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

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

[Laura Abbott]

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@codeaurora.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..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)
 	pfn &= (PAGES_PER_SECTION-1);
 	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 #else
-	pfn = pfn - zone->zone_start_pfn;
+	pfn = pfn - round_down(zone->zone_start_pfn, pageblock_nr_pages);
 	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 #endif /* CONFIG_SPARSEMEM */
 }
-- 
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Re: [PATCH v3] mm: Use aligned zone start for pfn_to_bitidx calculation

[Mel Gorman]

On Thu, Dec 06, 2012 at 01:11:41PM -0800, Laura Abbott wrote:
> 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@codeaurora.org>

Acked-by: Mel Gorman <mgorman@suse.de>

It's merge window at the moment so it's in danger of getting lost. What
I suggest you do is do is resend to Andrew with the same people cc'd
post-merge window so it'll be picked up in mmotm for the next cycle.

Thanks.

-- 
Mel Gorman
SUSE Labs