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

[Catalin Marinas]

On Tue, Oct 03, 2023 at 08:32:29AM +0100, Ryan Roberts wrote:
> On 02/10/2023 16:21, Catalin Marinas wrote:
> > On Fri, Sep 29, 2023 at 12:44:18PM +0100, Ryan Roberts wrote:
> >> 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.
> 
> This sounds like good idea to me - I'll implement it, assuming there is a next
> rev. (Or in the unlikely event that this is the only pending change, I'd rather
> defer it to when we actually need it with the contpte series).

Fine by me, at the moment there wouldn't be any user, so a patch on top
later would do.

> Side note: I don't think order-6 is ever a contpte size? Its order-4 for 4K,
> order-7 for 16k and order-5 for 64k.

Yes, it's order-4 for 4K pages (I was thinking too much of the "64" in 64KB).

-- 
Catalin