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

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2025/9/18 18:36, 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.
> 

Hi Dev，

> 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)
> +{
> +	pteval_t diff = oldval ^ newval;
> +
> +	/* invalid to valid transition requires no flush */
> +	if (!(oldval & PTE_VALID) || (oldval & PTE_PRESENT_INVALID))
> +		return false;
> +
> +	/* Transition in the SW bits and access flag requires no flush */
> +	diff &= ~(PTE_SWBITS_MASK | PTE_AF);
> +
> +	if (!diff)
> +		return false;
> +	return true;
> +}
> +

LibMicro mprotect testcase show 3~5% improvement with different size in
old kernel(we did this before, but only check PTE_VALID and 
PTE_PROT_NONE in our kernel), it seems that no one change other sw bit 
by mprotect?

Anyway, Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>

> +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