linux-kernel - 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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Date:	Wed,  1 Sep 2010 08:37:41 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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

> +#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);
> +
> +	/*
> +	 * While kswapd is awake, it is considered the zone is under some
> +	 * memory pressure. Under pressure, there is a risk that
> +	 * per-cpu-counter-drift will allow the min watermark to be breached
> +	 * potentially causing a live-lock. While kswapd is awake and
> +	 * free pages are low, get a better estimate for free pages
> +	 */
> +	if (nr_free_pages < zone->percpu_drift_mark &&
> +			!waitqueue_active(&zone->zone_pgdat->kswapd_wait)) {
> +		int cpu;
> +
> +		for_each_online_cpu(cpu) {
> +			struct per_cpu_pageset *pset;
> +
> +			pset = per_cpu_ptr(zone->pageset, cpu);
> +			nr_free_pages += pset->vm_stat_diff[NR_FREE_PAGES];

If my understanding is correct, we have no lock when reading pset->vm_stat_diff.
It mean nr_free_pages can reach negative value at very rarely race. boundary
check is necessary?




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