Re: [PATCH v2 1/3] arm64: mm: support batch clearing of the young flag for large folios

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

On 18/12/2025 07:15, Baolin Wang wrote:
> 
> 
> On 2025/12/17 23:43, Ryan Roberts wrote:
>> Sorry I'm a bit late to the party...
> 
> Never mind. It's not late and comments are always welcome :)
> 
>> On 11/12/2025 08:16, 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.
>>>
>>> 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          | 44 ++++++++++++++++++++++----------
>>>   2 files changed, 37 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>> index 0944e296dd4a..e03034683156 100644
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -1679,10 +1679,10 @@ 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);
>>> +extern int contpte_test_and_clear_young_ptes(struct vm_area_struct *vma,
>>> +                unsigned long addr, pte_t *ptep, unsigned int nr);
>>> +extern int contpte_clear_flush_young_ptes(struct vm_area_struct *vma,
>>> +                unsigned long addr, pte_t *ptep, unsigned int nr);
>>
>> The "contpte_" functions are intended to be private to the arm64 arch and should
>> be exposed via the generic APIs. But I don't see any generic batched API for
>> this, so you're only actually able to pass CONT_PTES as nr. Perhaps you're
>> planning to define "test_and_clear_young_ptes()" and "clear_flush_young_ptes()"
>> in later patches?
> 
> Right. This is a preparation patch, and will be used in patch 2.
> 
>>>   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,
>>> @@ -1854,7 +1854,7 @@ 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);
>>>   }
>>>     #define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
>>> @@ -1866,7 +1866,7 @@ 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);
>>>   }
>>>     #define wrprotect_ptes wrprotect_ptes
>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>>> index c0557945939c..19b122441be3 100644
>>> --- a/arch/arm64/mm/contpte.c
>>> +++ b/arch/arm64/mm/contpte.c
>>> @@ -488,8 +488,9 @@ 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
>>> @@ -500,39 +501,56 @@ int contpte_ptep_test_and_clear_young(struct
>>> vm_area_struct *vma,
>>>        * having to unfold.
>>>        */
>>>   +    unsigned long start = addr;
>>
>> Personally I wouldn't bother defining start - just reuse addr. You're
>> incrementing start in the below loop, so it's more appropriate to call it addr
>> anyway.
> 
> OK.
> 
>>> +    unsigned long end = start + nr * PAGE_SIZE;
>>>       int young = 0;
>>>       int i;
>>>   -    ptep = contpte_align_down(ptep);
>>> -    addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
>>> +    if (pte_cont(__ptep_get(ptep + nr - 1)))
>>> +        end = ALIGN(end, CONT_PTE_SIZE);
>>>   -    for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
>>> -        young |= __ptep_test_and_clear_young(vma, addr, ptep);
>>> +    if (pte_cont(__ptep_get(ptep))) {
>>> +        start = ALIGN_DOWN(start, CONT_PTE_SIZE);
>>> +        ptep = contpte_align_down(ptep);
>>> +    }
>>> +
>>> +    nr = (end - start) / PAGE_SIZE;
>>> +    for (i = 0; i < nr; i++, ptep++, start += PAGE_SIZE)
>>
>> Given you're now defining end, perhaps we don't need nr?
>>
>>     for (; addr != end; ptep++, addr += PAGE_SIZE)
>>         young |= __ptep_test_and_clear_young(vma, addr, ptep);
> 
> Yes, good point.
> 
>>> +        young |= __ptep_test_and_clear_young(vma, start, 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 start = addr;
>>> +        unsigned long end = start + nr * PAGE_SIZE;
>>> +
>>> +        if (pte_cont(__ptep_get(ptep + nr - 1)))
>>> +            end = ALIGN(end, CONT_PTE_SIZE);
>>> +
>>> +        if (pte_cont(__ptep_get(ptep)))
>>> +            start = ALIGN_DOWN(start, CONT_PTE_SIZE);
>>> +
>>
>> We now have this pattern of expanding contpte blocks up and down in 3 places.
>> Perhaps create a helper?
> 
> Sounds reasonable. How about the following helper?
> 
> static pte_t *contpte_align_addr_ptep(unsigned long *start, unsigned long *end,
>                                         pte_t *ptep, unsigned int nr)
> {
>         unsigned long end_addr = *start + nr * PAGE_SIZE;
> 
>         if (pte_cont(__ptep_get(ptep + nr - 1)))

I think this is safe but calling it out to check; you're not checking that the
pte is valid, so theoretically you could have a swap-entry here with whatever
overlays the contiguous bit set. So then you would incorrectly extend.

But I think it is safe because the expectation is that core-mm has already
checked that the whole range is present?

>                 *end = ALIGN(end_addr, CONT_PTE_SIZE);
> 
>         if (pte_cont(__ptep_get(ptep))) {
>                 *start = ALIGN_DOWN(*start, CONT_PTE_SIZE);
>                 ptep = contpte_align_down(ptep);
>         }
> 
>         return ptep;
> }

Looks good.