Re: [PATCH v2 4/5] dax/kmem: add sysfs interface for runtime hotplug state control

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 14 Jan 2026 18:50:20 -0500 Gregory Price <gourry@gourry.net> wrote:

> The dax kmem driver currently onlines memory automatically during
> probe using the system's default online policy but provides no way
> to control or query the entire region state at runtime.
> 
> There is no atomic to offline and remove memory blocks together.
> 
> Add a new 'hotplug' sysfs attribute that allows userspace to control
> and query the entire memory region state.
> 
> The interface supports the following states:
>   - "unplug": memory is offline and blocks are not present
>   - "online": memory is online as normal system RAM
>   - "online_movable": memory is online in ZONE_MOVABLE
> 
> Valid transitions:
>   - unplugged -> online
>   - unplugged -> online_movable
>   - online    -> unplugged
>   - online_movable -> unplugged
> 
> "offline" (memory blocks exist but are offline by default) is not
> supported because it's functionally equivalent to "unplugged" and
> entices races between offlining and unplugging.
> 
> The initial state after probe uses mhp_get_default_online_type() to
> preserve backwards compatibility - existing systems with auto-online
> policies will continue to work as before.
> 
> As with any hot-remove mechanism, the removal can fail and if rollback
> fails the system can be left in an inconsistent state.
> 
> Unbind Note:
>   We used to call remove_memory() during unbind, which would fire a
>   BUG() if any of the memory blocks were online at that time.  We lift
>   this into a WARN in the cleanup routine and don't attempt hotremove
>   if ->state is not DAX_KMEM_UNPLUGGED.
> 
>   The resources are still leaked but this prevents deadlock on unbind
>   if a memory region happens to be impossible to hotremove.
> 
> ...
>
> --- a/Documentation/ABI/testing/sysfs-bus-dax
> +++ b/Documentation/ABI/testing/sysfs-bus-dax
> @@ -151,3 +151,20 @@ Description:
>  		memmap_on_memory parameter for memory_hotplug. This is
>  		typically set on the kernel command line -
>  		memory_hotplug.memmap_on_memory set to 'true' or 'force'."
> +
> +What:		/sys/bus/dax/devices/daxX.Y/hotplug
> +Date:		January, 2026
> +KernelVersion:	v6.21
> +Contact:	nvdimm@lists.linux.dev
> +Description:
> +		(RW) Controls what hotplug state of the memory region.

s/what// ?

Maybe "Controls hotplug state of a dax memory region".

> +		Applies to all memory blocks associated with the device.
> +		Only applies to dax_kmem devices.
> +
> +                States: [unplugged, online, online_movable]
> +                Arguments:
> +		  "unplug": memory is offline and blocks are not present
> +		  "online": memory is online as normal system RAM
> +		  "online_movable": memory is online in ZONE_MOVABLE
> +

This is perhaps a little brief?  Is there more we can tell users about
what this is and how it behaves and why they might want to use it?