Re: [PATCH v7 02/33] arm64: mm: Avoid swapper block size when choosing vmemmap granularity

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 11/11/22 22:41, Ard Biesheuvel wrote:
> The logic to decide between PTE and PMD mappings in the vmemmap region
> is currently based on the granularity of the initial ID map but those
> things have little to do with each other.
> 
> The reason we use PMDs here on 4k pagesize kernels is because a struct
> page array describing a single section of memory takes up at least the
> size described by a PMD, and so mapping down to pages is pointless.
> 
> So use the correct conditional, and add a comment to clarify it.
> 
> This allows us to remove or rename the swapper block size related
> constants in the future.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>> ---

The patch LGTM in itself.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

>  arch/arm64/mm/mmu.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 757c2fe54d2e99f0..0c35e1f195678695 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1196,7 +1196,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>  
>  	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>  
> -	if (!ARM64_KERNEL_USES_PMD_MAPS)
> +	/*
> +	 * Use page mappings for the vmemmap region if the area taken up by a
> +	 * struct page array covering a single section is smaller than the area
> +	 * covered by a PMD.
> +	 */
> +	if (SECTION_SIZE_BITS - VMEMMAP_SHIFT < PMD_SHIFT)
>  		return vmemmap_populate_basepages(start, end, node, altmap);
>  
>  	do {

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