Re: [RFC][PATCH] make /proc/pid/pagemap work with huge pages and return page size
- From: Dave Hansen <haveblue@xxxxxxxxxx>
- Date: Mon, 25 Feb 2008 10:39:10 -0800
On Mon, 2008-02-25 at 13:09 +0100, Hans Rosenfeld wrote:
On Sat, Feb 23, 2008 at 10:31:01AM -0800, Dave Hansen wrote:
- 4 bits for the page size, with 0 meaning native page size (4k on x86,
8k on alpha, ...) and values 1-15 being specific to the architecture
(I used 1 for 2M, 2 for 4M and 3 for 1G for x86)
"Native page size" probably a bad idea. ppc64 can use 64k or 4k for its
"native" page size and has 16MB large pages (as well as some others).
To make it even more confusing, you can have a 64k kernel page size with
4k mmu mappings!
That said, this is a decent idea as long as we know that nobody will
ever have more than 16 page sizes.
Then a better way to encode the page size would be returning the page
shift. This would need 6 bits instead of 4, but it would probably be
enough for any 64 bit architecture.
That's a good point.
This is ok-ish, but I can't say I like it much. Especially the page size
field.
But I don't really have many ideas here. Perhaps having a bit saying
"this entry is really a continuation of the previous one". Then any page
size can be trivially represented. This might also make the code on both
sides simpler?
I don't like the idea of parsing thousands of entries just to find out
that I'm using a huge page. It would be much better to just get the page
size one way or the other in the first entry one reads.
Did you read my suggestion? We use one bit in the pte to specify that
its a large page mapping, then specify a mask to apply to the address to
get the *first* mapping of the large page, where you're find the actual
physical address. That keeps us from having to worry about specifying
*both* the page size and the pfn in the same pte.
+#ifdef CONFIG_X86
+ if (pmd_huge(*pmd)) {
+ struct ppte ppte = {
+ .paddr = pmd_pfn(*pmd) << PAGE_SHIFT,
+ .psize = (HPAGE_SHIFT == 22 ?
+ PM_PSIZE_4M : PM_PSIZE_2M),
+ .swap = 0,
+ .present = 1,
+ };
+
+ for(; addr != end; addr += PAGE_SIZE) {
+ err = add_to_pagemap(addr, ppte, pm);
+ if (err)
+ return err;
+ }
+ } else
+#endif
It's great to make this support huge pages, but things like this
probably need their own function. Putting an #ifdef in the middle of
here makes it a lot harder to read. Just think of when powerpc, ia64
and x86_64 get their grubby mitts in here. ;)
AFAIK the way huge pages are used on x86 differs much from other
architectures. While on x86 the address translation stops at some upper
table for a huge page, other architectures encode the page size in the
pte (at least the ones I looked at did). So pmd_huge() (and soon
pud_huge()) are very x86-specific and return just 0 on other archs,
I'm just asking that you please put this in a nice helper function to
hide it from the poor powerpc/ia64/mips... guys that don't want to see
x86 code cluttering up otherwise generic functions. Yes, the compiler
optimizes it away, but it still has a cost to my eyeballs. :)
I eagerly await your next patch!
-- Dave
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