ext4: import directory layout chapter from wiki page

Import the chapter about directory layout 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/directory.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+Directory Entries
+-----------------
+
+In an ext4 filesystem, a directory is more or less a flat file that maps
+an arbitrary byte string (usually ASCII) to an inode number on the
+filesystem. There can be many directory entries across the filesystem
+that reference the same inode number--these are known as hard links, and
+that is why hard links cannot reference files on other filesystems. As
+such, directory entries are found by reading the data block(s)
+associated with a directory file for the particular directory entry that
+is desired.
+
+Linear (Classic) Directories
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, each directory lists its entries in an “almost-linear”
+array. I write “almost” because it's not a linear array in the memory
+sense because directory entries are not split across filesystem blocks.
+Therefore, it is more accurate to say that a directory is a series of
+data blocks and that each block contains a linear array of directory
+entries. The end of each per-block array is signified by reaching the
+end of the block; the last entry in the block has a record length that
+takes it all the way to the end of the block. The end of the entire
+directory is of course signified by reaching the end of the file. Unused
+directory entries are signified by inode = 0. By default the filesystem
+uses ``struct ext4_dir_entry_2`` for directory entries unless the
+“filetype” feature flag is not set, in which case it uses
+``struct ext4_dir_entry``.
+
+The original directory entry format is ``struct ext4_dir_entry``, which
+is at most 263 bytes long, though on disk you'll need to reference
+``dirent.rec_len`` to know for sure.
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - inode
+     - Number of the inode that this directory entry points to.
+   * - 0x4
+     - \_\_le16
+     - rec\_len
+     - Length of this directory entry. Must be a multiple of 4.
+   * - 0x6
+     - \_\_le16
+     - name\_len
+     - Length of the file name.
+   * - 0x8
+     - char
+     - name[EXT4\_NAME\_LEN]
+     - File name.
+
+Since file names cannot be longer than 255 bytes, the new directory
+entry format shortens the rec\_len field and uses the space for a file
+type flag, probably to avoid having to load every inode during directory
+tree traversal. This format is ``ext4_dir_entry_2``, which is at most
+263 bytes long, though on disk you'll need to reference
+``dirent.rec_len`` to know for sure.
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - inode
+     - Number of the inode that this directory entry points to.
+   * - 0x4
+     - \_\_le16
+     - rec\_len
+     - Length of this directory entry.
+   * - 0x6
+     - \_\_u8
+     - name\_len
+     - Length of the file name.
+   * - 0x7
+     - \_\_u8
+     - file\_type
+     - File type code, see ftype_ table below.
+   * - 0x8
+     - char
+     - name[EXT4\_NAME\_LEN]
+     - File name.
+
+.. _ftype:
+
+The directory file type is one of the following values:
+
+.. list-table::
+   :widths: 1 79
+   :header-rows: 1
+
+   * - Value
+     - Description
+   * - 0x0
+     - Unknown.
+   * - 0x1
+     - Regular file.
+   * - 0x2
+     - Directory.
+   * - 0x3
+     - Character device file.
+   * - 0x4
+     - Block device file.
+   * - 0x5
+     - FIFO.
+   * - 0x6
+     - Socket.
+   * - 0x7
+     - Symbolic link.
+
+In order to add checksums to these classic directory blocks, a phony
+``struct ext4_dir_entry`` is placed at the end of each leaf block to
+hold the checksum. The directory entry is 12 bytes long. The inode
+number and name\_len fields are set to zero to fool old software into
+ignoring an apparently empty directory entry, and the checksum is stored
+in the place where the name normally goes. The structure is
+``struct ext4_dir_entry_tail``:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Size
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - det\_reserved\_zero1
+     - Inode number, which must be zero.
+   * - 0x4
+     - \_\_le16
+     - det\_rec\_len
+     - Length of this directory entry, which must be 12.
+   * - 0x6
+     - \_\_u8
+     - det\_reserved\_zero2
+     - Length of the file name, which must be zero.
+   * - 0x7
+     - \_\_u8
+     - det\_reserved\_ft
+     - File type, which must be 0xDE.
+   * - 0x8
+     - \_\_le32
+     - det\_checksum
+     - Directory leaf block checksum.
+
+The leaf directory block checksum is calculated against the FS UUID, the
+directory's inode number, the directory's inode generation number, and
+the entire directory entry block up to (but not including) the fake
+directory entry.
+
+Hash Tree Directories
+~~~~~~~~~~~~~~~~~~~~~
+
+A linear array of directory entries isn't great for performance, so a
+new feature was added to ext3 to provide a faster (but peculiar)
+balanced tree keyed off a hash of the directory entry name. If the
+EXT4\_INDEX\_FL (0x1000) flag is set in the inode, this directory uses a
+hashed btree (htree) to organize and find directory entries. For
+backwards read-only compatibility with ext2, this tree is actually
+hidden inside the directory file, masquerading as “empty” directory data
+blocks! It was stated previously that the end of the linear directory
+entry table was signified with an entry pointing to inode 0; this is
+(ab)used to fool the old linear-scan algorithm into thinking that the
+rest of the directory block is empty so that it moves on.
+
+The root of the tree always lives in the first data block of the
+directory. By ext2 custom, the '.' and '..' entries must appear at the
+beginning of this first block, so they are put here as two
+``struct ext4_dir_entry_2``\ s and not stored in the tree. The rest of
+the root node contains metadata about the tree and finally a hash->block
+map to find nodes that are lower in the htree. If
+``dx_root.info.indirect_levels`` is non-zero then the htree has two
+levels; the data block pointed to by the root node's map is an interior
+node, which is indexed by a minor hash. Interior nodes in this tree
+contains a zeroed out ``struct ext4_dir_entry_2`` followed by a
+minor\_hash->block map to find leafe nodes. Leaf nodes contain a linear
+array of all ``struct ext4_dir_entry_2``; all of these entries
+(presumably) hash to the same value. If there is an overflow, the
+entries simply overflow into the next leaf node, and the
+least-significant bit of the hash (in the interior node map) that gets
+us to this next leaf node is set.
