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

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Message-ID: <3911c239-8945-46aa-8ca8-52ea16221b1a@arm.com>
Date: Mon, 22 Sep 2025 09:37:14 +0530
From: Dev Jain <dev.jain@....com>
To: Anshuman Khandual <anshuman.khandual@....com>, catalin.marinas@....com,
 will@...nel.org
Cc: wangkefeng.wang@...wei.com, ryan.roberts@....com, baohua@...nel.org,
 pjaroszynski@...dia.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64/mm: Elide TLB flush in certain pte protection
 transitions


On 21/09/25 5:53 pm, Anshuman Khandual wrote:
> On 20/09/25 10:40 AM, 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
> The following transition does not require a TLB flush on all
> architectures ? In which case, should not this check be part
> of generic mprotect() itself.

You are probably correct - there should be some common transitions
like these which won't require flush on all arches. But that should
be part of a wider cleanup :) And, in the ppc version of this function,
I see that it returns true in the case of !radix_enabled(). So not
very sure.
  

>
> PROT_NONE ---> PROT_READ | PROT_WRITE
>> 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.
> Agreed, defining pte_needs_flush() on the platform does make
> sense, if it brings some perf improvement without additional
> cost.
>> 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.
> But that's a very specific workload intended to demonstrate
> the use case for this change. Hence the improvement claimed
> here is not representative of real world work loads.

True, but as I state, glibc malloc commonly uses this protection
transition to extend the heap.

>> Reviewed-by: Kefeng Wang <wangkefeng.wang@...wei.com>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>> mm-selftests pass.
>>
>> 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))
>> +		return false;
> Is not this true for all platforms which could be checked via
> pte_present() helper ? Hence should not this be moved inside
> the caller itself in generic MM ? I guess probably the above
> mentioned transition should be moved as well.
>
> PROT_NONE ---> PROT_READ | PROT_WRITE
>> +
>> +	/* 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