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diff --git a/Documentation/filesystems/ext4/group_descr.rst b/Documentation/filesystems/ext4/group_descr.rst new file mode 100644 index 000000000..7ba6114e7 --- /dev/null +++ b/Documentation/filesystems/ext4/group_descr.rst @@ -0,0 +1,173 @@ +.. 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: 8 8 24 40 + :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\_num+bg\_desc) if the + RO\_COMPAT\_GDT\_CSUM feature is set, or + crc32c(sb\_uuid+group\_num+bg\_desc) & 0xFFFF if the + RO\_COMPAT\_METADATA\_CSUM feature is set. The bg\_checksum + field in bg\_desc is skipped when calculating crc16 checksum, + and set to zero if crc32c checksum is used. + * - + - + - + - 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: 16 64 + :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). |