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

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

On Wed, 21 Nov 2012 22:52:29 +0800
Jiang Liu <liuj97@gmail.com> wrote:

> On 11/21/2012 03:19 AM, Andrew Morton wrote:
> > 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?
> 
> Hi Andrew,
> 	If there are holes within a zone, it may cost us one or two extra pages
> for each populated region within the zone due to alignment because memmap for 
> each populated regions may not naturally aligned on page boundary.

Right.  So with an additional pass across the zone and a bit of
arithmetic, we can calculate the exact space requirement for memmap?
No need for kludgy heuristics?

> Originally the ">> 4" heuristic is to trade off these extra memmap pages,
> especially for small zones linke DMA zone.

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