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

[Andrew Morton]

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?

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