Heterogeneous Memory Management Steps Up To Version 22
HMM v22 has some refcounting changes and other minor modifications, but overall there isn't too much churn compared to the earlier revisions to HMM years ago. HMM is primarily about allowing a process address space to be mirrored and allowing system memory to be transparently used by any device process. NVIDIA is really interested in this and has been working on these patches as well as support in their binary driver and pending patches to the Nouveau driver.
These latest HMM patches can be found on the kernel mailing list. Hopefully Heterogeneous Memory Management will be ready by the time of the Linux 4.13 merge window.