[bigodanktime]

> There is no copy with mmap

You are right, if you are directly modifying the mmaped region. I always internally model my data as staging my changes to be synchronized to the mmaped region, so thats my mistake there.

> the page is either unwritable or CoW.

This is not universally true, or maybe I'm confused on this statement. MAP_SHARED exists, but maybe you are referencing a specific kernels' implementation on how they achieve coherence between file backed shared memory regions in two processes? Im not sure.

> Darwin kernel does though.

Sure we can always point to a kernel that has has implemented some feature or another, which is why I said typically you don't see it.