Linux虛擬文件系統（安裝根文件系統）

安裝根文件系統式系統初始化的關鍵部分。Linux內核允許根文件系統放在很多不同的地方，比如硬盤分區、軟盤、通過NFS共享的遠程文件系統以及保存在ramdisk中。內核要在變量ROOT_DEV中尋找包含根文件系統的磁盤主設備號。當編譯內核時，或者像最初的啟動裝入程序傳遞一個合適的“root”選項時，根文件系統可以被指定為/dev目錄下的一個設備文件。

相關閱讀：

http://www.linuxidc.com/Linux/2012-02/53771.htm

安裝根文件系統分為兩個階段：

1，內核安裝特殊rootfs文件系統，該文件系統僅提供一個作為初始安裝點的空目錄

start_kernel()->vfs_caches_init()->mnt_init()->init_rootfs()

[cpp]

1. /*初始化根文件系統*/
2. int __init init_rootfs(void)
3. {
4. int err;
5. /*初始化ramfs_backing_dev_info*/
6. err = bdi_init(&ramfs_backing_dev_info);
7. if (err)
8. return err;
9. /*注冊rootfs_fs_type文件類型*/
10. err = register_filesystem(&rootfs_fs_type);
11. if (err)/*如果出錯，銷毀上面初始化的*/
12. bdi_destroy(&ramfs_backing_dev_info);
14. return err;
15. }

[cpp]

1. static struct backing_dev_info ramfs_backing_dev_info = {
2. .name = "ramfs",
3. .ra_pages = 0, /* No readahead */
4. .capabilities = BDI_CAP_NO_ACCT_AND_WRITEBACK |
5. BDI_CAP_MAP_DIRECT | BDI_CAP_MAP_COPY |
6. BDI_CAP_READ_MAP | BDI_CAP_WRITE_MAP | BDI_CAP_EXEC_MAP,
7. };

[cpp]

1. /**
2. * register_filesystem - register a new filesystem
3. * @fs: the file system structure
4. *
5. * Adds the file system passed to the list of file systems the kernel
6. * is aware of for mount and other syscalls. Returns 0 on success,
7. * or a negative errno code on an error.
8. *
9. * The &struct file_system_type that is passed is linked into the kernel
10. * structures and must not be freed until the file system has been
11. * unregistered.
12. */
13. /*注冊一個新的文件系統*/
14. int register_filesystem(struct file_system_type * fs)
15. {
16. int res = 0;
17. struct file_system_type ** p;
19. BUG_ON(strchr(fs->name, '.'));
20. if (fs->next)
21. return -EBUSY;
22. INIT_LIST_HEAD(&fs->fs_supers);
23. write_lock(&file_systems_lock);
24. /*從system_type鏈表中查找指定名稱的file_system_type*/
25. p = find_filesystem(fs->name, strlen(fs->name));
26. if (*p)
27. res = -EBUSY;
28. else
29. *p = fs;
30. write_unlock(&file_systems_lock);
31. return res;
32. }

根文件系統定義如下

[cpp]

1. static struct file_system_type rootfs_fs_type = {
2. .name = "rootfs",
3. .get_sb = rootfs_get_sb,
4. .kill_sb = kill_litter_super,
5. };

下面看看他的兩個函數

[cpp]

1. /*獲得根目錄的sb*/
2. static int rootfs_get_sb(struct file_system_type *fs_type,
3. int flags, const char *dev_name, void *data, struct vfsmount *mnt)
4. {
5. return get_sb_nodev(fs_type, flags|MS_NOUSER, data, ramfs_fill_super,
6. mnt);
7. }

[cpp]

1. int get_sb_nodev(struct file_system_type *fs_type,
2. int flags, void *data,
3. int (*fill_super)(struct super_block *, void *, int),
4. struct vfsmount *mnt)
5. {
6. int error;
7. /*獲得sb結構*/
8. struct super_block *s = sget(fs_type, NULL, set_anon_super, NULL);
10. if (IS_ERR(s))
11. return PTR_ERR(s);
13. s->s_flags = flags;
14. /*這裡實際調用ramfs_fill_super,對sb結構的屬性進行設置*/
15. error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
16. if (error) {
17. deactivate_locked_super(s);
18. return error;
19. }
20. s->s_flags |= MS_ACTIVE;
21. simple_set_mnt(mnt, s);/*設置mnt和sb關聯*/
22. return 0;
23. }

[cpp]

1. /**
2. * sget - find or create a superblock
3. * @type: filesystem type superblock should belong to
4. * @test: comparison callback
5. * @set: setup callback
6. * @data: argument to each of them
7. */
8. /*查找或創建一個sb結構*/
9. struct super_block *sget(struct file_system_type *type,
10. int (*test)(struct super_block *,void *),
11. int (*set)(struct super_block *,void *),
12. void *data)
13. {
14. struct super_block *s = NULL;
15. struct super_block *old;
16. int err;
18. retry:
19. spin_lock(&sb_lock);
20. if (test) {
21. list_for_each_entry(old, &type->fs_supers, s_instances) {
22. if (!test(old, data))
23. continue;
24. if (!grab_super(old))
25. goto retry;
26. if (s) {
27. up_write(&s->s_umount);
28. destroy_super(s);
29. }
30. return old;
31. }
32. }
33. if (!s) {/*如果找不到sb,從內存中申請一個*/
34. spin_unlock(&sb_lock);
35. s = alloc_super(type);
36. if (!s)
37. return ERR_PTR(-ENOMEM);
38. goto retry;
39. }
41. err = set(s, data);
42. if (err) {
43. spin_unlock(&sb_lock);
44. up_write(&s->s_umount);
45. destroy_super(s);
46. return ERR_PTR(err);
47. }
48. /*初始化得到的sb結構*/
49. s->s_type = type;
50. strlcpy(s->s_id, type->name, sizeof(s->s_id));
51. /*加入鏈表尾*/
52. list_add_tail(&s->s_list, &super_blocks);
53. list_add(&s->s_instances, &type->fs_supers);
54. spin_unlock(&sb_lock);
55. get_filesystem(type);
56. return s;
57. }