Surely it also marks the parent process' pages as COW too? If only the child is RO but the parent still has RW mappings to the same physical pages, writes from

Surely it also marks the parent process' pages as COW too? If only the child is RO but the parent still has RW mappings to the same physical pages, writes from the parent will be observed in the child, which is wrong for COW. You either have to copy the pages immediately (in which cases there's no COW) or you have to make all mappings to the physical page RO. The implications of this is that even if you immediately execve in the child, you still have to pay for the cost of setting COW on the entire address space and then later faulting on every single writable page in the parent process. The performance impact might not be massive, but it's not nothing.