From Linux 6.6, uffdio_register.iotcls contains a bit for _UFFDIO_POISON (code).
uffdio_register.iotcls
_UFFDIO_POISON
register() always fails as UnrecognizedIoctls(284) because struct IoctlFlags does not support UFFDIO_POISON yet.
UnrecognizedIoctls(284)
struct IoctlFlags
UFFDIO_POISON