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>

Documentation/filesystems/ext4/ondisk/globals.rst

@@ -7,3 +7,4 @@ The filesystem is sharded into a number of block groups, each of which
 have static metadata at fixed locations.
 
 .. include:: super.rst
+.. include:: group_descr.rst

Documentation/filesystems/ext4/ondisk/group_descr.rst

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