[置顶] 《Linux启动过程分析》内核挂载根文件系统

说明：本文基于Linux2.6.29内核分析；其他内核版本仅供参考。

Android2.3及Linux2.6.29内核模拟器版本编译与调试

  前边通过源码情景分析，看过了总线、设备、驱动及其发现机制，Linux2.6内核udev设备节点创建相关；对于文件系统，一直望而生畏，但内核学习、这部分又不可能绕的过去。目前对VFS中使用的hash表还未做研究，它在dentry和vfsmount下查找节点起关键作用；后边在做分析。下边将根文件系统挂载过程做简单分析：

一、rootfs的诞生

引子：

Linux一切皆文件的提出：在Linux中，普通文件、目录、字符设备、块设备、套接字等都以文件被对待；他们具体的类型及其操作不同，但需要向上层提供统一的操作接口。

虚拟文件系统VFS就是Linux内核中的一个软件层，向上给用户空间程序提供文件系统操作接口；向下允许不同的文件系统共存。所以，所有实际文件系统都必须实现VFS的结构封装。

矛盾的提出：

Linux系统中任何文件系统的挂载必须满足两个条件：挂载点和文件系统。

直接挂载nfs或flash文件系统有如下两个问题必须解决：

1.谁来提供挂载点？我们可以想象自己创建一个超级块(包含目录项和i节点)，这时挂载点不是就有了吗；很可惜，linux引入VFS(一切皆文件,所有类型文件系统必须提供一个VFS的软件层、以向上层提供统一接口)后该问题不能这么解决，因为挂载点必须关联到文件系统、也就是说挂载点必须属于某个文件系统。

2.怎样访问到nfs或flash上的文件系统？我们可以说直接访问设备驱动读取其上边的文件系统（设备上的文件系统是挂载在自己的根目录），不就可以了吗；别忘了还是Linux的VFS，设备访问也不例外。因为访问设备还是需要通过文件系统来访问它的挂载点，不能直接访问(要满足Linux的VFS架构，一切皆文件)。

所以，一句话：rootfs之所以存在，是因为需要在VFS机制下给系统提供最原始的挂载点。

如此矛盾，需要我们引入一种特殊文件系统：

1.它是系统自己创建并加载的第一个文件系统；该文件系统的挂载点就是它自己的根目录项。

2.该文件系统不能存在于nfs或flash上，因为如此将会陷入之前的矛盾。

rootfs的诞生：

上述问题需要我们创建具有如下三个特点的特殊文件系统：

1.它是系统自己创建并加载的第一个文件系统；

2.该文件系统的挂载点就是它自己的根目录项对象；

3.该文件系统仅仅存在于内存中。

  由以上分析可以看出，rootfs是Linux的VFS(一切皆文件，所有类型文件系统必须提供一个VFS的软件层、以向上层提供统一接口)存在的基石；二者关系密切。如果没有VFS机制，rootfs也就没有存在的必要；同样，如果没有rootfs、VFS机制也就不能实现。

  这就是两者之间的真正关系，之前看网上什么说法都有：有的只说关系密切，没有指明具体关系；有的干脆误人子弟，说VFS就是rootfs。

  其实，VFS是一种机制、是Linux下每一种文件系统（包括刚才说的rootfs，还有常见的ext3、yaffs等）都必须按照这个机制去实现的一种规范；而rootfs仅仅是符合VFS规范的而且又具有如上3个特点的一个文件系统。

  VFS是Linux文件系统实现必须遵循的一种机制，rootfs是一种具体实现的文件系统、Linux下所有文件系统的实现都必须符合VFS的机制(符合VFS的接口)；这就是二者的真正关系。

以下分析基于Android模拟器Linux2.6.29内核：

二、相关数据结构

Linux内核中current指针作为全局变量，使用非常广泛；例如：进程上下文中获取当前进程ID、任务调度，以及open等文件系统调用中路径搜索等；首先介绍下current结构体：

各个平台、各个内核版本中current的实现可能不同；但原理是一样的。该指针一般定义在具体平台的current.h头文件中，类型为struct task_struct：

#define current (get_current())
static inline struct task_struct *get_current(void)

include/linux/sched.h

struct task_struct {
  ......
  struct thread_info *thread_info;
  struct list_head tasks;
  pid_t pid;
  pid_t tgid;
  uid_t uid,euid,suid,fsuid;
  gid_t gid,egid,sgid,fsgid;
  struct fs_struct *fs;  //本节将大量使用这个
  struct files_struct *files;
  ......
}

1.文件系统注册

kernel/include/include/fs.h

struct file_system_type {
  const char *name; //文件系统名字；如:rootfs及ext3等
  int fs_flags;
  int (*get_sb) (struct file_system_type *, int, const char *, void *, struct vfsmount *);
  //安装/挂载文件系统时，会调用；获取超级块。
  void (*kill_sb) (struct super_block *);
  //卸载文件系统时会调用。
  struct module *owner;
  struct file_system_type * next;
  //指向下一个文件系统类型。
  struct list_head fs_supers;
  //同一个文件系统类型中所有超级块组成双向链表。
  struct lock_class_key s_lock_key;
  struct lock_class_key s_umount_key;

  struct lock_class_key i_lock_key;
  struct lock_class_key i_mutex_key;
  struct lock_class_key i_mutex_dir_key;
  struct lock_class_key i_alloc_sem_key;
};

