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

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

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)))
                 *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;
}