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -2766,8 +2766,13 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> > > >  			goto out;
> > > >  	}
> > > >  	/* Exhausted what can be done so it's blamo time */
> > > > -	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL))
> > > > +	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
> > > >  		*did_some_progress = 1;
> > > > +
> > > > +		if (gfp_mask & __GFP_NOFAIL)
> > > > +			page = get_page_from_freelist(gfp_mask, order,
> > > > +					ALLOC_NO_WATERMARKS, ac);
> > > > +	}
> > > >  out:
> > > >  	mutex_unlock(&oom_lock);
> > > >  	return page;
> > > 
> > > Well, sure, that's one way to do it, but for cpuset users, wouldn't this 
> > > lead to a depletion of the first system zone since you've dropped 
> > > ALLOC_CPUSET and are doing ALLOC_NO_WATERMARKS in the same call?  
> > 
> > Are you suggesting to do?
> > 		if (gfp_mask & __GFP_NOFAIL) {
> > 			page = get_page_from_freelist(gfp_mask, order,
> > 					ALLOC_NO_WATERMARKS|ALLOC_CPUSET, ac);
> > 			/*
> > 			 * fallback to ignore cpuset if our nodes are
> > 			 * depleted
> > 			 */
> > 			if (!page)
> > 				get_page_from_freelist(gfp_mask, order,
> > 					ALLOC_NO_WATERMARKS, ac);
> > 		}
> > 
> > I am not really sure this worth complication.
> 
> I'm objecting to the ability of a process that is doing a __GFP_NOFAIL 
> allocation, which has been disallowed access from allocating on certain 
> mems through cpusets, to cause an oom condition on those disallowed nodes, 
> yes.

That ability will be there even with the fallback mechanism. My primary
objections was that the fallback is unnecessarily complex without any
evidence that such a situation would happen in the real life often
enought to bother about it. __GFP_NOFAIL allocations are and should be
rare and any runaway triggerable from the userspace is a kernel bug.

Anyway, as you seem to feel really strongly about this I will post v2
with the above fallback. This is a superslow path anyway...

-- 
Michal Hocko
SUSE Labs

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