Re: shouldn't require a complete copy of the parent process's memory

Sorry what issue are you referring to? The conjecture so far is that there is a problem, since a forked process gets a copy of its parent's address space, which requires either reserving enough memory (RAM or swap) to provide that, or else overcommitting and crossing your fingers (with OOM killer for when you get it wrong). It is true that copy-on-write means the kernel doesn't have to copy all the pages straight away, but that is just a time saving; it doesn't resolve the underlying issue of needing overcommit.

vfork() doesn't make a copy of the address space and so doesn't require either over-caution or over-committing. But it has other limitations.