虚拟文件系统源码解析之初始化（基于linux1.2.13）

asmlinkage int sys_setup(void)
{
	static int callable = 1;

	if (!callable)
		return -1;
	callable = 0;

	device_setup();

#ifdef CONFIG_MINIX_FS
	register_filesystem(&(struct file_system_type)
		{minix_read_super, "minix", 1, NULL});
#endif

#ifdef CONFIG_EXT_FS
	register_filesystem(&(struct file_system_type)
		{ext_read_super, "ext", 1, NULL});
#endif
......
mount_root();
}

struct file_system_type {
	struct super_block *(*read_super) (struct super_block *, void *, int);
	char *name;
	int requires_dev;
	struct file_system_type * next;
};

int register_filesystem(struct file_system_type * fs)
{
	struct file_system_type ** tmp;

	if (!fs)
		return -EINVAL;
	if (fs->next)
		return -EBUSY;
	// tmp是二级指针，指向文件系统链表的头指针的地址
	tmp = &file_systems;
	// 遍历链表，直到尾部，插入新的节点
	while (*tmp) {
		// 判断是否已经注册了该文件系统
		if (strcmp((*tmp)->name, fs->name) == 0)
			return -EBUSY;
		// 指向当前节点的next域的地址，*tmp得到下一个被比较的节点
		tmp = &(*tmp)->next;
	}
	// 利用二级指针指针修改next域的内容，不需要使用->next = fs的形式
	*tmp = fs;
	return 0;
}

void mount_root(void)
{
	struct file_system_type * fs_type;
	struct super_block * sb;
	struct inode * inode, d_inode;
	struct file filp;
	int retval;
	// 清空超级块数组
	memset(super_blocks, 0, sizeof(super_blocks));
#ifdef CONFIG_BLK_DEV_FD
	if (MAJOR(ROOT_DEV) == FLOPPY_MAJOR) {
		printk(KERN_NOTICE "VFS: Insert root floppy and press ENTER\n");
		wait_for_keypress();
	}
#endif

	memset(&filp, 0, sizeof(filp));
	memset(&d_inode, 0, sizeof(d_inode));
	// 根设备号
	d_inode.i_rdev = ROOT_DEV;
	filp.f_inode = &d_inode;
	// 只读方式挂载
	if ( root_mountflags & MS_RDONLY)
		filp.f_mode = 1; /* read only */
	else
		filp.f_mode = 3; /* read write */
	// 暂时忽略
	retval = blkdev_open(&d_inode, &filp);
	if(retval == -EROFS){
		root_mountflags |= MS_RDONLY;
		filp.f_mode = 1;
		retval = blkdev_open(&d_inode, &filp);
	}

	for (fs_type = file_systems ; fs_type ; fs_type = fs_type->next) {
		if(retval)
			break;
		// 没有关联到设备则不需要往下执行，有些文件系统是没有对应的底层设备的
		if (!fs_type->requires_dev)
			continue;
		// 读根设备的超级块，设备的第一扇区是分区表，接着是超级块
		sb = read_super(ROOT_DEV,fs_type->name,root_mountflags,NULL,1);
		// 读取成功
		if (sb) {
			// 根节点
			inode = sb->s_mounted;
			inode->i_count += 3 ;	/* NOTE! it is logically used 4 times, not 1 */
			sb->s_covered = inode;
			sb->s_flags = root_mountflags;
			// 当前进程（init进程）的根目录和工作目录设置为根节点
			current->fs->pwd = inode;
			current->fs->root = inode;
			printk ("VFS: Mounted root (%s filesystem)%s.\n",
				fs_type->name,
				(sb->s_flags & MS_RDONLY) ? " readonly" : "");
			// 直接返回，即第一个读取成功的文件系统成为根文件系统
			return;
		}
	}
	panic("VFS: Unable to mount root fs on %02x:%02x",
		MAJOR(ROOT_DEV), MINOR(ROOT_DEV));
}

