btrfs broken --reflink=always

( strace cp -a --reflink=always xen-4.1.4 xen-4.1.4-brokenreflink 2>&1 ) > brokenreflink.log

An excerpt from the log:

lstat("xen-4.1.4/buildconfigs/mk.linux-2.6-xen", {st_mode=S_IFREG|0644, st_size=116, ...}) = 0
open("xen-4.1.4/buildconfigs/mk.linux-2.6-xen", O_RDONLY|O_NOFOLLOW) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=116, ...}) = 0
open("xen-4.1.4-brokenreflink/buildconfigs/mk.linux-2.6-xen", O_WRONLY|O_CREAT|O_EXCL, 0600) = 4
fstat(4, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
ioctl(4, 0x40049409, 0x3)               = -1 EINVAL (Invalid argument)
write(2, "cp: ", 4cp: )                     = 4
write(2, "failed to clone \342\200\230xen-4.1.4-bro"..., 126failed to clone ‘xen-4.1.4-brokenreflink/buildconfigs/mk.linux-2.6-xen’ from ‘xen-4.1.4/buildconfigs/mk.linux-2.6-xen’) = 126
write(2, ": Invalid argument", 18: Invalid argument)      = 18
write(2, "\n", 1
)                       = 1
close(4)                                = 0
close(3)                                = 0