linux-kernel - Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of pages occupied by memmap

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Message-Id: <20121120111942.c9596d3f.akpm@linux-foundation.org>
Date:	Tue, 20 Nov 2012 11:19:42 -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 v1 4/5] mm: provide more accurate estimation of
 pages occupied by memmap

On Tue, 20 Nov 2012 23:18:34 +0800
Jiang Liu <liuj97@...il.com> wrote:

> >> +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?
> Hi Andrew,
> 	A normal x86 platform always have some holes within the DMA ZONE,
> so the ">> 4" heuristic is to avoid applying this adjustment to the DMA
> ZONE on x86 platforms. 
> 	Because the memmap_size is just an estimation, I feel it's OK to
> remove the ">> 4" heuristic, that shouldn't affect much.

Again: 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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