Comment by ridiculous_fish

My (very possibly wrong) understanding is that xnu does CoW fork but doesn't overcommit, meaning that memory must be reserved (perhaps in swap) in case the pages need to be duplicated.

There's other complications relating to inheriting Mach ports and the mach_task <-> BSD process "duality" in xnu, which Linux doesn't have. I'd love for someone to chime in who knows more about how this stuff works.