Subject: Re: [v5 09/15] sparc64: optimized struct page zeroing

From: Sam Ravnborg <[email-masked]>
Date: Fri, 4 Aug 2017 07:37:01 +0200

Hi Pavel.

On Thu, Aug 03, 2017 at 05:23:47PM -0400, Pavel Tatashin wrote:
> Add an optimized mm_zero_struct_page(), so struct page's are zeroed without
> calling memset(). We do eight regular stores, thus avoid cost of membar.

The commit message does no longer reflect the implementation,
and should be updated.

> 
> Signed-off-by: Pavel Tatashin <[email-masked]>
> Reviewed-by: Steven Sistare <[email-masked]>
> Reviewed-by: Daniel Jordan <[email-masked]>
> Reviewed-by: Bob Picco <[email-masked]>
> ---
>  arch/sparc/include/asm/pgtable_64.h | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
> index 6fbd931f0570..be47537e84c5 100644
> --- a/arch/sparc/include/asm/pgtable_64.h
> +++ b/arch/sparc/include/asm/pgtable_64.h
> @@ -230,6 +230,38 @@ extern unsigned long _PAGE_ALL_SZ_BITS;
>  extern struct page *mem_map_zero;
>  #define ZERO_PAGE(vaddr)	(mem_map_zero)
>  
> +/* This macro must be updated when the size of struct page grows above 80
> + * or reduces below 64.
> + * The idea that compiler optimizes out switch() statement, and only
> + * leaves clrx instructions or memset() call.
> + */
> +#define	mm_zero_struct_page(pp) do {					\
> +	unsigned long *_pp = (void *)(pp);				\
> +									\
> +	/* Check that struct page is 8-byte aligned */			\
> +	BUILD_BUG_ON(sizeof(struct page) & 7);				\
Would also be good to catch if sizeof > 80 so we do not silently
migrate to the suboptimal version (silent at build time).
Can you at build time catch if size is no any of: 64, 72, 80
and simplify the below a little?

	Sam