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

[Lance Yang <lance.yang@linux.dev>]

On 2025/10/5 10:38, Zi Yan wrote:
> On 4 Oct 2025, at 22:35, Lance Yang wrote:
> 
>> On 2025/10/4 21:11, Dev Jain wrote:
>>>
>>> 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@linux.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@redhat.com>
>>>>>> Signed-off-by: Lance Yang <lance.yang@linux.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
>>
>> Yeah, VM_BUG_ON_FOLIO() is under CONFIG_DEBUG_VM, so it won't affect
>> production kernels.
> 
> Many distros enable it by default. For mm, we are moving away from
> using BUG_ON or VM_BUG_ON. No need to crash the system if it is possible
> to handle it gracefully.

Ah, good to know that, thanks!

> 
>>
>>> if you want to change this to WARN then you can do it at both places.
>>
>> It should flag such an impossible condition there during development.
>> So, I'd prefer to stick with VM_BUG_ON_FOLIO().
>>
>> @Wei please let me know if you feel strongly otherwise :)
> 
> 
> --
> Best Regards,
> Yan, Zi