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+.. SPDX-License-Identifier: GPL-2.0
+
+Super Block
+-----------
+
+The superblock records various information about the enclosing
+filesystem, such as block counts, inode counts, supported features,
+maintenance information, and more.
+
+If the sparse_super feature flag is set, redundant copies of the
+superblock and group descriptors are kept only in the groups whose group
+number is either 0 or a power of 3, 5, or 7. If the flag is not set,
+redundant copies are kept in all groups.
+
+The superblock checksum is calculated against the superblock structure,
+which includes the FS UUID.
+
+The ext4 superblock is laid out as follows in
+``struct ext4_super_block``:
+
+.. list-table::
+ :widths: 8 8 24 40
+ :header-rows: 1
+
+ * - Offset
+ - Size
+ - Name
+ - Description
+ * - 0x0
+ - __le32
+ - s_inodes_count
+ - Total inode count.
+ * - 0x4
+ - __le32
+ - s_blocks_count_lo
+ - Total block count.
+ * - 0x8
+ - __le32
+ - s_r_blocks_count_lo
+ - This number of blocks can only be allocated by the super-user.
+ * - 0xC
+ - __le32
+ - s_free_blocks_count_lo
+ - Free block count.
+ * - 0x10
+ - __le32
+ - s_free_inodes_count
+ - Free inode count.
+ * - 0x14
+ - __le32
+ - s_first_data_block
+ - First data block. This must be at least 1 for 1k-block filesystems and
+ is typically 0 for all other block sizes.
+ * - 0x18
+ - __le32
+ - s_log_block_size
+ - Block size is 2 ^ (10 + s_log_block_size).
+ * - 0x1C
+ - __le32
+ - s_log_cluster_size
+ - Cluster size is 2 ^ (10 + s_log_cluster_size) blocks if bigalloc is
+ enabled. Otherwise s_log_cluster_size must equal s_log_block_size.
+ * - 0x20
+ - __le32
+ - s_blocks_per_group
+ - Blocks per group.
+ * - 0x24
+ - __le32
+ - s_clusters_per_group
+ - Clusters per group, if bigalloc is enabled. Otherwise
+ s_clusters_per_group must equal s_blocks_per_group.
+ * - 0x28
+ - __le32
+ - s_inodes_per_group
+ - Inodes per group.
+ * - 0x2C
+ - __le32
+ - s_mtime
+ - Mount time, in seconds since the epoch.
+ * - 0x30
+ - __le32
+ - s_wtime
+ - Write time, in seconds since the epoch.
+ * - 0x34
+ - __le16
+ - s_mnt_count
+ - Number of mounts since the last fsck.
+ * - 0x36
+ - __le16
+ - s_max_mnt_count
+ - Number of mounts beyond which a fsck is needed.
+ * - 0x38
+ - __le16
+ - s_magic
+ - Magic signature, 0xEF53
+ * - 0x3A
+ - __le16
+ - s_state
+ - File system state. See super_state_ for more info.
+ * - 0x3C
+ - __le16
+ - s_errors
+ - Behaviour when detecting errors. See super_errors_ for more info.
+ * - 0x3E
+ - __le16
+ - s_minor_rev_level
+ - Minor revision level.
+ * - 0x40
+ - __le32
+ - s_lastcheck
+ - Time of last check, in seconds since the epoch.
+ * - 0x44
+ - __le32
+ - s_checkinterval
+ - Maximum time between checks, in seconds.
+ * - 0x48
+ - __le32
+ - s_creator_os
+ - Creator OS. See the table super_creator_ for more info.
+ * - 0x4C
+ - __le32
+ - s_rev_level
+ - Revision level. See the table super_revision_ for more info.
+ * - 0x50
+ - __le16
+ - s_def_resuid
+ - Default uid for reserved blocks.
+ * - 0x52
+ - __le16
+ - s_def_resgid
+ - Default gid for reserved blocks.
+ * -
+ -
+ -
+ - These fields are for EXT4_DYNAMIC_REV superblocks only.
+
+ Note: the difference between the compatible feature set and the
+ incompatible feature set is that if there is a bit set in the
+ incompatible feature set that the kernel doesn't know about, it should
+ refuse to mount the filesystem.
+
+ e2fsck's requirements are more strict; if it doesn't know
+ about a feature in either the compatible or incompatible feature set, it
+ must abort and not try to meddle with things it doesn't understand...
