Discussed on tech-kern:
https://mail-index.NetBSD.org/tech-kern/2020/01/13/msg025938.html
This was never (intentionally) enabled by default, and the design has
some shortcomings. You can get mostly the same results with ktrace,
as in usr.bin/make/filemon/filemon_ktrace.c which is now used instead
of filemon for make's meta mode.
If applications require higher performance than ktrace, or nesting
that ktrace doesn't support, we might consider adding something back
into the vfs system calls themselves, without hijacking the syscall
table. (Might want a more reliable output format too, e.g. one that
can handle newlines in file names.)
08a157de6e