On 11/20/2012 07:42 AM, Andrew Morton wrote:
> On Mon, 19 Nov 2012 00:07:29 +0800
> Jiang Liu <liu...@gmail.com> wrote:
>
>> If SPARSEMEM is enabled, it won't build page structures for
>> non-existing pages (holes) within a zone, so provide a more accurate
>> estimation of pages occupied by memmap if there are big holes within
>> the zone.
>>
>> And pages for highmem zones' memmap will be allocated from lowmem,
>> so charge nr_kernel_pages for that.
>>
>> ...
>>
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4435,6 +4435,22 @@ void __init set_pageblock_order(void)
>>
>> #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
>>
>> +static unsigned long calc_memmap_size(unsigned long spanned_pages,
>> + unsigned long present_pages)
>> +{
>> + unsigned long pages = spanned_pages;
>> +
>> + /*
>> + * Provide a more accurate estimation if there are big holes within
>> + * the zone and SPARSEMEM is in use.
>> + */
>> + if (spanned_pages > present_pages + (present_pages >> 4) &&
>> + IS_ENABLED(CONFIG_SPARSEMEM))
>> + pages = present_pages;
>> +
>> + return PAGE_ALIGN(pages * sizeof(struct page)) >> PAGE_SHIFT;
>> +}
>
> Please explain the ">> 4" heuristc more completely - preferably in both
> the changelog and code comments. Why can't we calculate this
> requirement exactly? That might require a second pass, but that's OK for
> code like this?
Hi Andrew,
A normal x86 platform always have some holes within the DMA ZONE,
so the ">> 4" heuristic is to avoid applying this adjustment to the DMA
ZONE on x86 platforms.
Because the memmap_size is just an estimation, I feel it's OK to
remove the ">> 4" heuristic, that shouldn't affect much.
Thanks
Gerry
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