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Message-ID: <b1075258-a42d-4e2f-95d7-5f8cb11f0d7d@arm.com>
Date: Wed, 17 Dec 2025 12:53:46 +0530
From: Dev Jain <dev.jain@....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, 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 17/12/25 12:39 pm, Baolin Wang wrote:
>
>
> On 2025/12/17 14:49, Dev Jain wrote:
>>
>> On 11/12/25 1:46 pm, 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
>>> 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
>>>
>>> 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);
>>> +}
>>> +
>>> #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; \
>>> })
>>> @@ -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);
>>
>> I may be hallucinating here but I am just trying to recall things - is this a
>> bug in
>> folio_pte_batch_flags()? A folio may not be naturally aligned in virtual
>> space and hence
>> we may cross the PTE table while batching across it, which can be fixed by
>> taking into
>> account pmd_addr_end() while computing max_nr.
>
> IMHO, the comments for the folio_pte_batch_flags() function have already made
> clear requirements for the caller to avoid such situations:
>
> "
> * @ptep must map any page of the folio. max_nr must be at least one and
> * must be limited by the caller so scanning cannot exceed a single VMA and
> * a single page table.
> "
>
> Additionally, Lance recently fixed a similar issue, see commit ddd05742b45b
> ("mm/rmap: fix potential out-of-bounds page table access during batched unmap").
Ah I see, all other users of the folio_pte_batch API constrain start and end
because they are already operating on a single PTE table. But for rmap code
this may not be the case.
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