+ * - 0x54
+ - __le32
+ - s_first_ino
+ - First non-reserved inode.
+ * - 0x58
+ - __le16
+ - s_inode_size
+ - Size of inode structure, in bytes.
+ * - 0x5A
+ - __le16
+ - s_block_group_nr
+ - Block group # of this superblock.
+ * - 0x5C
+ - __le32
+ - s_feature_compat
+ - Compatible feature set flags. Kernel can still read/write this fs even
+ if it doesn't understand a flag; fsck should not do that. See the
+ super_compat_ table for more info.
+ * - 0x60
+ - __le32
+ - s_feature_incompat
+ - Incompatible feature set. If the kernel or fsck doesn't understand one
+ of these bits, it should stop. See the super_incompat_ table for more
+ info.
+ * - 0x64
+ - __le32
+ - s_feature_ro_compat
+ - Readonly-compatible feature set. If the kernel doesn't understand one of
+ these bits, it can still mount read-only. See the super_rocompat_ table
+ for more info.
+ * - 0x68
+ - __u8
+ - s_uuid[16]
+ - 128-bit UUID for volume.
+ * - 0x78
+ - char
+ - s_volume_name[16]
+ - Volume label.
+ * - 0x88
+ - char
+ - s_last_mounted[64]
+ - Directory where filesystem was last mounted.
+ * - 0xC8
+ - __le32
+ - s_algorithm_usage_bitmap
+ - For compression (Not used in e2fsprogs/Linux)
+ * -
+ -
+ -
+ - Performance hints. Directory preallocation should only happen if the
+ EXT4_FEATURE_COMPAT_DIR_PREALLOC flag is on.
+ * - 0xCC
+ - __u8
+ - s_prealloc_blocks
+ - #. of blocks to try to preallocate for ... files? (Not used in
+ e2fsprogs/Linux)
+ * - 0xCD
+ - __u8
+ - s_prealloc_dir_blocks
+ - #. of blocks to preallocate for directories. (Not used in
+ e2fsprogs/Linux)
+ * - 0xCE
+ - __le16
+ - s_reserved_gdt_blocks
+ - Number of reserved GDT entries for future filesystem expansion.
+ * -
+ -
+ -
+ - Journalling support is valid only if EXT4_FEATURE_COMPAT_HAS_JOURNAL is
+ set.
+ * - 0xD0
+ - __u8
+ - s_journal_uuid[16]
+ - UUID of journal superblock
+ * - 0xE0
+ - __le32
+ - s_journal_inum
+ - inode number of journal file.
+ * - 0xE4
+ - __le32
+ - s_journal_dev
+ - Device number of journal file, if the external journal feature flag is
+ set.
+ * - 0xE8
+ - __le32
+ - s_last_orphan
+ - Start of list of orphaned inodes to delete.
+ * - 0xEC
+ - __le32
+ - s_hash_seed[4]
+ - HTREE hash seed.
+ * - 0xFC
+ - __u8
+ - s_def_hash_version
+ - Default hash algorithm to use for directory hashes. See super_def_hash_
+ for more info.
+ * - 0xFD
+ - __u8
+ - s_jnl_backup_type
+ - If this value is 0 or EXT3_JNL_BACKUP_BLOCKS (1), then the
+ ``s_jnl_blocks`` field contains a duplicate copy of the inode's
+ ``i_block[]`` array and ``i_size``.
+ * - 0xFE
+ - __le16
+ - s_desc_size
+ - Size of group descriptors, in bytes, if the 64bit incompat feature flag
+ is set.
+ * - 0x100
+ - __le32
+ - s_default_mount_opts
+ - Default mount options. See the super_mountopts_ table for more info.
+ * - 0x104
+ - __le32
+ - s_first_meta_bg
+ - First metablock block group, if the meta_bg feature is enabled.
+ * - 0x108
+ - __le32
+ - s_mkfs_time
+ - When the filesystem was created, in seconds since the epoch.
+ * - 0x10C
+ - __le32
+ - s_jnl_blocks[17]
+ - Backup copy of the journal inode's ``i_block[]`` array in the first 15
+ elements and i_size_high and i_size in the 16th and 17th elements,
+ respectively.
+ * -
+ -
+ -
+ - 64bit support is valid only if EXT4_FEATURE_COMPAT_64BIT is set.
