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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 20 Nov 2012 23:18:34 +0800
Jiang Liu <liuj97@gmail.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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