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Subject: Re: [PATCH 08/25] Calculate the preferred zone for allocation only
 once
From: Pekka Enberg <penberg@cs.helsinki.fi>
To: Mel Gorman <mel@csn.ul.ie>
Cc: Linux Memory Management List <linux-mm@kvack.org>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Lin Ming <ming.m.lin@intel.com>,
       Zhang Yanmin <yanmin_zhang@linux.intel.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <1240266011-11140-9-git-send-email-mel@csn.ul.ie>
References: <1240266011-11140-1-git-send-email-mel@csn.ul.ie>
	 <1240266011-11140-9-git-send-email-mel@csn.ul.ie>
Date: Tue, 21 Apr 2009 10:37:37 +0300
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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@csn.ul.ie>

Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>

> @@ -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.

> +
> +	/* 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;
>  }