Memfd is a mechanism similar to Android's Ashmem that allows zero-copy message passing in KDBUS. Memfd effectively comes down to just a chunk of memory with a file descriptor attached that can be passed to mmap(). The memfd_create() function returns a raw shmem file and there's optional support for sealing.
Memfd is needed by KDBUS for message passing and now the code -- after being public but out-of-tree for several months -- is finally mainline. As a result, the KDBUS code has been updated to take advantage of the mainline Linux 3.17 state.
While KDBUS wasn't merged for Linux 3.17, it seems to be getting closer to being in shape for merging. At the start of the year the developers expressed a goal of mainlining KDBUS in 2014 and now their next opportunity would be Linux 3.18 otherwise Linux 3.19 would be cutting it close to the end of the year.
Those wishing to checkout the latest KDBUS code can find it via Greg KH's GitHub.