linux-kernel - Re: [PATCH 2/3] page allocator: Do not allow interrupts to use ALLOC

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Date:	Tue, 27 Oct 2009 14:12:36 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Mel Gorman <mel@....ul.ie>, stable@...nel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Frans Pop <elendil@...net.nl>, Jiri Kosina <jkosina@...e.cz>,
	Sven Geggus <lists@...hsschwanzdomain.de>,
	Karol Lewandowski <karol.k.lewandowski@...il.com>,
	Tobias Oetiker <tobi@...iker.ch>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Stephan von Krawczynski <skraw@...net.com>,
	kernel-testers@...r.kernel.org, Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH 2/3] page allocator: Do not allow interrupts to use
 ALLOC_HARDER

On Tue, 27 Oct 2009, Andrew Morton wrote:

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index dfa4362..7f2aa3e 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1769,7 +1769,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> >  		 * See also cpuset_zone_allowed() comment in kernel/cpuset.c.
> >  		 */
> >  		alloc_flags &= ~ALLOC_CPUSET;
> > -	} else if (unlikely(rt_task(p)))
> > +	} else if (unlikely(rt_task(p)) && !in_interrupt())
> >  		alloc_flags |= ALLOC_HARDER;
> >  
> >  	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> 
> What are the runtime-observeable effects of this change?
> 

Giving rt tasks access to memory reserves is necessary to reduce latency, 
the privilege does not apply to interrupts that subsequently get run on 
the same cpu.

> The description is a bit waffly-sounding for a -stable backportable
> thing, IMO.  What reason do the -stable maintainers and users have to
> believe that this patch is needed, and an improvement?
> 

Allowing interrupts to allocate below the low watermark when not 
GFP_ATOMIC depletes memory reserves; this fixes an inconsistency 
introduced by the page allocator refactoring patchset that went into 
2.6.31.
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