Re: Re : Re : sparsemem usage

[Andy Whitcroft <apw@shadowen.org>]

moreau francis wrote:
> Alan Cox wrote:
> > Mapping out parts of a section is quite normal - think about the 640K to
> > 1Mb hole in PC memory space.
>
> OK. But I'm still worry. Please consider the following code
>
>        for (...; ...; ...) {
>                 [...]
>                 if (pfn_valid(i))
>                        num_physpages++;
>                 [...]
>         }
>
> In that case num_physpages won't store an accurate value. Still it will be
> used by the kernel to make some statistic assumptions on other kernel
> data structure sizes.

That would be incorrect usage.  pfn_valid() simply doesn't tell you if 
you have memory backing a pfn, it mearly means you can interrogate the 
page* for it.  A good example of code which counts pages in a region is 
in count_highmem_pages() which has a form as below:

			for (pfn = start; pfn < end; pfn++) {
 				if (!pfn_valid(pfn))
                                        continue;
                                page = pfn_to_page(pfn);
                                if (PageReserved(page))
                                        continue;
				num_physpages++;
			}

-apw
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