Date: Mon, 19 Nov 2012 15:42:40 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Jiang Liu <liuj97@gmail.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>, David Rientjes <rientjes@google.com>,
        Jiang Liu <jiang.liu@huawei.com>,
        Maciej Rutecki <maciej.rutecki@gmail.com>,
        Chris Clayton <chris2553@googlemail.com>,
        "Rafael J . Wysocki" <rjw@sisk.pl>, Mel Gorman <mgorman@suse.de>,
        Minchan Kim <minchan@kernel.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Michal Hocko <mhocko@suse.cz>, Jianguo Wu <wujianguo@huawei.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of
 pages occupied by memmap
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	<1353254850-27336-5-git-send-email-jiang.liu@huawei.com>
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On Mon, 19 Nov 2012 00:07:29 +0800
Jiang Liu <liuj97@gmail.com> 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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