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

From: Rashmica <[email-masked]>
Date: Fri, 19 Feb 2016 15:31:20 +1100

On 19/02/16 15:08, Balbir Singh wrote:
>
> 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)
I feel like someone more senior than me should weigh in on if this is 
worth doing...

> 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
Yup, you are right! Might have had a brain spaz...
>> 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?
Agreed.
>>   	default "9" if PPC64 && PPC_64K_PAGES
>>   	range 13 64 if PPC64 && !PPC_64K_PAGES
> Should this be fixed as well?
> range 9 13?
Agreed.
>>   	default "13" if PPC64 && !PPC_64K_PAGES
Should the default values remain as is, or follow the trend and be equal 
to the minimum value?
> Please check my calculations
>
> Reviewed-by: Balbir Singh <[email-masked]>
>
> Balbir Singh