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

From	"Suzuki K. Poulose" <Suzuki.Poulose@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCHv3 10/11] arm64: Add 16K page size support
Date	2015-10-15 17:50 +0200
Message-ID	<qjSX0-2mu-7@gated-at.bofh.it> (permalink)
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On 15/10/15 16:36, Steve Capper wrote:
> On 15 October 2015 at 15:48, Suzuki K. Poulose <Suzuki.Poulose@arm.com> wrote:
>> 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 ?
>
> Hi,
> My understanding is that 11 is a "good minimum" value for the page
> allocator with 4KB pages.
> (There are references to it being 10 in 2.4 kernels but raised to 11
> on 2.6 kernels?)
>
> We need to raise the minimum when we have a 16KB or 64KB PAGE_SIZE to
> be able allocate a 32MB or 512MB Transparent HugePages.
>

Thanks Steve, for the clarification. I will add the following comment
to the Kconfig

#
# 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
#
# However for 4K, we choose a higher default value 11 as opposed to 10, (giving us size 4M)
# matching the default value used by the generic code.
#


Thanks
Suzuki

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Thread

[PATCHv3 00/11] arm64: 16K translation granule support "Suzuki K. Poulose" <suzuki.poulose@arm.com> - 2015-10-14 13:30 +0200
  [PATCHv3 07/11] arm64: Kconfig: Fix help text about AArch32 support with 64K pages "Suzuki K. Poulose" <suzuki.poulose@arm.com> - 2015-10-14 13:30 +0200
    Re: [PATCHv3 07/11] arm64: Kconfig: Fix help text about AArch32  support with 64K pages Mark Rutland <mark.rutland@arm.com> - 2015-10-14 19:20 +0200
  [PATCHv3 10/11] arm64: Add 16K page size support "Suzuki K. Poulose" <suzuki.poulose@arm.com> - 2015-10-14 13:30 +0200
    Re: [PATCHv3 10/11] arm64: Add 16K page size support Jeremy Linton <jeremy.linton@arm.com> - 2015-10-14 17:50 +0200
      Re: [PATCHv3 10/11] arm64: Add 16K page size support "Suzuki K. Poulose" <Suzuki.Poulose@arm.com> - 2015-10-14 18:00 +0200
    Re: [PATCHv3 10/11] arm64: Add 16K page size support Mark Rutland <mark.rutland@arm.com> - 2015-10-15 16:10 +0200
      Re: [PATCHv3 10/11] arm64: Add 16K page size support "Suzuki K. Poulose" <Suzuki.Poulose@arm.com> - 2015-10-15 16:50 +0200
        Re: [PATCHv3 10/11] arm64: Add 16K page size support Steve Capper <steve.capper@linaro.org> - 2015-10-15 17:40 +0200
          Re: [PATCHv3 10/11] arm64: Add 16K page size support "Suzuki K. Poulose" <Suzuki.Poulose@arm.com> - 2015-10-15 17:50 +0200
  [PATCHv3 05/11] arm64: Handle 4 level page table for swapper "Suzuki K. Poulose" <suzuki.poulose@arm.com> - 2015-10-14 13:30 +0200
    Re: [PATCHv3 05/11] arm64: Handle 4 level page table for swapper Mark Rutland <mark.rutland@arm.com> - 2015-10-14 19:20 +0200

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