Re: [PATCH v10 00/13] khugepaged: mTHP support

David Hildenbrand
2025-09-01T16:46:58Z

On 29.08.25 03:55, Baolin Wang wrote:
>
>
> On 2025/8/28 18:48, Dev Jain wrote:
>>
>> On 28/08/25 3:16 pm, Baolin Wang wrote:
>>> (Sorry for chiming in late)
>>>
>>> On 2025/8/22 22:10, David Hildenbrand wrote:
>>>>>> Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1),
>>>>>> but not sure
>>>>>> if we have to add that for now.
>>>>>
>>>>> Yeah not so sure about this, this is a 'just have to know' too, and
>>>>> yes you
>>>>> might add it to the docs, but people are going to be mightily
>>>>> confused, esp if
>>>>> it's a calculated value.
>>>>>
>>>>> I don't see any other way around having a separate tunable if we
>>>>> don't just have
>>>>> something VERY simple like on/off.
>>>>
>>>> Yeah, not advocating that we add support for other values than 0/511,
>>>> really.
>>>>
>>>>>
>>>>> Also the mentioned issue sounds like something that needs to be
>>>>> fixed elsewhere
>>>>> honestly in the algorithm used to figure out mTHP ranges (I may be
>>>>> wrong - and
>>>>> happy to stand corrected if this is somehow inherent, but reallly
>>>>> feels that
>>>>> way).
>>>>
>>>> I think the creep is unavoidable for certain values.
>>>>
>>>> If you have the first two pages of a PMD area populated, and you
>>>> allow for at least half of the #PTEs to be non/zero, you'd collapse
>>>> first a
>>>> order-2 folio, then and order-3 ... until you reached PMD order.
>>>>
>>>> So for now we really should just support 0 / 511 to say "don't
>>>> collapse if there are holes" vs. "always collapse if there is at
>>>> least one pte used".
>>>
>>> If we only allow setting 0 or 511, as Nico mentioned before, "At 511,
>>> no mTHP collapses would ever occur anyway, unless you have 2MB
>>> disabled and other mTHP sizes enabled. Technically, at 511, only the
>>> highest enabled order would ever be collapsed."
>> I didn't understand this statement. At 511, mTHP collapses will occur if
>> khugepaged cannot get a PMD folio. Our goal is to collapse to the
>> highest order folio.
>
> Yes, I’m not saying that it’s incorrect behavior when set to 511. What I
> mean is, as in the example I gave below, users may only want to allow a
> large order collapse when the number of present PTEs reaches half of the
> large folio, in order to avoid RSS bloat.

How do these users control allocation at fault time where this parameter
is completely ignored?

--
Cheers

David / dhildenb