Re: [PATCH 1/2] cxl/region: Timeout auto region assembly waiting for endpoints

[Gregory Price <gourry@gourry.net>]

On Thu, Jan 29, 2026 at 08:58:09PM -0800, dan.j.williams@intel.com wrote:
> 
> > Cancel the timeout when all expected endpoints attach or the region is
> > unregistered for any reason.
> 
> Setting aside the above, this looks like policy, and every time I see
> policy the first question is "can userspace do it?". It would be
> straightforward for userspace to kick a 30 second watchdog upon each
> region KOBJ_ADD event. Each time that fires go cleanup partially
> assembled regions.
> 
> For example there is no automatic cleanup of partially assembled RAID
> arrays. So, precedent leans towards letting userspace decide what
> happens when composite devices fail assembly.

Sounds like there'll be a nasty race implied here.

Lets assume a kmem region that gets auto-onlined

0) Region is waiting for a device
   1) Final device arrives and starts probing, locking the region
   2) Userspace timeout occurs, firing a cleanup request
2) Region finishes probing
   2a) this creates the dax region
   2b) this creates the dax_kmem device
   2c) this may auto-hotplug into ZONE_NORMAL
   2d) kernel page gets allocated on the memory region
3) Userspace cleanup arrives to unbind
   3a) dax_kmem is online and can't hot-unplug
   3b) dax_kmem abandons hope and leaves the memory online
       (see dev_dax_kmem_remove and remove_memory)
   3c) region cleans up

Final state: region can't be rebound because memory is left online
and unassociated with any device

This will be hard to get right

~Gregory