[PATCH 03/11] mm: mmzone: introduce zone_pfn_same_memmap()

Michal Nazarewicz
2011-12-12T14:51:55Z

> On Fri, Nov 18, 2011 at 05:43:10PM +0100, Marek Szyprowski wrote:
>> From: Michal Nazarewicz <mina86@mina86.com>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 6afae0e..09c9702 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -111,7 +111,10 @@ skip:
>>
>> next:
>> pfn += isolated;
>> - page += isolated;
>> + if (zone_pfn_same_memmap(pfn - isolated, pfn))
>> + page += isolated;
>> + else
>> + page = pfn_to_page(pfn);
>> }

On Mon, 12 Dec 2011 15:19:53 +0100, Mel Gorman <mel@csn.ul.ie> wrote:
> Is this necessary?
>
> We are isolating pages, the largest of which is a MAX_ORDER_NR_PAGES
> page. [...]

On Mon, 12 Dec 2011 15:40:30 +0100, Mel Gorman <mel@csn.ul.ie> wrote:
> To be clear, I'm referring to a single page being isolated here. It may
> or may not be a high-order page but it's still going to be less then
> MAX_ORDER_NR_PAGES so you should be able check when a new block is
> entered and pfn_to_page is necessary.

Do you mean something like:

if (same pageblock)
just do arithmetic;
else
use pfn_to_page;

?

I've discussed it with Dave and he suggested that approach as an
optimisation since in some configurations zone_pfn_same_memmap()
is always true thus compiler will strip the else part, whereas
same pageblock test will be false on occasions regardless of kernel
configuration.

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