Re: [PATCH v6 7/9] arm64/mm: Override arch_wants_pte_order()

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Sep 29, 2023 at 12:44:18PM +0100, Ryan Roberts wrote:
> Define an arch-specific override of arch_wants_pte_order() so that when
> anon_orders=recommend is set, large folios will be allocated for
> anonymous memory with an order that is compatible with arm64's HPA uarch
> feature.
> 
> Reviewed-by: Yu Zhao <yuzhao@google.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 7f7d9b1df4e5..e3d2449dec5c 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1110,6 +1110,16 @@ extern pte_t ptep_modify_prot_start(struct vm_area_struct *vma,
>  extern void ptep_modify_prot_commit(struct vm_area_struct *vma,
>  				    unsigned long addr, pte_t *ptep,
>  				    pte_t old_pte, pte_t new_pte);
> +
> +#define arch_wants_pte_order arch_wants_pte_order
> +static inline int arch_wants_pte_order(void)
> +{
> +	/*
> +	 * Many arm64 CPUs support hardware page aggregation (HPA), which can
> +	 * coalesce 4 contiguous pages into a single TLB entry.
> +	 */
> +	return 2;
> +}

I haven't followed the discussions on previous revisions of this series
but I wonder why not return a bitmap from arch_wants_pte_order(). For
arm64 we may want an order 6 at some point (contiguous ptes) with a
fallback to order 2 as the next best.

-- 
Catalin