+ * - 0x150
+ - __le32
+ - s_blocks_count_hi
+ - High 32-bits of the block count.
+ * - 0x154
+ - __le32
+ - s_r_blocks_count_hi
+ - High 32-bits of the reserved block count.
+ * - 0x158
+ - __le32
+ - s_free_blocks_count_hi
+ - High 32-bits of the free block count.
+ * - 0x15C
+ - __le16
+ - s_min_extra_isize
+ - All inodes have at least # bytes.
+ * - 0x15E
+ - __le16
+ - s_want_extra_isize
+ - New inodes should reserve # bytes.
+ * - 0x160
+ - __le32
+ - s_flags
+ - Miscellaneous flags. See the super_flags_ table for more info.
+ * - 0x164
+ - __le16
+ - s_raid_stride
+ - RAID stride. This is the number of logical blocks read from or written
+ to the disk before moving to the next disk. This affects the placement
+ of filesystem metadata, which will hopefully make RAID storage faster.
+ * - 0x166
+ - __le16
+ - s_mmp_interval
+ - #. seconds to wait in multi-mount prevention (MMP) checking. In theory,
+ MMP is a mechanism to record in the superblock which host and device
+ have mounted the filesystem, in order to prevent multiple mounts. This
+ feature does not seem to be implemented...
+ * - 0x168
+ - __le64
+ - s_mmp_block
+ - Block # for multi-mount protection data.
+ * - 0x170
+ - __le32
+ - s_raid_stripe_width
+ - RAID stripe width. This is the number of logical blocks read from or
+ written to the disk before coming back to the current disk. This is used
+ by the block allocator to try to reduce the number of read-modify-write
+ operations in a RAID5/6.
+ * - 0x174
+ - __u8
+ - s_log_groups_per_flex
+ - Size of a flexible block group is 2 ^ ``s_log_groups_per_flex``.
+ * - 0x175
+ - __u8
+ - s_checksum_type
+ - Metadata checksum algorithm type. The only valid value is 1 (crc32c).
+ * - 0x176
+ - __le16
+ - s_reserved_pad
+ -
+ * - 0x178
+ - __le64
+ - s_kbytes_written
+ - Number of KiB written to this filesystem over its lifetime.
+ * - 0x180
+ - __le32
+ - s_snapshot_inum
+ - inode number of active snapshot. (Not used in e2fsprogs/Linux.)
+ * - 0x184
+ - __le32
+ - s_snapshot_id
+ - Sequential ID of active snapshot. (Not used in e2fsprogs/Linux.)
+ * - 0x188
+ - __le64
+ - s_snapshot_r_blocks_count
+ - Number of blocks reserved for active snapshot's future use. (Not used in
+ e2fsprogs/Linux.)
+ * - 0x190
+ - __le32
+ - s_snapshot_list
+ - inode number of the head of the on-disk snapshot list. (Not used in
+ e2fsprogs/Linux.)
+ * - 0x194
+ - __le32
+ - s_error_count
+ - Number of errors seen.
+ * - 0x198
+ - __le32
+ - s_first_error_time
+ - First time an error happened, in seconds since the epoch.
+ * - 0x19C
+ - __le32
+ - s_first_error_ino
+ - inode involved in first error.
+ * - 0x1A0
+ - __le64
+ - s_first_error_block
+ - Number of block involved of first error.
+ * - 0x1A8
+ - __u8
+ - s_first_error_func[32]
+ - Name of function where the error happened.
+ * - 0x1C8
+ - __le32
+ - s_first_error_line
+ - Line number where error happened.
+ * - 0x1CC
+ - __le32
+ - s_last_error_time
+ - Time of most recent error, in seconds since the epoch.
+ * - 0x1D0
+ - __le32
+ - s_last_error_ino
+ - inode involved in most recent error.
+ * - 0x1D4
+ - __le32
+ - s_last_error_line
+ - Line number where most recent error happened.
+ * - 0x1D8
+ - __le64
+ - s_last_error_block
+ - Number of block involved in most recent error.
+ * - 0x1E0
+ - __u8
+ - s_last_error_func[32]
+ - Name of function where the most recent error happened.
+ * - 0x200
+ - __u8
+ - s_mount_opts[64]
+ - ASCIIZ string of mount options.
