Back in 2014 is when longtime open-source graphics driver developer Jerome Glisse began pushing his patches for Heterogeneous Memory Management in the Linux kernel, while that work is still ongoing but has now been renewed.
We last heard an update on Heterogeneous Memory Management (HMM) last summer when Glisse sent out his latest patches. This big patch-set is a helper layer for a device wanting to mirror a process address space into their own MMU. HMM is designed for GPUs and others in needing to support OpenCL 2.0+ for mirroring a process address space. HMM also makes it possible for using the discrete GPU memory in a transparent manner to the application/game and other possible use-cases.