Re: [PATCH 2/6] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS

[Nitin Gupta]

On Fri, Sep 29, 2017 at 7:08 AM, Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> With boot-time switching between paging mode we will have variable
> MAX_PHYSMEM_BITS.
>
> Let's use the maximum variable possible for CONFIG_X86_5LEVEL=y
> configuration to define zsmalloc data structures.
>
> The patch introduces MAX_POSSIBLE_PHYSMEM_BITS to cover such case.
> It also suits well to handle PAE special case.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
> ---
>  arch/x86/include/asm/pgtable-3level_types.h |  1 +
>  arch/x86/include/asm/pgtable_64_types.h     |  2 ++
>  mm/zsmalloc.c                               | 13 +++++++------
>  3 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable-3level_types.h b/arch/x86/include/asm/pgtable-3level_types.h
> index b8a4341faafa..3fe1d107a875 100644
> --- a/arch/x86/include/asm/pgtable-3level_types.h
> +++ b/arch/x86/include/asm/pgtable-3level_types.h
> @@ -43,5 +43,6 @@ typedef union {
>   */
>  #define PTRS_PER_PTE   512
>
> +#define MAX_POSSIBLE_PHYSMEM_BITS      36
>
>  #endif /* _ASM_X86_PGTABLE_3LEVEL_DEFS_H */
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index 06470da156ba..39075df30b8a 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -39,6 +39,8 @@ typedef struct { pteval_t pte; } pte_t;
>  #define P4D_SIZE       (_AC(1, UL) << P4D_SHIFT)
>  #define P4D_MASK       (~(P4D_SIZE - 1))
>
> +#define MAX_POSSIBLE_PHYSMEM_BITS      52
> +
>  #else /* CONFIG_X86_5LEVEL */
>
>  /*
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 7c38e850a8fc..7bde01c55c90 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -82,18 +82,19 @@
>   * This is made more complicated by various memory models and PAE.
>   */
>
> -#ifndef MAX_PHYSMEM_BITS
> -#ifdef CONFIG_HIGHMEM64G
> -#define MAX_PHYSMEM_BITS 36
> -#else /* !CONFIG_HIGHMEM64G */
> +#ifndef MAX_POSSIBLE_PHYSMEM_BITS
> +#ifdef MAX_PHYSMEM_BITS
> +#define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
> +#else
>  /*
>   * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
>   * be PAGE_SHIFT
>   */
> -#define MAX_PHYSMEM_BITS BITS_PER_LONG
> +#define MAX_POSSIBLE_PHYSMEM_BITS BITS_PER_LONG
>  #endif
>  #endif
> -#define _PFN_BITS              (MAX_PHYSMEM_BITS - PAGE_SHIFT)
> +
> +#define _PFN_BITS              (MAX_POSSIBLE_PHYSMEM_BITS - PAGE_SHIFT)
>


I think we can avoid using this new constant in zsmalloc.

The reason for trying to save on MAX_PHYSMEM_BITS is just to gain more
bits for OBJ_INDEX_BITS which would reduce ZS_MIN_ALLOC_SIZE. However,
for all practical values of ZS_MAX_PAGES_PER_ZSPAGE, this min size
would remain 32 bytes.

So, we can unconditionally use MAX_PHYSMEM_BITS = BITS_PER_LONG and
thus OBJ_INDEX_BITS = PAGE_SHIFT.

- Nitin