+ * - 0x240
+ - __le32
+ - s_usr_quota_inum
+ - Inode number of user `quota <quota>`__ file.
+ * - 0x244
+ - __le32
+ - s_grp_quota_inum
+ - Inode number of group `quota <quota>`__ file.
+ * - 0x248
+ - __le32
+ - s_overhead_blocks
+ - Overhead blocks/clusters in fs. (Huh? This field is always zero, which
+ means that the kernel calculates it dynamically.)
+ * - 0x24C
+ - __le32
+ - s_backup_bgs[2]
+ - Block groups containing superblock backups (if sparse_super2)
+ * - 0x254
+ - __u8
+ - s_encrypt_algos[4]
+ - Encryption algorithms in use. There can be up to four algorithms in use
+ at any time; valid algorithm codes are given in the super_encrypt_ table
+ below.
+ * - 0x258
+ - __u8
+ - s_encrypt_pw_salt[16]
+ - Salt for the string2key algorithm for encryption.
+ * - 0x268
+ - __le32
+ - s_lpf_ino
+ - Inode number of lost+found
+ * - 0x26C
+ - __le32
+ - s_prj_quota_inum
+ - Inode that tracks project quotas.
+ * - 0x270
+ - __le32
+ - s_checksum_seed
+ - Checksum seed used for metadata_csum calculations. This value is
+ crc32c(~0, $orig_fs_uuid).
+ * - 0x274
+ - __u8
+ - s_wtime_hi
+ - Upper 8 bits of the s_wtime field.
+ * - 0x275
+ - __u8
+ - s_mtime_hi
+ - Upper 8 bits of the s_mtime field.
+ * - 0x276
+ - __u8
+ - s_mkfs_time_hi
+ - Upper 8 bits of the s_mkfs_time field.
+ * - 0x277
+ - __u8
+ - s_lastcheck_hi
+ - Upper 8 bits of the s_lastcheck field.
+ * - 0x278
+ - __u8
+ - s_first_error_time_hi
+ - Upper 8 bits of the s_first_error_time field.
+ * - 0x279
+ - __u8
+ - s_last_error_time_hi
+ - Upper 8 bits of the s_last_error_time field.
+ * - 0x27A
+ - __u8
+ - s_pad[2]
+ - Zero padding.
+ * - 0x27C
+ - __le16
+ - s_encoding
+ - Filename charset encoding.
+ * - 0x27E
+ - __le16
+ - s_encoding_flags
+ - Filename charset encoding flags.
+ * - 0x280
+ - __le32
+ - s_orphan_file_inum
+ - Orphan file inode number.
+ * - 0x284
+ - __le32
+ - s_reserved[94]
+ - Padding to the end of the block.
+ * - 0x3FC
+ - __le32
+ - s_checksum
+ - Superblock checksum.
+
+.. _super_state:
+
+The superblock state is some combination of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0x0001
+ - Cleanly umounted
+ * - 0x0002
+ - Errors detected
+ * - 0x0004
+ - Orphans being recovered
+
+.. _super_errors:
+
+The superblock error policy is one of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 1
+ - Continue
+ * - 2
+ - Remount read-only
+ * - 3
+ - Panic
+
+.. _super_creator:
+
+The filesystem creator is one of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0
+ - Linux
+ * - 1
+ - Hurd
+ * - 2
+ - Masix
+ * - 3
+ - FreeBSD
+ * - 4
+ - Lites
+
+.. _super_revision:
+
+The superblock revision is one of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0
+ - Original format
+ * - 1
+ - v2 format w/ dynamic inode sizes
+
+Note that ``EXT4_DYNAMIC_REV`` refers to a revision 1 or newer filesystem.
+
+.. _super_compat:
+
+The superblock compatible features field is a combination of any of the
+following:
+
+.. list-table::
+ :widths: 16 64
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0x1
+ - Directory preallocation (COMPAT_DIR_PREALLOC).
+ * - 0x2
+ - “imagic inodes”. Not clear from the code what this does
+ (COMPAT_IMAGIC_INODES).
+ * - 0x4
+ - Has a journal (COMPAT_HAS_JOURNAL).
+ * - 0x8
+ - Supports extended attributes (COMPAT_EXT_ATTR).
+ * - 0x10
+ - Has reserved GDT blocks for filesystem expansion
+ (COMPAT_RESIZE_INODE). Requires RO_COMPAT_SPARSE_SUPER.
