Linux虚拟文件系统（安装根文件系统）

安装根文件系统式系统初始化的关键部分。Linux内核允许根文件系统放在很多不同的地方，比如硬盘分区、软盘、通过NFS共享的远程文件系统以及保存在ramdisk中。内核要在变量ROOT_DEV中寻找包含根文件系统的磁盘主设备号。当编译内核时，或者像最初的启动装入程序传递一个合适的“root”选项时，根文件系统可以被指定为/dev目录下的一个设备文件。

相关阅读：

http://www./Linux/2012-02/53771.htm

安装根文件系统分为两个阶段：

1，内核安装特殊rootfs文件系统，该文件系统仅提供一个作为初始安装点的空目录

start_kernel()->vfs_caches_init()->mnt_init()->init_rootfs()

/*初始化根文件系统*/
int __init init_rootfs(void)
{
    int err;
    /*初始化ramfs_backing_dev_info*/
    err = bdi_init(&ramfs_backing_dev_info);
    if (err)
        return err;
    /*注册rootfs_fs_type文件类型*/
    err = register_filesystem(&rootfs_fs_type);
    if (err)/*如果出错，销毁上面初始化的*/
        bdi_destroy(&ramfs_backing_dev_info);

    return err;
}

static struct backing_dev_info ramfs_backing_dev_info = {
    .name       = "ramfs",
    .ra_pages   = 0,    /* No readahead */
    .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK |
              BDI_CAP_MAP_DIRECT | BDI_CAP_MAP_COPY |
              BDI_CAP_READ_MAP | BDI_CAP_WRITE_MAP | BDI_CAP_EXEC_MAP,
};

/**
 *  register_filesystem - register a new filesystem
 *  @fs: the file system structure
 *
 *  Adds the file system passed to the list of file systems the kernel
 *  is aware of for mount and other syscalls. Returns 0 on success,
 *  or a negative errno code on an error.
 *
 *  The &struct file_system_type that is passed is linked into the kernel
 *  structures and must not be freed until the file system has been
 *  unregistered.
 */
 /*注册一个新的文件系统*/
int register_filesystem(struct file_system_type * fs)
{
    int res = 0;
    struct file_system_type ** p;

    BUG_ON(strchr(fs->name, '.'));
    if (fs->next)
        return -EBUSY;
    INIT_LIST_HEAD(&fs->fs_supers);
    write_lock(&file_systems_lock);
    /*从system_type链表中查找指定名称的file_system_type*/
    p = find_filesystem(fs->name, strlen(fs->name));
    if (*p)
        res = -EBUSY;
    else
        *p = fs;
    write_unlock(&file_systems_lock);
    return res;
}

根文件系统定义如下

static struct file_system_type rootfs_fs_type = {
    .name       = "rootfs",
    .get_sb     = rootfs_get_sb,
    .kill_sb    = kill_litter_super,
};

下面看看他的两个函数

/*获得根目录的sb*/
static int rootfs_get_sb(struct file_system_type *fs_type,
    int flags, const char *dev_name, void *data, struct vfsmount *mnt)
{
    return get_sb_nodev(fs_type, flags|MS_NOUSER, data, ramfs_fill_super,
                mnt);
}

int get_sb_nodev(struct file_system_type *fs_type,
    int flags, void *data,
    int (*fill_super)(struct super_block *, void *, int),
    struct vfsmount *mnt)
{
    int error;
    /*获得sb结构*/
    struct super_block *s = sget(fs_type, NULL, set_anon_super, NULL);

    if (IS_ERR(s))
        return PTR_ERR(s);

    s->s_flags = flags;
    /*这里实际调用ramfs_fill_super,对sb结构的属性进行设置*/
    error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
    if (error) {
        deactivate_locked_super(s);
        return error;
    }
    s->s_flags |= MS_ACTIVE;
    simple_set_mnt(mnt, s);/*设置mnt和sb关联*/
    return 0;
}

/**
 *  sget    -   find or create a superblock
 *  @type:  filesystem type superblock should belong to
 *  @test:  comparison callback
 *  @set:   setup callback
 *  @data:  argument to each of them
 */
 /*查找或创建一个sb结构*/
struct super_block *sget(struct file_system_type *type,
            int (*test)(struct super_block *,void *),
            int (*set)(struct super_block *,void *),
            void *data)
{
    struct super_block *s = NULL;
    struct super_block *old;
    int err;

retry:
    spin_lock(&sb_lock);
    if (test) {
        list_for_each_entry(old, &type->fs_supers, s_instances) {
            if (!test(old, data))
                continue;
            if (!grab_super(old))
                goto retry;
            if (s) {
                up_write(&s->s_umount);
                destroy_super(s);
            }
            return old;
        }
    }
    if (!s) {/*如果找不到sb,从内存中申请一个*/
        spin_unlock(&sb_lock);
        s = alloc_super(type);
        if (!s)
            return ERR_PTR(-ENOMEM);
        goto retry;
    }

    err = set(s, data);
    if (err) {
        spin_unlock(&sb_lock);
        up_write(&s->s_umount);
        destroy_super(s);
        return ERR_PTR(err);
    }
    /*初始化得到的sb结构*/
    s->s_type = type;
    strlcpy(s->s_id, type->name, sizeof(s->s_id));
    /*加入链表尾*/
    list_add_tail(&s->s_list, &super_blocks);
    list_add(&s->s_instances, &type->fs_supers);
    spin_unlock(&sb_lock);
    get_filesystem(type);
    return s;
}