Re: [PATCH v2 4/6] mm: add a batched helper to clear the young flag for large folios

[David Hildenbrand (Arm)]

On 2/27/26 10:44, Baolin Wang wrote:
> Currently, MGLRU will call ptep_test_and_clear_young_notify() to check and
> clear the young flag for each PTE sequentially, which is inefficient for
> large folios reclamation.
> 
> Moreover, on Arm64 architecture, which supports contiguous PTEs, the Arm64-
> specific ptep_test_and_clear_young() already implements an optimization to
> clear the young flags for PTEs within a contiguous range. However, this is not
> sufficient. Similar to the Arm64 specific clear_flush_young_ptes(), we can
> extend this to perform batched operations for the entire large folio (which
> might exceed the contiguous range: CONT_PTE_SIZE).
> 
> Thus, we can introduce a new batched helper: test_and_clear_young_ptes() and
> its wrapper test_and_clear_young_ptes_notify() which are consistent with the
> existing functions, to perform batched checking of the young flags for large
> folios, which can help improve performance during large folio reclamation when
> MGLRU is enabled. 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@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/pgtable.h | 38 ++++++++++++++++++++++++++++++++++++++
>  mm/internal.h           | 16 +++++++++++-----
>  2 files changed, 49 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 776993d4567b..29bd9fd04e1e 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1103,6 +1103,44 @@ static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
>  }
>  #endif
>  
> +#ifndef test_and_clear_young_ptes
> +/**
> + * test_and_clear_young_ptes - Mark PTEs that map consecutive pages of the same
> + *			       folio as old
> + * @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_test_and_clear_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.
> + *
> + * Returns: whether any PTE was young.
> + */
> +static inline int test_and_clear_young_ptes(struct vm_area_struct *vma,
> +					    unsigned long addr, pte_t *ptep,
> +					    unsigned int nr)

Two tabs ...


What happened to using a boolen as return type and for "int young"?

> +{
> +	int young = 0;
> +
> +	for (;;) {
> +		young |= ptep_test_and_clear_young(vma, addr, ptep);
> +		if (--nr == 0)
> +			break;
> +		ptep++;
> +		addr += PAGE_SIZE;
> +	}
> +
> +	return young;
> +}
> +#endif


-- 
Cheers,

David