Re: Re : Re : Re : sparsemem usage

From: Andy Whitcroft
Date: Thu Aug 03 2006 - 09:16:06 EST

moreau francis wrote:
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))
page = pfn_to_page(pfn);
if (PageReserved(page))

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.

The semantics and meaning of both pfn_valid() and PageReserved() are the same in all three memory models, just not what you need them to be for your pfn_valid() loop to tell you how many real frames there are.
I do not believe it is correct to say that your loop would give you the number of physical pages under FLATMEM. If there are any gaps at all (such as there is for IO space just below 1MB) that will pass pfn_valid(), and yet does _not_ have any real memory associated with it.
With FLATMEM you will get pfn_valid() passing on non-memory pages.

I have to re-iterate pfn_valid() does not mean pfn_valid_memory(), it means pfn_valid_memmap(). If you want to know if a page is valid and memory (at least on x86) you could use:

if (pfn_valid(pfn) && page_is_ram(pfn)) {

It is rare you care how many real page frames there are in the system. You are more interested in how many usable frames there are. Such as for use in sizing hashes or caches. The reserved pages should be excluded in this calculation. ACPI pages, BIOS pages and the like simply are no interest.

I don't see anywhere in the kernel using that construct to work out how many pages there are in the system. Mostly we have architectual information to tell us what real physical pages exist in the system such as the srat or e820 etc. If we really care about real page counts at that accuracy we have those to refer to.

Do you have a usage model in which we really care about the number of pages in the system to that level of accuracy?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at