Re: [PATCH 1/2] powerpc/mm/64s: Drop pgd_huge()

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Michael Ellerman <mpe@ellerman.id.au> writes:

>
> On powerpc there are two ways for huge pages to be represented in the
> top level page table, aka PGD (Page Global Directory).
>
> If the address space mapped by an individual PGD entry does not
> correspond to a given huge page size, then the PGD entry points to a
> non-standard page table, known as a "hugepd" (Huge Page Directory).
> The hugepd contains some number of huge page PTEs sufficient to map the
> address space with the given huge page size.
>
> On the other hand, if the address space mapped by an individual PGD
> entry does correspond exactly to a given huge page size, that PGD entry
> is used to directly encode the huge page PTE in place. In this case the
> pgd_huge() wrapper indicates to generic code that the PGD entry is
> actually a huge page PTE.
>
> This commit deals with the pgd_huge() case only, it does nothing with
> respect to the hugepd case.
>
> Over time the size of the virtual address space supported on powerpc has
> increased several times, which means the location at which huge pages
> can sit in the tree has also changed. There have also been new huge page
> sizes added, with the introduction of the Radix MMU.
>
> On Power9 and later with the Radix MMU, the largest huge page size in
> any implementation is 1GB.
>
> Since the introduction of Radix, 1GB entries have been supported at the
> PUD level, with both 4K and 64K base page size. Radix has never had a
> supported huge page size at the PGD level.
>
> On Power8 or earlier, which uses the Hash MMU, or Power9 or later with
> the Hash MMU enabled, the largest huge page size is 16GB.
>
> Using the Hash MMU and a base page size of 4K, 16GB has never been a
> supported huge page size at the PGD level, due to the geometry being
> incompatible. The two supported huge page sizes (16M & 16GB) both use
> the hugepd format.
>
> Using the Hash MMU and a base page size of 64K, 16GB pages were
> supported in the past at the PGD level.
>
> However in commit ba95b5d03596 ("powerpc/mm/book3s/64: Rework page table
> geometry for lower memory usage") the page table layout was reworked to
> shrink the size of the PGD.
>
> As a result the 16GB page size now fits at the PUD level when using 64K
> base page size.
>
> Therefore there are no longer any supported configurations where
> pgd_huge() can be true, so drop the definitions for pgd_huge(), and
> fallback to the generic definition which is always false.
>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

> Fixes: ba95b5d03596 ("powerpc/mm/book3s/64: Rework page table geometry for lower memory usage")
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/include/asm/book3s/64/pgtable-4k.h  | 10 ----------
>  arch/powerpc/include/asm/book3s/64/pgtable-64k.h |  9 ---------
>  2 files changed, 19 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> index 4e697bc2f4cd..48f21820afe2 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
> @@ -26,16 +26,6 @@ static inline int pud_huge(pud_t pud)
>  	return 0;
>  }
>  
> -static inline int pgd_huge(pgd_t pgd)
> -{
> -	/*
> -	 * leaf pte for huge page
> -	 */
> -	if (radix_enabled())
> -		return !!(pgd_raw(pgd) & cpu_to_be64(_PAGE_PTE));
> -	return 0;
> -}
> -#define pgd_huge pgd_huge
>  /*
>   * With radix , we have hugepage ptes in the pud and pmd entries. We don't
>   * need to setup hugepage directory for them. Our pte and page directory format
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable-64k.h b/arch/powerpc/include/asm/book3s/64/pgtable-64k.h
> index 34d1018896b3..2fce3498b000 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable-64k.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable-64k.h
> @@ -30,15 +30,6 @@ static inline int pud_huge(pud_t pud)
>  	return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PTE));
>  }
>  
> -static inline int pgd_huge(pgd_t pgd)
> -{
> -	/*
> -	 * leaf pte for huge page
> -	 */
> -	return !!(pgd_raw(pgd) & cpu_to_be64(_PAGE_PTE));
> -}
> -#define pgd_huge pgd_huge
> -
>  /*
>   * With 64k page size, we have hugepage ptes in the pgd and pmd entries. We don't
>   * need to setup hugepage directory for them. Our pte and page directory format
> -- 
> 2.37.2