On Fri, Jan 22, 2016 at 09:13:25AM +0900, Tsutomu Itoh wrote:

ENOTSUPP should not be returned to the user program.
 (cf. include/linux/errno.h)
Therefore, EOPNOTSUPP is used instead of ENOTSUPP.  

I'm not sure that EOPNOTSUPP is the right error code here. I'd rather
make it EINVAL, the bug we're talking about is a new type of inode
outside of the S_IFMT bits. "not supported" would mean we don't support
it but we could in theory.