Re: [PATCH 6/8] mm: Store compound_dtor / compound_order as bytes

[Michal Hocko <mhocko@kernel.org>]

On Sat 16-12-17 08:44:23, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@microsoft.com>
> 
> Neither of these values get even close to 256; compound_dtor is
> currently at a maximum of 3, and compound_order can't be over 64.
> No machine has inefficient access to bytes since EV5, and while
> those are still supported, we don't optimise for them any more.

Hmm, so the improvement is the ifdef-ery removale, right? Beucase this
will not shrink the structure size AFAICS. I think that the former is
a sufficient justification. Maybe you should spell it out.

> Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/mm_types.h | 15 ++-------------
>  1 file changed, 2 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5521c9799c50..1a3ba1f1605d 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -136,19 +136,8 @@ struct page {
>  			unsigned long compound_head; /* If bit zero is set */
>  
>  			/* First tail page only */
> -#ifdef CONFIG_64BIT
> -			/*
> -			 * On 64 bit system we have enough space in struct page
> -			 * to encode compound_dtor and compound_order with
> -			 * unsigned int. It can help compiler generate better or
> -			 * smaller code on some archtectures.
> -			 */
> -			unsigned int compound_dtor;
> -			unsigned int compound_order;
> -#else
> -			unsigned short int compound_dtor;
> -			unsigned short int compound_order;
> -#endif
> +			unsigned char compound_dtor;
> +			unsigned char compound_order;
>  		};
>  
>  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
> -- 
> 2.15.1
> 
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-- 
Michal Hocko
SUSE Labs

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