《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：

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#define current (get_current())

static inline struct task_struct *get_current(void)

include/linux/sched.h

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

[cpp] view
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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

[cpp] view
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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：

[cpp] view
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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

[cpp] view
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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）

[cpp] view
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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

[cpp] view
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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：

[plain] view
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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：

[plain] view
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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：

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/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

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#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

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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

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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

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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

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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