2.文件系统挂载vfsmount(struct vfsmount)：

  本质上，mount操作的过程就是新建一个vfsmount结构，然后将此结构和挂载点(目录项对象)关联。关联之后，目录查找时就能沿着vfsmount挂载点一级级向下查找文件了。
对于每一个mount的文件系统，都由一个vfsmount实例来表示。

kernel/include/linux/mount.h

struct vfsmount {
  struct list_head mnt_hash; //内核通过哈希表对vfsmount进行管理
  struct vfsmount *mnt_parent;	//指向父文件系统对应的vfsmount
  struct dentry *mnt_mountpoint; //指向该文件系统挂载点对应的目录项对象dentry
  struct dentry *mnt_root; //该文件系统对应的设备根目录dentry
  struct super_block *mnt_sb; //指向该文件系统对应的超级块
  struct list_head mnt_mounts; 
  struct list_head mnt_child;  //同一个父文件系统中的所有子文件系统通过该字段链接成双联表
  int mnt_flags;
  /* 4 bytes hole on 64bits arches */
  const char *mnt_devname;	/* Name of device e.g. /dev/dsk/hda1 */
  struct list_head mnt_list;  //所有已挂载文件系统的vfsmount结构通过该字段链接在一起
  struct list_head mnt_expire;	/* link in fs-specific expiry list */
  struct list_head mnt_share;	/* circular list of shared mounts */
  struct list_head mnt_slave_list;/* list of slave mounts */
  struct list_head mnt_slave;	/* slave list entry */
  struct vfsmount *mnt_master;	/* slave is on master->mnt_slave_list */
  struct mnt_namespace *mnt_ns;	/* containing namespace */
  int mnt_id;			/* mount identifier */
  int mnt_group_id;		/* peer group identifier */
  /*
  * We put mnt_count & mnt_expiry_mark at the end of struct vfsmount
  * to let these frequently modified fields in a separate cache line
  * (so that reads of mnt_flags wont ping-pong on SMP machines)
  */
  atomic_t mnt_count;
  int mnt_expiry_mark;		/* true if marked for expiry */
  int mnt_pinned;
  int mnt_ghosts;
  /*
  * This value is not stable unless all of the mnt_writers[] spinlocks
  * are held, and all mnt_writer[]s on this mount have 0 as their ->count
  */
  atomic_t __mnt_writers;
};

3.超级块(struct super_bloc)：

kernel/include/linux/fs.h

struct super_block {
  struct list_head	s_list;		/* Keep this first */
  dev_t			s_dev;		/* search index; _not_ kdev_t */
  unsigned long		s_blocksize;
  unsigned char		s_blocksize_bits;
  unsigned char		s_dirt;
  unsigned long long	s_maxbytes;	/* Max file size */
  struct file_system_type	*s_type; //文件系统类型
  //(kernel/include/linux/fs.h,struct file_system_type)
  const struct super_operations	*s_op;
  struct dquot_operations	*dq_op;
  struct quotactl_ops	*s_qcop;
  const struct export_operations *s_export_op;
  unsigned long		s_flags;
  unsigned long		s_magic;
  struct dentry		*s_root;  //超级块要指向目录项对象
  struct rw_semaphore	s_umount;
  struct mutex		s_lock;
  int			s_count;
  int			s_need_sync_fs;
  atomic_t		s_active;
#ifdef CONFIG_SECURITY
  void                    *s_security;
#endif
  struct xattr_handler	**s_xattr;

  struct list_head	s_inodes;	/* all inodes */
  struct list_head	s_dirty;	/* dirty inodes */
  struct list_head	s_io;		/* parked for writeback */
  struct list_head	s_more_io;	/* parked for more writeback */
  struct hlist_head	s_anon;//哈希表头		/* anonymous dentries for (nfs) exporting */
  struct list_head	s_files;
  /* s_dentry_lru and s_nr_dentry_unused are protected by dcache_lock */
  struct list_head	s_dentry_lru;	/* unused dentry lru */
  int			s_nr_dentry_unused;	/* # of dentry on lru */

  struct block_device	*s_bdev;
  struct mtd_info		*s_mtd;
  struct list_head	s_instances;
  struct quota_info	s_dquot;	/* Diskquota specific options */

  int			s_frozen;
  wait_queue_head_t	s_wait_unfrozen;

  char s_id[32];				/* Informational name */

  void 			*s_fs_info;	/* Filesystem private info */
  fmode_t			s_mode;

  /*
  * The next field is for VFS *only*. No filesystems have any business
  * even looking at it. You had been warned.
  */
  struct mutex s_vfs_rename_mutex;	/* Kludge */

  /* Granularity of c/m/atime in ns.Cannot be worse than a second */
  u32		   s_time_gran;

  /*
  * Filesystem subtype.  If non-empty the filesystem type field
  * in /proc/mounts will be "type.subtype"
  */
  char *s_subtype;

