| .. 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. |
