linux-kernel - Re: [PATCH 08/14] Memory compaction core

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Date:	Thu, 8 Apr 2010 17:59:54 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 08/14] Memory compaction core

On Fri, Apr 02, 2010 at 05:02:42PM +0100, Mel Gorman wrote:
> This patch is the core of a mechanism which compacts memory in a zone by
> relocating movable pages towards the end of the zone.
> 

When merging compaction and transparent huge pages, Andrea spotted and
fixed this problem in his tree but it should go to mmotm as well.

Thanks Andrea.

==== CUT HERE ====
mm,compaction: page buddy can go away before reading page_order while isolating pages for migration

From: Andrea Arcangeli <aarcange@...hat.com>

zone->lock isn't held so the optimisation is unsafe. The page could be
allocated between when PageBuddy is checked and page-order is called. The
scanner will harmlessly walk the other free pages so let's just skip this
optimization.

This is a fix to the patch "Memory compaction core".

[mel@....ul.ie: Expanded the changelog]
Signed-off-by: Mel Gorman <mel@....ul.ie>

---
 mm/compaction.c |    4 +---
 1 files changed, 1 insertions(+), 3 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index dadad52..4fb33f6 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -262,10 +262,8 @@ static unsigned long isolate_migratepages(struct zone *zone,
 
 		/* Get the page and skip if free */
 		page = pfn_to_page(low_pfn);
-		if (PageBuddy(page)) {
-			low_pfn += (1 << page_order(page)) - 1;
+		if (PageBuddy(page))
 			continue;
-		}
 
 		/* Try isolate the page */
 		if (__isolate_lru_page(page, ISOLATE_BOTH, 0) != 0)
--
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