  /*
  * Saved mount options for lazy filesystems using
  * generic_show_options()
  */
  char *s_options;

  /*
  * storage for asynchronous operations
  */
  struct list_head s_async_list;
};

4.目录索引节点(struct inode)：

kernel/include/linux/fs.h

struct inode {
  struct hlist_node	i_hash; //哈希表节点
  struct list_head	i_list;
  struct list_head	i_sb_list;
  struct list_head	i_dentry;
  unsigned long		i_ino;
  atomic_t		i_count;
  unsigned int		i_nlink;
  uid_t			i_uid;
  gid_t			i_gid;
  dev_t			i_rdev;
  u64			i_version;
  loff_t			i_size;
#ifdef __NEED_I_SIZE_ORDERED
  seqcount_t		i_size_seqcount;
#endif
  struct timespec		i_atime;
  struct timespec		i_mtime;
  struct timespec		i_ctime;
  unsigned int		i_blkbits;
  blkcnt_t		i_blocks;
  unsigned short          i_bytes;
  umode_t			i_mode;
  spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
  struct mutex		i_mutex;
  struct rw_semaphore	i_alloc_sem;
  const struct inode_operations	*i_op;
  const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */
  struct super_block	*i_sb;
  struct file_lock	*i_flock;
  struct address_space	*i_mapping;
  struct address_space	i_data;
#ifdef CONFIG_QUOTA
  struct dquot		*i_dquot[MAXQUOTAS];
#endif
  struct list_head	i_devices;
  union {
    struct pipe_inode_info	*i_pipe;
    struct block_device	*i_bdev;
    struct cdev		*i_cdev;
  };
  int			i_cindex;

  __u32			i_generation;

#ifdef CONFIG_DNOTIFY
  unsigned long		i_dnotify_mask; /* Directory notify events */
  struct dnotify_struct	*i_dnotify; /* for directory notifications */
#endif

#ifdef CONFIG_INOTIFY
  struct list_head	inotify_watches; /* watches on this inode */
  struct mutex		inotify_mutex;	/* protects the watches list */
#endif

  unsigned long		i_state;
  unsigned long		dirtied_when;	/* jiffies of first dirtying */

  unsigned int		i_flags;

  atomic_t		i_writecount;
#ifdef CONFIG_SECURITY
  void			*i_security;
#endif
  void			*i_private; /* fs or device private pointer */
};

5.目录项对象(struct dentry)：

kernel/include/linux/dcache.h

struct dentry {
  atomic_t d_count;
  unsigned int d_flags;		/* protected by d_lock */
  spinlock_t d_lock;		/* per dentry lock */
  int d_mounted;
  struct inode *d_inode; //目录项对象与目录索引的关联		
  /* Where the name belongs to - NULL is
  * negative */
  /*
  * The next three fields are touched by __d_lookup.  Place them here
  * so they all fit in a cache line.
  */
  struct hlist_node d_hash; //哈希表节点	/* lookup hash list */
  struct dentry *d_parent; //目录项对象的父亲	/* parent directory */
  struct qstr d_name; //d_name.name这个是文件名，目录对象与目录名的关联

  struct list_head d_lru;		/* LRU list */
  /*
  * d_child and d_rcu can share memory
  */
  union {
    struct list_head d_child;	/* child of parent list */
    struct rcu_head d_rcu;
  } d_u;
  struct list_head d_subdirs;	/* our children */
  struct list_head d_alias;	/* inode alias list */
  unsigned long d_time;		/* used by d_revalidate */
  struct dentry_operations *d_op;
  struct super_block *d_sb; //指向文件系统的超级块/* The root of the dentry tree */
  void *d_fsdata;			/* fs-specific data */

  unsigned char d_iname[DNAME_INLINE_LEN_MIN];	/* small names */
};

其他：

include/linux/fs.h

struct file {
  /*
  * fu_list becomes invalid after file_free is called and queued via
  * fu_rcuhead for RCU freeing
  */
  union {
    struct list_head	fu_list;
    struct rcu_head 	fu_rcuhead;
  } f_u;
  struct path		f_path;  //重要！！！记录挂载信息和目录项信息
#define f_dentry	f_path.dentry
#define f_vfsmnt	f_path.mnt
  const struct file_operations	*f_op;
  atomic_long_t		f_count;
  unsigned int 		f_flags;
  fmode_t			f_mode;
  loff_t			f_pos;
  struct fown_struct	f_owner;
  const struct cred	*f_cred;
  struct file_ra_state	f_ra;

  u64			f_version;
#ifdef CONFIG_SECURITY
  void			*f_security;
#endif
  /* needed for tty driver, and maybe others */
  void			*private_data;

