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Date: Wed, 1 Sep 2010 19:43:41 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Mel Gorman <mel@csn.ul.ie>
cc: Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel List <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Minchan Kim <minchan.kim@gmail.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Subject: Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate of
 NR_FREE_PAGES when memory is low and kswapd is awake
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On Tue, 31 Aug 2010, Mel Gorman wrote:

> +#ifdef CONFIG_SMP
> +/* Called when a more accurate view of NR_FREE_PAGES is needed */
> +unsigned long zone_nr_free_pages(struct zone *zone)
> +{
> +	unsigned long nr_free_pages = zone_page_state(zone, NR_FREE_PAGES);

You cannot call zone_page_state here because zone_page_state clips the
counter at zero. The nr_free_pages needs to reflect the unclipped state
and then the deltas need to be added. Then the clipping at zero can be
done.