(done) 梳理 xv6-lab-2023 fs.img 生成过程，以及 xv6 磁盘结构

由于写 LAB8 时调试文件系统并发 bug 遇到了很大困难，这里决定读读 xv6 文件系统源码，来帮助调试

先运行 make qemu，看看编译日志中哪里出现了 fs.img：

猜测：mkfs 是一个命令，他会把后边的一堆参数都编译到 fs.img。

后面那一堆参数会成为 fs.img 中的 “用户程序”。

再搜 mkfs：

这里可以看到 mkfs 其实是 mkfs.c 的直接编译产物

---

解析 mkfs.c

从注释来看，可知 xv6 磁盘结构的数据排布：

// Disk layout:
// [ boot block | sb block | log | inode blocks | free bit map | data blocks ]
// 磁盘结构：启动块，超级块，日志块，inode块，位图块，数据块

超级块的结构如下(定义于 fs.h)：

// Disk layout:
// [ boot block | super block | log | inode blocks |
//                                          free bit map | data blocks]
//
// mkfs computes the super block and builds an initial file system. The
// super block describes the disk layout:
struct superblock {uint magic;        // Must be FSMAGICuint size;         // Size of file system image (blocks)uint nblocks;      // Number of data blocksuint ninodes;      // Number of inodes.uint nlog;         // Number of log blocksuint logstart;     // Block number of first log blockuint inodestart;   // Block number of first inode blockuint bmapstart;    // Block number of first free map block
};

磁盘上的 inode 定义如下：

// On-disk inode structure
struct dinode {short type;           // File typeshort major;          // Major device number (T_DEVICE only)short minor;          // Minor device number (T_DEVICE only)short nlink;          // Number of links to inode in file systemuint size;            // Size of file (bytes)uint addrs[NDIRECT+1];   // Data block addresses
};

阅读 mkfs.c 源码，可以理解这个代码在做什么：
1.写超级块到 fs.img
2.写根目录 inode 到 fs.img
3.写 . 和 … 节点 inodes 到 / 目录
4.把命令行参数 fs.img 后边的所有文件拷贝、写入到 fs.img 的根目录内