#ifdef CONFIG_EPOLL
  /* Used by fs/eventpoll.c to link all the hooks to this file */
  struct list_head	f_ep_links;
  spinlock_t		f_ep_lock;
#endif /* #ifdef CONFIG_EPOLL */
  struct address_space	*f_mapping;
#ifdef CONFIG_DEBUG_WRITECOUNT
  unsigned long f_mnt_write_state;
#endif
};

include/linux/fs_struct.h

struct fs_struct {
  atomic_t count;
  rwlock_t lock;
  int umask;
  struct path root, pwd; //重要！！！记录挂载信息和目录项信息
};

include/linux/namei.h

struct nameidata {
  struct path	path;  //重要！！！记录挂载信息和目录项信息
  struct qstr	last;  //重要！！！记录目录名
  unsigned int	flags;
  int		last_type;
  unsigned	depth;
  char *saved_names[MAX_NESTED_LINKS + 1];

  /* Intent data */
  union {
    struct open_intent open;
  } intent;
};

include/linux/path.h

struct path {
  struct vfsmount *mnt; //重要！！！记录文件系统挂载信息
  struct dentry *dentry;  //重要！！！记录目录项信息
};

include/linux/dcache.h

struct qstr {
  unsigned int hash;
  unsigned int len;
  const unsigned char *name;//重要！！！目录/文件名字，如"/","tank1"等具体的文件名
};

三、注册/创建、安装/挂载rootfs，并调用set_fs_root设置系统current的根文件系统为rootfs

过程：

第一步：建立rootfs文件系统；

第二步：调用其get_sb函数(对于rootfs这种内存/伪文件系统是get_sb_nodev，实际文件系统比如ext2等是get_sb_bdev)、建立超级块(包含目录项和i节点)；

第三步：挂载该文件系统(该文件系统的挂载点指向该文件系统超级块的根目录项)；

第四步：将系统current的根文件系统和根目录设置为rootfs和其根目录。

kernel/init/main.c

asmlinkage void __init start_kernel(void)
{
  setup_arch(&command_line);//解析uboot命令行，实际文件系统挂载需要
  parse_args("Booting kernel", static_command_line, __start___param,
		   __stop___param - __start___param,
		   &unknown_bootoption);
  vfs_caches_init(num_physpages);
}

kernel/fs/dcache.c

void __init vfs_caches_init(unsigned long mempages)
{
  mnt_init();
  bdev_cache_init(); //块设备文件创建
  chrdev_init();//字符设备文件创建
}

kernel/fs/namespace.c

void __init mnt_init(void)
{
  init_rootfs(); //向内核注册rootfs
  init_mount_tree();//重要！！！rootfs根目录的建立以及rootfs文件系统的挂载；设置系统current根目录和根文件系统为rootfs
}

下边分两步：

1.向内核注册rootfs虚拟文件系统init_rootfs

kernel/fs/ramfs/inode.c

int __init init_rootfs(void)
{
  err = register_filesystem(&rootfs_fs_type);
}
static struct file_system_type rootfs_fs_type = {
  .name		= "rootfs",
  .get_sb		= rootfs_get_sb,
  .kill_sb	= kill_litter_super,
};

2.建立rootfs的根目录，并将rootfs挂载到自己的根目录；设置系统current根目录和根文件系统

kernel/fs/namespace.c

static void __init init_mount_tree(void)
{
  struct vfsmount *mnt;
  struct mnt_namespace *ns;
  struct path root;
  //创建rootfs的vfsmount结构，建立rootfs的超级块、并将rootfs挂载到自己的根目录。
  /*
  mnt->mnt_mountpoint = mnt->mnt_root = dget(sb->s_root)，而该mnt和自己的sb是关联的；
  所以，是把rootfs文件系统挂载到了自己对应的超级块的根目录上。
  这里也是实现的关键：一般文件系统的挂载是调用do_mount->do_new_mount而该函数中首先调用do_kern_mount，这时mnt->mnt_mountpoint = mnt->mnt_root；但后边
  它还会调用do_add_mount->graft_tree->attach_recursive_mnt如下代码mnt_set_mountpoint(dest_mnt, dest_dentry, source_mnt)改变了其挂载点！！！
  */
  mnt = do_kern_mount("rootfs", 0, "rootfs", NULL);
  list_add(&mnt->mnt_list, &ns->list);
  ns->root = mnt; //将创建好的mnt加入系统当前
  mnt->mnt_ns = ns;

  init_task.nsproxy->mnt_ns = ns; //设置进程的命名空间
  get_mnt_ns(ns);

  root.mnt = ns->root; //文件系统为rootfs，相当与root.mnt = mnt;
  root.dentry = ns->root->mnt_root;//目录项为根目录项，相当与root.dentry = mnt->mnt_root;

  //设置系统current的pwd目录和文件系统
  set_fs_pwd(current->fs, &root);
  //设置系统current根目录,根文件系统。这个是关键！！！整个内核代码最多只有两处调用
  set_fs_root(current->fs, &root);  
}

