linux-kernel - Re: [PATCH 08/25] Calculate the preferred zone for allocation only once

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Date:	Tue, 21 Apr 2009 09:27:32 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 08/25] Calculate the preferred zone for allocation only
	once

On Tue, Apr 21, 2009 at 10:37:37AM +0300, Pekka Enberg wrote:
> Hi Mel,
> 
> On Mon, 2009-04-20 at 23:19 +0100, Mel Gorman wrote:
> > get_page_from_freelist() can be called multiple times for an
> > allocation.
> > Part of this calculates the preferred_zone which is the first usable
> > zone in the zonelist. This patch calculates preferred_zone once.
> > 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> 
> Reviewed-by: Pekka Enberg <penberg@...helsinki.fi>
> 

Thanks

> > @@ -1772,11 +1774,20 @@ __alloc_pages_nodemask(gfp_t gfp_mask,
> > unsigned int order,
> >  	if (unlikely(!zonelist->_zonerefs->zone))
> >  		return NULL;
> >  
> > +	/* The preferred zone is used for statistics later */
> > +	(void)first_zones_zonelist(zonelist, high_zoneidx, nodemask,
> > +							&preferred_zone);
> > +	if (!preferred_zone)
> > +		return NULL;
> 
> You might want to add an explanation to the changelog why this change is
> safe. It looked like a functional change at first glance and it was
> pretty difficult to convince myself that __alloc_pages_slowpath() will
> always return NULL when there's no preferred zone because of the other
> cleanups in this patch series.
> 

Is this better?

get_page_from_freelist() can be called multiple times for an allocation.
Part of this calculates the preferred_zone which is the first usable zone in
the zonelist but the zone depends on the GFP flags specified at the beginning
of the allocation call. This patch calculates preferred_zone once. It's safe
to do this because if preferred_zone is NULL at the start of the call, no
amount of direct reclaim or other actions will change the fact the allocation
will fail.

> > +
> > +	/* First allocation attempt */
> >  	page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask, order,
> > -			zonelist, high_zoneidx, ALLOC_WMARK_LOW|ALLOC_CPUSET);
> > +			zonelist, high_zoneidx, ALLOC_WMARK_LOW|ALLOC_CPUSET,
> > +			preferred_zone);
> >  	if (unlikely(!page))
> >  		page = __alloc_pages_slowpath(gfp_mask, order,
> > -				zonelist, high_zoneidx, nodemask);
> > +				zonelist, high_zoneidx, nodemask,
> > +				preferred_zone);
> >  
> >  	return page;
> >  }
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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