If you create a seed device with btrfstune and then try to add a small (32MB) device to it, the kernel will oops and the attempt to add the device will fail. I have attached a transcript of the shell session that demonstrates this. (This was generated by piping UML to tee and then cleaning up the file. I removed some control characters, but the backtrace is still intact.) This bug os triggered by a 32MB device, but not a 64MB one. This bug is still present with the rc6 kernel. Note that, in earlier kernels, the attempt to remount read-write would succeed, but process attempting to access the filesystem would hang forever, resisting even SIGKILL.