At the suggestion of LKML, I have spent today working on making blk-cgroup, also known as blkiocg, into a module. it has not gone entirely smoothly, as there is an already existing module (cfq-iosched) that depends on it, so the process has involved a good amount of figuring out how module dependencies (in terms of symbol-loading and the language of the kbuild system) work.
Now that I got it finally to build, boot, and load the modules (modprobe even figured out the dependency for me, it was great), I discovered that the kernel panics when trying to unload the subsystem (with what looks like a SIGILL in some memory management code). So, GDBing it up, I have discovered that it is difficult to deal with code that's loaded at runtime that isn't a standard dynamic library.
0x000000006005146a 3504 ss->destroy(ss, dummytop);
0x00000000628202ad in ?? ()
(gdb) break block/blk-cgroup.c:207
No source file named block/blk-cgroup.c.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb) disassemble 0x628202ad
No function contains specified address.
(gdb) disassemble 0x628202ad 0x62820363
Dump of assembler code from 0x628202ad to 0x62820363:
0x00000000628202ad: push %rbp
And you thought 213's bomblab would never be useful!