Subject: Re: [PATCH 1/7] powerpc/mm: 64-bit 4k: use page-sized PMDs

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 19 May 2011 07:32:41 +1000

On Wed, 2011-05-18 at 16:04 -0500, Scott Wood wrote:
> This allows a virtual page table to be used at the PMD rather than
> the PTE level.
> 
> Rather than adjust the constant in pgd_index() (or ignore it, as
> too-large values don't hurt as long as overly large addresses aren't
> passed in), go back to using PTRS_PER_PGD.  The overflow comment seems to
> apply to a very old implementation of free_pgtables that used pgd_index()
> (unfortunately the commit message, if you seek it out in the historic
> tree, doesn't mention any details about the overflow).  The existing
> value was numerically indentical to the old 4K-page PTRS_PER_PGD, so
> using it shouldn't produce an overflow where it's not otherwise possible.
> 
> Also get rid of the incorrect comment at the top of pgtable-ppc64-4k.h.

Why do you want to create a virtual page table at the PMD level ? Also,
you are changing the geometry of the page tables which I think we don't
want. We chose that geometry so that the levels match the segment sizes
on server, I think it may have an impact with the hugetlbfs code (check
with David), it also was meant as a way to implement shared page tables
on hash64 tho we never published that.

Cheers,
Ben.

> Signed-off-by: Scott Wood <[email-masked]>
> ---
>  arch/powerpc/include/asm/pgtable-ppc64-4k.h |   12 ++++--------
>  arch/powerpc/include/asm/pgtable-ppc64.h    |    3 +--
>  2 files changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/pgtable-ppc64-4k.h b/arch/powerpc/include/asm/pgtable-ppc64-4k.h
> index 6eefdcf..194005e 100644
> --- a/arch/powerpc/include/asm/pgtable-ppc64-4k.h
> +++ b/arch/powerpc/include/asm/pgtable-ppc64-4k.h
> @@ -1,14 +1,10 @@
>  #ifndef _ASM_POWERPC_PGTABLE_PPC64_4K_H
>  #define _ASM_POWERPC_PGTABLE_PPC64_4K_H
> -/*
> - * Entries per page directory level.  The PTE level must use a 64b record
> - * for each page table entry.  The PMD and PGD level use a 32b record for
> - * each entry by assuming that each entry is page aligned.
> - */
> +
>  #define PTE_INDEX_SIZE  9
> -#define PMD_INDEX_SIZE  7
> +#define PMD_INDEX_SIZE  9
>  #define PUD_INDEX_SIZE  7
> -#define PGD_INDEX_SIZE  9
> +#define PGD_INDEX_SIZE  7
>  
>  #ifndef __ASSEMBLY__
>  #define PTE_TABLE_SIZE	(sizeof(pte_t) << PTE_INDEX_SIZE)
> @@ -19,7 +15,7 @@
>  
>  #define PTRS_PER_PTE	(1 << PTE_INDEX_SIZE)
>  #define PTRS_PER_PMD	(1 << PMD_INDEX_SIZE)
> -#define PTRS_PER_PUD	(1 << PMD_INDEX_SIZE)
> +#define PTRS_PER_PUD	(1 << PUD_INDEX_SIZE)
>  #define PTRS_PER_PGD	(1 << PGD_INDEX_SIZE)
>  
>  /* PMD_SHIFT determines what a second-level page table entry can map */
> diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h b/arch/powerpc/include/asm/pgtable-ppc64.h
> index 2b09cd5..8bd1cd9 100644
> --- a/arch/powerpc/include/asm/pgtable-ppc64.h
> +++ b/arch/powerpc/include/asm/pgtable-ppc64.h
> @@ -181,8 +181,7 @@
>   * Find an entry in a page-table-directory.  We combine the address region
>   * (the high order N bits) and the pgd portion of the address.
>   */
> -/* to avoid overflow in free_pgtables we don't use PTRS_PER_PGD here */
> -#define pgd_index(address) (((address) >> (PGDIR_SHIFT)) & 0x1ff)
> +#define pgd_index(address) (((address) >> (PGDIR_SHIFT)) & (PTRS_PER_PGD - 1))
>  
>  #define pgd_offset(mm, address)	 ((mm)->pgd + pgd_index(address))
>