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Message-ID: <4413d7f4-7734-4715-a450-2ba09db5ed22@lucifer.local>
Date: Mon, 15 Dec 2025 12:22:58 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, david@...nel.org, catalin.marinas@....com,
will@...nel.org, ryan.roberts@....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, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] mm: rmap: support batched checks of the
references for large folios
On Thu, Dec 11, 2025 at 04:16:55PM +0800, 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 Arm architecture, which supports contiguous PTEs, there is already
arm64 you mean :)
> 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.
>
> Performance testing:
> Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to
> reclaim 8G file-backed folios via the memory.reclaim interface. I can observe
> 33% performance improvement on my Arm64 32-core server (and 10%+ improvement
> on my X86 machine). Meanwhile, the hotspot folio_check_references() dropped
> from approximately 35% to around 5%.
>
> W/o patchset:
> real 0m1.518s
> user 0m0.000s
> sys 0m1.518s
>
> W/ patchset:
> real 0m1.018s
> user 0m0.000s
> sys 0m1.018s
That's nice!
Have you performed the same kind of performance testing on non-arm64? As in the
past we've had a batch optimisation go horribly wrong on non-arm64 even if it
was ok on arm64 :)
>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
> arch/arm64/include/asm/pgtable.h | 11 +++++++++++
> include/linux/mmu_notifier.h | 9 +++++----
> include/linux/pgtable.h | 19 +++++++++++++++++++
> mm/rmap.c | 22 ++++++++++++++++++++--
> 4 files changed, 55 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e03034683156..a865bd8c46a3 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1869,6 +1869,17 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
> return contpte_clear_flush_young_ptes(vma, addr, ptep, CONT_PTES);
> }
>
> +#define clear_flush_young_ptes clear_flush_young_ptes
> +static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep,
> + unsigned int nr)
> +{
> + if (likely(nr == 1))
> + return __ptep_clear_flush_young(vma, addr, ptep);
> +
> + return contpte_clear_flush_young_ptes(vma, addr, ptep, nr);
> +}
Hmm again this is a weird way of exposing a contepte-specific function, you
really need to rework that as discussed in patch 1/3.
It seems to me we can share code to avoid this.
> +
> #define wrprotect_ptes wrprotect_ptes
> static __always_inline void wrprotect_ptes(struct mm_struct *mm,
> unsigned long addr, pte_t *ptep, unsigned int nr)
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index d1094c2d5fb6..be594b274729 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) \
> ({ \
> 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; \
> })
An aside, but I wonder why this needs to be a (pretty disgusting) macro?
>
> @@ -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 b13b6f42be3c..c7d0fd228cb7 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -947,6 +947,25 @@ static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
> }
> #endif
>
> +#ifndef clear_flush_young_ptes
> +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
> +
> /*
> * 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..ec232165c47d 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 +876,21 @@ 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);
I do wish we could put this fiddly logic into a helper for each place in
which we do similar kind 'end of the PTE table, maximum number we could
have' logic.
> + }
NIT but we're running into pretty long lines here.
> +
> + ptes += nr;
> if (ptep_clear_flush_young_notify(vma, address,
> - pvmw.pte))
> + pvmw.pte, nr))
> referenced++;
I find this referenced logic weird, it seems like it should be a boolean,
but this is outside the scope of your patch here :)
> + /* 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 +900,11 @@ static bool folio_referenced_one(struct folio *folio,
> WARN_ON_ONCE(1);
> }
>
> - pra->mapcount--;
> + pra->mapcount -= nr;
> + if (ptes == pvmw.nr_pages) {
> + page_vma_mapped_walk_done(&pvmw);
> + break;
> + }
> }
>
> if (referenced)
> --
> 2.47.3
>
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