Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of pages occupied by memmap
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2012-11-19 23:42:43
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On Mon, 19 Nov 2012 00:07:29 +0800 Jiang Liu [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>