ext4: Orphan file documentation

Add documentation about the orphan file feature.

Reviewed-by: Theodore Ts'o <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>

Documentation/filesystems/ext4/globals.rst

···
 .. include:: bitmaps.rst
 .. include:: mmp.rst
 .. include:: journal.rst
+.. include:: orphan.rst

Documentation/filesystems/ext4/inodes.rst

···
 modification time (mtime), and deletion time (dtime). The four fields
 are 32-bit signed integers that represent seconds since the Unix epoch
 (1970-01-01 00:00:00 GMT), which means that the fields will overflow in
-January 2038. For inodes that are not linked from any directory but are
-still open (orphan inodes), the dtime field is overloaded for use with
-the orphan list. The superblock field ``s_last_orphan`` points to the
-first inode in the orphan list; dtime is then the number of the next
-orphaned inode, or zero if there are no more orphans.
+January 2038. If the filesystem does not have orphan_file feature, inodes
+that are not linked from any directory but are still open (orphan inodes) have
+the dtime field overloaded for use with the orphan list. The superblock field
+``s_last_orphan`` points to the first inode in the orphan list; dtime is then
+the number of the next orphaned inode, or zero if there are no more orphans.
 
 If the inode structure size ``sb->s_inode_size`` is larger than 128
 bytes and the ``i_inode_extra`` field is large enough to encompass the

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.

Documentation/filesystems/ext4/special_inodes.rst

···
    * - 11
      - Traditional first non-reserved inode. Usually this is the lost+found directory. See s\_first\_ino in the superblock.
 
+Note that there are also some inodes allocated from non-reserved inode numbers
+for other filesystem features which are not referenced from standard directory
+hierarchy. These are generally reference from the superblock. They are:
+
+.. list-table::
+   :widths: 20 50
+   :header-rows: 1
+
+   * - Superblock field
+     - Description
+
+   * - s\_lpf\_ino
+     - Inode number of lost+found directory.
+   * - s\_prj\_quota\_inum
+     - Inode number of quota file tracking project quotas
+   * - s\_orphan\_file\_inum
+     - Inode number of file tracking orphan inodes.

Documentation/filesystems/ext4/super.rst

···
      - Filename charset encoding flags.
    * - 0x280
      - \_\_le32
-     - s\_reserved[95]
+     - s\_orphan\_file\_inum
+     - Orphan file inode number.
+   * - 0x284
+     - \_\_le32
+     - s\_reserved[94]
      - Padding to the end of the block.
    * - 0x3FC
      - \_\_le32
···
        the journal, JBD2 incompat feature
        (JBD2\_FEATURE\_INCOMPAT\_FAST\_COMMIT) gets
        set (COMPAT\_FAST\_COMMIT).
+   * - 0x1000
+     - Orphan file allocated. This is the special file for more efficient
+       tracking of unlinked but still open inodes. When there may be any
+       entries in the file, we additionally set proper rocompat feature
+       (RO\_COMPAT\_ORPHAN\_PRESENT).
 
 .. _super_incompat:
 
···
      - Filesystem tracks project quotas. (RO\_COMPAT\_PROJECT)
    * - 0x8000
      - Verity inodes may be present on the filesystem. (RO\_COMPAT\_VERITY)
+   * - 0x10000
+     - Indicates orphan file may have valid orphan entries and thus we need
+       to clean them up when mounting the filesystem
+       (RO\_COMPAT\_ORPHAN\_PRESENT).
 
 .. _super_def_hash: