Re: [PATCH v2 03/14] arm64: reword ARCH_FORCE_MAX_ORDER prompt and help text

[Zi Yan <ziy@nvidia.com>]

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On 24 Mar 2023, at 1:22, Mike Rapoport wrote:

> From: "Mike Rapoport (IBM)" <rppt@kernel.org>
>
> The prompt and help text of ARCH_FORCE_MAX_ORDER are not even close to
> describe this configuration option.
>
> Update both to actually describe what this option does.
>
> Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  arch/arm64/Kconfig | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7324032af859..cdaf52ac4018 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1487,24 +1487,24 @@ config XEN
>  # 16K |       27          |      14      |       13        |         11         |
>  # 64K |       29          |      16      |       13        |         13         |
>  config ARCH_FORCE_MAX_ORDER
> -	int "Maximum zone order" if EXPERT && (ARM64_4K_PAGES || ARM64_16K_PAGES)
> +	int "Order of maximal physically contiguous allocations" if ARM64_4K_PAGES || ARM64_16K_PAGES

It seems that "if EXPERT" was dropped by accident here.

>  	default "13" if ARM64_64K_PAGES
>  	default "11" if ARM64_16K_PAGES
>  	default "10"
>  	help
> -	  The kernel memory allocator divides physically contiguous memory
> -	  blocks into "zones", where each zone is a power of two number of
> -	  pages.  This option selects the largest power of two that the kernel
> -	  keeps in the memory allocator.  If you need to allocate very large
> -	  blocks of physically contiguous memory, then you may need to
> -	  increase this value.
> +	  The kernel page allocator limits the size of maximal physically
> +	  contiguous allocations. The limit is called MAX_ORDER and it
> +	  defines the maximal power of two of number of pages that can be
> +	  allocated as a single contiguous block. This option allows
> +	  overriding the default setting when ability to allocate very
> +	  large blocks of physically contiguous memory is required.
>
> -	  We make sure that we can allocate up to a HugePage size for each configuration.
> -	  Hence we have :
> -		MAX_ORDER = PMD_SHIFT - PAGE_SHIFT  => PAGE_SHIFT - 3
> +	  The maximal size of allocation cannot exceed the size of the
> +	  section, so the value of MAX_ORDER should satisfy
>
> -	  However for 4K, we choose a higher default value, 10 as opposed to 9, giving us
> -	  4M allocations matching the default size used by generic code.
> +	    MAX_ORDER + PAGE_SHIFT <= SECTION_SIZE_BITS
> +
> +	  Don't change if unsure.
>
>  config UNMAP_KERNEL_AT_EL0
>  	bool "Unmap kernel when running in userspace (aka \"KAISER\")" if EXPERT
> -- 
> 2.35.1


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Best Regards,
Yan, Zi

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