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

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

On 2/9/26 5:09 PM, David Hildenbrand (Arm) wrote:
> 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.

The handling of ZONE_DEVICE memory in check_pte() makes me me doubt my 
earlier understanding. And I think you are right.

	} else if (pte_present(ptent)) {
		pfn = pte_pfn(ptent);
	} else {
		const softleaf_t entry = softleaf_from_pte(ptent);

		/* Handle un-addressable ZONE_DEVICE memory */
		if (!softleaf_is_device_private(entry) &&
		    !softleaf_is_device_exclusive(entry))
			return false;

		pfn = softleaf_to_pfn(entry);
	}


> 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

Thanks for clarifying. So I can drop the pte valid check.