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
