ext4: Orphan file documentation

Add documentation about the orphan file feature.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210816095713.16537-4-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 52 insertions, 0 deletions

diff --git a/Documentation/filesystems/ext4/orphan.rst b/Documentation/filesystems/ext4/orphan.rst
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+.. 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.