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

Baolin Wang
2025-09-02T02:28:26Z

On 2025/9/2 00:46, David Hildenbrand wrote:
> 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?

Sorry, I did not get your point. Why does the 'max_pte_none' need to
control allocation at fault time? Could you be more specific? Thanks.