Re: [RFC PATCH] memory,memory_hotplug: allow restricting memory blocks to zone movable

[Gregory Price <gourry@gourry.net>]

On Tue, Jan 06, 2026 at 04:24:21PM +0100, David Hildenbrand (Red Hat) wrote:
> > +/*
> > + * Restrict hotplugged memory blocks to ZONE_MOVABLE only.
> > + *
> > + * During offlining of hotplugged memory which was originally onlined
> > + * as ZONE_MOVABLE, userland services may detect blocks going offline
> > + * and automatically re-online them into ZONE_NORMAL or lower.  When
> > + * this happens it may become permanently incapable of being removed.
> 
> If it's really only that, we could also look into simply making a re-online
> without a specific mode ("online") to use the previous mode.
> 
> We could glue that to the "contig-zones" policy only, to not affect
> "auto-movable".
> 
> That is, remember the zone to which it was previously onlined, and then
> simply re-online to that one.
> 

I know we do this in memory_hotplug.c to rollback to prior state.

I did notice in... i think it was either memory.c or hotplug.c... that
we end up setting mem->online_type=MMOP_OFFLINE after comping an online
operation.  That seemed confusing and maybe we can use that to store the
current state.

I'm not against this idea, but it also makes the sysfs a little more
confusing (`echo online` now does different things based on prior
state).  I preferred just failing if the block wasn't compatible with
the zone (maybe making it more clear with a dmesg print?)

Anyway, let me know what your preference is, happy to pivot however.

Hopefully Hannes can add additional feedback and guidance here.

~Gregory