注释后的源码如下：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <assert.h>#define stat xv6_stat  // avoid clash with host struct stat
#include "kernel/types.h"
#include "kernel/fs.h"
#include "kernel/stat.h"
#include "kernel/param.h"#ifndef static_assert
#define static_assert(a, b) do { switch (0) case 0: case (a): ; } while (0)
#endif#define NINODES 200// Disk layout:
// [ boot block | sb block | log | inode blocks | free bit map | data blocks ]
// 磁盘结构：启动块，超级块，日志块，inode块，位图块，数据块int nbitmap = FSSIZE/(BSIZE*8) + 1; // 一共有 FSSIZE 个块需要记录，也就需要 FSSIZE 个 bits 需要记录。这里计算存放 FSSIZE 个 bits 所需的块数量
int ninodeblocks = NINODES / IPB + 1; // xv6 文件系统提供 200 个 inode，这里计算存放所有 inodes 需要的磁盘块数
int nlog = LOGSIZE; // 日志部分包含 30 个磁盘块
int nmeta;    // Number of meta blocks (boot, sb, nlog, inode, bitmap)
int nblocks;  // Number of data blocks// 文件系统文件描述符
int fsfd;
struct superblock sb;
char zeroes[BSIZE];
uint freeinode = 1;
uint freeblock;void balloc(int);
void wsect(uint, void*);
void winode(uint, struct dinode*);
void rinode(uint inum, struct dinode *ip);
void rsect(uint sec, void *buf);
uint ialloc(ushort type);
void iappend(uint inum, void *p, int n);
void die(const char *);// convert to riscv byte order
ushort
xshort(ushort x)
{ushort y;uchar *a = (uchar*)&y;a[0] = x;a[1] = x >> 8;return y;
}uint
xint(uint x)
{uint y;uchar *a = (uchar*)&y;a[0] = x;a[1] = x >> 8;a[2] = x >> 16;a[3] = x >> 24;return y;
}int
main(int argc, char *argv[])
{int i, cc, fd;uint rootino, inum, off;struct dirent de;char buf[BSIZE];struct dinode din;static_assert(sizeof(int) == 4, "Integers must be 4 bytes!");if(argc < 2){fprintf(stderr, "Usage: mkfs fs.img files...\n");exit(1);}assert((BSIZE % sizeof(struct dinode)) == 0);assert((BSIZE % sizeof(struct dirent)) == 0);// 打开 fs.img，文件描述符为 fsfd (全局变量)fsfd = open(argv[1], O_RDWR|O_CREAT|O_TRUNC, 0666);// 打不开就报错if(fsfd < 0)die(argv[1]);// 1 fs block = 1 disk sector// 元数据块数量 = 启动块 + 超级块 + 日志块 + inode块 + bitmap块nmeta = 2 + nlog + ninodeblocks + nbitmap;// 数据块数量 = FS总数量 - 元数据块数量nblocks = FSSIZE - nmeta;// 记录超级块元数据sb.magic = FSMAGIC;sb.size = xint(FSSIZE);sb.nblocks = xint(nblocks);sb.ninodes = xint(NINODES);sb.nlog = xint(nlog);sb.logstart = xint(2);sb.inodestart = xint(2+nlog);sb.bmapstart = xint(2+nlog+ninodeblocks);printf("nmeta %d (boot, super, log blocks %u inode blocks %u, bitmap blocks %u) blocks %d total %d\n",nmeta, nlog, ninodeblocks, nbitmap, nblocks, FSSIZE);// 计算第一个 datablock 的位置freeblock = nmeta;     // the first free block that we can allocate// 对文件系统所有的块写入全零for(i = 0; i < FSSIZE; i++)wsect(i, zeroes);// 对文件系统第一个 sector 写入 superblock 的内容memset(buf, 0, sizeof(buf));memmove(buf, &sb, sizeof(sb));wsect(1, buf);// 为根目录分配一个 inode (会写入磁盘)rootino = ialloc(T_DIR);assert(rootino == ROOTINO);// 为根目录写入 . inodebzero(&de, sizeof(de));de.inum = xshort(rootino);strcpy(de.name, ".");iappend(rootino, &de, sizeof(de));// 为根目录写入 .. inodebzero(&de, sizeof(de));de.inum = xshort(rootino);strcpy(de.name, "..");iappend(rootino, &de, sizeof(de));// 把命令行参数后面跟着的所有文件拷贝到 fs.img 里，分配 inodefor(i = 2; i < argc; i++){// get rid of "user/"char *shortname;if(strncmp(argv[i], "user/", 5) == 0)shortname = argv[i] + 5;elseshortname = argv[i];assert(index(shortname, '/') == 0);if((fd = open(argv[i], 0)) < 0)die(argv[i]);// Skip leading _ in name when writing to file system.// The binaries are named _rm, _cat, etc. to keep the// build operating system from trying to execute them// in place of system binaries like rm and cat.if(shortname[0] == '_')shortname += 1;inum = ialloc(T_FILE);bzero(&de, sizeof(de));de.inum = xshort(inum);strncpy(de.name, shortname, DIRSIZ);iappend(rootino, &de, sizeof(de));while((cc = read(fd, buf, sizeof(buf))) > 0)iappend(inum, buf, cc);close(fd);}// fix size of root inode dir// 配置好跟文件系统的 inode 的元数据rinode(rootino, &din);off = xint(din.size);off = ((off/BSIZE) + 1) * BSIZE;din.size = xint(off);winode(rootino, &din);balloc(freeblock);exit(0);
}void
wsect(uint sec, void *buf)
{if(lseek(fsfd, sec * BSIZE, 0) != sec * BSIZE)die("lseek");if(write(fsfd, buf, BSIZE) != BSIZE)die("write");
}void
winode(uint inum, struct dinode *ip)
{char buf[BSIZE];uint bn;struct dinode *dip;bn = IBLOCK(inum, sb);rsect(bn, buf);dip = ((struct dinode*)buf) + (inum % IPB);*dip = *ip;wsect(bn, buf);
}void
rinode(uint inum, struct dinode *ip)
{char buf[BSIZE];uint bn;struct dinode *dip;bn = IBLOCK(inum, sb);rsect(bn, buf);dip = ((struct dinode*)buf) + (inum % IPB);*ip = *dip;
}void
rsect(uint sec, void *buf)
{if(lseek(fsfd, sec * BSIZE, 0) != sec * BSIZE)die("lseek");if(read(fsfd, buf, BSIZE) != BSIZE)die("read");
}uint
ialloc(ushort type)
{uint inum = freeinode++;struct dinode din;bzero(&din, sizeof(din));din.type = xshort(type);din.nlink = xshort(1);din.size = xint(0);winode(inum, &din);return inum;
}void
balloc(int used)
{uchar buf[BSIZE];int i;printf("balloc: first %d blocks have been allocated\n", used);assert(used < BSIZE*8);bzero(buf, BSIZE);for(i = 0; i < used; i++){buf[i/8] = buf[i/8] | (0x1 << (i%8));}printf("balloc: write bitmap block at sector %d\n", sb.bmapstart);wsect(sb.bmapstart, buf);
}#define min(a, b) ((a) < (b) ? (a) : (b))void
iappend(uint inum, void *xp, int n)
{char *p = (char*)xp;uint fbn, off, n1;struct dinode din;char buf[BSIZE];uint indirect[NINDIRECT];uint x;rinode(inum, &din);off = xint(din.size);// printf("append inum %d at off %d sz %d\n", inum, off, n);while(n > 0){fbn = off / BSIZE;assert(fbn < MAXFILE);if(fbn < NDIRECT){if(xint(din.addrs[fbn]) == 0){din.addrs[fbn] = xint(freeblock++);}x = xint(din.addrs[fbn]);} else {if(xint(din.addrs[NDIRECT]) == 0){din.addrs[NDIRECT] = xint(freeblock++);}rsect(xint(din.addrs[NDIRECT]), (char*)indirect);if(indirect[fbn - NDIRECT] == 0){indirect[fbn - NDIRECT] = xint(freeblock++);wsect(xint(din.addrs[NDIRECT]), (char*)indirect);}x = xint(indirect[fbn-NDIRECT]);}n1 = min(n, (fbn + 1) * BSIZE - off);rsect(x, buf);bcopy(p, buf + off - (fbn * BSIZE), n1);wsect(x, buf);n -= n1;off += n1;p += n1;}din.size = xint(off);winode(inum, &din);
}// 报错
void
die(const char *s)
{perror(s);exit(1);
}

---