memo: do_fork()

Flow of switching a process address space.

do_fork() @kernel/fork.c
|-> copy_process() @kernel/fork.c
   |-> copy_mm() @kernel/fork.c
       |-> dup_mm() @kernel/fork.c
           |-> mm_init() @kernel/fork.c
               |-> mm_alloc_pgd() @kernel/fork.c
                   |-> pgd_alloc() @arch/x86/mm/pgtable.c