Re: [PATCH v6 1/5] mm: rmap: support batched checks of the references for large folios

[Barry Song]

On Mon, Feb 9, 2026 at 10:07 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> 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 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.
>
> While we are at it, rename ptep_clear_flush_young_notify() to
> clear_flush_young_ptes_notify() to indicate that this is a batch operation.
>
> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

LGTM,

Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

> ---
>  include/linux/mmu_notifier.h |  9 +++++----
>  include/linux/pgtable.h      | 35 +++++++++++++++++++++++++++++++++++
>  mm/rmap.c                    | 28 +++++++++++++++++++++++++---
>  3 files changed, 65 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index d1094c2d5fb6..07a2bbaf86e9 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 clear_flush_young_ptes_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 clear_flush_young_ptes_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 21b67d937555..a50df42a893f 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1068,6 +1068,41 @@ static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
>  }
>  #endif
>
> +#ifndef clear_flush_young_ptes
> +/**
> + * clear_flush_young_ptes - Mark PTEs that map consecutive pages of the same
> + *                         folio as old and flush the TLB.
> + * @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)
> +{
> +       int young = 0;
> +
> +       for (;;) {
> +               young |= ptep_clear_flush_young(vma, addr, ptep);
> +               if (--nr == 0)
> +                       break;
> +               ptep++;
> +               addr += PAGE_SIZE;
> +       }
> +
> +       return young;
> +}
> +#endif

We might have an opportunity to batch the TLB synchronization,
using flush_tlb_range() instead of calling flush_tlb_page()
one by one. Not sure the benefit would be significant though,
especially if only one entry among nr has the young bit set.

Best Regards
Barry