Re: [LSF/MM/BPF TOPIC][RFC PATCH v4 00/27] Private Memory Nodes (w/ Compressed RAM)

Gregory Price
2026-04-17T15:07:58Z

On Fri, Apr 17, 2026 at 11:50:58AM +0200, David Hildenbrand (Arm) wrote:
> On 4/16/26 03:24, Gregory Price wrote:
> > On Wed, Apr 15, 2026 at 12:47:50PM -0700, Frank van der Linden wrote:
> >>
> > 1GB ZONE_MOVABLE HugeTLBFS Pages is an example weird carve-out, because
> > the memory is in ZONE_MOVABLE to help make 1GB allocations more
> > reliable, but 1GB movable pages were removed from the kernel because
> > they're not easily migrated (and therefore may block hot-unplug).
> >
> > (Thankfully they're back now, so VMs can live on this memory :P)
>
> Heh, but longterm-pinning would fail on them (making vfio with VMs
> angry). Similar to CMA hugetlb.
>

Yeah, depends how you configure things. As long as you expose those
pages on a separate memfd and online it in ZONE_MOVABLE in the guest
to avoid vfio from touching it - you can have your cake and eat it too.

It's a bit of bodge but it works.

However...

> In the latter case, we should have a way to identify "this allocation is
> actually from the CMA owner, so longterm pinning is perfectly fine".
> Checking the CMA alloc state would be one approach, but that's rather
> nasty. I guess there would be ways to make that work.
>
> I'd assume that people barely rely on 1GB ZONE_MOVABLE HugeTLBFS Pages
> (iow, mixing kernel-cmdline ZONE_MOVABLE creation with kernel-cmdline
> hugetlb reservation).
>
> I'll note that there was long long ago a proposal of converting
> ZONE_MOVABLE to "sticky-movable" page blocks. It wouldn't really solve
> this problem, though, where the early boot code just does something
> that's rather stupid.
>

I have been toying with hotpluggable CMA regions.

Interesting opportunity:

Hotplug on a private node w/ (RECLAIM | DEMOTION | CMA | HUGETLBFS)

Now you have exactly two enabled consumers:
1) HugeTLBFS
2) vmscan.c demotion logic

In this regard, HugeTLBFS is the only one that can reach these pages in
a way that could result in the pages being pinned.

All other pages on the node are - by definition - movable, because they
can only reach the node via migration (demotion).

The system can't do fallback allocations to the node, so it operates a
bit slower as a general purpose memory pool - but if you decide you want
to optimize for that you can unplug/hotplug the memory back to a normal
node in ZONE_MOVABLE - without rebooting.

~Gregory