linux-kernel - Re: [PATCH 2/2] vmscan: don't subtraction of unsined

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Date:	Fri, 25 Jun 2010 09:17:03 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 2/2] vmscan: don't subtraction of unsined 

On Fri, 25 Jun 2010, KOSAKI Motohiro wrote:

> 'slab_reclaimable' and 'nr_pages' are unsigned. so, subtraction is
> unsafe.

Why? We are subtracting the current value of NR_SLAB_RECLAIMABLE from the
earlier one. The result can be negative (maybe concurrent allocations) and
then the nr_reclaimed gets decremented instead. This is  okay since we
have not reached our goal then of reducing the number of reclaimable slab
pages on the zone.

> @@ -2622,17 +2624,21 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  		 * Note that shrink_slab will free memory on all zones and may
>  		 * take a long time.
>  		 */
> -		while (shrink_slab(sc.nr_scanned, gfp_mask, lru_pages) &&
> -			zone_page_state(zone, NR_SLAB_RECLAIMABLE) >
> -				slab_reclaimable - nr_pages)

The comparison could be a problem here. So

			zone_page_state(zone, NR_SLAB_RECLAIMABLE) + nr_pages >
				slab_reclaimable

?

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