On Tue, Oct 22, 2024 at 11:33:07AM +0200, David Hildenbrand wrote: > On 20.09.24 11:06, Gregory Price wrote: > > > The only concern is when insufficient ZONE_NORMAL exists to support > > ZONE_MOVABLE capacity - but this is unlikely to be the general scenario AND > > can be mitigated w/ existing mechanisms. > > It might be worthwhile looking at > Documentation/admin-guide/mm/memory-hotplug.rst "auto-movable" memory > onlining polciy. It might not fit all sue cases, though (just like > ZONE_MOVABLE doesn't) > I managed to miss auto-movable in my last pass through there. Though for our use-case, forcibly preventing ZONE_NORMAL for all CXL is the preferred option in an effort to keep as much kernel resources out of high latency memory. So I think we're just going to end up using memhp_default_state, and that'll be mostly fine. > > > > Manually onlined capacity defaults to ZONE_MOVABLE. > > > > It would be nice to make this behavior consistent, since the general opinion > > appears to be that this capacity should default to ZONE_MOVABLE. > > It's much easier to shoot yourself into the foot with ZONE_MOVABLE, that's > why the default can be adjusted manually using "online_movable" with e.g., > memhp_default_state. > > It's all a bit complicated, because there are various use cases and > mechanisms for memory hotplug ... IIRC RHEL defaults with its udev rules to > "ZONE_MOVABLE" on bare metal and "ZONE_NORMAL" in VMs. Except on s390, where > we default to "offline" (standby memory ....). > > I once worked on a systemd unit to make this configuration easier (and avoid > udev rules), and possibly more "automatic" depending on the detected > environment. > Appreciate the additional context, thanks! ~Gregory > -- > Cheers, > > David / dhildenb >
