[geofft]

It certainly is permitted to fail by spec. The goal of the entire overcommit / OOM killer business is to yield better real-world outcomes.

If I have overcommit off, and a program that allocates more than half of my physical memory (RAM + swap), it can't fork - even if it's going to immediately exec a tiny program. If I have overcommit on, it can fork and exec, and nobody gets OOM killed.

The tradeoff is that, if I have overcommit on and the child process starts modifying every page instead of exec'ing, then the OOM killer triggers. The bet is that a) probably the child process will be killed, since it has the most physical memory in use (assuming the parent process has not touched every page it has allocated), so this isn't worse than preventing the child process in the first place, and b) this situation is rare.