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

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Message-ID: <20251004094217.bah5q2zxczrqontm@master>
Date: Sat, 4 Oct 2025 09:42:17 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Dev Jain <dev.jain@....com>
Cc: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
	david@...hat.com, lorenzo.stoakes@...cle.com, ziy@...dia.com,
	baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
	npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
	ioworker0@...il.com, richard.weiyang@...il.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new 2/2] mm/khugepaged: merge PTE scanning logic into
 a new helper

On Fri, Oct 03, 2025 at 10:35:12PM +0530, 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

This is true for the first loop, but we will unlock/lock mmap and revalidate
vma before isolation.

>an anonymous vma, is it even possible to hit if (!folio_test_anon(folio))?
>In which case we can replace this with VM_BUG_ON_FOLIO and abstract away
>till the folio_maybe_mapped_shared() block?

But it looks still valid, since hugepage_vma_revalidate() will check the vma
is still anonymous vma after grab the mmap lock again.

My concern is would VM_BUG_ON_FOLIO() be too heavy? How about warn on and
return?

-- 
Wei Yang
Help you, Help me