mount

如果一种文件系统的类型曾经在内核注册过，这就说明允许挂载并且使用这个文件系统。

register_filesystem(&ext4_fs_type);

static struct file_system_type ext4_fs_type = {
  .owner    = THIS_MODULE,
  .name    = "ext4",
  .mount    = ext4_mount,
  .kill_sb  = kill_block_super,
  .fs_flags  = FS_REQUIRES_DEV,
};

SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name, char __user *, type, unsigned long, flags, void __user *, data)
{
......
  ret = do_mount(kernel_dev, dir_name, kernel_type, flags, options);
......
}

//接下来的调用链为：do_mount->do_new_mount->vfs_kern_mount

struct vfsmount *
vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
{
......
  mnt = alloc_vfsmnt(name);
......
  root = mount_fs(type, flags, name, data);
......
  mnt->mnt.mnt_root = root;
  mnt->mnt.mnt_sb = root->d_sb;
  mnt->mnt_mountpoint = mnt->mnt.mnt_root;
  mnt->mnt_parent = mnt;
  list_add_tail(&mnt->mnt_instance, &root->d_sb->s_mounts);
  return &mnt->mnt;
}

vfs_kern_mount 先是创建 struct mount 结构，每个挂载的文件系统都对应于这样一个结构。

struct mount {
  struct hlist_node mnt_hash;
  struct mount *mnt_parent; // 装载点所在的父文件系统
  struct dentry *mnt_mountpoint; // 装载点在父文件系统中的dentry
  struct vfsmount mnt;
  union {
    struct rcu_head mnt_rcu;
    struct llist_node mnt_llist;
  };
  struct list_head mnt_mounts;  /* list of children, anchored here */
  struct list_head mnt_child;  /* and going through their mnt_child */
  struct list_head mnt_instance;  /* mount instance on sb->s_mounts */
  const char *mnt_devname;  /* Name of device e.g. /dev/dsk/hda1 */
  struct list_head mnt_list;
......
} __randomize_layout;

struct vfsmount {
  // 当前文件系统根目录的dentry
  struct dentry *mnt_root;  /* root of the mounted tree */
  struct super_block *mnt_sb;  /* pointer to superblock */
  int mnt_flags;
} __randomize_layout;

调用 mount_fs 挂载文件系统。

struct dentry *
mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
{
  struct dentry *root;
  struct super_block *sb;
......
  root = type->mount(type, flags, name, data); //比如ext4_mount
......
  sb = root->d_sb;
......
}