以下着重分析do_kern_mount函数，它实现了rootfs在自己根目录上的挂载：

kernel/fs/super.c

struct vfsmount *
do_kern_mount(const char *fstype, int flags, const char *name, void *data)
{
  mnt = vfs_kern_mount(type, flags, name, data);
  return mnt;
}

kernel/fs/super.c

struct vfsmount *
vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
{
  mnt = alloc_vfsmnt(name); //建立并填充vfsmount
  error = type->get_sb(type, flags, name, data, mnt);//为文件系统建立并填充超级块(主要是其dentry和inode)，建立rootfs根目录
  mnt->mnt_mountpoint = mnt->mnt_root; //文件系统挂载点目录,其实就是刚才建立的”/”目录。挂载点就是自己！！！！
  mnt->mnt_parent = mnt; //父对象是自己！！！！
  return mnt;
}

kernel/fs/ramfs/inode.c

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);
}

kernel/fs/super.c

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)
{
  struct super_block *s = sget(fs_type, NULL, set_anon_super, NULL);
  //在内存中分配一个超级块
  error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
  //执行回调，填充超级块，并建立根目录项及对应i节点
  /*
  kernel/fs/ramfs/inode.c
  static int ramfs_fill_super(struct super_block * sb, void * data, int silent)
  {
    struct inode * inode;
    struct dentry * root;
    sb->s_maxbytes = MAX_LFS_FILESIZE;
    sb->s_blocksize = PAGE_CACHE_SIZE;
    sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
    sb->s_magic = RAMFS_MAGIC;
    sb->s_op = &ramfs_ops;
    //static const struct super_operations ramfs_ops;
    sb->s_time_gran = 1;
    //建立根目录索引节点，我们最终的目标是要找到目录项对象关联的索引节点。
    //根目录索引节点会有自己的ops。
    inode = ramfs_get_inode(sb, S_IFDIR | 0755, 0); 
    //ramfs_get_inode
    kernel/fs/ramfs/inode.c
    struct inode *ramfs_get_inode(struct super_block *sb, int mode, dev_t dev)
    {
      struct inode * inode = new_inode(sb);
      switch (mode & S_IFMT) {  //判断文件类型
        default:
	  init_special_inode(inode, mode, dev);
	  //init_special_inode
          void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
          {
	    inode->i_mode = mode;
	    if (S_ISCHR(mode)) {//字符设备文件
	      inode->i_fop = &def_chr_fops;
	      inode->i_rdev = rdev;
	    } else if (S_ISBLK(mode)) {//块设备文件
	      inode->i_fop = &def_blk_fops;
	      inode->i_rdev = rdev;
	    } else if (S_ISFIFO(mode))
	      inode->i_fop = &def_fifo_fops;
	    else if (S_ISSOCK(mode)) //网络设备文件
	      inode->i_fop = &bad_sock_fops;
	    else
	      printk(KERN_DEBUG "init_special_inode: bogus i_mode (%o) for"
				  " inode %s:%lu\n", mode, inode->i_sb->s_id,
				  inode->i_ino);
           }
          //init_special_inode end
          break;
        case S_IFREG: //普通文件
          inode->i_op = &ramfs_file_inode_operations;  //索引节点的操作方法
          inode->i_fop = &ramfs_file_operations;  //缺省普通文件的操作方法
          break;
        case S_IFDIR:  //目录文件
          inode->i_op = &ramfs_dir_inode_operations;
          //ramfs_dir_inode_operations
          static const struct inode_operations ramfs_dir_inode_operations;
          kernel/include/linux/fs.h
          struct inode_operations {
            int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
            int (*mkdir) (struct inode *,struct dentry *,int);
	    int (*rmdir) (struct inode *,struct dentry *);
	    int (*mknod) (struct inode *,struct dentry *,int,dev_t);
          }
          //ramfs_dir_inode_operations end
          inode->i_fop = &simple_dir_operations;  //目录文件的操作方法
          inc_nlink(inode);
          break;
      }
    }
    //ramfs_get_inode end
    //建立根目录目录对象，目录项对象的存在主要是为了我们进行路径的查找。
    root = d_alloc_root(inode);   
    //d_alloc_root
    kernel/fs/dcache.c
    struct dentry * d_alloc_root(struct inode * root_inode)
    {
      struct dentry *res = NULL;
      static const struct qstr name = { .name = "/", .len = 1 };
      res = d_alloc(NULL, &name);
      res->d_sb = root_inode->i_sb; //指向该文件系统的超级块
      res->d_parent = res;  //根目录的父亲是它自己
      d_instantiate(res, root_inode); //关联 dentry 和 inode
    }
    //d_alloc_root end
    sb->s_root = root;  //超级块的s_root指向刚建立的根目录对象。
  }
  */
  return simple_set_mnt(mnt, s); //关联超级块(包含目录项dentry和i节点inode)和vfsmount
}

kernel/fs/namespace.c

int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
  printk("TK-------_>>>>>>>namespace.c>>>>simple_set_mnt\n");//add by tankai
  mnt->mnt_sb = sb;  //对 mnt_sb超级块指针附值
  mnt->mnt_root = dget(sb->s_root); //对mnt_root指向的根目录赋值
  return 0;
}

至此，rootfs文件系统建立、并且挂载于自己超级块(包括目录项dentry和i节点inod)对应的目录项，设置了系统current根目录和根文件系统、pwd的目录和文件系统。

