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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