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

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Message-ID: <a36c9776-d158-4add-86d0-c41a09e4a255@linux.dev>
Date: Sat, 4 Oct 2025 11:03:11 +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 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);

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