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Message-ID: <7391d74f-782c-4d5a-9f9d-02afcefdbf87@linux.alibaba.com>
Date: Tue, 16 Dec 2025 11:47:00 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 2025/12/15 20:22, Lorenzo Stoakes wrote:
> 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 :)
Right. Will make it clear.
>> 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 :)
Yes, seems you missed my test results for the x86 machine in the commit
message :)
"I can observe 33% performance improvement on my Arm64 32-core server
(and 10%+ improvement on my X86 machine)."
>> 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.
Sorry I don't think so. This is the current way of exposing a contpte
for Arm64. Please take a look at set_ptes(), clear_full_ptes(),
wrprotect_ptes() and so on (in this file).
>> #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?
Um, I can send a follow-up to clean up all these related macros.
>> @@ -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.
Um, the logic is already simple, and I don’t think adding a new helper
would improve readability. If some code can reuse this logic, we can
factor it out into a helper at that point.
>> + }
>
> NIT but we're running into pretty long lines here.
OK. Will fix this.
>> +
>> + 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 :)
Right. Thanks for reviewing.
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