UML inside UML doesn't work for me:

  $ linux
  Locating the bottom of the address space ... do_syscall_stub : ret = -1, offset = 1052680, data = 18ca9008
  do_syscall_stub: syscall 192 failed, return value = 0xffffffff, expected return value = 0x1000
      syscall parameters: 0x1000 0x1000 0x7 0x11 0x3 0x2449
  Failed to flush page for address 0x1000
  Killed

I was fairly certain I had done it before. I did some Googling to sanity-check myself and found, "UML can be compiled to run within UML" [1]. I don't seem to recall any unusual compiling options to make it work, but then that was a long time ago, as was that document, I suspect (although it has no dates). Maybe it's broken...

[1] http://user-mode-linux.sourceforge.net/old/UserModeLinux-HOW...