Kernel development is not for people who lack persistence; changes can take
a number of revisions and a lot of time to make it into a mainline
release. Even so, the story of the Landlock security module, developed by
Mickaël Salaün, seems like an extreme case; this code was merged for 5.13 after
more than five years of development and 34 versions of the patch set.
This sandboxing mechanism has evolved considerably since LWN covered version 3 of the patch set in
2016, so a look at what Landlock has become is warranted.

Source: LWN.net – [$] Landlock (finally) sets sail