========================================

释放Initramfs到rootfs；如果Initramfs中有init，这种情况比较特殊、rootfs就是最后系统使用的根文件系统。

而且此时，不需要在单独烧录根文件系统的img；此时，根文件系统就是内核uImage的一部分。当然，缺陷就是该文件系统运行时的介质是ramdisk即内存盘、它不再与磁盘对应；因此，此时修改根目录下的文件将不被得到保存。它的内核配置项为：CONFIG_INITRAMFS_SOURCE。实际项目中会经常碰到。

make menuconfig->General setup->Initial RAM filesystem and RAM disk(initramfs/initrd) support
底下的Initramfs source file(s)填写根文件系统的路径，如：../out/target/product/tclm6/root；不填的话，将导致initrd或磁盘文件系统的挂载（因为下边将会看到，内核将找不到“/init”）。

对应内核源码：

kernel/init/main.c

static int __init kernel_init(void * unused){
  ......
  do_basic_setup();  //初始化设备驱动，加载静态内核模块；释放Initramfs到rootfs
  /*
  kernel/init/initramfs.c
  rootfs_initcall(populate_rootfs);
  static int __init populate_rootfs(void)
  {
    printk(KERN_INFO "checking if image is initramfs...");
    err = unpack_to_rootfs((char *)initrd_start,
                        initrd_end - initrd_start, 1); //释放ramdisk到rootfs
  }
  */
  ......
  if (!ramdisk_execute_command)  ramdisk_execute_command = "/init";
  if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
    ramdisk_execute_command = NULL;
    //如果此时rootfs中没有init，则加载initfd、nfs或磁盘文件系统
    //也即磁盘的文件系统挂载至rootfs的/root目录，并设置系统current对应的根目录项为磁盘根目录项、系统current根文件系统为磁盘文件系统
    //至此，rootfs对于以后所有进程而言、已被隐藏。
    prepare_namespace(); 
  }
  init_post(); //启动init进程
  ......
}

看看init_post实现：

static noinline int init_post(void)
{
  if (ramdisk_execute_command) {  //Initramfs从这里启动init
    run_init_process(ramdisk_execute_command);
    printk(KERN_WARNING "Failed to execute %s\n", ramdisk_execute_command);
  }
  //initrd、nfs和磁盘都是从如下启动的init
  if (execute_command) {
    run_init_process(execute_command);
    printk(KERN_WARNING "Failed to execute %s.  Attempting "
					"defaults...\n", execute_command);
  }
  //一般执行如下
  run_init_process("/sbin/init");
  run_init_process("/etc/init");
  run_init_process("/bin/init");
  run_init_process("/bin/sh");
}

四、挂载实际文件系统至rootfs，并调用set_fs_root设置为系统current的根文件系统

下边从uboot启动内核参数的角度来简单说明：

以下三种情况都是将文件系统挂载到rootfs的/root目录，并将系统current的根目录切换为/root、系统current的根文件系统切换为磁盘文件系统。

kernel/init/do_mounts.c

void __init prepare_namespace(void)
{
  if (initrd_load()) //如果挂载initrd并执行成功，则不再挂载磁盘文件系统
    goto out;
  if (saved_root_name[0]) {
    root_device_name = saved_root_name;
    if (!strncmp(root_device_name, "mtd", 3) ||
        !strncmp(root_device_name, "ubi", 3)) {
      mount_block_root(root_device_name, root_mountflags); //启动时root=参数，如《四.2》中“root=/dev/mtdblock0”
      goto out;
    }
    ROOT_DEV = name_to_dev_t(root_device_name);
    if (strncmp(root_device_name, "/dev/", 5) == 0)
      root_device_name += 5;
  }
  mount_root(); //将实际文件系统挂载到rootfs的/root目录
out:
  //sys_mount(".", "/", NULL, MS_MOVE, NULL); 这句话无关紧要，影响理解；屏蔽不影响功能
  sys_chroot(".");  //将当前目录(/root)设置为系统current根目录，磁盘文件系统设置为系统current根文件系统。
}

下边分两步解释mount_root()和sys_chroot(".")调用：

1.将nfs或磁盘文件系统挂载至rootfs的/root目录（以磁盘为例）

void __init mount_root(void)
{
  if (mount_nfs_root())  //如果网络文件系统挂载成功，则nfs作为根文件系统
    return;
  //挂载磁盘文件系统为根文件系统
  //在rootfs中建立/dev/root设备文件
  create_dev("/dev/root", ROOT_DEV);  //在rootfs中建立/dev/root设备文件，也就是/dev/mtdblock0设备。
  //挂载/dev/root到rootfs的/root目录
  mount_block_root("/dev/root", root_mountflags);
}
void __init mount_block_root(char *name, int flags)
{
  int err = do_mount_root(name, p, flags, root_mount_data);  
}
static int __init do_mount_root(char *name, char *fs, int flags, void *data)
{
  int err = sys_mount(name, "/root", fs, flags, data);//将/dev/root挂载到/root
  sys_chdir("/root"); //系统current->fs->pwd为当前目录/root
  ROOT_DEV = current->fs->pwd.mnt->mnt_sb->s_dev;
  return 0;
}

