Re: [RFC][PATCH] make /proc/pid/pagemap work with huge pages andreturn page size

From: Matt Mackall
Date: Fri Feb 22 2008 - 21:18:53 EST

(sorry for the delay, travelling)

On Wed, 2008-02-20 at 14:57 +0100, Hans Rosenfeld 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.

Fair.

> 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

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?

> 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.
>
> Signed-Off-By: Hans Rosenfeld <hans.rosenfeld@xxxxxxx>
>
> ---
> fs/proc/task_mmu.c | 68 +++++++++++++++++++++++++++++------------
> include/asm-x86/pgtable.h | 2 +
> include/asm-x86/pgtable_64.h | 1 -
> 3 files changed, 50 insertions(+), 21 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 49958cf..58af588 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -527,16 +527,23 @@ struct pagemapread {
> char __user *out, *end;
> };
>
> -#define PM_ENTRY_BYTES sizeof(u64)
> -#define PM_RESERVED_BITS 3
> -#define PM_RESERVED_OFFSET (64 - PM_RESERVED_BITS)
> -#define PM_RESERVED_MASK (((1LL<<PM_RESERVED_BITS)-1) << PM_RESERVED_OFFSET)
> -#define PM_SPECIAL(nr) (((nr) << PM_RESERVED_OFFSET) | PM_RESERVED_MASK)
> -#define PM_NOT_PRESENT PM_SPECIAL(1LL)
> -#define PM_SWAP PM_SPECIAL(2LL)
> -#define PM_END_OF_BUFFER 1
> -
> -static int add_to_pagemap(unsigned long addr, u64 pfn,
> +struct ppte {
> + uint64_t paddr:58;
> + uint64_t psize:4;
> + uint64_t swap:1;
> + uint64_t present:1;
> +};
> +
> +#ifdef CONFIG_X86
> +#define PM_PSIZE_1G 3
> +#define PM_PSIZE_4M 2
> +#define PM_PSIZE_2M 1
> +#endif
> +
> +#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
> 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;
> }
> diff --git a/include/asm-x86/pgtable.h b/include/asm-x86/pgtable.h
> index 174b877..76bc8a8 100644
> --- a/include/asm-x86/pgtable.h
> +++ b/include/asm-x86/pgtable.h
> @@ -181,6 +181,8 @@ static inline pmd_t pfn_pmd(unsigned long page_nr, pgprot_t pgprot)
> pgprot_val(pgprot)) & __supported_pte_mask);
> }
>
> +#define pmd_pfn(x) ((pmd_val(x) & __PHYSICAL_MASK) >> PAGE_SHIFT)
> +
> static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
> {
> pteval_t val = pte_val(pte);
> diff --git a/include/asm-x86/pgtable_64.h b/include/asm-x86/pgtable_64.h
> index 02bd4aa..094a538 100644
> --- a/include/asm-x86/pgtable_64.h
> +++ b/include/asm-x86/pgtable_64.h
> @@ -216,7 +216,6 @@ static inline int pud_large(pud_t pte)
> #define pmd_none(x) (!pmd_val(x))
> #define pmd_present(x) (pmd_val(x) & _PAGE_PRESENT)
> #define pfn_pmd(nr,prot) (__pmd(((nr) << PAGE_SHIFT) | pgprot_val(prot)))
> -#define pmd_pfn(x) ((pmd_val(x) & __PHYSICAL_MASK) >> PAGE_SHIFT)
>
> #define pte_to_pgoff(pte) ((pte_val(pte) & PHYSICAL_PAGE_MASK) >> PAGE_SHIFT)
> #define pgoff_to_pte(off) ((pte_t) { .pte = ((off) << PAGE_SHIFT) | _PAGE_FILE })
> --
> 1.5.3.7
>
--
Mathematics is the supreme nostalgia of our time.

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