Module cow 

Copy-on-Write (COW) support for fork()

This module provides the core functionality for copy-on-write memory management, allowing efficient fork() implementation by sharing pages between parent and child until one of them writes to a shared page.

Design

• Pages are marked as COW by clearing the WRITABLE bit and setting a software COW flag
• The COW flag is stored in bit 9 of the PTE (available for software use on x86_64)
• When a write fault occurs on a COW page:
  1. If refcount > 1: allocate new frame, copy content, update PTE
  2. If refcount == 1: just mark page as writable (no copy needed)

SMP Safety

• Atomic refcount per frame (lock-free inc/dec)
• COW_LOCK protects flag changes and frame free
• TLB shootdown is performed after modifying page table flags

Functions

frame_clear_cow

Clear COW flag on a frame (SMP-safe).

frame_dec_ref

Performs the frame dec ref operation.

frame_get_refcount

Performs the frame get refcount operation.

frame_inc_ref

Performs the frame inc ref operation.

frame_is_cow

Check if a frame is marked as COW (lock-free read).

frame_is_dll

Check if a frame is a DLL frame (lock-free read).

frame_set_cow

Set COW flag on a frame (SMP-safe).

frame_set_dll

Mark a frame as DLL (shared, never COW) (SMP-safe).