On Wed, 14 Jan 2026 18:50:20 -0500 Gregory Price <[email protected]> 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: [email protected] > +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?
