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

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Message-ID: <20151126093427.GA7953@dhcp22.suse.cz>
Date:	Thu, 26 Nov 2015 10:34:27 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
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 Wed 25-11-15 12:57:08, David Rientjes wrote:
> On Wed, 25 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. __GFP_NOFAIL should be
relatively rare and nodes are rarely depeleted so much that
ALLOC_NO_WATERMARKS wouldn't be able to allocate from the first zone in
the zone list. I mean I have no problem to do the above it just sounds
overcomplicating the situation without making practical difference.
If you and others insist I can resping the patch though.
-- 
Michal Hocko
SUSE Labs
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