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

[Usama Arif <usamaarif642@gmail.com>]

On 04/02/2026 03:08, 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.


Yeah this was definitely missing. I was manually aligning the fault address in selftest
and benchmarks with the trick used in other selftests
(((unsigned long)addr + PUD_SIZE - 1) & ~(PUD_SIZE - 1))

Thanks for pointing this out! This is basically what I wanted with the RFC, to find out
what I am missing and not testing. Will look into VFIO and DAX as you mentioned as well.

> 
> 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.
> 

Thanks!

>>
>>>
>>> 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 :)


Yes, Thanks for this! It would be really helpful to discuss in a call. I didn't
know there was a meeting but have requested details (date/time) in another thread.

> 
> 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.
> 

Yes agreed! I will definitely need your and others guidance on what needs to be
properly refractored so that this fits well with the current code.

> 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
> in general want to make sure it plays nice.
> 

Will do!


>>
>> 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