[$] Controlling device peer-to-peer access from user space

The recent addition of support for direct (peer-to-peer) operations between PCIe devices in thekernel has opened the door for different use cases. The initial workconcentrated on in-kernel support and the NVMe subsystem; it alsoadded support for memory regions that can be used for such transfers.Ji(C)rime Glisse recently proposedtwo extensions that would allow the mapping of those regions into userspace and mapping device files between two devices. The resulting discussion surprisingly led to consideration of thefuture of core kernel structures dealing with memory management.