You don't need to re-license the BSD code to GPL as that code is not a derived from the GPL'd bcache. Distributing binaries is the only tricky bit: a kernel wit

You don't need to re-license the BSD code to GPL as that code is not a derived from the GPL'd bcache. Distributing binaries is the only tricky bit: a kernel with GPL code would need to be distributed as GPL. This can be solved with loadable kernel modules. There isn't much stopping anyone from using the code from a license perspective. The problem is this sort of code is strongly tied to the operating system, and porting it would require significant effort, if even feasible at all.