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

[Usama Arif]

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.

> 
> 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.

Thanks for the reviews! Really do apprecaite it!

[1] https://lore.kernel.org/all/20f92576-e932-435f-bb7b-de49eb84b012@xxxxxxxxx/#t
[2] https://lore.kernel.org/all/3561FD10-664D-42AA-8351-DE7D8D49D42E@xxxxxxxxxx/