linux-kernel - Re: [PATCH 1/1] arm64: reduce section size for sparsemem

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Message-ID: <20210120174933.GA20981@willie-the-truck>
Date:   Wed, 20 Jan 2021 17:49:33 +0000
From:   Will Deacon <will@...nel.org>
To:     Sudarshan Rajagopalan <sudaraja@...eaurora.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        anshuman.khandual@....com, david@...hat.com,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mark Rutland <mark.rutland@....com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH 1/1] arm64: reduce section size for sparsemem

On Fri, Jan 08, 2021 at 03:16:00PM -0800, Sudarshan Rajagopalan wrote:
> Reducing the section size helps reduce wastage of reserved memory
> for huge memory holes in sparsemem model. But having a much smaller
> section size bits could break PMD mappings for vmemmap and wouldn't
> accomodate the highest order page for certain page size granule configs.
> It is determined that SECTION_SIZE_BITS of 27 (128MB) could be ideal
> default value for 4K_PAGES that gives least section size without breaking
> PMD based vmemmap mappings. For simplicity, 16K_PAGES could follow the
> same as 4K_PAGES. And the least SECTION_SIZE_BITS for 64K_PAGES is 29
> that could accomodate MAX_ORDER.
> 
> Signed-off-by: Sudarshan Rajagopalan <sudaraja@...eaurora.org>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Cc: Mike Rapoport <rppt@...ux.ibm.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  arch/arm64/include/asm/sparsemem.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
> index 1f43fcc79738..ff08ff6b677c 100644
> --- a/arch/arm64/include/asm/sparsemem.h
> +++ b/arch/arm64/include/asm/sparsemem.h
> @@ -7,7 +7,13 @@
>  
>  #ifdef CONFIG_SPARSEMEM
>  #define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
> -#define SECTION_SIZE_BITS	30
> -#endif
> +
> +#if defined(CONFIG_ARM64_4K_PAGES) || defined(CONFIG_ARM64_16K_PAGES)
> +#define SECTION_SIZE_BITS 27
> +#else
> +#define SECTION_SIZE_BITS 29
> +#endif /* CONFIG_ARM64_4K_PAGES || CONFIG_ARM64_16K_PAGES */
> +
> +#endif /* CONFIG_SPARSEMEM*/

Please can you repost this in light of the comments from Anshuman?

Thanks,

Will