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Date: Sat, 23 Feb 2008 00:06:26 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Hans Rosenfeld" <hans.rosenfeld@....com>
Cc: "Matt Mackall" <mpm@...enic.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] make /proc/pid/pagemap work with huge pages and
return page size
On Wed, 20 Feb 2008 14:57:43 +0100 "Hans Rosenfeld" <hans.rosenfeld@....com> wrote:
> The current code for /proc/pid/pagemap does not work with huge pages (on
> x86). The code will make no difference between a normal pmd and a huge
> page pmd, trying to parse the contents of the huge page as ptes. Another
> problem is that there is no way to get information about the page size a
> specific mapping uses.
>
> Also, the current way the "not present" and "swap" bits are encoded in
> the returned pfn isn't very clean, especially not if this interface is
> going to be extended.
>
> I propose to change /proc/pid/pagemap to return a pseudo-pte instead of
> just a raw pfn. The pseudo-pte will contain:
>
> - 58 bits for the physical address of the first byte in the page, even
> less bits would probably be sufficient for quite a while
>
> - 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)
>
> - a "swap" bit indicating that a not present page is paged out, with the
> physical address field containing page file number and block number
> just like before
>
> - a "present" bit just like in a real pte
>
> By shortening the field for the physical address, some more interesting
> information could be included, like read/write permissions and the like.
> The page size could also be returned directly, 6 bits could be used to
> express any page shift in a 64 bit system, but I found the encoded page
> size more useful for my specific use case.
>
>
> The attached patch changes the /proc/pid/pagemap code to use such a
> pseudo-pte. The huge page handling is currently limited to 2M/4M pages
> on x86, 1G pages will need some more work. To keep the simple mapping of
> virtual addresses to file index intact, any huge page pseudo-pte is
> replicated in the user buffer to map the equivalent range of small
> pages.
>
> Note that I had to move the pmd_pfn() macro from asm-x86/pgtable_64.h to
> asm-x86/pgtable.h, it applies to both 32 bit and 64 bit x86.
>
> Other architectures will probably need other changes to support huge
> pages and return the page size.
>
> I think that the definition of the pseudo-pte structure and the page
> size codes should be made available through a header file, but I didn't
> do this for now.
>
If we're going to do this, we need to do it *fast*. Once 2.6.25 goes out
our hands are tied.
That means talking with the maintainers of other hugepage-capable
architectures.
> +struct ppte {
> + uint64_t paddr:58;
> + uint64_t psize:4;
> + uint64_t swap:1;
> + uint64_t present:1;
> +};
This is part of the exported kernel interface and hence should be in a
header somewhere, shouldn't it? The old stuff should have been too.
u64 is a bit more conventional than uint64_t, and if we move this to a
userspace-visible header then __u64 is the type to use, I think. Although
one would expect uint64_t to be OK as well.
> +#ifdef CONFIG_X86
> +#define PM_PSIZE_1G 3
> +#define PM_PSIZE_4M 2
> +#define PM_PSIZE_2M 1
> +#endif
No, we should factor this correctly and get the CONFIG_X86 stuff out of here.
> +#define PM_ENTRY_BYTES sizeof(struct ppte)
> +#define PM_END_OF_BUFFER 1
> +
> +static int add_to_pagemap(unsigned long addr, struct ppte ppte,
> struct pagemapread *pm)
> {
> /*
> @@ -545,13 +552,13 @@ static int add_to_pagemap(unsigned long addr, u64 pfn,
> * the pfn.
> */
> if (pm->out + PM_ENTRY_BYTES >= pm->end) {
> - if (copy_to_user(pm->out, &pfn, pm->end - pm->out))
> + if (copy_to_user(pm->out, &ppte, pm->end - pm->out))
> return -EFAULT;
> pm->out = pm->end;
> return PM_END_OF_BUFFER;
> }
>
> - if (put_user(pfn, pm->out))
> + if (copy_to_user(pm->out, &ppte, sizeof(ppte)))
> return -EFAULT;
> pm->out += PM_ENTRY_BYTES;
> return 0;
> @@ -564,7 +571,7 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
> unsigned long addr;
> int err = 0;
> for (addr = start; addr < end; addr += PAGE_SIZE) {
> - err = add_to_pagemap(addr, PM_NOT_PRESENT, pm);
> + err = add_to_pagemap(addr, (struct ppte) {0, 0, 0, 0}, pm);
> if (err)
> break;
> }
> @@ -574,7 +581,7 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
> u64 swap_pte_to_pagemap_entry(pte_t pte)
> {
> swp_entry_t e = pte_to_swp_entry(pte);
> - return PM_SWAP | swp_type(e) | (swp_offset(e) << MAX_SWAPFILES_SHIFT);
> + return swp_type(e) | (swp_offset(e) << MAX_SWAPFILES_SHIFT);
> }
>
> static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> @@ -584,16 +591,37 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> pte_t *pte;
> int err = 0;
>
> +#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
You can remove the ifdefs here and then for each hugepage-capable
architecture add a
struct ppte pmd_to_ppte(pmd_t *pmd);
and call that. Something like that.
ppte isn't a very well-chosen name, especially if we export this to
userspace.
> for (; addr != end; addr += PAGE_SIZE) {
> - u64 pfn = PM_NOT_PRESENT;
> + struct ppte ppte = { 0, 0, 0, 0};
> +
> pte = pte_offset_map(pmd, addr);
> - if (is_swap_pte(*pte))
> - pfn = swap_pte_to_pagemap_entry(*pte);
> - else if (pte_present(*pte))
> - pfn = pte_pfn(*pte);
> + if (is_swap_pte(*pte)) {
> + ppte.swap = 1;
> + ppte.paddr = swap_pte_to_pagemap_entry(*pte);
> + } else if (pte_present(*pte)) {
> + ppte.present = 1;
> + ppte.paddr = pte_pfn(*pte) << PAGE_SHIFT;
> + }
> /* unmap so we're not in atomic when we copy to userspace */
> pte_unmap(pte);
> - err = add_to_pagemap(addr, pfn, pm);
> + err = add_to_pagemap(addr, ppte, pm);
> if (err)
> return err;
> }
Matt? Help?
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