linux-kernel - Re: Re : Re : sparsemem usage

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Date:	Thu, 03 Aug 2006 10:47:02 +0100
From:	Andy Whitcroft <apw@...dowen.org>
To:	moreau francis <francis_moreau2000@...oo.fr>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: Re : Re : sparsemem usage

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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