ext4: import group descriptors chapter from wiki page
Import the chapter about group descriptors from the on-disk format wiki page into the kernel documentation. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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Darrick J. Wong
Theodore Ts'o
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3db42a2139
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3c6ba09dd3
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@ -7,3 +7,4 @@ The filesystem is sharded into a number of block groups, each of which
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have static metadata at fixed locations.
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have static metadata at fixed locations.
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.. include:: super.rst
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.. include:: super.rst
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.. include:: group_descr.rst
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@ -0,0 +1,170 @@
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.. SPDX-License-Identifier: GPL-2.0
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Block Group Descriptors
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-----------------------
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Each block group on the filesystem has one of these descriptors
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associated with it. As noted in the Layout section above, the group
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descriptors (if present) are the second item in the block group. The
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standard configuration is for each block group to contain a full copy of
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the block group descriptor table unless the sparse\_super feature flag
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is set.
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Notice how the group descriptor records the location of both bitmaps and
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the inode table (i.e. they can float). This means that within a block
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group, the only data structures with fixed locations are the superblock
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and the group descriptor table. The flex\_bg mechanism uses this
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property to group several block groups into a flex group and lay out all
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of the groups' bitmaps and inode tables into one long run in the first
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group of the flex group.
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If the meta\_bg feature flag is set, then several block groups are
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grouped together into a meta group. Note that in the meta\_bg case,
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however, the first and last two block groups within the larger meta
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group contain only group descriptors for the groups inside the meta
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group.
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flex\_bg and meta\_bg do not appear to be mutually exclusive features.
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In ext2, ext3, and ext4 (when the 64bit feature is not enabled), the
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block group descriptor was only 32 bytes long and therefore ends at
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bg\_checksum. On an ext4 filesystem with the 64bit feature enabled, the
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block group descriptor expands to at least the 64 bytes described below;
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the size is stored in the superblock.
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If gdt\_csum is set and metadata\_csum is not set, the block group
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checksum is the crc16 of the FS UUID, the group number, and the group
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descriptor structure. If metadata\_csum is set, then the block group
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checksum is the lower 16 bits of the checksum of the FS UUID, the group
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number, and the group descriptor structure. Both block and inode bitmap
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checksums are calculated against the FS UUID, the group number, and the
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entire bitmap.
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The block group descriptor is laid out in ``struct ext4_group_desc``.
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.. list-table::
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:widths: 1 1 1 77
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:header-rows: 1
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* - Offset
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- Size
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- Name
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- Description
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* - 0x0
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- \_\_le32
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- bg\_block\_bitmap\_lo
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- Lower 32-bits of location of block bitmap.
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* - 0x4
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- \_\_le32
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- bg\_inode\_bitmap\_lo
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- Lower 32-bits of location of inode bitmap.
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* - 0x8
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- \_\_le32
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- bg\_inode\_table\_lo
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- Lower 32-bits of location of inode table.
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* - 0xC
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- \_\_le16
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- bg\_free\_blocks\_count\_lo
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- Lower 16-bits of free block count.
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* - 0xE
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- \_\_le16
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- bg\_free\_inodes\_count\_lo
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- Lower 16-bits of free inode count.
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* - 0x10
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- \_\_le16
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- bg\_used\_dirs\_count\_lo
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- Lower 16-bits of directory count.
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* - 0x12
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- \_\_le16
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- bg\_flags
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- Block group flags. See the bgflags_ table below.
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* - 0x14
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- \_\_le32
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- bg\_exclude\_bitmap\_lo
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- Lower 32-bits of location of snapshot exclusion bitmap.
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* - 0x18
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- \_\_le16
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- bg\_block\_bitmap\_csum\_lo
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- Lower 16-bits of the block bitmap checksum.
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* - 0x1A
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- \_\_le16
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- bg\_inode\_bitmap\_csum\_lo
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- Lower 16-bits of the inode bitmap checksum.
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* - 0x1C
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- \_\_le16
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- bg\_itable\_unused\_lo
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- Lower 16-bits of unused inode count. If set, we needn't scan past the
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``(sb.s_inodes_per_group - gdt.bg_itable_unused)``\ th entry in the
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inode table for this group.
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* - 0x1E
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- \_\_le16
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- bg\_checksum
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- Group descriptor checksum; crc16(sb\_uuid+group+desc) if the
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RO\_COMPAT\_GDT\_CSUM feature is set, or crc32c(sb\_uuid+group\_desc) &
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0xFFFF if the RO\_COMPAT\_METADATA\_CSUM feature is set.
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* -
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-
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-
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- These fields only exist if the 64bit feature is enabled and s_desc_size
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> 32.
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* - 0x20
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- \_\_le32
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- bg\_block\_bitmap\_hi
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- Upper 32-bits of location of block bitmap.
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* - 0x24
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- \_\_le32
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- bg\_inode\_bitmap\_hi
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- Upper 32-bits of location of inodes bitmap.
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* - 0x28
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- \_\_le32
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- bg\_inode\_table\_hi
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- Upper 32-bits of location of inodes table.
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* - 0x2C
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- \_\_le16
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- bg\_free\_blocks\_count\_hi
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- Upper 16-bits of free block count.
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* - 0x2E
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- \_\_le16
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- bg\_free\_inodes\_count\_hi
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- Upper 16-bits of free inode count.
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* - 0x30
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- \_\_le16
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- bg\_used\_dirs\_count\_hi
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- Upper 16-bits of directory count.
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* - 0x32
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- \_\_le16
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- bg\_itable\_unused\_hi
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- Upper 16-bits of unused inode count.
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* - 0x34
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- \_\_le32
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- bg\_exclude\_bitmap\_hi
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- Upper 32-bits of location of snapshot exclusion bitmap.
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* - 0x38
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- \_\_le16
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- bg\_block\_bitmap\_csum\_hi
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- Upper 16-bits of the block bitmap checksum.
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* - 0x3A
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- \_\_le16
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- bg\_inode\_bitmap\_csum\_hi
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- Upper 16-bits of the inode bitmap checksum.
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* - 0x3C
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- \_\_u32
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- bg\_reserved
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- Padding to 64 bytes.
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.. _bgflags:
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Block group flags can be any combination of the following:
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.. list-table::
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:widths: 1 79
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- inode table and bitmap are not initialized (EXT4\_BG\_INODE\_UNINIT).
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* - 0x2
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- block bitmap is not initialized (EXT4\_BG\_BLOCK\_UNINIT).
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* - 0x4
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- inode table is zeroed (EXT4\_BG\_INODE\_ZEROED).
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