linux-kernel - [PATCH 1/2][BUGFIX] fix next active pageblock calculation

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Message-Id: <20100903121159.30099a9c.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 3 Sep 2010 12:11:59 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Haicheng Li <haicheng.li@...ux.intel.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mel Gorman <mel@...ux.vnet.ibm.com>
Subject: [PATCH 1/2][BUGFIX] fix next active pageblock calculation

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

next_active_pageblock() is for finding next _used_ freeblock. It skips
several blocks when it finds there are a chunk of free pages lager than
pageblock. But it has 2 bugs.

  1. We have no lock. page_order(page) - pageblock_order can be minus.
  2. pageblocks_stride += is wrong. it should skip page_order(p) of pages.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
---
 mm/memory_hotplug.c |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

Index: mmotm-0827/mm/memory_hotplug.c
===================================================================
--- mmotm-0827.orig/mm/memory_hotplug.c
+++ mmotm-0827/mm/memory_hotplug.c
@@ -584,19 +584,19 @@ static inline int pageblock_free(struct 
 /* Return the start of the next active pageblock after a given page */
 static struct page *next_active_pageblock(struct page *page)
 {
-	int pageblocks_stride;
-
 	/* Ensure the starting page is pageblock-aligned */
 	BUG_ON(page_to_pfn(page) & (pageblock_nr_pages - 1));
 
-	/* Move forward by at least 1 * pageblock_nr_pages */
-	pageblocks_stride = 1;
-
 	/* If the entire pageblock is free, move to the end of free page */
-	if (pageblock_free(page))
-		pageblocks_stride += page_order(page) - pageblock_order;
+	if (pageblock_free(page)) {
+		int order;
+		/* be careful. we don't have locks, page_order can be changed.*/
+		order = page_order(page);
+		if (order > pageblock_order)
+			return page + (1 << order);
+	}
 
-	return page + (pageblocks_stride * pageblock_nr_pages);
+	return page + pageblock_nr_pages;
 }
 
 /* Checks if this range of memory is likely to be hot-removable. */

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