Messages in this thread | | | Date | Tue, 20 Nov 2012 11:19:42 -0800 | From | Andrew Morton <> | Subject | Re: [RFT PATCH v1 4/5] mm: provide more accurate estimation of pages occupied by memmap |
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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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