Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 173 insertions, 0 deletions

diff --git a/Documentation/filesystems/ext4/group_descr.rst b/Documentation/filesystems/ext4/group_descr.rst
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+.. 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).