+ * - 0x20
+ - Has directory indices (COMPAT_DIR_INDEX).
+ * - 0x40
+ - “Lazy BG”. Not in Linux kernel, seems to have been for uninitialized
+ block groups? (COMPAT_LAZY_BG)
+ * - 0x80
+ - “Exclude inode”. Not used. (COMPAT_EXCLUDE_INODE).
+ * - 0x100
+ - “Exclude bitmap”. Seems to be used to indicate the presence of
+ snapshot-related exclude bitmaps? Not defined in kernel or used in
+ e2fsprogs (COMPAT_EXCLUDE_BITMAP).
+ * - 0x200
+ - Sparse Super Block, v2. If this flag is set, the SB field s_backup_bgs
+ points to the two block groups that contain backup superblocks
+ (COMPAT_SPARSE_SUPER2).
+ * - 0x400
+ - Fast commits supported. Although fast commits blocks are
+ backward incompatible, fast commit blocks are not always
+ present in the journal. If fast commit blocks are present in
+ 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:
+
+The superblock incompatible features field is a combination of any of the
+following:
+
+.. list-table::
+ :widths: 16 64
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0x1
+ - Compression (INCOMPAT_COMPRESSION).
+ * - 0x2
+ - Directory entries record the file type. See ext4_dir_entry_2 below
+ (INCOMPAT_FILETYPE).
+ * - 0x4
+ - Filesystem needs recovery (INCOMPAT_RECOVER).
+ * - 0x8
+ - Filesystem has a separate journal device (INCOMPAT_JOURNAL_DEV).
+ * - 0x10
+ - Meta block groups. See the earlier discussion of this feature
+ (INCOMPAT_META_BG).
+ * - 0x40
+ - Files in this filesystem use extents (INCOMPAT_EXTENTS).
+ * - 0x80
+ - Enable a filesystem size of 2^64 blocks (INCOMPAT_64BIT).
+ * - 0x100
+ - Multiple mount protection (INCOMPAT_MMP).
+ * - 0x200
+ - Flexible block groups. See the earlier discussion of this feature
+ (INCOMPAT_FLEX_BG).
+ * - 0x400
+ - Inodes can be used to store large extended attribute values
+ (INCOMPAT_EA_INODE).
+ * - 0x1000
+ - Data in directory entry (INCOMPAT_DIRDATA). (Not implemented?)
+ * - 0x2000
+ - Metadata checksum seed is stored in the superblock. This feature enables
+ the administrator to change the UUID of a metadata_csum filesystem
+ while the filesystem is mounted; without it, the checksum definition
+ requires all metadata blocks to be rewritten (INCOMPAT_CSUM_SEED).
+ * - 0x4000
+ - Large directory >2GB or 3-level htree (INCOMPAT_LARGEDIR). Prior to
+ this feature, directories could not be larger than 4GiB and could not
+ have an htree more than 2 levels deep. If this feature is enabled,
+ directories can be larger than 4GiB and have a maximum htree depth of 3.
+ * - 0x8000
+ - Data in inode (INCOMPAT_INLINE_DATA).
+ * - 0x10000
+ - Encrypted inodes are present on the filesystem. (INCOMPAT_ENCRYPT).
+
+.. _super_rocompat:
+
+The superblock read-only compatible features field is a combination of any of
+the following:
+
+.. list-table::
+ :widths: 16 64
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0x1
+ - Sparse superblocks. See the earlier discussion of this feature
+ (RO_COMPAT_SPARSE_SUPER).
+ * - 0x2
+ - This filesystem has been used to store a file greater than 2GiB
+ (RO_COMPAT_LARGE_FILE).
+ * - 0x4
+ - Not used in kernel or e2fsprogs (RO_COMPAT_BTREE_DIR).
+ * - 0x8
+ - This filesystem has files whose sizes are represented in units of
+ logical blocks, not 512-byte sectors. This implies a very large file
+ indeed! (RO_COMPAT_HUGE_FILE)
+ * - 0x10
+ - Group descriptors have checksums. In addition to detecting corruption,
+ this is useful for lazy formatting with uninitialized groups
+ (RO_COMPAT_GDT_CSUM).
+ * - 0x20
+ - Indicates that the old ext3 32,000 subdirectory limit no longer applies
+ (RO_COMPAT_DIR_NLINK). A directory's i_links_count will be set to 1
+ if it is incremented past 64,999.
