linux-kernel - Re: [PATCH mm-new 2/2] mm/khugepaged: merge PTE scanning logic into a new helper

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Message-ID: <44d529b6-a538-4131-a00c-76c73ae49d81@linux.dev>
Date: Sat, 4 Oct 2025 13:24:27 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Dev Jain <dev.jain@....com>
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 ioworker0@...il.com, david@...hat.com, richard.weiyang@...il.com,
 linux-kernel@...r.kernel.org, lorenzo.stoakes@...cle.com,
 linux-mm@...ck.org, akpm@...ux-foundation.org
Subject: Re: [PATCH mm-new 2/2] mm/khugepaged: merge PTE scanning logic into a
 new helper



On 2025/10/4 12:42, Dev Jain wrote:
> 
> On 04/10/25 8:33 am, Lance Yang wrote:
>>
>>
>> On 2025/10/4 01:05, Dev Jain wrote:
>>>
>>> On 02/10/25 1:02 pm, Lance Yang wrote:
>>>> From: Lance Yang <lance.yang@...ux.dev>
>>>>
>>>> As David suggested, the PTE scanning logic in hpage_collapse_scan_pmd()
>>>> and __collapse_huge_page_isolate() was almost duplicated.
>>>>
>>>> This patch cleans things up by moving all the common PTE checking logic
>>>> into a new shared helper, thp_collapse_check_pte().
>>>>
>>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>>>> ---
>>>
>>> In hpage_collapse_scan_pmd(), we enter with mmap lock held, so for
>>> an anonymous vma, is it even possible to hit if (! 
>>> folio_test_anon(folio))?
>>
>> Ah, indeed :)
>>
>>> In which case we can replace this with VM_BUG_ON_FOLIO and abstract away
>>> till the folio_maybe_mapped_shared() block?
>>
>> So you meant something like this:
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 2a897cfb1d03..ef87d7fe3d50 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1374,11 +1374,7 @@ static int hpage_collapse_scan_pmd(struct 
>> mm_struct *mm,
>>                         goto out_unmap;
>>                 }
>>                 folio = page_folio(page);
>> -
>> -               if (!folio_test_anon(folio)) {
>> -                       result = SCAN_PAGE_ANON;
>> -                       goto out_unmap;
>> -               }
>> +               VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
> 
> Yes, I would suggest a different patch making this change, then in the
> 
> last patch you can abstract away till the shared statistics line since that
> 
> much code will become common between scan_pmd and _isolate.

Nice. Makes sense to me!

I'll rework the series into three patches then:

1) The if-else-if-else-if optimization
2) Convert the !folio_test_anon() check to a VM_BUG_ON_FOLIO()
3) The final merge of the scanning logic into a new helper

Thanks,
Lance

> 
> 
>>
>>                 /*
>>                  * We treat a single page as shared if any part of the 
>> THP
>>
>>