n.b. turning over-commit off comes with its own set of problems, such as causing programs to fail long before all memory has really been exhausted.

For example, fork() will have to ensure that there is enough memory for a complete copy of the running process. If you have a large process, e.g. using 4GB of a 8GB machine, then fork() won't be able to run, even if you just want to fork and run a tiny program.

With over-commit turned on, the fork() would work (because of copy-on-write) and the program could then happily exec() the new program.

The work-around is to allocate huge amounts of swap space so that the OS can be confident that it can reserve all the potentially required memory from fork(), even if it never normally has to use all that memory.