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