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

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Message-ID: <f235a557-d031-4fcd-8c68-81d9ab7b54e0@arm.com>
Date: Sat, 4 Oct 2025 18:41:48 +0530
From: Dev Jain <dev.jain@....com>
To: Wei Yang <richard.weiyang@...il.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,
 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 04/10/25 3:12 pm, Wei Yang wrote:
> 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?

Frankly I do not have much opinion on the BUG_ON/WARN_ON debate since I haven't
properly understood that, but this BUG_ON is under CONFIG_DEBUG_VM anways. But
if you want to change this to WARN then you can do it at both places.

>