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

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

Sorry I'm a bit late to the party...


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?

>  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.

> +	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);

> +		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?

Thanks,
Ryan

>  		/*
>  		 * 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, start, end,
>  					 PAGE_SIZE, true, 3);
>  	}
>  
>  	return young;
>  }
> -EXPORT_SYMBOL_GPL(contpte_ptep_clear_flush_young);
> +EXPORT_SYMBOL_GPL(contpte_clear_flush_young_ptes);
>  
>  void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
>  					pte_t *ptep, unsigned int nr)