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

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

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.

>  }
>  
>  #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?

Thanks,
Ryan

>  	}
>  
>  	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)