Documentation/filesystems/ext4/orphan.rst

.. SPDX-License-Identifier: GPL-2.0

Orphan file
-----------

In unix there can inodes that are unlinked from directory hierarchy but that
are still alive because they are open. In case of crash the filesystem has to
clean up these inodes as otherwise they (and the blocks referenced from them)
would leak. Similarly if we truncate or extend the file, we need not be able
to perform the operation in a single journalling transaction. In such case we
track the inode as orphan so that in case of crash extra blocks allocated to
the file get truncated.

Traditionally ext4 tracks orphan inodes in a form of single linked list where
superblock contains the inode number of the last orphan inode (s\_last\_orphan
field) and then each inode contains inode number of the previously orphaned
inode (we overload i\_dtime inode field for this). However this filesystem
global single linked list is a scalability bottleneck for workloads that result
in heavy creation of orphan inodes. When orphan file feature
(COMPAT\_ORPHAN\_FILE) is enabled, the filesystem has a special inode
(referenced from the superblock through s\_orphan_file_inum) with several
blocks. Each of these blocks has a structure:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - Array of \_\_le32 entries
     - Orphan inode entries
     - Each \_\_le32 entry is either empty (0) or it contains inode number of
       an orphan inode.
   * - blocksize - 8
     - \_\_le32
     - ob\_magic
     - Magic value stored in orphan block tail (0x0b10ca04)
   * - blocksize - 4
     - \_\_le32
     - ob\_checksum
     - Checksum of the orphan block.

When a filesystem with orphan file feature is writeably mounted, we set
RO\_COMPAT\_ORPHAN\_PRESENT feature in the superblock to indicate there may
be valid orphan entries. In case we see this feature when mounting the
filesystem, we read the whole orphan file and process all orphan inodes found
there as usual. When cleanly unmounting the filesystem we remove the
RO\_COMPAT\_ORPHAN\_PRESENT feature to avoid unnecessary scanning of the orphan
file and also make the filesystem fully compatible with older kernels.