2.将当前目录/root设置为系统current根目录，磁盘文件系统设置为系统current根文件系统

分析sys_chroot(".")：见Linux内核编程之C语言预处理功能与宏

fs/open.c

SYSCALL_DEFINE1(chroot, const char __user *, filename)
{
  struct path path;
  error = user_path_dir(filename, &path);
  //这才是完成切换的关键！！！！整个内核代码只有两处调用
  set_fs_root(current->fs, &path); 
}

注意，如下情况：rootfs特殊文件系统没有被卸载，他只是隐藏在基于磁盘的根文件系统下了。

initrd作为根文件系统

setenv bootargs root=/dev/ram0 initrd=0x2800000,24M rootfstype=ext2 mem=64M console=ttyAMA0

参数说明：

root：用来指定rootfs的位置。

rootfstype：用来指定文件系统的类型。

nfs作为根文件系统

setenv bootargs root=/dev/nfs nfsroot=192.168.1.7:/opt/yz/nfs,rw ip=192.168.1.160 mem=64M console=ttyAMA0

参数说明：

nfsroot：文件系统在哪台主机的哪个目录下。

ip：指定系统启动之后网卡的ip地址。

flash作为根文件系统

setenv bootargs root=/dev/mtdblock0 mem=16M mtdparts=armflash.1:4M@0x400000(jffs2) macaddr=9854 rootfstype=jffs2 console=ttyAMA0

参数说明：

mtdparts：根文件系统在flash中的位置。

总结：rootfs永远不会被卸载，它只是被隐藏了。在用户空间下，更多地情况是只能见到rootfs这棵大树的一叶，而且还是被安装过文件系统了的。

五、其他说明

至于在mirco2440下mount出的结果：

rootfs on / type rootfs (rw)
/dev/root on / type yaffs (rw,relatime)
none on /proc type proc (rw,relatime)
none on /sys type sysfs (rw,relatime)
none on /proc/bus/usb type usbfs (rw,relatime)
none on /dev type ramfs (rw,relatime)
none on /dev/pts type devpts (rw,relatime,mode=622)
tmpfs on /dev/shm type tmpfs (rw,relatime)
none on /tmp type ramfs (rw,relatime)
none on /var type ramfs (rw,relatime)

从log中体会一下这个过程：

s3c2410-rtc s3c2410-rtc: setting system clock to 2006-04-16 22:15:34 UTC (1145225734)
TK------->>>>>init/main.c>>>>>>kernel_init>>before>prepare_namespace
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is rootfs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is rootfs
##################################################################################################################
TK------->>>>>init/do_mounts.c>>>>>>prepare_namespace>>before>mount_root
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is rootfs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is rootfs
##################################################################################################################
yaffs: dev is 32505859 name is "mtdblock3"
yaffs: passed flags ""
yaffs: Attempting MTD mount on 31.3, "mtdblock3"
yaffs_read_super: isCheckpointed 0
VFS: Mounted root (yaffs filesystem) on device 31:3.
TK------->>>>>init/do_mounts.c>>>>>>prepare_namespace>>before>sys_mount
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is rootfs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
TK------->>>>>init/do_mounts.c>>>>>>prepare_namespace>>before>sys_chroot
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is rootfs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
TK------->>>>>fs/open.c>>>>>>SYSCALL_DEFINE1(chroot>>before>set_fs_root
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is rootfs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
TK------->>>>>init/do_mounts.c>>>>>>SYSCALL_DEFINE1(chroot>>after>set_fs_root
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
TK------->>>>>init/do_mounts.c>>>>>>prepare_namespace>>after>sys_chroot
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
TK------->>>>>init/main.c>>>>>>kernel_init>>after>prepare_namespace
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
##################################################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is root
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is yaffs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs
##################################################################################################################
Freeing init memory: 156K
[16/Apr/2006:14:15:35 +0000] boa: server version Boa/0.94.13
[16/Apr/2006:14:15:35 +0000] boa: server built Mar 26 2009 at 15:28:42.
[16/Apr/2006:14:15:35 +0000] boa: starting server pid=681, port 80
                        
Try to bring eth0 interface up......eth0: link down
Done

Please press Enter to activate this console. 
[root@FriendlyARM /]# mount
rootfs on / type rootfs (rw)
/dev/root on / type yaffs (rw,relatime)
none on /proc type proc (rw,relatime)
none on /sys type sysfs (rw,relatime)
none on /proc/bus/usb type usbfs (rw,relatime)
none on /dev type ramfs (rw,relatime)
none on /dev/pts type devpts (rw,relatime,mode=622)
tmpfs on /dev/shm type tmpfs (rw,relatime)
none on /tmp type ramfs (rw,relatime)
none on /var type ramfs (rw,relatime)
[root@FriendlyARM /]# 

ubuntu下mount出的结果：

/dev/sda5 on / type ext3 (rw,errors=remount-ro,commit=0)
proc on /proc type proc (rw,noexec,nosuid,nodev)
sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
fusectl on /sys/fs/fuse/connections type fusectl (rw)
none on /sys/kernel/debug type debugfs (rw)
none on /sys/kernel/security type securityfs (rw)
udev on /dev type devtmpfs (rw,mode=0755)
devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
none on /run/shm type tmpfs (rw,nosuid,nodev)
/dev/sda7 on /home type ext3 (rw,commit=0)
binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,noexec,nosuid,nodev)
rpc_pipefs on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw)
nfsd on /proc/fs/nfsd type nfsd (rw)
gvfs-fuse-daemon on /home/tankai/.gvfs type fuse.gvfs-fuse-daemon (rw,nosuid,nodev,user=tankai)

