Virtual-memory systems provide a great deal of flexibility in how memory
can be mapped and protected. Unfortunately, memory-management flexibility
can also be useful to attackers bent on compromising a system. In the
OpenBSD world, a new system call is being added to reduce this flexibility;
it is, though, a system call that almost no code is expected to use.

Source: LWN.net – [$] mimmutable() for OpenBSD