linux-kernel - Re: [PATCH v3] mm: Use aligned zone start for pfn_to

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Message-ID: <20121213112058.GA9887@suse.de>
Date:	Thu, 13 Dec 2012 11:20:58 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Laura Abbott <lauraa@...eaurora.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v3] mm: Use aligned zone start for pfn_to_bitidx
 calculation

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@...eaurora.org>

Acked-by: Mel Gorman <mgorman@...e.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
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