Copy-on-Write

Linux fork Uses Copy-on-Write to Delay Private Page Copies

fork() creates a new process with a virtual address space derived from the caller, but Linux does not need to duplicate every private physical page at the instant the syscall returns. For ordinary private writable mappings, the kernel can arrange parent and child page tables so both processes initially refer to the same physical memory while writes are constrained by copy-on-write state. This design makes process creation proportional to page-table and kernel bookkeeping rather than to the full amount of resident private data. Physical copying is deferred until a write requires the two address spaces to diverge.