[RESEND][PATCH v3] mm: Use aligned zone start for pfn_to_bitidx calculation

From: Laura Abbott
Date: Sat Jan 05 2013 - 14:29:16 EST

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@xxxxxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
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..b6a2510 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)
return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
- pfn = pfn - zone->zone_start_pfn;
+ pfn = pfn - round_down(zone->zone_start_pfn, pageblock_nr_pages);
return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
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