CS433 Homework 9 on Creating Your Own File System

Andrew Sohn, Computer Science Department, New Jersey Institute of Technology

• Write a module to create a small file system called "myfs" that does only a few operations including read and write. First, define your file system type using:

  static struct file_system_type myfs_type = {
  	.owner 		= THIS_MODULE,
  	.name		= "myfs",
          .mount          = myfs_mount,
  	.kill_sb	= kill_block_super,
          .fs_flags       = FS_REQUIRES_DEV,
  };
  

  See static struct file_system_type ext4_fs_type defined in fs/ext4/super.c as an example.
• Write a module to register/unregister your own file system:

  module_init(myfs_init_fs)
  module_exit(mtfs_exit_fs)
  

  myfs_init_fs will register myfs_type defined above while myfs_exit_fs will unregister.
  See module_init(ext4_init_fs) and module_exit(ext4_exit_fs) defined in fs/ext4/super.c as an example. 3318-4292
• Implement myfs_mount() defined in myfs_type just like ext4_mount, except you need to pass your own func pointer myfs_fill_super which will be called in mount_bdev().
• As we discussed in class, global variable super_blocks holds a list of super blocks using list_head and mount_bdev() gets a superblock which needs to be filled by myfs_fill_super. Hence, you need to implement myfs_fill_super(). Start with simple_fill_super() as an example to implement your own. ext4_fill_super is almost 1000 lines which you don't want to bother at this moment.
• Add your own super_operations to super_block and again the starting point is simple_super_operations while "super_operations ext4_sops" exists for ext4. simple_super_operations has nothing much in it so you need to add a couple of fields such as .alloc_inode by consulting ext4_ops.

  static struct super_operations myfs_super_operations = {
          .statfs = simple_statfs,
          .alloc_inode = ext4_alloc_inode,
  };
  

• Add inode_operations to super_block as well. Again as simple_dir_inode_operations has not much in it, you need to check with ext4_ops to add a few fields.

  static struct inode_operations myfs_inode_operations = {
          .lookup = simple_lookup,
          .link = simple_link,
          .mkdir = simple_mkdir,
  };
  

• Do similiar for basic file_operations for myfs_file_ops with a couple of additional fields:

  static struct file_operations myfs_file_operations = {
  	.open	= myfs_open,
  	.read 	= myfs_read_file,
  	.write  = myfs_write_file,
  };
  

• Don't forget to put in a magic number to distinguish your file system with others. See #define EXT4_SUPER_MAGIC 0xEF53 as an example.

  #define MYFS_SUPER_MAGIC 0x???? 

• Afer all is said and done, /sbin/insmod the ko file and "mount -t myfs myfs /mnt";.
• Issue the following commands:
  • cat /proc/filesystems;
  • ls -l /mnt;
  • mount;
  • cat /mnt/myfile0;
  • stat -f myfile0;
  • ln /mnt/myfile0 /mnt/link0 ...
• Show all the output results including linking