I believe (per the stuff at the bottom of https://www.kernel.org/doc/Documentation/vm/overcommit-accou... ) that the kernel does the accounting of how much memory the new child process needs and will fail the fork() if there isn't enough. All the COW pages should be in the "shared anonymous" category so get counted once per user (i.e. once for the parent process, once for the child), ensuring that the COW copy can't fail if the fork succeeded.