Re: [PATCH v4 3/5] arm64: mm: support batch clearing of the young flag for large folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 1/2/26 8:12 PM, Ryan Roberts wrote:
> On 25/12/2025 02:48, Baolin Wang wrote:
>>
>>
>> On 2025/12/24 22:07, Ryan Roberts wrote:
>>> On 23/12/2025 05:48, Baolin Wang wrote:
>>>> Currently, contpte_ptep_test_and_clear_young() and
>>>> contpte_ptep_clear_flush_young()
>>>> only clear the young flag and flush TLBs for PTEs within the contiguous range.
>>>> To support batch PTE operations for other sized large folios in the following
>>>> patches, adding a new parameter to specify the number of PTEs that map
>>>> consecutive
>>>> pages of the same large folio in a single VMA and a single page table.
>>>>
>>>> While we are at it, rename the functions to maintain consistency with other
>>>> contpte_*() functions.
>>>>
>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>> ---
>>>>    arch/arm64/include/asm/pgtable.h | 12 ++++++------
>>>>    arch/arm64/mm/contpte.c          | 33 ++++++++++++++++++--------------
>>>>    2 files changed, 25 insertions(+), 20 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>>> index 445e18e92221..d5fbe72e820a 100644
>>>> --- a/arch/arm64/include/asm/pgtable.h
>>>> +++ b/arch/arm64/include/asm/pgtable.h
>>>> @@ -1648,10 +1648,10 @@ extern void contpte_clear_full_ptes(struct mm_struct
>>>> *mm, unsigned long addr,
>>>>    extern pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
>>>>                    unsigned long addr, pte_t *ptep,
>>>>                    unsigned int nr, int full);
>>>> -extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
>>>> -                unsigned long addr, pte_t *ptep);
>>>> -extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
>>>> -                unsigned long addr, pte_t *ptep);
>>>> +int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma,
>>>> +                unsigned long addr, pte_t *ptep, unsigned int nr);
>>>> +int contpte_clear_flush_young_ptes(struct vm_area_struct *vma,
>>>> +                unsigned long addr, pte_t *ptep, unsigned int nr);
>>>>    extern void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
>>>>                    pte_t *ptep, unsigned int nr);
>>>>    extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
>>>> @@ -1823,7 +1823,7 @@ static inline int ptep_test_and_clear_young(struct
>>>> vm_area_struct *vma,
>>>>        if (likely(!pte_valid_cont(orig_pte)))
>>>>            return __ptep_test_and_clear_young(vma, addr, ptep);
>>>>    -    return contpte_ptep_test_and_clear_young(vma, addr, ptep);
>>>> +    return contpte_test_and_clear_young_ptes(vma, addr, ptep, CONT_PTES);
>>>
>>> As per your fixup patch, I agree that nr should be 1 here, not CONT_PTES.
>>
>> Yes.
>>
>>>>    }
>>>>      #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
>>>> @@ -1835,7 +1835,7 @@ static inline int ptep_clear_flush_young(struct
>>>> vm_area_struct *vma,
>>>>        if (likely(!pte_valid_cont(orig_pte)))
>>>>            return __ptep_clear_flush_young(vma, addr, ptep);
>>>>    -    return contpte_ptep_clear_flush_young(vma, addr, ptep);
>>>> +    return contpte_clear_flush_young_ptes(vma, addr, ptep, CONT_PTES);
>>>
>>> And same here.
>>>
>>>>    }
>>>>      #define wrprotect_ptes wrprotect_ptes
>>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>>>> index e4ddeb46f25d..b929a455103f 100644
>>>> --- a/arch/arm64/mm/contpte.c
>>>> +++ b/arch/arm64/mm/contpte.c
>>>> @@ -508,8 +508,9 @@ pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm,
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(contpte_get_and_clear_full_ptes);
>>>>    -int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
>>>> -                    unsigned long addr, pte_t *ptep)
>>>> +int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma,
>>>> +                    unsigned long addr, pte_t *ptep,
>>>> +                    unsigned int nr)
>>>>    {
>>>>        /*
>>>>         * ptep_clear_flush_young() technically requires us to clear the access
>>>> @@ -518,41 +519,45 @@ int contpte_ptep_test_and_clear_young(struct
>>>> vm_area_struct *vma,
>>>>         * contig range when the range is covered by a single folio, we can get
>>>>         * away with clearing young for the whole contig range here, so we avoid
>>>>         * having to unfold.
>>>> +     *
