linux-kernel - [PATCH v2] mm: Use aligned zone start for pfn_to

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Message-Id: <1354824324-21993-1-git-send-email-lauraa@codeaurora.org>
Date:	Thu,  6 Dec 2012 12:05:24 -0800
From:	Laura Abbott <lauraa@...eaurora.org>
To:	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	Laura Abbott <lauraa@...eaurora.org>
Subject: [PATCH v2] mm: Use aligned zone start for pfn_to_bitidx calculation

The current calculation in pfn_to_bitidx assumes that
(pfn - zone->zone_start_pfn) >> pageblock_order will return the
same bit for all pfn in a pageblock. If zone_start_pfn is not
aligned to pageblock_nr_pages, this may not always be correct.

Consider the following with pageblock order = 10, zone start 2MB:

pfn     | pfn - zone start | (pfn - zone start) >> page block order
----------------------------------------------------------------
0x26000 | 0x25e00	   |  0x97
0x26100 | 0x25f00	   |  0x97
0x26200 | 0x26000	   |  0x98
0x26300 | 0x26100	   |  0x98

This means that calling {get,set}_pageblock_migratetype on a single
page will not set the migratetype for the full block. Fix this by
rounding down zone_start_pfn when doing the bitidx calculation.

Signed-off-by: Laura Abbott <lauraa@...eaurora.org>
---
 mm/page_alloc.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 92dd060..2e06abd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5422,7 +5422,7 @@ static inline int pfn_to_bitidx(struct zone *zone, unsigned long pfn)
 	pfn &= (PAGES_PER_SECTION-1);
 	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 #else
-	pfn = pfn - zone->zone_start_pfn;
+	pfn = pfn - round_down(zone->start_pfn, pageblock_nr_pages);
 	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 #endif /* CONFIG_SPARSEMEM */
 }
-- 
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
hosted by The Linux Foundation

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