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

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Date:	Mon, 30 Nov 2015 14:17:03 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...nel.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, oom: Give __GFP_NOFAIL allocations access to
 memory reserves

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.
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