>>>> +     * The 'nr' means consecutive (present) PTEs that map consecutive pages
>>>> +     * of the same large folio in a single VMA and a single page table.
>>>>         */
>>>>    +    unsigned long end = addr + nr * PAGE_SIZE;
>>>>        int young = 0;
>>>> -    int i;
>>>>    -    ptep = contpte_align_down(ptep);
>>>> -    addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>>> -
>>>> -    for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
>>>> +    ptep = contpte_align_addr_ptep(&addr, &end, ptep, nr);
>>>> +    for (; addr != end; ptep++, addr += PAGE_SIZE)
>>>>            young |= __ptep_test_and_clear_young(vma, addr, ptep);
>>>>          return young;
>>>>    }
>>>> -EXPORT_SYMBOL_GPL(contpte_ptep_test_and_clear_young);
>>>> +EXPORT_SYMBOL_GPL(contpte_test_and_clear_young_ptes);
>>>>    -int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
>>>> -                    unsigned long addr, pte_t *ptep)
>>>> +int contpte_clear_flush_young_ptes(struct vm_area_struct *vma,
>>>> +                unsigned long addr, pte_t *ptep,
>>>> +                unsigned int nr)
>>>>    {
>>>>        int young;
>>>>    -    young = contpte_ptep_test_and_clear_young(vma, addr, ptep);
>>>> +    young = contpte_test_and_clear_young_ptes(vma, addr, ptep, nr);
>>>>          if (young) {
>>>> +        unsigned long end = addr + nr * PAGE_SIZE;
>>>> +
>>>> +        contpte_align_addr_ptep(&addr, &end, ptep, nr);
>>>>            /*
>>>>             * See comment in __ptep_clear_flush_young(); same rationale for
>>>>             * eliding the trailing DSB applies here.
>>>>             */
>>>> -        addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>>> -        __flush_tlb_range_nosync(vma->vm_mm, addr, addr + CONT_PTE_SIZE,
>>>> +        __flush_tlb_range_nosync(vma->vm_mm, addr, end,
>>>>                         PAGE_SIZE, true, 3);
>>>
>>> Hmm... The requirement is that we must flush the _page_ if clearing access for a
>>> pte that does not have the contiguous bit set, or we must flush the _contpte
>>> block_ if clearing access for a pte that does have the contiguous bit set.
>>>
>>> With your changes, you may call for a large range that covers multiple contpte
>>> blocks but only has a single pte in a single contpte block for which the access
>>> bit was previously set. But that will cause flushing the TLB for the full range.
>>> Could this cause a performance issue? Yes, no, maybe... I think it's unlikely
>>> but I wouldn't rule it out in some edge case.
>>>
>>> I wonder if it's better to track the sub-ranges where access was cleared and
>>> only issue tlbi for those sub-ranges? Probably just keep it simple (the way you
>>> have done it) until/unless we see an actual problem?
>>
>> Good question. Indeed, as you said, we flush the TLB per folio now, which might
>> increase the flush range. However, I think this approach is relatively
>> reasonable for now.
>>
>> First, the mm-core also tracks the access status per folio, and it's really
>> unnecessary to add excessive complexity to track the access status of sub-pages
>> (or sub-ranges). I can already imagine that tracking the access status for each
>> cont-block range as well as for non-cont pages across the entire large folio
>> range, which can be too complicated.
>>
>> Second, __flush_tlb_range_nosync() is a lightweight flush. I quickly ran a
>> measurement on my machine and found that the overhead of
>> __flush_tlb_range_nosync() barely changes between nr=16 and nr=256 (both are
>> around 40 ns).
> 
> I'm not concerned about the direct cost of the flush; I agree it should be
> lightweight given we elide the trailing DSB. (although there is a possible case
> on older HW that doesn't support TLBI-by-range where this will be converted to
> multiple TLBI-by-page instructions and that can cause stalls if there are too
> many of them).
> 
> My concern was the opportunity cost of evicting the entries for all the
> non-accessed parts of the folio from the TLB. But of course, I'm talking
> nonsense because the architecture does not allow caching non-accessed entries in
> the TLB.

Ah, now I understand your concern:). Yes, agree.

> So doesn't sound like a problem; I think we can ignore this. Sorry for the noise.

OK. No worries.

>> Therefore, I would still prefer to keep the logic here simple.
> 
> Agreed.

Thanks for your reviewing and valuable input.