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

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Message-ID: <f9f62e40-92a6-4ee8-aba0-6bb9b2929f41@arm.com>
Date: Sat, 4 Oct 2025 10:12:08 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <lance.yang@...ux.dev>
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 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.


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