How to configure special root filesystem on boot( c.f) Understanding the Linux Kernel 2.6.11)

special_rootfs.yml

# start_kernel() -> vfs_caches_init() -> mnt_init() -> init_rootfs, init_mount_tree の中身
- init_rootfs(): register_filesystem(&rootfs_fs_type); append (struct file_system)rootfs_fs_type global variable to (struct file_system*)file_systems.
  - find_filesystem("rootfs") and return the end of `(struct file_system*)file_systems`
  - append (struct file_system)rootfs_fs_type to the previous result
- init_mount_tree: create `struct vfsmount` element
  - vfsmount = do_kern_mount(fstype="rootfs", flags=0, name="rootfs", data=NULL)
    - get_fs_type("rootfs"): get (struct file_system*)rootfs_fs_type
    - vfsmount = alloc_vfsmnt("rootfs"): allocate&initialize `struct vfsmount` element
      - vfsmount = kmem_cache_alloc(mnt_cache, GFP_KERNEL): allocate `struct vfsmount`
      - initialize vfsmount->mnt_list: (later, link to `namespace.list` as the head)
      - mnt->mnt_devname = "rootfs"
    - super_block = type->get_sb(type, flags, name, data): (=rootfs_get_sb) allocate&configure `struct super_block` element
      - get_sb_nodev(rootfs_fs_type, MS_NOUSER, NULL, ramfs_fill_super):
        - super_block = sget(rootfs_fs_type, NULL, set_anon_super, NULL): alloc &  configure`struct super_block`.
          - super_block = alloc_super(): allocate and initialize `struct super_block`
            - super_block = kmalloc(sizeof(struct super_block),  GFP_USER): allocate `struct super_block`
            - initialize super_block->s_(files|instances|inodes|anon) and so on
          - set_anon_super(super_block, data: void*): fill in super_block->s_dev(device number).
            - idr_pre_get(&unnamed_dev_idr, GFP_ATOMIC): unnamed_dev_idr is `struct idr`
              - idr_layer = kmem_cache_alloc(idr_layer_cache, GFP_ATOMIC);
              - free_layer(unnamed_dev_idr, idr_layer): link idr_layer to unnamed_dev_idr
            - idr_get_new(&unnamed_dev_idr, NULL, &dev): get newid and put it in dev
              - idr_get_new_above_int(unnamed_dev_idr, ptr, 0)
                - unnamed_dev_idr->top = alloc_layer(unnamed_dev_idr)
                - newid = sub_alloc(unnamed_dev_idr, NULL, &0): get unique id
            - super_block->s_dev = MKDEV(major=0, minor=(dev & 0xFFF));
          - super_block->s_type = rootfs_fs_type;
          - list_add_tail(&super_block->s_list, &super_blocks); (struct list_head)super_blocks
          - list_add(&super_block->s_instances, &rootfs_fs_type->fs_supers);
        - ramfs_fill_super(super_block, (void*)data=NULL, (int)silent=0): (silent is not used)
          - root_inode = ramfs_get_inode(super_block, S_IFDIR | 0755, 0): allocate & initialize `struct inode`
            - root_inode = alloc_inode(super_block);
              - root_inode = (struct inode *)kmem_cache_alloc(inode_cachep, SLAB_KERNEL);
                - INIT_LIST_HEAD(&root_inode->i_dentry); && INIT_HLIST_NODE(&root_inode->i_hash): exec init_once() -> inode_init_once() as ctor
              - root_inode->i_sb = super_block;
              - root_inode->i_mapping = root_inode->i_data: root_inode->i_data is `struct address_space` object and initialize
              - initialize inode object
            - list_add(&root_inode->i_list, &inode_in_use);
            - list_add(&root_inode->i_sb_list, &sb->s_inodes);
          - struct dentry* dentry_root = d_alloc_root(root_inode): allocate & initialize `struct dentry`
            - dentry_root = d_alloc(NULL, &name="/"):
              - dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL):
              - initialize dentry->(d_inode|d_parent|d_sb|d_hash|d_lru|d_subdirs|d_alias|dentry_child)
            - dentry_root->d_sb = root_inode->i_sb(=super_block) && dentry_root->d_parent=dentry_root
            - d_instantiate(dentry_root, root_inode):
              - list_add(&dentry->d_alias, &inode->i_dentry): d_alias is “in use” dentry object
              - entry->d_inode = root_inode: d_inode is "unused" dentry object
          - super_block->s_root = (struct dentry*)root: (later, link to `vfsmount->mnt_root` which is the root directory of the filesystem)
    - vfsmount->mnt_(sb|root|mount_point) = (super_block|super_block->s_root|super_block->s_root)
    - vfsmount->mnt_namespace = init_task->namespace;
  - namespace = kmalloc(sizeof(*namespace), GFP_KERNEL): alloc element of `struct namespace`
  - link between vfsmount(root_mount) and namespace
  - (struct task_struct)(init_task).namespace = namespace;
  - (struct vfsmount)current->fs->pwdmnt = namespace->root
  - (struct dentry)current->fs->pwd = namespace->root->mnt_root

This is a overview image!