linux-kernel - Re: [PATCHv3 10/11] arm64: Add 16K page size support

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Message-ID: <561FBCC9.2000106@arm.com>
Date:	Thu, 15 Oct 2015 15:48:41 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	catalin.marinas@....com, will.deacon@....com,
	steve.capper@...aro.org, marc.zyngier@....com,
	ard.biesheuvel@...aro.org, christoffer.dall@...aro.org,
	Jeremy Linton <jeremy.linton@....com>
Subject: Re: [PATCHv3 10/11] arm64: Add 16K page size support

On 15/10/15 15:06, Mark Rutland wrote:
> Hi,
>

I have fixed all the nits locally. Thanks for pointing them out.

>>   config FORCE_MAX_ZONEORDER
>>   	int
>>   	default "14" if (ARM64_64K_PAGES && TRANSPARENT_HUGEPAGE)
>> +	default "12" if (ARM64_16K_PAGES && TRANSPARENT_HUGEPAGE)
>>   	default "11"
>
> I'm a little lost here. How are these numbers derived?
>

I struggled to find the right value for 16K. Thanks to Steve Capper
for the following explanation. I will add it as a comment.

All allocations from the buddy allocator have to have compound order
strictly less than MAX_ORDER. i.e, the maximum allocation size is
(MAX_ORDER - 1) PAGES. To align with the transparent huge page size,
we get :

  (MAX_ORDER - 1) + PAGE_SHIFT = PMD_SHIFT

Which gives us:

MAX_ORDER = PAGE_SHIFT - 3 + PAGE_SHIFT - PAGE_SHIFT + 1
           = PAGE_SHIFT - 2

That raises an interesting question about the selection of the value
for 4K. Shouldn't that be 10 instead of 11 ?

Steve ?

>> -#ifdef CONFIG_ARM64_64K_PAGES
>> +#if	defined(CONFIG_ARM64_64K_PAGES)
>>   #define NR_FIX_BTMAPS		4
>> +#elif	defined (CONFIG_ARM64_16K_PAGES)
>> +#define NR_FIX_BTMAPS		16
>>   #else
>>   #define NR_FIX_BTMAPS		64
>>   #endif
>
> We could include <linux/sizes.h> and simplify this to:
>
> #define NR_FIX_BTMAPS (SZ_256K / PAGE_SIZE)
>
> Which works for me locally.

Nice cleanup. I will pick that as a separate patch in the series.

>
>> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
>> index 5eac6a2..90c7ff2 100644
>> --- a/arch/arm64/include/asm/thread_info.h
>> +++ b/arch/arm64/include/asm/thread_info.h
>> @@ -25,6 +25,8 @@
>>
>>   #ifdef CONFIG_ARM64_4K_PAGES
>>   #define THREAD_SIZE_ORDER	2
>> +#elif defined(CONFIG_ARM64_16K_PAGES)
>> +#define THREAD_SIZE_ORDER	0
>>   #endif
>>   #define THREAD_SIZE		16384
>
> The above looks correct.
>
> As an open/general question, why do both THREAD_SIZE_ORDER and
> THREAD_SIZE exist? One really should be defined in terms of the other.

I think its mainly for choosing the mechanism for stack allocation. If it
is a multiple of a page, you allocate a page. If not, uses a kmem_cache.


>>   #define id_aa64mmfr0_tgran_shift	ID_AA64MMFR0_TGRAN4_SHIFT
>>   #define id_aa64mmfr0_tgran_on		ID_AA64MMFR0_TGRAN4_ON
>
> I assume you'll s/ON/SUPPORTED/ per comments in another thread.
>

Yes

Thanks
Suzuki

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