Re: [PATCH] mm/zsmalloc: simplify zs_max_alloc_size handling

Minchan Kim
2017-06-30T01:24:39Z

Hi Jerome,

On Wed, Jun 28, 2017 at 10:14:20AM +0200, Jerome Marchand wrote:
> Commit 40f9fb8cffc6 ("mm/zsmalloc: support allocating obj with size of
> ZS_MAX_ALLOC_SIZE") fixes a size calculation error that prevented
> zsmalloc to allocate an object of the maximal size
> (ZS_MAX_ALLOC_SIZE). I think however the fix is unneededly
> complicated.
>
> This patch replaces the dynamic calculation of zs_size_classes at init
> time by a compile time calculation that uses the DIV_ROUND_UP() macro
> already used in get_size_class_index().
>
> Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
> ---
> mm/zsmalloc.c | 52 +++++++++++++++-------------------------------------
> 1 file changed, 15 insertions(+), 37 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index d41edd2..134024b 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -116,6 +116,11 @@
> #define OBJ_INDEX_BITS (BITS_PER_LONG - _PFN_BITS - OBJ_TAG_BITS)
> #define OBJ_INDEX_MASK ((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
>
> +#define FULLNESS_BITS 2
> +#define CLASS_BITS 8
> +#define ISOLATED_BITS 3
> +#define MAGIC_VAL_BITS 8
> +
> #define MAX(a, b) ((a) >= (b) ? (a) : (b))
> /* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
> #define ZS_MIN_ALLOC_SIZE \
> @@ -137,6 +142,8 @@
> * (reason above)
> */
> #define ZS_SIZE_CLASS_DELTA (PAGE_SIZE >> CLASS_BITS)
> +#define ZS_SIZE_CLASSES DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
> + ZS_SIZE_CLASS_DELTA)

#define ZS_SIZE_CLASSES (DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
ZS_SIZE_CLASS_DELTA) + 1)

I think it should add +1 to cover ZS_MIN_ALLOC_SIZE.
Otherwise, looks good to me.

Thanks.