The description of this security feature:
https://lwn.net/Articles/730006/
It needs support from the userspace side:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a5c03c31af2291f13689d11760c0b59fb70c9a5a
Improve the comments about the userspace support by the way.