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Message-ID: <ec49f0fe-9df8-4762-b315-240cbb1ed3ce@arm.com>
Date: Wed, 24 Dec 2025 13:24:39 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
david@...nel.org, catalin.marinas@....com, will@...nel.org
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, riel@...riel.com,
harry.yoo@...cle.com, jannh@...gle.com, willy@...radead.org,
baohua@...nel.org, dev.jain@....com, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/5] mm: rmap: support batched checks of the references
for large folios
On 23/12/2025 05:48, Baolin Wang 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.
>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
With the 2 niggles below addressed:
Reviewed-by: Ryan Roberts <ryan.roberts@....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..dbbdcef4abf1 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) \
I think I previously suggested that this should be renamed to
clear_flush_young_ptes_notify() given that it is now a batch operation. Were
others against that or did you forget?
> ({ \
> 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 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 2f0dd3a4ace1..fcf7a7820061 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1087,6 +1087,41 @@ 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)
> +{
> + int young;
> +
> + young = ptep_clear_flush_young(vma, addr, ptep);
> + while (--nr) {
> + ptep++;
> + addr += PAGE_SIZE;
> + young |= ptep_clear_flush_young(vma, addr, ptep);
> + }
I think it's better to avoid the two ptep_clear_flush_young() calls if we can.
Personally I think we should just go for the simple:
for (i = 0; i < nr; ++i, ++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;
>
> 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)
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