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

[RESEND][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>
Acked-by: Mel Gorman <mgorman@suse.de>
---
 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: [RESEND][PATCH v3] mm: Use aligned zone start for pfn_to_bitidx calculation

[Andrew Morton]

On Sat,  5 Jan 2013 11:28:31 -0800
Laura Abbott <lauraa@codeaurora.org> 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.

What are the user-visible effects of this bug?

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

[Laura Abbott]

On 1/7/2013 2:31 PM, Andrew Morton wrote:
> On Sat,  5 Jan 2013 11:28:31 -0800
> Laura Abbott <lauraa@codeaurora.org> 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.
>
> What are the user-visible effects of this bug?
>

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

I'm not sure about effect outside of CMA.

Laura
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