Subject: Re: [PATCH] powerpc/Kconfig: Update config option based on page size.

From: Balbir Singh <[email-masked]>
Date: Fri, 19 Feb 2016 15:08:21 +1100

On 19/02/16 12:55, Rashmica Gupta wrote:
> Currently on PPC64 changing kernel pagesize from 4K to 64K leaves
> FORCE_MAX_ZONEORDER set to 13 - which produces a compile error.
>
> The error occurs because of the following constraint (from
> include/linux/mmzone.h) being violated:
>
> 	MAX_ORDER -1 + PAGESHIFT <= SECTION_SIZE_BITS.
IA64 has this cool hack

      12 #ifdef CONFIG_FORCE_MAX_ZONEORDER
      13 #if ((CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS)
      14 #undef SECTION_SIZE_BITS
      15 #define SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT)
      16 #endif

But coming back (we can revisit the SECTION_SIZE_BITS definition later)

MAX_ORDER -1 + 16 <= 24 for 64 K

and

MAX_ORDER -1 + 12 < = 24 for 4K

Your calculations are correct
>
> Expanding this out, we get:
>
> 	FORCE_MAX_ZONEBITS <= 25 - PAGESHIFT,
>
> which requires, for a 64K page, FORCE_MAX_ZONEBITS <= 9. Thus
> set max value of FORCE_MAX_ZONEORDER for 64K pages to 9.
>
> Also, check the minimum value:
> In include/linux/huge_mm.h, we have the constraint HPAGE_PMD_ORDER <
> MAX_ORDER which expands out to:
>
> 	PTE_INDEX_SIZE < FORCE_MAX_ZONEORDER.

> PTE_INDEX_SIZE is:
> 	9 (4k hash or no hash 4K pgtable) or
> 	8 (64K hash or no hash 64K pgtable).
> Thus a min value of 9 for 64K pages is reasonable.

For 4K pages we end up with

9 < FORCE_MAX_ZONE_ORDER
FORCE_MAX_ZONE_ORDER -1 + 12 < = 24

The range is 9 to 13

For 64K we end up with
8 < FORCE_MAX_ZONE_ORDER
FORCE_MAX_ZONE_ORDER -1 + 16 <= 24 or FORCE_MAX_ZONE_ORDER <= 9

The range is really between 8 and 9 unless we tweak the SECTION_SIZE_BITS
> So, update the range of FORCE_MAX_ZONEORDER from 9-64 to 9-9.
>
> Signed-off-by: Rashmica Gupta <[email-masked]>
> ---
>  arch/powerpc/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index e4824fd04bb7..3bd3465b93ba 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -585,7 +585,7 @@ endchoice
>  
>  config FORCE_MAX_ZONEORDER
>  	int "Maximum zone order"
> -	range 9 64 if PPC64 && PPC_64K_PAGES
> +	range 9 9 if PPC64 && PPC_64K_PAGES
range 8 9?
>  	default "9" if PPC64 && PPC_64K_PAGES
>  	range 13 64 if PPC64 && !PPC_64K_PAGES
Should this be fixed as well?
range 9 13?
>  	default "13" if PPC64 && !PPC_64K_PAGES

Please check my calculations

Reviewed-by: Balbir Singh <[email-masked]>

Balbir Singh