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+.. SPDX-License-Identifier: GPL-2.0
+
+Checksums
+---------
+
+Starting in early 2012, metadata checksums were added to all major ext4
+and jbd2 data structures. The associated feature flag is metadata_csum.
+The desired checksum algorithm is indicated in the superblock, though as
+of October 2012 the only supported algorithm is crc32c. Some data
+structures did not have space to fit a full 32-bit checksum, so only the
+lower 16 bits are stored. Enabling the 64bit feature increases the data
+structure size so that full 32-bit checksums can be stored for many data
+structures. However, existing 32-bit filesystems cannot be extended to
+enable 64bit mode, at least not without the experimental resize2fs
+patches to do so.
+
+Existing filesystems can have checksumming added by running
+``tune2fs -O metadata_csum`` against the underlying device. If tune2fs
+encounters directory blocks that lack sufficient empty space to add a
+checksum, it will request that you run ``e2fsck -D`` to have the
+directories rebuilt with checksums. This has the added benefit of
+removing slack space from the directory files and rebalancing the htree
+indexes. If you _ignore_ this step, your directories will not be
+protected by a checksum!
+
+The following table describes the data elements that go into each type
+of checksum. The checksum function is whatever the superblock describes
+(crc32c as of October 2013) unless noted otherwise.
+
+.. list-table::
+ :widths: 20 8 50
+ :header-rows: 1
+
+ * - Metadata
+ - Length
+ - Ingredients
+ * - Superblock
+ - __le32
+ - The entire superblock up to the checksum field. The UUID lives inside
+ the superblock.
+ * - MMP
+ - __le32
+ - UUID + the entire MMP block up to the checksum field.
+ * - Extended Attributes
+ - __le32
+ - UUID + the entire extended attribute block. The checksum field is set to
+ zero.
+ * - Directory Entries
+ - __le32
+ - UUID + inode number + inode generation + the directory block up to the
+ fake entry enclosing the checksum field.
+ * - HTREE Nodes
+ - __le32
+ - UUID + inode number + inode generation + all valid extents + HTREE tail.
+ The checksum field is set to zero.
+ * - Extents
+ - __le32
+ - UUID + inode number + inode generation + the entire extent block up to
+ the checksum field.
+ * - Bitmaps
+ - __le32 or __le16
+ - UUID + the entire bitmap. Checksums are stored in the group descriptor,
+ and truncated if the group descriptor size is 32 bytes (i.e. ^64bit)
+ * - Inodes
+ - __le32
+ - UUID + inode number + inode generation + the entire inode. The checksum
+ field is set to zero. Each inode has its own checksum.
+ * - Group Descriptors
+ - __le16
+ - If metadata_csum, then UUID + group number + the entire descriptor;
+ else if gdt_csum, then crc16(UUID + group number + the entire
+ descriptor). In all cases, only the lower 16 bits are stored.
+