Re: [PATCH] arm64/mm: Elide TLB flush in certain pte protection transitions

[Ryan Roberts <ryan.roberts@arm.com>]

On 18/09/2025 11:36, Dev Jain wrote:
> Currently arm64 does an unconditional TLB flush in mprotect(). This is not
> required for some cases, for example, when changing from PROT_NONE to
> PROT_READ | PROT_WRITE (a real usecase - glibc malloc does this to emulate
> growing into the non-main heaps), and unsetting uffd-wp in a range.
> 
> Therefore, implement pte_needs_flush() for arm64, which is already
> implemented by some other arches as well.
> 
> Running a userspace program changing permissions back and forth between
> PROT_NONE and PROT_READ | PROT_WRITE, and measuring the average time taken
> for the none->rw transition, I get a reduction from 3.2 microseconds to
> 2.95 microseconds, giving an 8.5% improvement.
> 
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
> mm-selftests pass. Based on 6.17-rc6.
> 
>  arch/arm64/include/asm/tlbflush.h | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
> index 18a5dc0c9a54..4a566d589100 100644
> --- a/arch/arm64/include/asm/tlbflush.h
> +++ b/arch/arm64/include/asm/tlbflush.h
> @@ -524,6 +524,35 @@ static inline void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *b
>  {
>  	__flush_tlb_range_nosync(mm, start, end, PAGE_SIZE, true, 3);
>  }
> +
> +static inline bool __pte_flags_need_flush(pteval_t oldval, pteval_t newval)

ptdesc_t is the preferred any-level type.

> +{
> +	pteval_t diff = oldval ^ newval;
> +
> +	/* invalid to valid transition requires no flush */
> +	if (!(oldval & PTE_VALID) || (oldval & PTE_PRESENT_INVALID))

Is the PTE_PRESENT_INVALID really required? If the oldval was invalid, there
can't be a TLB entry for it, so no flush is required; that's it, I think?

In fact, PTE_PRESENT_INVALID is overlaid with PTE_NG; it only means
PTE_PRESENT_INVALID when PTE_INVALID=0, so I think this is broken as is. Valid
user-space PTEs always have PTE_NG set, so you will never flush.

> +		return false;
> +
> +	/* Transition in the SW bits and access flag requires no flush */
> +	diff &= ~(PTE_SWBITS_MASK | PTE_AF);

Could you explain your thinking on why PTE_AF changes don't need a flush? I
would have thought if we want to clear the access flag, that would definitely
require a flush? Otherwise how would the MMU know to set the acccess bit on next
access if it already has a TLB entry?

> +
> +	if (!diff)
> +		return false;
> +	return true;

Perhaps just "return !!diff;" here?

Thanks,
Ryan


> +}
> +
> +static inline bool pte_needs_flush(pte_t oldpte, pte_t newpte)
> +{
> +	return __pte_flags_need_flush(pte_val(oldpte), pte_val(newpte));
> +}
> +#define pte_needs_flush pte_needs_flush
> +
> +static inline bool huge_pmd_needs_flush(pmd_t oldpmd, pmd_t newpmd)
> +{
> +	return __pte_flags_need_flush(pmd_val(oldpmd), pmd_val(newpmd));
> +}
> +#define huge_pmd_needs_flush huge_pmd_needs_flush
> +
>  #endif
>  
>  #endif