Re: [PATCH v3 1/5] mm: rmap: support batched checks of the references for large folios

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Baolin Wang <baolin.wang@linux.alibaba.com> [251219 01:03]:
> 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 Arm64 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. And it will be overridden by the architecture
> that implements a more efficient batch operation in the following patches.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  include/linux/mmu_notifier.h |  9 +++++----
>  include/linux/pgtable.h      | 35 +++++++++++++++++++++++++++++++++++
>  mm/rmap.c                    | 29 +++++++++++++++++++++++++++--
>  3 files changed, 67 insertions(+), 6 deletions(-)
> 
> 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);	\

Did you mean nr * PAGE_SIZE here?  I think it should be __nr or ___nr?
I think this nr variable works because it exists where this macro is
expanded?

I am also not sure why you have ___nr  at all?

>  	__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..7e659f4171e2 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -947,6 +947,41 @@ static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
>  }
>  #endif
>  
> +#ifndef clear_flush_young_ptes
> +/**
> + * clear_flush_young_ptes - Clear the access bit and perform a TLB flush for PTEs
> + *			    that map consecutive pages of the same folio.
> + * @vma: The virtual memory area the pages are mapped into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of entries to clear access bit.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_clear_flush_young().
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might be write-protected.
> + *
> + * Context: The caller holds the page table lock.  The PTEs map consecutive
> + * pages that belong to the same folio.  The PTEs are all in the same PMD.
> + */
> +static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
> +					 unsigned long addr, pte_t *ptep,
> +					 unsigned int nr)
This is where nr is defined                         ^^^^^

> +{
> +	int young;
> +
> +	young = ptep_clear_flush_young(vma, addr, ptep);
> +	while (--nr) {
> +		ptep++;
> +		addr += PAGE_SIZE;
> +		young |= ptep_clear_flush_young(vma, addr, ptep);
> +	}
> +
> +	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..a0fc05f5966f 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;
and here            ^^^

>  
>  	while (page_vma_mapped_walk(&pvmw)) {
>  		address = pvmw.address;
> +		nr = 1;
>  
>  		if (vma->vm_flags & VM_LOCKED) {
>  			ptes++;
> @@ -874,9 +876,24 @@ 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);
> +				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 +903,15 @@ static bool folio_referenced_one(struct folio *folio,
>  			WARN_ON_ONCE(1);
>  		}
>  
> -		pra->mapcount--;
> +		pra->mapcount -= nr;
> +		/*
> +		 * If we are sure that we batched the entire folio,
> +		 * we can just optimize and stop right here.
> +		 */
> +		if (ptes == pvmw.nr_pages) {
> +			page_vma_mapped_walk_done(&pvmw);
> +			break;
> +		}
>  	}
>  
>  	if (referenced)
> -- 
> 2.47.3
> 
>