Linux Device Drivers Development

The copy-on-write (CoW) case

CoW (heavily used with fork()) is a kernel feature that does not allocate several times the memory for data shared by two or more processes, until a process touches it (writes into it); in this case, memory is allocated for its private copy. The following shows how a page fault handler manages CoW (one-page case study):

1. A PTE is added to the process page table, and marked as unwritable.
2. The mapping will result in a VMA creation in the process VMA list. The page is added to that VMA and that VMA is marked as writable.
3. On page access (at the first write), the fault handler notices the difference, which means this is a CoW. It will then allocate a physical page, which is assigned to the PTE added before, update ...