Re: [PATCH v2 2/3] mm: rmap: support batched checks of the references for large folios

[Dev Jain <dev.jain@arm.com>]

On 11/12/25 1:46 pm, Baolin Wang wrote:
> Currently, folio_referenced_one() always checks the young flag for each PTE
> sequentially, which is inefficient for large folios. This inefficiency is
> especially noticeable when reclaiming clean file-backed large folios, where
> folio_referenced() is observed as a significant performance hotspot.
>
> Moreover, on Arm architecture, which supports contiguous PTEs, there is already
> an optimization to clear the young flags for PTEs within a contiguous range.
> However, this is not sufficient. We can extend this to perform batched operations
> for the entire large folio (which might exceed the contiguous range: CONT_PTE_SIZE).
>
> Introduce a new API: clear_flush_young_ptes() to facilitate batched checking
> of the young flags and flushing TLB entries, thereby improving performance
> during large folio reclamation.
>
> Performance testing:
> Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to
> reclaim 8G file-backed folios via the memory.reclaim interface. I can observe
> 33% performance improvement on my Arm64 32-core server (and 10%+ improvement
> on my X86 machine). Meanwhile, the hotspot folio_check_references() dropped
> from approximately 35% to around 5%.
>
> W/o patchset:
> real	0m1.518s
> user	0m0.000s
> sys	0m1.518s
>
> W/ patchset:
> real	0m1.018s
> user	0m0.000s
> sys	0m1.018s
>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  arch/arm64/include/asm/pgtable.h | 11 +++++++++++
>  include/linux/mmu_notifier.h     |  9 +++++----
>  include/linux/pgtable.h          | 19 +++++++++++++++++++
>  mm/rmap.c                        | 22 ++++++++++++++++++++--
>  4 files changed, 55 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e03034683156..a865bd8c46a3 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1869,6 +1869,17 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
>  	return contpte_clear_flush_young_ptes(vma, addr, ptep, CONT_PTES);
>  }
>  
> +#define clear_flush_young_ptes clear_flush_young_ptes
> +static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
> +					unsigned long addr, pte_t *ptep,
> +					unsigned int nr)
> +{
> +	if (likely(nr == 1))
> +		return __ptep_clear_flush_young(vma, addr, ptep);
> +
> +	return contpte_clear_flush_young_ptes(vma, addr, ptep, nr);
> +}
> +
>  #define wrprotect_ptes wrprotect_ptes
>  static __always_inline void wrprotect_ptes(struct mm_struct *mm,
>  				unsigned long addr, pte_t *ptep, unsigned int nr)
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index d1094c2d5fb6..be594b274729 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -515,16 +515,17 @@ static inline void mmu_notifier_range_init_owner(
>  	range->owner = owner;
>  }
>  
> -#define ptep_clear_flush_young_notify(__vma, __address, __ptep)		\
> +#define ptep_clear_flush_young_notify(__vma, __address, __ptep, __nr)	\
>  ({									\
>  	int __young;							\
>  	struct vm_area_struct *___vma = __vma;				\
>  	unsigned long ___address = __address;				\
> -	__young = ptep_clear_flush_young(___vma, ___address, __ptep);	\
> +	unsigned int ___nr = __nr;					\
> +	__young = clear_flush_young_ptes(___vma, ___address, __ptep, ___nr);	\
>  	__young |= mmu_notifier_clear_flush_young(___vma->vm_mm,	\
>  						  ___address,		\
>  						  ___address +		\
> -							PAGE_SIZE);	\
> +						nr * PAGE_SIZE);	\
>  	__young;							\
>  })
>  
> @@ -650,7 +651,7 @@ static inline void mmu_notifier_subscriptions_destroy(struct mm_struct *mm)
>  
>  #define mmu_notifier_range_update_to_read_only(r) false
>  
> -#define ptep_clear_flush_young_notify ptep_clear_flush_young
> +#define ptep_clear_flush_young_notify clear_flush_young_ptes
>  #define pmdp_clear_flush_young_notify pmdp_clear_flush_young
>  #define ptep_clear_young_notify ptep_test_and_clear_young
>  #define pmdp_clear_young_notify pmdp_test_and_clear_young
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index b13b6f42be3c..c7d0fd228cb7 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -947,6 +947,25 @@ static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
>  }
>  #endif
>  
> +#ifndef clear_flush_young_ptes
> +static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
> +					 unsigned long addr, pte_t *ptep,
> +					 unsigned int nr)
> +{
> +	int young = 0;
> +
> +	for (;;) {
> +		young |= ptep_clear_flush_young(vma, addr, ptep);
> +		if (--nr == 0)
> +			break;
> +		ptep++;
> +		addr += PAGE_SIZE;
> +	}
> +
> +	return young;
> +}
> +#endif
> +
>  /*
>   * On some architectures hardware does not set page access bit when accessing
>   * memory page, it is responsibility of software setting this bit. It brings
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d6799afe1114..ec232165c47d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -827,9 +827,11 @@ static bool folio_referenced_one(struct folio *folio,
>  	struct folio_referenced_arg *pra = arg;
>  	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
>  	int ptes = 0, referenced = 0;
> +	unsigned int nr;
>  
>  	while (page_vma_mapped_walk(&pvmw)) {
>  		address = pvmw.address;
> +		nr = 1;
>  
>  		if (vma->vm_flags & VM_LOCKED) {
>  			ptes++;
> @@ -874,9 +876,21 @@ static bool folio_referenced_one(struct folio *folio,
>  			if (lru_gen_look_around(&pvmw))
>  				referenced++;
>  		} else if (pvmw.pte) {
> +			if (folio_test_large(folio)) {
> +				unsigned long end_addr = pmd_addr_end(address, vma->vm_end);

I may be hallucinating here but I am just trying to recall things - is this a bug in
folio_pte_batch_flags()? A folio may not be naturally aligned in virtual space and hence
we may cross the PTE table while batching across it, which can be fixed by taking into
account pmd_addr_end() while computing max_nr.

> +				unsigned int max_nr = (end_addr - address) >> PAGE_SHIFT;
> +				pte_t pteval = ptep_get(pvmw.pte);
> +
> +				nr = folio_pte_batch(folio, pvmw.pte, pteval, max_nr);
> +			}
> +
> +			ptes += nr;
>  			if (ptep_clear_flush_young_notify(vma, address,
> -						pvmw.pte))
> +						pvmw.pte, nr))
>  				referenced++;
> +			/* Skip the batched PTEs */
> +			pvmw.pte += nr - 1;
> +			pvmw.address += (nr - 1) * PAGE_SIZE;
>  		} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
>  			if (pmdp_clear_flush_young_notify(vma, address,
>  						pvmw.pmd))
> @@ -886,7 +900,11 @@ static bool folio_referenced_one(struct folio *folio,
>  			WARN_ON_ONCE(1);
>  		}
>  
> -		pra->mapcount--;
> +		pra->mapcount -= nr;
> +		if (ptes == pvmw.nr_pages) {
> +			page_vma_mapped_walk_done(&pvmw);
> +			break;
> +		}
>  	}
>  
>  	if (referenced)