Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of pagesoccupied by memmap

[Jiang Liu]

On 11/20/2012 07:42 AM, Andrew Morton wrote:
> On Mon, 19 Nov 2012 00:07:29 +0800
> Jiang Liu <liuj97@xxxxxxxxx> 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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