Re: [RESEND] [PATCH v2] arm64/mm: Elide TLB flush in certain pte protection transitions

[Dev Jain <dev.jain@arm.com>]

On 04/11/25 5:45 am, Anshuman Khandual wrote:
>
> On 17/10/25 9:32 PM, 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
>> 2.85 microseconds, giving a 12.3% improvement.
>>
>> Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>> mm-selftests pass. Based on 6.18-rc1.
>>
>> v1->v2:
>>   - Drop PTE_PRESENT_INVALID and PTE_AF checks, use ptdesc_t instead of
>>     pteval_t, return !!diff (Ryan)
>>
>>   arch/arm64/include/asm/tlbflush.h | 27 +++++++++++++++++++++++++++
>>   1 file changed, 27 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
>> index 18a5dc0c9a54..40df783ba09a 100644
>> --- a/arch/arm64/include/asm/tlbflush.h
>> +++ b/arch/arm64/include/asm/tlbflush.h
>> @@ -524,6 +524,33 @@ 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(ptdesc_t oldval, ptdesc_t newval)
>> +{
>> +	ptdesc_t diff = oldval ^ newval;
>> +
>> +	/* invalid to valid transition requires no flush */
>> +	if (!(oldval & PTE_VALID))
> Using pte_valid() helper would be better.
>
> 	if (!pte_valid(oldval))
> 		return false;

Unfortunately, that would need asm/pgtable.h, which already includes asm/tlbflush.h,
leading to circular include.

If I put all this code into asm/pgtable.h, then pte_valid() needs a pte_t or a pmd_t
to work on, which forces us to break away from the nice abstraction of directing
both pte_needs_flush and huge_pmd_needs_flush to __pte_flags_need_flush.


>> +		return false;
>> +
>> +	/* Transition in the SW bits requires no flush */
>> +	diff &= ~PTE_SWBITS_MASK;
>> +
>> +	return !!diff;
>> +}
>> +
>> +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