// 读设备对应的超级块
static struct super_block * read_super(dev_t dev,char *name,int flags,
				       void *data, int silent)
{
	struct super_block * s;
	struct file_system_type *type;

	if (!dev)
		return NULL;
	check_disk_change(dev);
	// 有则直接返回，初始化的时候还没有
	s = get_super(dev);
	if (s)
		return s;
	// 否则根据name在文件系统链表中（在系统初始化时建立的）找到对应的文件系统节点，里面有一个read_super函数
	if (!(type = get_fs_type(name))) {
		printk("VFS: on device %d/%d: get_fs_type(%s) failed\n",
						MAJOR(dev), MINOR(dev), name);
		return NULL;
	}
	// 在超级块数组中找到一个slot
	for (s = 0+super_blocks ;; s++) {
		if (s >= NR_SUPER+super_blocks)
			return NULL;
		if (!s->s_dev)
			break;
	}
	// 赋值给超级块节点的字段
	s->s_dev = dev;
	s->s_flags = flags;
	// 调底层的文件系统到硬盘去读取超级块内容，比如ext文件系统，ext2文件系统等等都定义了该函数。
	if (!type->read_super(s,data, silent)) {
		s->s_dev = 0;
		return NULL;
	}
	s->s_dev = dev;
	s->s_covered = NULL;
	s->s_rd_only = 0;
	s->s_dirt = 0;
	s->s_type = type;
	return s;
}

struct super_block *ext_read_super(struct super_block *s,void *data,
				   int silent)
{
	struct buffer_head *bh;
	struct ext_super_block *es;
	int dev = s->s_dev,block;

	lock_super(s);
	set_blocksize(dev, BLOCK_SIZE);
	// 读取设备的内容，即超级块的内容
	if (!(bh = bread(dev, 1, BLOCK_SIZE))) {
		s->s_dev=0;
		unlock_super(s);
		printk("EXT-fs: unable to read superblock\n");
		return NULL;
	}
	// 文件系统的一些属性
	es = (struct ext_super_block *) bh->b_data;
	s->s_blocksize = 1024;
	s->s_blocksize_bits = 10;
	s->u.ext_sb.s_ninodes = es->s_ninodes;
	s->u.ext_sb.s_nzones = es->s_nzones;
	s->u.ext_sb.s_firstdatazone = es->s_firstdatazone;
	s->u.ext_sb.s_log_zone_size = es->s_log_zone_size;
	s->u.ext_sb.s_max_size = es->s_max_size;
	s->s_magic = es->s_magic;
	s->u.ext_sb.s_firstfreeblocknumber = es->s_firstfreeblock;
	s->u.ext_sb.s_freeblockscount = es->s_freeblockscount;
	s->u.ext_sb.s_firstfreeinodenumber = es->s_firstfreeinode;
	s->u.ext_sb.s_freeinodescount = es->s_freeinodescount;
	brelse(bh);
	if (s->s_magic != EXT_SUPER_MAGIC) {
		s->s_dev = 0;
		unlock_super(s);
		if (!silent)
			printk("VFS: Can't find an extfs filesystem on dev 0x%04x.\n",
				   dev);
		return NULL;
	}
	if (!s->u.ext_sb.s_firstfreeblocknumber)
		s->u.ext_sb.s_firstfreeblock = NULL;
	else
		if (!(s->u.ext_sb.s_firstfreeblock = bread(dev,
			s->u.ext_sb.s_firstfreeblocknumber, BLOCK_SIZE))) {
			printk("ext_read_super: unable to read first free block\n");
			s->s_dev = 0;
			unlock_super(s);
			return NULL;
		}
	if (!s->u.ext_sb.s_firstfreeinodenumber)
		s->u.ext_sb.s_firstfreeinodeblock = NULL;
	else {
		block = 2 + (s->u.ext_sb.s_firstfreeinodenumber - 1) / EXT_INODES_PER_BLOCK;
		if (!(s->u.ext_sb.s_firstfreeinodeblock = bread(dev, block, BLOCK_SIZE))) {
			printk("ext_read_super: unable to read first free inode block\n");
			brelse(s->u.ext_sb.s_firstfreeblock);
			s->s_dev = 0;
			unlock_super (s);
			return NULL;
		}
	}
	unlock_super(s);
	/* set up enough so that it can read an inode */
	s->s_dev = dev;
	// 操作函数集
	s->s_op = &ext_sops;
	// 读取根节点
	if (!(s->s_mounted = iget(s,EXT_ROOT_INO))) {
		s->s_dev=0;
		printk("EXT-fs: get root inode failed\n");
		return NULL;
	}
	return s;
}