    一个还显示rootfs，一个干脆不显示。这个无关紧要。可能micro2440中执行mount命令还会将系统current根目录的父节点也显示出来；而ubuntu下不会再去关心系统current根目录的父节点。但所有的文件搜索，内核都是从系统current根目录开始向下查找的，因此、可以说我们不能在访问rootfs中除了作为系统current根文件系统之外的其他任何节点。

六、测试用例，说明系统current的文件系统布局，不管在那个目录、其根都不会改变：

hello.c

#include <linux/syscalls.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/smp_lock.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/acct.h>
#include <linux/capability.h>
#include <linux/cpumask.h>
#include <linux/module.h>
#include <linux/sysfs.h>
#include <linux/seq_file.h>
#include <linux/mnt_namespace.h>
#include <linux/namei.h>
#include <linux/security.h>
#include <linux/mount.h>
#include <linux/ramfs.h>
#include <linux/log2.h>
#include <linux/idr.h>
#include <asm/uaccess.h>
#include <asm/unistd.h>
//#include "pnode.h"
//#include "internal.h"


#include <linux/init.h>
#include <linux/module.h>
MODULE_LICENSE("Dual BSD/GPL");

static int hello_init(void)
{
    printk(KERN_ALERT "Hello, world\n");
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is %s\n",current->fs->root.dentry->d_name.name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is %s\n",current->fs->root.dentry->d_sb->s_type->name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is %s\n",current->fs->root.mnt->mnt_mountpoint->d_name.name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is %s\n",current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is %s\n",current->fs->root.mnt->mnt_root->d_name.name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is %s\n",current->fs->root.mnt->mnt_root->d_sb->s_type->name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is %s\n",current->fs->root.mnt->mnt_sb->s_type->name);
    printk("########################################################################################\n");
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is %s\n",current->fs->pwd.dentry->d_name.name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is %s\n",current->fs->pwd.dentry->d_sb->s_type->name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is %s\n",current->fs->pwd.mnt->mnt_mountpoint->d_name.name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is %s\n",current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is %s\n",current->fs->pwd.mnt->mnt_root->d_name.name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is %s\n",current->fs->pwd.mnt->mnt_root->d_sb->s_type->name);
    printk("TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is %s\n",current->fs->pwd.mnt->mnt_sb->s_type->name);
    return 0;
}

static void hello_exit(void)
{
    printk(KERN_ALERT"Goodbye, cruel world\n");
}

module_init(hello_init);
module_exit(hello_exit);

Makefile

ifneq ($(KERNELRELEASE),)
obj-m:=hello.o
else
KERNELDIR:=/home/android2.3/android2.3_kernel/
PWD:=$(shell pwd)
default:
	$(MAKE) -C $(KERNELDIR)  M=$(PWD) modules
clean:
	rm -rf *.o *.mod.c *.mod.o *.ko
endif

make生成hello.ko

先看下文件系统布局：

mount

rootfs / rootfs ro 0 0
tmpfs /dev tmpfs rw,mode=755 0 0
devpts /dev/pts devpts rw,mode=600 0 0
proc /proc proc rw 0 0
sysfs /sys sysfs rw 0 0
none /acct cgroup rw,cpuacct 0 0
tmpfs /mnt/asec tmpfs rw,mode=755,gid=1000 0 0
tmpfs /mnt/obb tmpfs rw,mode=755,gid=1000 0 0
none /dev/cpuctl cgroup rw,cpu 0 0
/dev/block/mtdblock0 /system yaffs2 rw 0 0
/dev/block/mtdblock1 /data yaffs2 rw,nosuid,nodev 0 0
/dev/block/mtdblock2 /cache yaffs2 rw,nosuid,nodev 0 0
/dev/block/vold/179:0 /mnt/sdcard vfat rw,dirsync,nosuid,nodev,noexec,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
/dev/block/vold/179:0 /mnt/secure/asec vfat rw,dirsync,nosuid,nodev,noexec,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
tmpfs /mnt/sdcard/.android_secure tmpfs ro,size=0k,mode=000 0 0

1.放入/data/下运行insmod hello.ko rmmod hello.ko

Hello, world
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is rootfs
########################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is yaffs2
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is data
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is yaffs2
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs2
Goodbye, cruel world

2.放在/sdcard/tank/下运行insmod hello.ko rmmod hello.ko

Hello, world
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.dentry->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_root->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_sb->s_type->name is rootfs
########################################################################################
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_name.name is tank
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is vfat
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is sdcard
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfs
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_name.name is /
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_root->d_sb->s_type->name is vfat
TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is vfat
Goodbye, cruel world

由此证明；current->fs->root就是系统承认的根文件系统。