linux-kernel - [ 043/128] mm: use aligned zone start for pfn_to

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Message-Id: <20130203144647.879496757@decadent.org.uk>
Date:	Sun, 03 Feb 2013 15:47:27 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Laura Abbott <lauraa@...eaurora.org>,
	Mel Gorman <mgorman@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [ 043/128] mm: use aligned zone start for pfn_to_bitidx calculation

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Laura Abbott <lauraa@...eaurora.org>

commit c060f943d0929f3e429c5d9522290584f6281d6e upstream.

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.

For our use case, the effects of this bug were mostly tied to the fact
that CMA allocations would either take a long time or fail to happen.
Depending on the driver using CMA, this could result in anything from
visual glitches to application failures.

Signed-off-by: Laura Abbott <lauraa@...eaurora.org>
Acked-by: Mel Gorman <mgorman@...e.de>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/page_alloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bc6cc0e..c957805 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5604,7 +5604,7 @@ 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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