linux-kernel - [PATCH] mm: make nr_scan_try

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Date:	Thu, 20 Aug 2009 10:52:09 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Avi Kivity <avi@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"menage@...gle.com" <menage@...gle.com>
Subject: [PATCH] mm: make nr_scan_try_batch() more safe on races

nr_scan_try_batch() can be called concurrently on the same zone
and the non-atomic calculations can go wrong. This is not a big
problem as long as the errors are small and won't impact the
balanced zone aging noticeably.

@nr_to_scan could be much larger values than @swap_cluster_max.
So don't store such large values to *nr_saved_scan directly,
which helps reducing possible errors on races.

CC: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
 mm/vmscan.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

--- linux.orig/mm/vmscan.c	2009-08-20 10:46:30.000000000 +0800
+++ linux/mm/vmscan.c	2009-08-20 10:49:36.000000000 +0800
@@ -1496,15 +1496,14 @@ static unsigned long nr_scan_try_batch(u
 				       unsigned long *nr_saved_scan,
 				       unsigned long swap_cluster_max)
 {
-	unsigned long nr;
-
-	*nr_saved_scan += nr_to_scan;
-	nr = *nr_saved_scan;
+	unsigned long nr = *nr_saved_scan + nr_to_scan;
 
 	if (nr >= swap_cluster_max)
 		*nr_saved_scan = 0;
-	else
+	else {
+		*nr_saved_scan = nr;
 		nr = 0;
+	}
 
 	return nr;
 }
--
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