Message-ID: <26e6828e-78f7-4454-abaa-334257a8f8c2@redhat.com>
Date: Thu, 4 Sep 2025 20:56:45 +0200
From: David Hildenbrand <david@...hat.com>
To: Nico Pache <npache@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support

On 04.09.25 04:44, Nico Pache wrote:
> On Thu, Aug 21, 2025 at 10:55 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
>>
>> On Thu, Aug 21, 2025 at 10:46:18AM -0600, Nico Pache wrote:
>>>>>>> Thanks and I"ll have a look, but this series is unmergeable with a broken
>>>>>>> default in
>>>>>>> /sys/kernel/mm/transparent_hugepage/khugepaged/mthp_max_ptes_none_ratio
>>>>>>> sorry.
>>>>>>>
>>>>>>> We need to have a new tunable as far as I can tell. I also find the use of
>>>>>>> this PMD-specific value as an arbitrary way of expressing a ratio pretty
>>>>>>> gross.
>>>>>> The first thing that comes to mind is that we can pin max_ptes_none to
>>>>>> 255 if it exceeds 255. It's worth noting that the issue occurs only
>>>>>> for adjacently enabled mTHP sizes.
>>>>
>>>> No! Presumably the default of 511 (for PMDs with 512 entries) is set for a
>>>> reason, arbitrarily changing this to suit a specific case seems crazy no?
>>> We wouldn't be changing it for PMD collapse, just for the new
>>> behavior. At 511, no mTHP collapses would ever occur anyways, unless
>>> you have 2MB disabled and other mTHP sizes enabled. Technically at 511
>>> only the highest enabled order always gets collapsed.
>>>
>>> Ive also argued in the past that 511 is a terrible default for
>>> anything other than thp.enabled=always, but that's a whole other can
>>> of worms we dont need to discuss now.
>>>
>>> with this cap of 255, the PMD scan/collapse would work as intended,
>>> then in mTHP collapses we would never introduce this undesired
>>> behavior. We've discussed before that this would be a hard problem to
>>> solve without introducing some expensive way of tracking what has
>>> already been through a collapse, and that doesnt even consider what
>>> happens if things change or are unmapped, and rescanning that section
>>> would be helpful. So having a strictly enforced limit of 255 actually
>>> seems like a good idea to me, as it completely avoids the undesired
>>> behavior and does not require the admins to be aware of such an issue.
>>>
>>> Another thought similar to what (IIRC) Dev has mentioned before, if we
>>> have max_ptes_none > 255 then we only consider collapses to the
>>> largest enabled order, that way no creep to the largest enabled order
>>> would occur in the first place, and we would get there straight away.
>>>
>>> To me one of these two solutions seem sane in the context of what we
>>> are dealing with.
>>>>
>>>>>>
>>>>>> ie)
>>>>>> if order!=HPAGE_PMD_ORDER && khugepaged_max_ptes_none > 255
>>>>>>        temp_max_ptes_none = 255;
>>>>> Oh and my second point, introducing a new tunable to control mTHP
>>>>> collapse may become exceedingly complex from a tuning and code
>>>>> management standpoint.
>>>>
>>>> Umm right now you hve a ratio expressed in PTES per mTHP * ((PTEs per PMD) /
>>>> PMD) 'except please don't set to the usual default when using mTHP' and it's
>>>> currently default-broken.
>>>>
>>>> I'm really not sure how that is simpler than a seprate tunable that can be
>>>> expressed as a ratio (e.g. percentage) that actually makes some kind of sense?
>>> I agree that the current tunable wasn't designed for this, but we
>>> tried to come up with something that leverages the tunable we have to
>>> avoid new tunables and added complexity.
>>>>
>>>> And we can make anything workable from a code management point of view by
>>>> refactoring/developing appropriately.
>>> What happens if max_ptes_none = 0 and the ratio is 50% - 1 pte
>>> (ideally the max number)? seems like we would be saying we want no new
>>> none pages, but also to allow new none pages. To me that seems equally
>>> broken and more confusing than just taking a scale of the current
>>> number (now with a cap).
>>>
>>>
>>
>> The one thing we absolutely cannot have is a default that causes this
>> 'creeping' behaviour. This feels like shipping something that is broken and
>> alluding to it in the documentation.
> Ok I've put a lot of thought and time into this and came up with a solution.
> 
> Here is what I currently have tested and would like to proposing:
> 
> - Expand bitmap to HPAGE_PMD_NR (512)*, this increases the accuracy of
> the max_pte_none handling, and removes a lot of inaccuracies caused by
> the compression into 128 bits that was being done. This also makes the
> code a lot easier to understand.

That sounds good to me. Should make the code easier as well.

> 
> - When attempting mTHP level collapses cap max_ptes_none to 255 to
> prevent the creep issue

I guess the documentation would then state something like

* When collapsing smaller THPs, "max_ptes_none" is scaled proportional
   to the THP size.
* When collapsing smaller THPs, "max_ptes_none" may be internally
   capped at 255 if it exceeds 255 but is not set to the default (511).

Not 100% a fan of all of that, but maybe the only option when wanting to 
avoid other toggles.

The only alternative would really be respecting only 0/511 for mTHP, and 
not doing any scaling. That would obviously make the documentation 
easier and would allow us to revisit that later. The documentation would be:

* When collapsing smaller THPs, "max_ptes_none" may be interpreted as
   "0"  when set to a value different to the default (511). This behavior
   might change in the future.

> 
> Ive tested this and found this performs better than my previous
> version, allows for more granular control via max_ptes_none, and
> prevents the creep issue without any admin knowledge needed.

How would this interact with the shrinker once extended to mTHP? Would 
your RFC patch be sufficient for that or would we actually also want to 
cap? I haven't; fully thought this through yet. I'd assume we would not 
want to cap here. Which makes the doc weird as well, lol.

-- 
Cheers

David / dhildenb

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