1. 创建big.c
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#include "types.h"
#include "stat.h"
#include "user.h"
#include "fcntl.h"

int
main()
{
  char buf[512];
  int fd, i, sectors;

  fd = open("big.file", O_CREATE | O_WRONLY);
  if(fd < 0){
    printf(2, "big: cannot open big.file for writing\n");
    exit();
  }

  sectors = 0;
  while(1){
    *(int*)buf = sectors;
    int cc = write(fd, buf, sizeof(buf));
    if(cc <= 0)
      break;
    sectors++;
    if(sectors % 100 == 0)
      printf(1, ".");
  }

  printf(1, "\nwrote %d sectors\n", sectors);
  close(fd);

  fd = open("big.file", O_RDONLY);
  if(fd < 0){
    printf(2, "big: cannot open big.file for reading\n");
    exit();
  }
  for(i = 0; i < sectors; i++){
    int cc = read(fd, buf, sizeof(buf));
    if(cc <= 0){
      printf(2, "big: read error at sector %d\n", i);
      exit();
    }
    if(*(int*)buf != i){
      printf(2, "big: read the wrong data at sector %d\n", i);
      exit();
    }
  }
  printf(1, "done.\n");
  exit();
}
  1. 修改Makefile,将CPUS := 2改为CPUS := 1，在QEMUOPTS前添加QEMUEXTRA = -snapshot，在UPROGS添加_big\
  2. 在param.h更改为：#define FSSIZE 20000
  3. 修改fs.h
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#define NDIRECT 11
#define MAXFILE (NDIRECT + NINDIRECT + NINDIRECT*NINDIRECT)
uint addrs[NDIRECT+2];
  1. 修改fs.c
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// 替换bmap函数
static uint
bmap(struct inode *ip, uint bn)
{
  uint addr, *a;
  struct buf *bp;

  // Direct blocks (0 to NDIRECT-1)
  if(bn < NDIRECT) {
    if((addr = ip->addrs[bn]) == 0)
      ip->addrs[bn] = addr = balloc(ip->dev);
    return addr;
  }
  bn -= NDIRECT;

  // Singly-indirect block (NDIRECT to NDIRECT+NINDIRECT-1)
  if(bn < NINDIRECT) {
    // Load singly-indirect block
    if((addr = ip->addrs[NDIRECT]) == 0)
      ip->addrs[NDIRECT] = addr = balloc(ip->dev);
    bp = bread(ip->dev, addr);
    a = (uint*)bp->data;
    if((addr = a[bn]) == 0) {
      a[bn] = addr = balloc(ip->dev);
      log_write(bp);
    }
    brelse(bp);
    return addr;
  }
  bn -= NINDIRECT;

  // Doubly-indirect block (NDIRECT+NINDIRECT to MAXFILE-1)
  if(bn < NINDIRECT * NINDIRECT) {
    // Load doubly-indirect block
    if((addr = ip->addrs[NDIRECT+1]) == 0)
      ip->addrs[NDIRECT+1] = addr = balloc(ip->dev);
    bp = bread(ip->dev, addr);
    a = (uint*)bp->data;
    
    uint first_idx = bn / NINDIRECT;   // Index into doubly-indirect block
    uint second_idx = bn % NINDIRECT;  // Index into singly-indirect block
    
    if((addr = a[first_idx]) == 0) {
      a[first_idx] = addr = balloc(ip->dev);
      log_write(bp);
    }
    brelse(bp);
    
    // Load the singly-indirect block
    bp = bread(ip->dev, addr);
    a = (uint*)bp->data;
    if((addr = a[second_idx]) == 0) {
      a[second_idx] = addr = balloc(ip->dev);
      log_write(bp);
    }
    brelse(bp);
    return addr;
  }
  
  panic("bmap: out of range");
  return 0;
}

// 替换itrunc函数
void
itrunc(struct inode *ip)
{
  int i, j, k;
  struct buf *bp, *bp2;
  uint *a, *a2;

  // Free direct blocks
  for(i = 0; i < NDIRECT; i++) {
    if(ip->addrs[i]) {
      bfree(ip->dev, ip->addrs[i]);
      ip->addrs[i] = 0;
    }
  }

  // Free singly-indirect block
  if(ip->addrs[NDIRECT]) {
    bp = bread(ip->dev, ip->addrs[NDIRECT]);
    a = (uint*)bp->data;
    for(j = 0; j < NINDIRECT; j++) {
      if(a[j])
        bfree(ip->dev, a[j]);
    }
    brelse(bp);
    bfree(ip->dev, ip->addrs[NDIRECT]);
    ip->addrs[NDIRECT] = 0;
  }

  // Free doubly-indirect block
  if(ip->addrs[NDIRECT+1]) {
    bp = bread(ip->dev, ip->addrs[NDIRECT+1]);
    a = (uint*)bp->data;
    for(j = 0; j < NINDIRECT; j++) {
      if(a[j]) {
        bp2 = bread(ip->dev, a[j]);
        a2 = (uint*)bp2->data;
        for(k = 0; k < NINDIRECT; k++) {
          if(a2[k])
            bfree(ip->dev, a2[k]);
        }
        brelse(bp2);
        bfree(ip->dev, a[j]);
      }
    }
    brelse(bp);
    bfree(ip->dev, ip->addrs[NDIRECT+1]);
    ip->addrs[NDIRECT+1] = 0;
  }

  ip->size = 0;
  iupdate(ip);
}
  1. 编译测试
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make clean
make
make qemu
big