Re: [RFC 00/12] mm: PUD (1GB) THP implementation

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

On 04/02/26 5:20 pm, Dev Jain wrote:
> On 04/02/26 4:38 pm, Lorenzo Stoakes wrote:
>> On Tue, Feb 03, 2026 at 05:00:10PM -0800, Usama Arif wrote:
>>> On 02/02/2026 03:20, Lorenzo Stoakes wrote:
>>>> OK so this is somewhat unexpected :)
>>>>
>>>> It would have been nice to discuss it in the THP cabal or at a conference
>>>> etc. so we could discuss approaches ahead of time. Communication is important,
>>>> especially with major changes like this.
>>> Makes sense!
>>>
>>>> And PUD THP is especially problematic in that it requires pages that the page
>>>> allocator can't give us, presumably you're doing something with CMA and... it's
>>>> a whole kettle of fish.
>>> So we dont need CMA. It helps ofcourse, but we don't *need* it.
>>> Its summarized in the first reply I gave to Zi in [1]:
>>>
>>>> It's also complicated by the fact we _already_ support it in the DAX, VFIO cases
>>>> but it's kinda a weird sorta special case that we need to keep supporting.
>>>>
>>>> There's questions about how this will interact with khugepaged, MADV_COLLAPSE,
>>>> mTHP (and really I want to see Nico's series land before we really consider
>>>> this).
>>> So I have numbers and experiments for page faults which are in the cover letter,
>>> but not for khugepaged. I would be very surprised (although pleasently :)) if
>>> khugepaged by some magic finds 262144 pages that meets all the khugepaged requirements
>>> to collapse the page. In the basic infrastructure support which this series is adding,
>>> I want to keep khugepaged collapse disabled for 1G pages. This is also the initial
>>> approach that was taken in other mTHP sizes. We should go slow with 1G THPs.
>> Yes we definitely want to limit to page faults for now.
>>
>> But keep in mind for that to be viable you'd surely need to update who gets
>> appropriate alignment in __get_unmapped_area()... not read through series far
>> enough to see so not sure if you update that though!
>>
>> I guess that'd be the sanest place to start, if an allocation _size_ is aligned
>> 1 GB, then align the unmapped area _address_ to 1 GB for maximum chance of 1 GB
>> fault-in.
>>
>> Oh by the way I made some rough THP notes at
>> https://publish.obsidian.md/mm/Transparent+Huge+Pages+(THP) which are helpful
>> for reminding me about what does what where, useful for a top-down view of how
>> things are now.
>>
>>>> So overall, I want to be very cautious and SLOW here. So let's please not drop
>>>> the RFC tag until David and I are ok with that?
>>>>
>>>> Also the THP code base is in _dire_ need of rework, and I don't really want to
>>>> add major new features without us paying down some technical debt, to be honest.
>>>>
>>>> So let's proceed with caution, and treat this as a very early bit of
>>>> experimental code.
>>>>
>>>> Thanks, Lorenzo
>>> Ack, yeah so this is mainly an RFC to discuss what the major design choices will be.
>>> I got a kernel with selftests for allocation, memory integrity, fork, partial munmap,
>>> mprotect, reclaim and migration passing and am running them with DEBUG_VM to make sure
>>> we dont get the VM bugs/warnings and the numbers are good, so just wanted to share it
>>> upstream and get your opinions! Basically try and trigger a discussion similar to what
>>> Zi asked in [2]! And also if someone could point out if there is something fundamental
>>> we are missing in this series.
>> Well that's fair enough :)
>>
>> But do come to a THP cabal so we can chat, face-to-face (ok, digital face to
>> digital face ;). It's usually a force-multiplier I find, esp. if multiple people
>> have input which I think is the case here. We're friendly :)
>>
>> In any case, conversations are already kicking off so that's definitely positive!
>>
>> I think we will definitely get there with this at _some point_ but I would urge
>> patience and also I really want to underline my desire for us in THP to start
>> paying down some of this technical debt.
>>
>> I know people are already making efforts (Vernon, Luiz), and sorry that I've not
>> been great at review recently (should be gradually increasing over time), but I
>> feel that for large features to be added like this now we really do require some
>> refactoring work before we take it.
>>
>> We definitely need to rebase this once Nico's series lands (should do next
>> cycle) and think about how it plays with this, I'm not sure if arm64 supports
>> mTHP between PMD and PUD size (Dev? Do you know?) so maybe that one is moot, but
> arm64 does support cont mappings at the PMD level. Currently, they are supported
> for kernel pagetables, and hugetlbpages. You may search around for "CONT_PMD" in
> the codebase. Hence it only supports cont PMD in the "static" case, there is
> no dynamic folding/unfolding of the cont bit at the PMD level, which mTHP requires.
>
> I see that this patchset splits PUD all the way down to PTEs. If we were to split
> it down to PMD, and add arm64 support for dynamic cont mappings at the PMD level,
> it will be nicer. But I guess there is some mapcount/rmap stuff involved
> here stopping us from doing that :(

Hmm, this won't make a difference w.r.t cont PMD. If we were to split PUD folio
down to PMD folios, we won't get cont PMD. But yes, in general PMD mappings
are nicer.

>
>> in general want to make sure it plays nice.
>>
>>> Thanks for the reviews! Really do apprecaite it!
>> No worries! :)
>>
>>> [1] https://lore.kernel.org/all/20f92576-e932-435f-bb7b-de49eb84b012@gmail.com/#t
>>> [2] https://lore.kernel.org/all/3561FD10-664D-42AA-8351-DE7D8D49D42E@nvidia.com/
>> Cheers, Lorenzo
>>