Re: [PATCH v3] mm: Use aligned zone start for pfn_to_bitidxcalculation

From: Mel Gorman
Date: Thu Dec 13 2012 - 06:20:51 EST

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@xxxxxxxxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

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.


Mel Gorman
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