# lab 6 Copt On Write
##
Whenever there is a fork, we copy parent's memory to child
```c
// Copy user memory from parent to child.
if(uvmcopy(p->pagetable, np->pagetable, p->sz) < 0){
freeproc(np);
release(&np->lock);
return -1;
}
```
after then, child may call exec
```c++
if((pagetable = proc_pagetable(p)) == 0)
// ...
// Commit to the user image.
oldpagetable = p->pagetable;
p->pagetable = pagetable;
p->sz = sz;
p->trapframe->epc = elf.entry; // initial program counter = main
p->trapframe->sp = sp; // initial stack pointer
proc_freepagetable(oldpagetable, oldsz);
```
this allocate a new page table, load to memory and discard old page table, mapping memory
We can improve
1. share memory with parent at first place and set to read only
2. when need to modify content, trigger page fault, allocate a new page, copy page fault content to new page, set to read/write
3. what if the child process exit, but is still using parent memory space -> reference count
For vm.c/uvmcopy
1. mark COW page
2. disable PTE_W
3. mapping
when modified child, it trigger page fault because of read-only
For trap.c/usertrap
1. store instruction page fault(sscause = 15)
2. cowaaloc()
if this is a cow situation:
allocate new page for child
copy content from parent
free old memory mapping
clear COW bit
copyout will also use cow mechanism
if there is a exit in child process, make sure do not effect parent
use refernce count
```c
struct {
struct spinlock lock;
int count[(PGROUNDUP(PHYSTOP) - KERNBASE)/PGSIZE];
} refcnt;
```