After fork, why can a child write a private mapping without changing the parent’s value?

After fork, why can a child write a private mapping without changing the parent’s value?

1. The pipe automatically restores the parent page.
2. The child receives a new virtual address for every page.
3. Copy-on-write preserves private semantics by separating backing when a shared page is written.
4. Private mappings are always copied eagerly before fork returns.