【MIT-OS6.S081作业5.1】Lab5-lazy page allocation-Eliminate allocation from sbrk()

本文记录MIT-OS6.S081 Lab5 lazy page allocation 的Eliminate allocation from sbrk()函数的实现过程

文章目录

[1. 作业要求](#1. 作业要求)
- [Eliminate allocation from sbrk() (easy)](#Eliminate allocation from sbrk() (easy))
[2. 实现过程](#2. 实现过程)
- [2.1 代码实现](#2.1 代码实现)

1. 作业要求

Eliminate allocation from sbrk() (easy)

Your first task is to delete page allocation from the sbrk(n) system call implementation, which is the function sys_sbrk() in sysproc.c. The sbrk(n) system call grows the process's memory size by n bytes, and then returns the start of the newly allocated region (i.e., the old size). Your new sbrk(n) should just increment the process's size (myproc()->sz) by n and return the old size. It should not allocate memory -- so you should delete the call to growproc() (but you still need to increase the process's size!).

Try to guess what the result of this modification will be: what will break?

Make this modification, boot xv6, and type echo hi to the shell. You should see something like this:

c 复制代码

init: starting sh
$ echo hi
usertrap(): unexpected scause 0x000000000000000f pid=3
            sepc=0x0000000000001258 stval=0x0000000000004008
va=0x0000000000004000 pte=0x0000000000000000
panic: uvmunmap: not mapped

The "usertrap(): ..." message is from the user trap handler in trap.c; it has caught an exception that it does not know how to handle. Make sure you understand why this page fault occurs. The "stval=0x0...04008" indicates that the virtual address that caused the page fault is 0x4008.

2. 实现过程

2.1 代码实现

Your new sbrk(n) should just increment the process's size (myproc()->sz) by n and return the old size. It should not allocate memory -- so you should delete the call to growproc() (but you still need to increase the process's size!).

这个提示的意思是只增加进程的sz，返回旧的size，但是不分配内存。

我们看一下原来的sys_sbrk函数，调用了growproc函数来对内存进行重新分配，返回旧的大小。

c 复制代码

uint64
sys_sbrk(void)
{
  int addr;
  int n;

  if(argint(0, &n) < 0)
    return -1;
  addr = myproc()->sz;
  if(growproc(n) < 0)
    return -1;
  return addr;
}

进入growproc:

c 复制代码

int
growproc(int n)
{
  uint sz;
  struct proc *p = myproc();

  sz = p->sz;
  if(n > 0){
    if((sz = uvmalloc(p->pagetable, sz, sz + n)) == 0) {
      return -1;
    }
  } else if(n < 0){
    sz = uvmdealloc(p->pagetable, sz, sz + n);
  }
  p->sz = sz;
  return 0;
}

uvmalloc底层会调用kalloc函数来分配内存。仿照growproc的思路我们重写一下sys_sbrk:

c 复制代码

uint64
sys_sbrk(void)
{
  uint64 oldSize;
  int n;

  if(argint(0, &n) < 0)
    return -1;
  struct proc *p = myproc();
  oldSize = p->sz;
  if(n > 0){
    p->sz += n;
  } else if(n < 0){
    p->sz = uvmdealloc(p->pagetable, p->sz, p->sz + n);
  }
  return oldSize;
}

编译通过，测试echo hi，如题打印：

完活！还有bug下一节继续讲怎么实现Lazy allocation。