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

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Date:	Thu, 3 Aug 2006 12:46:28 +0000 (GMT)
From:	moreau francis <francis_moreau2000@...oo.fr>
To:	Andy Whitcroft <apw@...dowen.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org
Subject: Re : Re : Re : sparsemem usage

Andy Whitcroft wrote:
> 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++;
>             }
> 
num_physpages would still not give the right total number of pages in the
system. It will report a value smaller than the size of all memories which can
be suprising, depending on how it is used. In my mind I thought that it should
store the number of all pages in the system (reserved + free + ...).

Futhermore for flatmem model, my example that count the number of physical
pages is valid: reserved pages are really pages that are in used by the kernel.
But it's not valid anymore for sparsemem model. For consistency and code
sharing, I would make the same meaning of pfn_valid() and PageReserved() for
both models.

Francis


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