linux-kernel - Re: [PATCH v5 4/5] arm64: mm: implement the architecture-specific clear_flush_young

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Message-ID: <6817c7b9-e67c-46b6-b3b5-1bc3d9eed4a2@linux.alibaba.com>
Date: Mon, 9 Feb 2026 18:13:19 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: "David Hildenbrand (Arm)" <david@...nel.org>, Chris Mason <clm@...a.com>
Cc: akpm@...ux-foundation.org, catalin.marinas@....com, will@...nel.org,
 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, dev.jain@....com,
 linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/5] arm64: mm: implement the architecture-specific
 clear_flush_young_ptes()



On 2/9/26 5:55 PM, David Hildenbrand (Arm) wrote:
> On 2/9/26 10:36, Baolin Wang wrote:
>>
>>
>> On 2/9/26 5:09 PM, David Hildenbrand (Arm) wrote:
>>> On 1/29/26 02:42, Baolin Wang wrote:
>>>>
>>>>
>>>>
>>>> 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.
> 
> We should probably add a safety check in folio_referenced(), warning
> if we would ever get a ZONE_DEVICE folio passed.
> 
> Can someone look into that ? :)

Sure, I can take a close look and address that in my follow-up patchset.