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

[David Hildenbrand (Arm)]

On 4/16/26 03:24, Gregory Price wrote:
> On Wed, Apr 15, 2026 at 12:47:50PM -0700, Frank van der Linden wrote:
>>
>> This has been a really great discussion. I just wanted to add a few
>> points that I think I have mentioned in other forums, but not here.
>>
>> In essence, this is a discussion about memory properties and the level
>> at which they should be dealt with. Right now there are basically 3
>> levels: pageblocks, zones and nodes. While these levels exist for good
>> reasons, they also sometimes lead to issues. There's duplication of
>> functionality. MIGRATE_CMA and ZONE_MOVABLE both implement the same
>> basic property, but at different levels (attempts have been made to
>> merge them, but it didn't work out).
> 
> I have made this observation as well.  ZONEs in particular are a bit
> odd because they're somehow simultaneously too broad and too narrow in
> terms of what they control and what they're used for.
> 
> 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.

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.


-- 
Cheers,

David