Re: [PATCH 8/8] dax/kmem: add memory notifier to block external state changes

[Gregory Price <gourry@gourry.net>]

On Wed, Jan 14, 2026 at 10:44:08AM +0100, David Hildenbrand (Red Hat) wrote:
> On 1/14/26 09:52, Gregory Price wrote:
> > Add a memory notifier to prevent external operations from changing the
> > online/offline state of memory blocks managed by dax_kmem. This ensures
> > state changes only occur through the driver's hotplug sysfs interface,
> > providing consistent state tracking and preventing races with auto-online
> > policies or direct memory block sysfs manipulation.
> > 
> > The notifier uses a transition protocol with memory barriers:
> >    - Before initiating a state change, set target_state then in_transition
> >    - Use a barrier to ensure target_state is visible before in_transition
> >    - The notifier checks in_transition, then uses barrier before reading
> >      target_state to ensure proper ordering on weakly-ordered architectures
> > 
> > The notifier callback:
> >    - Returns NOTIFY_DONE for non-overlapping memory (not our concern)
> >    - Returns NOTIFY_BAD if in_transition is false (block external ops)
> >    - Validates the memory event matches target_state (MEM_GOING_ONLINE
> >      for online operations, MEM_GOING_OFFLINE for offline/unplug)
> >    - Returns NOTIFY_OK only for driver-initiated operations with matching
> >      target_state
> > 
> > This prevents scenarios where:
> >    - Auto-online policies re-online memory the driver is trying to offline
> 
> Is this still a problem when using offline_and_remove_memory() ?
> 

I just remembered another reason I did this:  

echo offline > memoryN/state

This leaves the dax/hotplug state in an inconsistent state.

if you do the above for every block in a dax region, `daxN.M/hotplug`
still shows up as online.

This just hard-locks the state to consistent (unless an online/offline
fails along with its rollback).

The additional complexity seemed warranted for that, but if you're happy
to leave users to their footguns I'm not going to argue it.

---

I just realized this breaks the current ndctl pattern and would force
ndctl to convert to `hotplug` since memory block onlining will fail.

~Gregory