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

[Laura Abbott <lauraa@codeaurora.org>]

On 12/6/2012 2:12 AM, Mel Gorman wrote:
> On Tue, Dec 04, 2012 at 02:10:01PM -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. The correct
>> fix is to round down zone_start_pfn for the bit index calculation.
>> Rather than do this calculation everytime, store this precalcualted
>> algined start in the zone structure to allow the actual start_pfn to
>> be used elsewhere.
>>
>> Change-Id: I13e2f53f50db294f38ec86138c17c6fe29f0ee82
>> Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
>
> Hi Laura,
>
> There should be no need to add a new zone field. It's probably ok in terms
> of functionality but it does mean that we have to worry about things like
> hotplug (FWIW, should be fine) and the memory overhead is added even on
> CONFIG_SPARSEMEM where it is not needed. Instead, mask out the lower bits
> in pfn_to_bitidx() using the same round_down trick you already do. The
> cost is negligible.
>
> Thanks.
>

I was debating if storing the size was actually necessary. I'll resubmit 
with the calculation done directly in the function.

Thanks,
Laura

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