6.S081-2021-Lab5: Copy-on-Write Fork for xv6

The problem 问题

The fork() system call in xv6 copies all of the parent process’s user-space memory into the child. If the parent is large, copying can take a long time. Worse, the work is often largely wasted; for example, a fork() followed by exec() in the child will cause the child to discard the copied memory, probably without ever using most of it. On the other hand, if both parent and child use a page, and one or both writes it, a copy is truly needed.
xv6中的fork()系统调用将父进程的所有用户空间内存复制到子进程中。如果父进程较大，则复制可能需要很长时间。更糟糕的是，这项工作经常造成大量浪费；例如，子进程中的fork()后跟exec()将导致子进程丢弃复制的内存，而其中的大部分可能都从未使用过。另一方面，如果父子进程都使用一个页面，并且其中一个或两个对该页面有写操作，则确实需要复制。

The solution 解决方案

The goal of copy-on-write (COW) fork() is to defer allocating and copying physical memory pages for the child until the copies are actually needed, if ever.
copy-on-write (COW) fork()的目标是推迟到子进程实际需要物理内存拷贝时再进行分配和复制物理内存页面。

COW fork() creates just a pagetable for the child, with PTEs for user memory pointing to the parent’s physical pages. COW fork() marks all the user PTEs in both parent and child as not writable. When either process tries to write one of these COW pages, the CPU will force a page fault. The kernel page-fault handler detects this case, allocates a page of physical memory for the faulting process, copies the original page into the new page, and modifies the relevant PTE in the faulting process to refer to the new page, this time with the PTE marked writeable. When the page fault handler returns, the user process will be able to write its copy of the page.
COW fork()只为子进程创建一个页表，用户内存的PTE指向父进程的物理页。COW fork()将父进程和子进程中的所有用户PTE标记为不可写。当任一进程试图写入其中一个COW页时，CPU将强制产生页面错误。内核页面错误处理程序检测到这种情况将为出错进程分配一页物理内存，将原始页复制到新页中，并修改出错进程中的相关PTE指向新的页面，将PTE标记为可写。当页面错误处理程序返回时，用户进程将能够写入其页面副本。

COW fork() makes freeing of the physical pages that implement user memory a little trickier. A given physical page may be referred to by multiple processes’ page tables, and should be freed only when the last reference disappears.
COW fork()将使得释放用户内存的物理页面变得更加棘手。给定的物理页可能会被多个进程的页表引用，并且只有在最后一个引用消失时才应该被释放。

Implement copy-on write(hard)

Your task is to implement copy-on-write fork in the xv6 kernel. You are done if your modified kernel executes both the cowtest and usertests programs successfully.
您的任务是在xv6内核中实现copy-on-write fork。如果修改后的内核同时成功执行cowtest和usertests程序就完成了。

To help you test your implementation, we’ve provided an xv6 program called cowtest (source in user/cowtest.c). cowtest runs various tests, but even the first will fail on unmodified xv6. Thus, initially, you will see:
为了帮助测试你的实现方案，我们提供了一个名为cowtest的xv6程序（源代码位于user/cowtest.c）。cowtest运行各种测试，但在未修改的xv6上，即使是第一个测试也会失败。因此，最初您将看到：

$ cowtest
simple: fork() failed
$ 

The “simple” test allocates more than half of available physical memory, and then fork()s. The fork fails because there is not enough free physical memory to give the child a complete copy of the parent’s memory.
“simple”测试分配超过一半的可用物理内存，然后执行一系列的fork()。fork失败的原因是没有足够的可用物理内存来为子进程提供父进程内存的完整副本。

When you are done, your kernel should pass all the tests in both cowtest and usertests. That is:
完成本实验后，内核应该通过cowtest和usertests中的所有测试。即：

$ cowtest
simple: ok
simple: ok
three: zombie!
ok
three: zombie!
ok
three: zombie!
ok
file: ok
ALL COW TESTS PASSED
$ usertests
...
ALL TESTS PASSED
$

Here’s a reasonable plan of attack.

这是一个合理的攻克计划：

Modify uvmcopy() to map the parent’s physical pages into the child, instead of allocating new pages. Clear PTE_W in the PTEs of both child and parent.
修改uvmcopy()将父进程的物理页映射到子进程，而不是分配新页。在子进程和父进程的PTE中清除PTE_W标志。
原本uvmcopy作用是将父进程的页表复制到子进程。

//vm.c
int
uvmcopy(pagetable_t old, pagetable_t new, uint64 sz)
{
   
   
  pte_t *pte;
  uint64 pa, i;
  uint flags;
  char *mem;

  for(i = 0; i < sz; i += PGSIZE){
   
   
    if((pte = walk(old, i, 0)) == 0)
      panic("uvmcopy: pte should exist");
    if((*pte & PTE_V) == 0)
      panic("uvmcopy: page not present");
    pa = PTE2PA(*pte);
    *pte=*pte & ~PTE_W;//freewalk中有类似用法，清除写权限
    flags = PTE_FLAGS(*pte);
    // if((mem = kalloc()) == 0)//新页表页(大小4096)第0个位置上的物理地址
    //   goto err;
    // memmove(mem, (char*)pa, PGSIZE);
    if(mappages(new, i, PGSIZE, (uint64)pa, flags) != 0){
   
   //为起始地址为va的虚拟地址创建pte，该虚拟地址引用起始地址为pa的物理地址。Va和size可能没有对齐页面。成功时返回0，如果walk()无法分配所需的页表页则返回-1。
      // kfree(mem);
      goto err;
    }
  }
  return 0;

 err:
  uvmunmap(new, 0, i / PGSIZE, 1);
  return -1;
}

Modify usertrap() to recognize page faults. When a page-fault occurs on a COW page, allocate a new page with kalloc(), copy the old page to the new page, and install the new page in the PTE with PTE_W set.
修改usertrap()以识别页面错误。当COW页面出现页面错误时，使用kalloc()分配一个新页面，并将旧页面复制到新页面，然后将新页面添加到PTE中并设置PTE_W。

识别页面错误，上一个lab（2021年的任务书删了）的内容

//vm.c
// PTE是否是COW映射
int is_cow_fault(pagetable_t pagetable,uint64 va){
   
   
  if(va>=MAXVA){
   
   
    return 0;
  }
  pte_t *pte;
  pte = walk(pagetable, va, 0);
  if((*pte &am