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

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Message-Id: <20130107143128.face9220.akpm@linux-foundation.org>
Date:	Mon, 7 Jan 2013 14:31:28 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Laura Abbott <lauraa@...eaurora.org>
Cc:	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [RESEND][PATCH v3] mm: Use aligned zone start for pfn_to_bitidx
 calculation

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