1.. SPDX-License-Identifier: GPL-2.0
2
3Orphan file
4-----------
5
6In unix there can inodes that are unlinked from directory hierarchy but that
7are still alive because they are open. In case of crash the filesystem has to
8clean up these inodes as otherwise they (and the blocks referenced from them)
9would leak. Similarly if we truncate or extend the file, we need not be able
10to perform the operation in a single journalling transaction. In such case we
11track the inode as orphan so that in case of crash extra blocks allocated to
12the file get truncated.
13
14Traditionally ext4 tracks orphan inodes in a form of single linked list where
15superblock contains the inode number of the last orphan inode (s\_last\_orphan
16field) and then each inode contains inode number of the previously orphaned
17inode (we overload i\_dtime inode field for this). However this filesystem
18global single linked list is a scalability bottleneck for workloads that result
19in heavy creation of orphan inodes. When orphan file feature
20(COMPAT\_ORPHAN\_FILE) is enabled, the filesystem has a special inode
21(referenced from the superblock through s\_orphan_file_inum) with several
22blocks. Each of these blocks has a structure:
23
24.. list-table::
25   :widths: 8 8 24 40
26   :header-rows: 1
27
28   * - Offset
29     - Type
30     - Name
31     - Description
32   * - 0x0
33     - Array of \_\_le32 entries
34     - Orphan inode entries
35     - Each \_\_le32 entry is either empty (0) or it contains inode number of
36       an orphan inode.
37   * - blocksize - 8
38     - \_\_le32
39     - ob\_magic
40     - Magic value stored in orphan block tail (0x0b10ca04)
41   * - blocksize - 4
42     - \_\_le32
43     - ob\_checksum
44     - Checksum of the orphan block.
45
46When a filesystem with orphan file feature is writeably mounted, we set
47RO\_COMPAT\_ORPHAN\_PRESENT feature in the superblock to indicate there may
48be valid orphan entries. In case we see this feature when mounting the
49filesystem, we read the whole orphan file and process all orphan inodes found
50there as usual. When cleanly unmounting the filesystem we remove the
51RO\_COMPAT\_ORPHAN\_PRESENT feature to avoid unnecessary scanning of the orphan
52file and also make the filesystem fully compatible with older kernels.
53