Re: [PATCH v5 08/19] selftests/mm: add folio-order check for address ranges

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/10/26 9:04 PM, Kiryl Shutsemau wrote:
> On Thu, Sep 10, 2026 at 07:14:42PM +0800, Baolin Wang wrote:
>>
>>
>> On 9/10/26 6:45 PM, Kiryl Shutsemau wrote:
>>> On Wed, Sep 09, 2026 at 06:01:13PM +0800, Baolin Wang wrote:
>>>>
>>>>
>>>> On 9/8/26 8:50 PM, Kiryl Shutsemau wrote:
>>>>> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>>>>>
>>>>> An mTHP collapse test needs to know that a range is backed by folios of the
>>>>> target order, and that they sit where a collapse would put them.  Nothing
>>>>> answers that today: is_backed_by_folio() classifies the folio behind a
>>>>> single page, and check_huge_anon() reads smaps AnonHugePages, which only
>>>>> accounts PMD mappings.
>>>>
>>>> Have you checked check_large_folios() in vm_util.c? It seems to meet your
>>>> requirements, or am I missing something?
>>>
>>> The changelog is out of date, sorry: since 6dedaf0d46a9 check_huge_anon()
>>> counts mTHP folios through check_large_folios() rather than reading
>>> smaps, and it is what check_huge() already uses for mthp_khugepaged.
>>>
>>> It could do this job too, called once per window.
>>>
>>> What is_range_backed_by_order() adds is alignment check: the folio has
>>> to sit at the window start. And check_large_folios() reopens two fds per
>>> per call which kinda wasteful.
>>
>> OK. But can we extend the check_huge_xxx() functions to meet your
>> requirements, for example by adding a check_aligned_huge_xxx() that wraps
>> the underlying implementation of check_large_folios()?
>>
>> I still find it confusing that khugepaged.c has two separate sets of
>> functions for checking large folios.
> 
> Agreed that two sets is one too many.  Yeoreum's series rewrites
> check_huge_xxx() on top of pagemap and kpageflags, which is what
> is_backed_by_folio() already walks, so once that lands the two can meet:
> the mTHP branch of check_huge() checks each window for a folio of the
> order at its start, and is_range_backed_by_order() goes away or becomes
> its internal.  That also makes the generic mTHP cases stricter for free.

Look forward to seeing this happen:)

> I would rather do that as a follow-up than fold it into either series
> now, since both touch the same functions.

Sure.