Message-ID: <ed784d6c-6c2a-4e0b-ad2f-e953bf310ce8@redhat.com>
Date: Fri, 12 Sep 2025 19:53:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Kiryl Shutsemau <kas@...nel.org>, Nico Pache <npache@...hat.com>,
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On 12.09.25 17:51, Lorenzo Stoakes wrote:
> On Fri, Sep 12, 2025 at 05:45:26PM +0200, David Hildenbrand wrote:
>> On 12.09.25 17:41, Kiryl Shutsemau wrote:
>>> On Fri, Sep 12, 2025 at 04:56:47PM +0200, David Hildenbrand wrote:
>>>> On 12.09.25 16:35, Kiryl Shutsemau wrote:
>>>>> On Fri, Sep 12, 2025 at 04:28:09PM +0200, David Hildenbrand wrote:
>>>>>> On 12.09.25 15:47, David Hildenbrand wrote:
>>>>>>> On 12.09.25 14:19, Kiryl Shutsemau wrote:
>>>>>>>> On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote:
>>>>>>>>> The following series provides khugepaged with the capability to collapse
>>>>>>>>> anonymous memory regions to mTHPs.
>>>>>>>>>
>>>>>>>>> To achieve this we generalize the khugepaged functions to no longer depend
>>>>>>>>> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
>>>>>>>>> pages that are occupied (!none/zero). After the PMD scan is done, we do
>>>>>>>>> binary recursion on the bitmap to find the optimal mTHP sizes for the PMD
>>>>>>>>> range. The restriction on max_ptes_none is removed during the scan, to make
>>>>>>>>> sure we account for the whole PMD range. When no mTHP size is enabled, the
>>>>>>>>> legacy behavior of khugepaged is maintained. max_ptes_none will be scaled
>>>>>>>>> by the attempted collapse order to determine how full a mTHP must be to be
>>>>>>>>> eligible for the collapse to occur. If a mTHP collapse is attempted, but
>>>>>>>>> contains swapped out, or shared pages, we don't perform the collapse. It is
>>>>>>>>> now also possible to collapse to mTHPs without requiring the PMD THP size
>>>>>>>>> to be enabled.
>>>>>>>>>
>>>>>>>>> When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on
>>>>>>>>> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for
>>>>>>>>> mTHP collapses to prevent collapse "creep" behavior. This prevents
>>>>>>>>> constantly promoting mTHPs to the next available size, which would occur
>>>>>>>>> because a collapse introduces more non-zero pages that would satisfy the
>>>>>>>>> promotion condition on subsequent scans.
>>>>>>>>
>>>>>>>> Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count
>>>>>>>> all-zeros 4k as none_or_zero? It mirrors the logic of shrinker.
>>>>>>>
>>>>>>> BTW, I thought further about this and I agree: if we count zero-filled
>>>>>>> pages towards none_or_zero one we can avoid the "creep" problem.
>>>>>>>
>>>>>>> The scanning-for-zero part is rather nasty, though.
>>>>>>
>>>>>> Aaand, thinking again from the other direction, this would mean that just
>>>>>> because pages became zero after some time that we would no longer collapse
>>>>>> because none_or_zero would then be higher. Hm ....
>>>>>>
>>>>>> How I hate all of this so very very much :)
>>>>>
>>>>> This is not new. Shrinker has the same problem: it cannot distinguish
>>>>> between hot 4k that happened to be zero from the 4k that is there just
>>>>> because of we faulted in 2M a time.
>>>>
>>>> Right. And so far that problem is isolated to the shrinker.
>>>>
>>>> To me so far "none_or_zero" really meant "will I consume more memory when
>>>> collapsing". That's not true for zero-filled pages, obviously.
>>>
>>> Well, KSM can reclaim these zero-filled memory until we collapse it.
>>
>> KSM is used so rarely (for good reasons) that I would never ever build an
>> argument based on its existence :P
>>
>> But yes: during the very first shrinker discussion I raised that KSM can do
>> the same thing. Obviously that was not good enough.
>>
>> --
>> Cheers
>>
>> David / dhildenb
>>
> 
> With all this stuff said, do we have an actual plan for what we intend to do
> _now_?

Oh no, no I have to use my brain and it's Friday evening.

> 
> As Nico has implemented a basic solution here that we all seem to agree is not
> what we want.
> 
> Without needing special new hardware or major reworks, what would this parameter
> look like?
> 
> What would the heuristics be? What about the eagerness scales?
> 
> I'm but a simple kernel developer, 

:)

and interested in simple pragmatic stuff :)
> do you have a plan right now David?

Ehm, if you ask me that way ...

> 
> Maybe we can start with something simple like a rough percentage per eagerness
> entry that then gets scaled based on utilisation?

... I think we should probably:

1) Start with something very simple for mTHP that doesn't lock us into any particular direction.

2) Add an "eagerness" parameter with fixed scale and use that for mTHP as well

3) Improve that "eagerness" algorithm using a dynamic scale or #whatever

4) Solve world peace and world hunger

5) Connect it all to memory pressure / reclaim / shrinker / heuristics / hw hotness / #whatever


I maintain my initial position that just using

max_ptes_none == 511 -> collapse mTHP always
max_ptes_none != 511 -> collapse mTHP only if we all PTEs are non-none/zero

As a starting point is probably simple and best, and likely leaves room for any
changes later.


Of course, we could do what Nico is proposing here, as 1) and change it all later.

It's just when it comes to documenting all that stuff in patch #15 that I feel like
"alright, we shouldn't be doing it longterm like that, so let's not make anybody
depend on any weird behavior here by over-domenting it".

I mean

"
+To prevent "creeping" behavior where collapses continuously promote to larger
+orders, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on 4K page size), it is
+capped to HPAGE_PMD_NR/2 - 1 for mTHP collapses. This is due to the fact
+that introducing more than half of the pages to be non-zero it will always
+satisfy the eligibility check on the next scan and the region will be collapse.
"

Is just way, way to detailed.

I would just say "The kernel might decide to use a more conservative approach
when collapsing smaller THPs" etc.


Thoughts?

-- 
Cheers

David / dhildenb

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