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

Baolin Wang
2026-02-10T02:01:11Z

On 2/10/26 9:53 AM, Andrew Morton wrote:
> On Mon, 9 Feb 2026 22:07:23 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> 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@linux.alibaba.com/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@linux.alibaba.com/

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;
> _