| .. 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: | |
| ============= ================ =============== =============================== | |
| Offset Type Name Description | |
| ============= ================ =============== =============================== | |
| 0x0 Array of Orphan inode Each __le32 entry is either | |
| __le32 entries entries 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. |