This tries to read /proc/getpid()/maps and mlock all pages in those
mapped regions.

Well, it tries to do better than page by page; we try chunks of 32KB
until they fail (32KB is a traditional kernel default for RLIMIT_MEMLOCK).

Unfortunately, this seems to work for the first many pages, but then it
starts failing in mapped libraries.  Hah!  Who knows why.  I tried sleeping
in case it was some kind of race in munlock*(); no joy.  Maybe RLIMITS
are ratelimited too :)