Re: [PATCH v4 1/5] mm: rmap: support batched checks of the references for large folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Hi Ryan,

On 2025/12/24 21:24, Ryan Roberts wrote:
> 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@linux.alibaba.com>
> 
> With the 2 niggles below addressed:
> 
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

Thanks.

>> ---
>>   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?

Sorry, I missed this comment. Yes, sounds reasonable to me. Will do in 
the next version.

>>   ({									\
>>   	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);

ACK. Thanks for reviewing.