[PATCH] powerpc/mm/64: Document the sizes of/sizes mapped by Pxx_INDEX_SIZE

[Aneesh Kumar K.V]

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

> Add comments describing the size in bytes of the various levels of the
> page table tree, and the size of the virtual address space mapped by
> each level, to make it clear what the sizes are without having to also
> look up other definitions.
>
> The code that calculates the sizes actually uses sizeof(pgd_t) etc.,
> so in theory these comments could skew vs the code, but the size of
> pgd_t etc. is unlikely to change very often.
>

This makes it soo much easier to follow the page table mapping w.r.t hugepages.

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

> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> ---
>  arch/powerpc/include/asm/book3s/64/hash-4k.h   | 8 ++++----
>  arch/powerpc/include/asm/book3s/64/hash-64k.h  | 9 +++++----
>  arch/powerpc/include/asm/book3s/64/radix-4k.h  | 9 +++++----
>  arch/powerpc/include/asm/book3s/64/radix-64k.h | 8 ++++----
>  4 files changed, 18 insertions(+), 16 deletions(-)
>
> Someone *please* check my math :)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/hash-4k.h b/arch/powerpc/include/asm/book3s/64/hash-4k.h
> index cf5ba5254299..54fab723a8c7 100644
> --- a/arch/powerpc/include/asm/book3s/64/hash-4k.h
> +++ b/arch/powerpc/include/asm/book3s/64/hash-4k.h
> @@ -2,10 +2,10 @@
>  #ifndef _ASM_POWERPC_BOOK3S_64_HASH_4K_H
>  #define _ASM_POWERPC_BOOK3S_64_HASH_4K_H
>
> -#define H_PTE_INDEX_SIZE  9
> -#define H_PMD_INDEX_SIZE  7
> -#define H_PUD_INDEX_SIZE  9
> -#define H_PGD_INDEX_SIZE  9
> +#define H_PTE_INDEX_SIZE  9  // size: 8B << 9 = 4KB, maps: 2^9 x   4KB =   2MB
> +#define H_PMD_INDEX_SIZE  7  // size: 8B << 7 = 1KB, maps: 2^7 x   2MB = 256MB
> +#define H_PUD_INDEX_SIZE  9  // size: 8B << 9 = 4KB, maps: 2^9 x 256MB = 128GB
> +#define H_PGD_INDEX_SIZE  9  // size: 8B << 9 = 4KB, maps: 2^9 x 128GB =  64TB
>
>  /*
>   * Each context is 512TB. But on 4k we restrict our max TASK size to 64TB
> diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> index f82ee8a3b561..81f4eb6e7da4 100644
> --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
> +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> @@ -2,10 +2,11 @@
>  #ifndef _ASM_POWERPC_BOOK3S_64_HASH_64K_H
>  #define _ASM_POWERPC_BOOK3S_64_HASH_64K_H
>
> -#define H_PTE_INDEX_SIZE  8
> -#define H_PMD_INDEX_SIZE  10
> -#define H_PUD_INDEX_SIZE  10
> -#define H_PGD_INDEX_SIZE  8
> +#define H_PTE_INDEX_SIZE   8  // size: 8B <<  8 = 2KB, maps 2^8  x 64KB = 16MB
> +#define H_PMD_INDEX_SIZE  10  // size: 8B << 10 = 8KB, maps 2^10 x 16MB = 16GB
> +#define H_PUD_INDEX_SIZE  10  // size: 8B << 10 = 8KB, maps 2^10 x 16GB = 16TB
> +#define H_PGD_INDEX_SIZE   8  // size: 8B <<  8 = 2KB, maps 2^8  x 16TB =  4PB
> +
>
>  /*
>   * Each context is 512TB size. SLB miss for first context/default context
> diff --git a/arch/powerpc/include/asm/book3s/64/radix-4k.h b/arch/powerpc/include/asm/book3s/64/radix-4k.h
> index 863c3e8286fb..d5f5ab73dc7f 100644
> --- a/arch/powerpc/include/asm/book3s/64/radix-4k.h
> +++ b/arch/powerpc/include/asm/book3s/64/radix-4k.h
> @@ -5,10 +5,11 @@
>  /*
>   * For 4K page size supported index is 13/9/9/9
>   */
> -#define RADIX_PTE_INDEX_SIZE  9  /* 2MB huge page */
> -#define RADIX_PMD_INDEX_SIZE  9  /* 1G huge page */
> -#define RADIX_PUD_INDEX_SIZE	 9
> -#define RADIX_PGD_INDEX_SIZE  13
> +#define RADIX_PTE_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x    4K =   2MB
> +#define RADIX_PMD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   2MB =   1GB
> +#define RADIX_PUD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   1GB = 512GB
> +#define RADIX_PGD_INDEX_SIZE  13  // size: 8B << 13 = 64KB, maps 2^13 x 512GB =   4PB
> +
>  /*
>   * One fragment per per page
>   */
> diff --git a/arch/powerpc/include/asm/book3s/64/radix-64k.h b/arch/powerpc/include/asm/book3s/64/radix-64k.h
> index ccb78ca9d0c5..54e33828b0fb 100644
> --- a/arch/powerpc/include/asm/book3s/64/radix-64k.h
> +++ b/arch/powerpc/include/asm/book3s/64/radix-64k.h
> @@ -5,10 +5,10 @@
>  /*
>   * For 64K page size supported index is 13/9/9/5
>   */
> -#define RADIX_PTE_INDEX_SIZE  5  /* 2MB huge page */
> -#define RADIX_PMD_INDEX_SIZE  9  /* 1G huge page */
> -#define RADIX_PUD_INDEX_SIZE	 9
> -#define RADIX_PGD_INDEX_SIZE  13
> +#define RADIX_PTE_INDEX_SIZE   5  // size: 8B <<  5 = 256B, maps 2^5  x   64K =   2MB
> +#define RADIX_PMD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   2MB =   1GB
> +#define RADIX_PUD_INDEX_SIZE   9  // size: 8B <<  9 =  4KB, maps 2^9  x   1GB = 512GB
> +#define RADIX_PGD_INDEX_SIZE  13  // size: 8B << 13 = 64KB, maps 2^13 x 512GB =   4PB
>
>  /*
>   * We use a 256 byte PTE page fragment in radix
> -- 
> 2.20.1