[$] Heterogeneous memory management and MMU notifiers

Heterogeneous memory management (HMM) is a relatively new kernel subsystemthat allows the system to manage peripherals (such as graphics processors)that have their own memory-management units. In two sessions during thememory-management track of the 2018 Linux Storage,Filesystem, and Memory-Management Summit, HMM creator Ji(C)rime Glisseprovided an update on the status of this subsystem and where it is going,along with a more detailed look at the memory-management unit (MMU)notifiers mechanism on which it depends.