linux-kernel - Re: [PATCH v2 03/14] arm64: reword ARCH_FORCE_MAX

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Date:   Fri, 24 Mar 2023 09:48:49 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "David S. Miller" <davem@...emloft.net>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Guo Ren <guoren@...nel.org>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Max Filippov <jcmvbkbc@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Rich Felker <dalias@...c.org>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mm@...ck.org,
        linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v2 03/14] arm64: reword ARCH_FORCE_MAX_ORDER prompt and
 help text

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

> From: "Mike Rapoport (IBM)" <rppt@...nel.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@...nel.org>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.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


--
Best Regards,
Yan, Zi

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