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

[Dev Jain <dev.jain@arm.com>]

On 22/08/25 8:19 pm, Lorenzo Stoakes wrote:
> On Fri, Aug 22, 2025 at 04:10:35PM +0200, 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.
> Yeah I'm fine with 0/511.
>
>>> 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.
> Feels like we should be looking at this in reverse? What's the largest, then
> next largest, then etc.?
>
> Surely this is the sensible way of doing it?

What David means to say is, for example, suppose all orders are enabled,
and we fail to collapse for order-9, then order-8, then order-7, and so on,
*only* because the distribution of ptes did not obey the scaled max_ptes_none.
Let order-4 collapse succeed.

Next time, khugepaged comes and tries for order-9, fails, then order-8, fails and
so on. Then it checks for order-5, and it comes under the scaled max_ptes_none constraint
only because the previous cycle's order-4 collapse changed the ptes' distribution.
  

>
>> 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".
> Yes.
>
>>>> Because, as raised in the past, I'm afraid nobody on this earth has a clue how
>>>> to set this parameter to values different to 0 (don't waste memory with khugepaged)
>>>> and 511 (page fault behavior).
>>> Yup
>>>
>>>>
>>>> If any other value is set, essentially
>>>> 	pr_warn("Unsupported 'max_ptes_none' value for mTHP collapse");
>>>>
>>>> for now and just disable it.
>>> Hmm but under what circumstances? I would just say unsupported value not mention
>>> mTHP or people who don't use mTHP might find that confusing.
>> Well, we can check whether any mTHP size is enabled while the value is set
>> to something unexpected. We can then even print the problematic sizes if we
>> have to.
> Ack
>
>> We could also just just say that if the value is set to something else than
>> 511 (which is the default), it will be treated as being "0" when collapsing
>> mthp, instead of doing any scaling.
> Or we could make it an error to set anything but 0, 511, but on the other hand
> that's likely to break userspace so yeah probably not.
>
> Maybe have a warning saying 'this is no longer supported and will be ignored'
> then set the value to 0 for anything but 511 or 0.
>
> Then can remove the warning later.
>
> By having 0/511 we can really simplify the 'scaling' logic too which would be
> fantastic! :)

FWIW here was my implementation of this thing, for ease of everyone:
https://lore.kernel.org/all/20250211111326.14295-17-dev.jain@arm.com/

>
> Cheers, Lorenzo