+ * - 0x40
+ - Indicates that large inodes exist on this filesystem
+ (RO_COMPAT_EXTRA_ISIZE).
+ * - 0x80
+ - This filesystem has a snapshot (RO_COMPAT_HAS_SNAPSHOT).
+ * - 0x100
+ - `Quota <Quota>`__ (RO_COMPAT_QUOTA).
+ * - 0x200
+ - This filesystem supports “bigalloc”, which means that file extents are
+ tracked in units of clusters (of blocks) instead of blocks
+ (RO_COMPAT_BIGALLOC).
+ * - 0x400
+ - This filesystem supports metadata checksumming.
+ (RO_COMPAT_METADATA_CSUM; implies RO_COMPAT_GDT_CSUM, though
+ GDT_CSUM must not be set)
+ * - 0x800
+ - Filesystem supports replicas. This feature is neither in the kernel nor
+ e2fsprogs. (RO_COMPAT_REPLICA)
+ * - 0x1000
+ - Read-only filesystem image; the kernel will not mount this image
+ read-write and most tools will refuse to write to the image.
+ (RO_COMPAT_READONLY)
+ * - 0x2000
+ - 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:
+
+The ``s_def_hash_version`` field is one of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0x0
+ - Legacy.
+ * - 0x1
+ - Half MD4.
+ * - 0x2
+ - Tea.
+ * - 0x3
+ - Legacy, unsigned.
+ * - 0x4
+ - Half MD4, unsigned.
+ * - 0x5
+ - Tea, unsigned.
+
+.. _super_mountopts:
+
+The ``s_default_mount_opts`` field is any combination of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0x0001
+ - Print debugging info upon (re)mount. (EXT4_DEFM_DEBUG)
+ * - 0x0002
+ - New files take the gid of the containing directory (instead of the fsgid
+ of the current process). (EXT4_DEFM_BSDGROUPS)
+ * - 0x0004
+ - Support userspace-provided extended attributes. (EXT4_DEFM_XATTR_USER)
+ * - 0x0008
+ - Support POSIX access control lists (ACLs). (EXT4_DEFM_ACL)
+ * - 0x0010
+ - Do not support 32-bit UIDs. (EXT4_DEFM_UID16)
+ * - 0x0020
+ - All data and metadata are commited to the journal.
+ (EXT4_DEFM_JMODE_DATA)
+ * - 0x0040
+ - All data are flushed to the disk before metadata are committed to the
+ journal. (EXT4_DEFM_JMODE_ORDERED)
+ * - 0x0060
+ - Data ordering is not preserved; data may be written after the metadata
+ has been written. (EXT4_DEFM_JMODE_WBACK)
+ * - 0x0100
+ - Disable write flushes. (EXT4_DEFM_NOBARRIER)
+ * - 0x0200
+ - Track which blocks in a filesystem are metadata and therefore should not
+ be used as data blocks. This option will be enabled by default on 3.18,
+ hopefully. (EXT4_DEFM_BLOCK_VALIDITY)
+ * - 0x0400
+ - Enable DISCARD support, where the storage device is told about blocks
+ becoming unused. (EXT4_DEFM_DISCARD)
+ * - 0x0800
+ - Disable delayed allocation. (EXT4_DEFM_NODELALLOC)
+
+.. _super_flags:
+
+The ``s_flags`` field is any combination of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0x0001
+ - Signed directory hash in use.
+ * - 0x0002
+ - Unsigned directory hash in use.
+ * - 0x0004
+ - To test development code.
+
+.. _super_encrypt:
+
+The ``s_encrypt_algos`` list can contain any of the following:
+
+.. list-table::
+ :widths: 8 72
+ :header-rows: 1
+
+ * - Value
+ - Description
+ * - 0
+ - Invalid algorithm (ENCRYPTION_MODE_INVALID).
+ * - 1
+ - 256-bit AES in XTS mode (ENCRYPTION_MODE_AES_256_XTS).
+ * - 2
+ - 256-bit AES in GCM mode (ENCRYPTION_MODE_AES_256_GCM).
+ * - 3
+ - 256-bit AES in CBC mode (ENCRYPTION_MODE_AES_256_CBC).
+
+Total size of the superblock is 1024 bytes.