《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_heads_list;/* Keep this first */  dev_ts_dev;/* search index; _not_ kdev_t */  unsigned longs_blocksize;  unsigned chars_blocksize_bits;  unsigned chars_dirt;  unsigned long longs_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 longs_flags;  unsigned longs_magic;  struct dentry*s_root;  //超级块要指向目录项对象  struct rw_semaphores_umount;  struct mutexs_lock;  ints_count;  ints_need_sync_fs;  atomic_ts_active;#ifdef CONFIG_SECURITY  void                    *s_security;#endif  struct xattr_handler**s_xattr;  struct list_heads_inodes;/* all inodes */  struct list_heads_dirty;/* dirty inodes */  struct list_heads_io;/* parked for writeback */  struct list_heads_more_io;/* parked for more writeback */  struct hlist_heads_anon;//哈希表头/* anonymous dentries for (nfs) exporting */  struct list_heads_files;  /* s_dentry_lru and s_nr_dentry_unused are protected by dcache_lock */  struct list_heads_dentry_lru;/* unused dentry lru */  ints_nr_dentry_unused;/* # of dentry on lru */  struct block_device*s_bdev;  struct mtd_info*s_mtd;  struct list_heads_instances;  struct quota_infos_dquot;/* Diskquota specific options */  ints_frozen;  wait_queue_head_ts_wait_unfrozen;  char s_id[32];/* Informational name */  void *s_fs_info;/* Filesystem private info */  fmode_ts_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_nodei_hash; //哈希表节点  struct list_headi_list;  struct list_headi_sb_list;  struct list_headi_dentry;  unsigned longi_ino;  atomic_ti_count;  unsigned inti_nlink;  uid_ti_uid;  gid_ti_gid;  dev_ti_rdev;  u64i_version;  loff_ti_size;#ifdef __NEED_I_SIZE_ORDERED  seqcount_ti_size_seqcount;#endif  struct timespeci_atime;  struct timespeci_mtime;  struct timespeci_ctime;  unsigned inti_blkbits;  blkcnt_ti_blocks;  unsigned short          i_bytes;  umode_ti_mode;  spinlock_ti_lock;/* i_blocks, i_bytes, maybe i_size */  struct mutexi_mutex;  struct rw_semaphorei_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_spacei_data;#ifdef CONFIG_QUOTA  struct dquot*i_dquot[MAXQUOTAS];#endif  struct list_headi_devices;  union {    struct pipe_inode_info*i_pipe;    struct block_device*i_bdev;    struct cdev*i_cdev;  };  inti_cindex;  __u32i_generation;#ifdef CONFIG_DNOTIFY  unsigned longi_dnotify_mask; /* Directory notify events */  struct dnotify_struct*i_dnotify; /* for directory notifications */#endif#ifdef CONFIG_INOTIFY  struct list_headinotify_watches; /* watches on this inode */  struct mutexinotify_mutex;/* protects the watches list */#endif  unsigned longi_state;  unsigned longdirtied_when;/* jiffies of first dirtying */  unsigned inti_flags;  atomic_ti_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_headfu_list;    struct rcu_head fu_rcuhead;  } f_u;  struct pathf_path;  //重要！！！记录挂载信息和目录项信息#define f_dentryf_path.dentry#define f_vfsmntf_path.mnt  const struct file_operations*f_op;  atomic_long_tf_count;  unsigned int f_flags;  fmode_tf_mode;  loff_tf_pos;  struct fown_structf_owner;  const struct cred*f_cred;  struct file_ra_statef_ra;  u64f_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_headf_ep_links;  spinlock_tf_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 pathpath;  //重要！！！记录挂载信息和目录项信息  struct qstrlast;  //重要！！！记录目录名  unsigned intflags;  intlast_type;  unsigneddepth;  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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 0VFS: 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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->root.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 yaffsTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is rootTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffsTK---->>>>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 downDonePlease press Enter to activate this console. [root@FriendlyARM /]# mountrootfs 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 ($(KERNELRELEASE),)obj-m:=hello.oelseKERNELDIR:=/home/android2.3/android2.3_kernel/PWD:=$(shell pwd)default:$(MAKE) -C $(KERNELDIR)  M=$(PWD) modulesclean:rm -rf *.o *.mod.c *.mod.o *.koendif

make生成hello.ko

         先看下文件系统布局：

mount

rootfs / rootfs ro 0 0tmpfs /dev tmpfs rw,mode=755 0 0devpts /dev/pts devpts rw,mode=600 0 0proc /proc proc rw 0 0sysfs /sys sysfs rw 0 0none /acct cgroup rw,cpuacct 0 0tmpfs /mnt/asec tmpfs rw,mode=755,gid=1000 0 0tmpfs /mnt/obb tmpfs rw,mode=755,gid=1000 0 0none /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 0tmpfs /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, worldTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 yaffs2TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is dataTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 yaffs2TK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is yaffs2Goodbye, cruel world

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

Hello, worldTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 rootfsTK---->>>>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 tankTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.dentry->d_sb->s_type->name is vfatTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_name.name is sdcardTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_mountpoint->d_sb->s_type->name is rootfsTK---->>>>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 vfatTK---->>>>init/main.c>>>kernel_init>>>>current->fs->pwd.mnt->mnt_sb->s_type->name is vfatGoodbye, cruel world

         由此证明；current->fs->root就是系统承认的根文件系统。