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.

As pm215 states, it doubles your memory commit. It's somewhat common for large programs/runtimes that may fork at runtime to spawn an intermediary process during startup to use for runtime forks to avoid the cost of CoW on memory and mapppings and etc where the CoW isn't needed or desirable; but redis has to fork the actual service process because it uses CoW to effectively snapshot memory.