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

[David Hildenbrand <david@redhat.com>]

On 02.09.25 04:28, Baolin Wang wrote:
> 
> 
> 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.

The comment over khugepaged_max_ptes_none gives a hint:

/*
  * default collapse hugepages if there is at least one pte mapped like
  * it would have happened if the vma was large enough during page
  * fault.
  *
  * Note that these are only respected if collapse was initiated by khugepaged.
  */

In the common case (for anything that really cares about RSS bloat) you will just a
get a THP during page fault and consequently RSS bloat.

As raised in my other reply, the only documented reason to set max_ptes_none=0 seems
to be when an application later (after once possibly getting a THP already during
page faults) did some MADV_DONTNEED and wants to control the usage of THPs itself using
MADV_COLLAPSE.

It's a questionable use case, that already got more problematic with mTHP and page
table reclaim.

Let me explain:

Before mTHP, if someone would MADV_DONTNEED (resulting in
a page table with at least one pte_none entry), there would have been no way we would
get memory over-allocated afterwards with max_ptes_none=0.

(1) Page faults would spot "there is a page table" and just fallback to order-0 pages.
(2) khugepaged was told to not collapse through max_ptes_none=0.

But now:

(A) With mTHP during page-faults, we can just end up over-allocating memory in such
     an area again: page faults will simply spot a bunch of pte_nones around the fault area
     and install an mTHP.

(B) With page table reclaim (when zapping all PTEs in a table at once), we will reclaim the
     page table. The next page fault will just try installing a PMD THP again, because there is
     no PTE table anymore.

So I question the utility of max_ptes_none. If you can't tame page faults, then there is only
limited sense in taming khugepaged. I think there is vale in setting max_ptes_none=0 for some
corner cases, but I am yet to learn why max_ptes_none=123 would make any sense.


-- 
Cheers

David / dhildenb