Re: 2.6.17-mm1

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2006/6/22, Mel Gorman <[email protected]>:
On Thu, 22 Jun 2006, Franck Bui-Huu wrote:

> Mel Gorman wrote:
>> On Thu, 22 Jun 2006, Franck Bui-Huu wrote:
>>>
>>> Should ARCH_PFN_OFFSET macro be used instead in order to make pfn/page
>>> convertions work when node 0 start offset do not start at 0 ?
>>>
>>
>> What happens if you have ARCH_PFN_OFFSET as
>>
>> #define ARCH_PFN_OFFSET (0UL)
>>
>> ?
>
> It's the default value (see memory_model.h). It means that pfn start
> for node 0 is 0, therefore your physical memory address starts at 0.
>

I know, but what I'm getting at is that ARCH_PFN_OFFSET may be unnecessary
with flatmem-relax-requirement-for-memory-to-start-at-pfn-0.patch applied.

yes it seems so. But ARCH_PFN_OFFSET has been used before your patch
has been sent. So your patch seems to be incomplete...

ARCH_PFN_OFFSET is used as

#define page_to_pfn(page)       ((unsigned long)((page) - mem_map) + \
                                  ARCH_PFN_OFFSET)

because it knew that the map may not start at PFN 0. With
flatmem-relax-requirement-for-memory-to-start-at-pfn-0.patch, the map will
start at PFN 0 even if physical memory does not start until later.


well your approach's trick is on the mem_map address whereas
ARCH_PFN_OFFSET's one is on the computation of the index. Your
solution may result in smaller kernel (when ARCH_PFN_OFFSET != 0)
because in your case page/pfn conversion is simpler.

Maybe in your patch instead of doing:

       map -= pgdat->node_start_pfn;

you could do:

       map -= ARCH_OFFSET_PFN;

--
              Franck
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