Re: [PATCH v5 4/5] arm64: mm: implement the architecture-specific clear_flush_young_ptes()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 1/29/26 02:42, Baolin Wang wrote:
> 
> 
> On 1/28/26 7:47 PM, Chris Mason wrote:
>> Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
>>> Implement the Arm64 architecture-specific clear_flush_young_ptes() to 
>>> enable
>>> batched checking of young flags and TLB flushing, 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%.
>>
>> Hi everyone, I ran mm-new through my AI review prompts and this one was
>> flagged.  AI review below:
>>
>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/ 
>>> asm/pgtable.h
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -1838,6 +1838,17 @@ static inline int 
>>> ptep_clear_flush_young(struct vm_area_struct *vma,
>>>       return contpte_clear_flush_young_ptes(vma, addr, ptep, 1);
>>>   }
>>>
>>> +#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 && !pte_cont(__ptep_get(ptep))))
>>> +        return __ptep_clear_flush_young(vma, addr, ptep);
>>
>> Should this be checking !pte_valid_cont() instead of !pte_cont()?
>>
>> The existing ptep_clear_flush_young() above uses !pte_valid_cont() to
>> determine when to take the fast path. The new function only checks
>> !pte_cont(), which differs when handling non-present PTEs.
>>
>> Non-present PTEs (device-private, device-exclusive) can reach
>> clear_flush_young_ptes() through folio_referenced_one()->
>> clear_flush_young_ptes_notify(). These entries may have bit 52 set as
>> part of their encoding, but they aren't valid contiguous mappings.
>>
>> With the current check, wouldn't such entries incorrectly trigger the
>> contpte path and potentially cause contpte_clear_flush_young_ptes() to
>> process additional unrelated PTEs beyond the intended single entry?
> 
> Indeed. I previously discussed with Ryan whether using pte_cont() was 
> enough, and we believed that invalid PTEs wouldn’t have the PTE_CONT bit 
> set. But we clearly missed the device-folio cases. Thanks for reporting.
> 
> Andrew, could you please squash the following fix into this patch? If 
> you prefer a new version, please let me know. Thanks.

Isn't the real problem that we should never ever ever ever, try clearing 
the young bit on a non-present pte?

See damon_ptep_mkold() how that is handled with the flushing/notify.

There needs to be a pte_present() check in the caller.


BUT

I recall that folio_referenced() should never apply to ZONE_DEVICE 
folios. folio_referenced() is only called from memory reclaim code, and 
ZONE_DEVICE pages never get reclaimed through vmscan.c

-- 
Cheers,

David