It's not really wrong. For something like redis, you could potentially fork and the child gets stuck for a long time and in the meantime the whole cache in the parent is rewritten. In that case, even though the cache is fixed size / no new allocations, all of the pages are touched and so the total used memory is double from before the fork. If you want to guarantee allocation failures rather than demand paging failures, and you don't have enough ram/swap to back twice the allocations, you must fail the fork.

On the other hand, if you have a pretty good idea that the child will finish persisting and exit before the cache is fully rewritten, double is too much. There's not really a mechanism for that though. Even if you could set an optimistic multiplier for multiple mapped CoW pages, you're back to demand paging failures --- although maybe it's still worthwhile.