Re: [PATCH v6 0/5] support batch checking of references and unmapping for large folios

[Baolin Wang]

On 2/10/26 9:53 AM, Andrew Morton wrote:
> On Mon,  9 Feb 2026 22:07:23 +0800 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 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).
> >
> > Similar to folio_referenced_one(), we can also apply batched unmapping for large
> > file folios to optimize the performance of file folio reclamation. By supporting
> > batched checking of the young flags, flushing TLB entries, and unmapping, I can
> > observed a significant performance improvements in my performance tests for file
> > folios reclamation. Please check the performance data in the commit message of
> > each patch.
>
> Thanks, I updated mm.git to this version.  Below is how v6 altered
> mm.git.
>
> I notice that this fix:
>
> https://lore.kernel.org/all/de141225-a0c1-41fd-b3e1-bcab09827ddd@xxxxxxxxxxxxxxxxx/T/#u
>
> was not carried forward.  Was this deliberate?

Yes. After discussing with David[1], we believe the original patch is 
correct, so the 'fix' is unnecessary.

[1] 
https://lore.kernel.org/all/280ae63e-d66e-438f-8045-6c870420fe76@xxxxxxxxxxxxxxxxx/

The following diff looks good to me. Thanks.

> Also, regarding the 80-column tricks in folio_referenced_one(): we're
> allowed to do this ;)
>
>
> 				unsigned long end_addr;
> 				unsigned int max_nr;
>
> 				end_addr = pmd_addr_end(address, vma->vm_end);
> 				max_nr = (end_addr - address) >> PAGE_SHIFT;
>
>
>
>
>   arch/arm64/include/asm/pgtable.h |    2 +-
>   include/linux/pgtable.h          |   16 ++++++++++------
>   mm/rmap.c                        |    9 +++------
>   3 files changed, 14 insertions(+), 13 deletions(-)
>
> --- a/arch/arm64/include/asm/pgtable.h~b
> +++ a/arch/arm64/include/asm/pgtable.h
> @@ -1843,7 +1843,7 @@ static inline int clear_flush_young_ptes
>   					 unsigned long addr, pte_t *ptep,
>   					 unsigned int nr)
>   {
> -	if (likely(nr == 1 && !pte_valid_cont(__ptep_get(ptep))))
> +	if (likely(nr == 1 && !pte_cont(__ptep_get(ptep))))
>   		return __ptep_clear_flush_young(vma, addr, ptep);
>   
>   	return contpte_clear_flush_young_ptes(vma, addr, ptep, nr);
> --- a/include/linux/pgtable.h~b
> +++ a/include/linux/pgtable.h
> @@ -1070,8 +1070,8 @@ static inline void wrprotect_ptes(struct
>   
>   #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.
> + * 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.
> @@ -1087,13 +1087,17 @@ static inline void wrprotect_ptes(struct
>    * 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)
> +		unsigned long addr, pte_t *ptep, unsigned int nr)
>   {
> -	int i, young = 0;
> +	int young = 0;
>   
> -	for (i = 0; i < nr; ++i, ++ptep, addr += PAGE_SIZE)
> +	for (;;) {
>   		young |= ptep_clear_flush_young(vma, addr, ptep);
> +		if (--nr == 0)
> +			break;
> +		ptep++;
> +		addr += PAGE_SIZE;
> +	}
>   
>   	return young;
>   }
> --- a/mm/rmap.c~b
> +++ a/mm/rmap.c
> @@ -963,10 +963,8 @@ static bool folio_referenced_one(struct
>   				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;
> +				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,
> @@ -974,8 +972,7 @@ static bool folio_referenced_one(struct
>   			}
>   
>   			ptes += nr;
> -			if (clear_flush_young_ptes_notify(vma, address,
> -						pvmw.pte, nr))
> +			if (clear_flush_young_ptes_notify(vma, address, pvmw.pte, nr))
>   				referenced++;
>   			/* Skip the batched PTEs */
>   			pvmw.pte += nr - 1;
> _