linux-kernel - Re: [RFT PATCH v2 4/5] mm: provide more accurate estimation of pages occupied by memmap

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Date:	Wed, 28 Nov 2012 15:52:21 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Wen Congyang <wency@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Chris Clayton <chris2553@...glemail.com>,
	"Rafael J . Wysocki" <rjw@...k.pl>, Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan@...nel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	Jianguo Wu <wujianguo@...wei.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFT PATCH v2 4/5] mm: provide more accurate estimation of
 pages occupied by memmap

On Wed, 21 Nov 2012 23:09:46 +0800
Jiang Liu <liuj97@...il.com> wrote:

> Subject: Re: [RFT PATCH v2 4/5] mm: provide more accurate estimation of pages occupied by memmap

How are people to test this?  "does it boot"?

> 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 bigger 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
> @@ -4442,6 +4442,26 @@ 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 holes within
> +	 * the zone and SPARSEMEM is in use. If there are holes within the
> +	 * zone, each populated memory region may cost us one or two extra
> +	 * memmap pages due to alignment because memmap pages for each
> +	 * populated regions may not naturally algined on page boundary.
> +	 * So the (present_pages >> 4) heuristic is a tradeoff for that.
> +	 */
> +	if (spanned_pages > present_pages + (present_pages >> 4) &&
> +	    IS_ENABLED(CONFIG_SPARSEMEM))
> +		pages = present_pages;
> +
> +	return PAGE_ALIGN(pages * sizeof(struct page)) >> PAGE_SHIFT;
> +}
> +

I spose we should do this, although it makes no difference as the
compiler will inline calc_memmap_size() into its caller:

--- a/mm/page_alloc.c~mm-provide-more-accurate-estimation-of-pages-occupied-by-memmap-fix
+++ a/mm/page_alloc.c
@@ -4526,8 +4526,8 @@ 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)
+static unsigned long __paginginit calc_memmap_size(unsigned long spanned_pages,
+						   unsigned long present_pages)
 {
 	unsigned long pages = spanned_pages;
 

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