+
+To traverse the directory as a htree, the code calculates the hash of
+the desired file name and uses it to find the corresponding block
+number. If the tree is flat, the block is a linear array of directory
+entries that can be searched; otherwise, the minor hash of the file name
+is computed and used against this second block to find the corresponding
+third block number. That third block number will be a linear array of
+directory entries.
+
+To traverse the directory as a linear array (such as the old code does),
+the code simply reads every data block in the directory. The blocks used
+for the htree will appear to have no entries (aside from '.' and '..')
+and so only the leaf nodes will appear to have any interesting content.
+
+The root of the htree is in ``struct dx_root``, which is the full length
+of a data block:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Type
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - dot.inode
+     - inode number of this directory.
+   * - 0x4
+     - \_\_le16
+     - dot.rec\_len
+     - Length of this record, 12.
+   * - 0x6
+     - u8
+     - dot.name\_len
+     - Length of the name, 1.
+   * - 0x7
+     - u8
+     - dot.file\_type
+     - File type of this entry, 0x2 (directory) (if the feature flag is set).
+   * - 0x8
+     - char
+     - dot.name[4]
+     - “.\\0\\0\\0”
+   * - 0xC
+     - \_\_le32
+     - dotdot.inode
+     - inode number of parent directory.
+   * - 0x10
+     - \_\_le16
+     - dotdot.rec\_len
+     - block\_size - 12. The record length is long enough to cover all htree
+       data.
+   * - 0x12
+     - u8
+     - dotdot.name\_len
+     - Length of the name, 2.
+   * - 0x13
+     - u8
+     - dotdot.file\_type
+     - File type of this entry, 0x2 (directory) (if the feature flag is set).
+   * - 0x14
+     - char
+     - dotdot\_name[4]
+     - “..\\0\\0”
+   * - 0x18
+     - \_\_le32
+     - struct dx\_root\_info.reserved\_zero
+     - Zero.
+   * - 0x1C
+     - u8
+     - struct dx\_root\_info.hash\_version
+     - Hash type, see dirhash_ table below.
+   * - 0x1D
+     - u8
+     - struct dx\_root\_info.info\_length
+     - Length of the tree information, 0x8.
+   * - 0x1E
+     - u8
+     - struct dx\_root\_info.indirect\_levels
+     - Depth of the htree. Cannot be larger than 3 if the INCOMPAT\_LARGEDIR
+       feature is set; cannot be larger than 2 otherwise.
+   * - 0x1F
+     - u8
+     - struct dx\_root\_info.unused\_flags
+     -
+   * - 0x20
+     - \_\_le16
+     - limit
+     - Maximum number of dx\_entries that can follow this header, plus 1 for
+       the header itself.
+   * - 0x22
+     - \_\_le16
+     - count
+     - Actual number of dx\_entries that follow this header, plus 1 for the
+       header itself.
+   * - 0x24
+     - \_\_le32
+     - block
+     - The block number (within the directory file) that goes with hash=0.
+   * - 0x28
+     - struct dx\_entry
+     - entries[0]
+     - As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.
+
+.. _dirhash:
+
+The directory hash is one of the following values:
+
+.. list-table::
+   :widths: 1 79
+   :header-rows: 1
+
+   * - Value
+     - Description
+   * - 0x0
+     - Legacy.
+   * - 0x1
+     - Half MD4.
+   * - 0x2
+     - Tea.
+   * - 0x3
+     - Legacy, unsigned.
+   * - 0x4
+     - Half MD4, unsigned.
+   * - 0x5
+     - Tea, unsigned.
+
+Interior nodes of an htree are recorded as ``struct dx_node``, which is
+also the full length of a data block:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Type
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - fake.inode
+     - Zero, to make it look like this entry is not in use.
+   * - 0x4
+     - \_\_le16
+     - fake.rec\_len
+     - The size of the block, in order to hide all of the dx\_node data.
+   * - 0x6
+     - u8
+     - name\_len
+     - Zero. There is no name for this “unused” directory entry.
+   * - 0x7
+     - u8
+     - file\_type
+     - Zero. There is no file type for this “unused” directory entry.
+   * - 0x8
+     - \_\_le16
+     - limit
+     - Maximum number of dx\_entries that can follow this header, plus 1 for
+       the header itself.
+   * - 0xA
+     - \_\_le16
+     - count
+     - Actual number of dx\_entries that follow this header, plus 1 for the
+       header itself.
+   * - 0xE
+     - \_\_le32
+     - block
+     - The block number (within the directory file) that goes with the lowest
+       hash value of this block. This value is stored in the parent block.
+   * - 0x12
+     - struct dx\_entry
+     - entries[0]
+     - As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.
+
+The hash maps that exist in both ``struct dx_root`` and
+``struct dx_node`` are recorded as ``struct dx_entry``, which is 8 bytes
+long:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Type
+     - Name
+     - Description
+   * - 0x0
+     - \_\_le32
+     - hash
+     - Hash code.
+   * - 0x4
+     - \_\_le32
+     - block
+     - Block number (within the directory file, not filesystem blocks) of the
+       next node in the htree.
+
+(If you think this is all quite clever and peculiar, so does the
+author.)
+
+If metadata checksums are enabled, the last 8 bytes of the directory
+block (precisely the length of one dx\_entry) are used to store a
+``struct dx_tail``, which contains the checksum. The ``limit`` and
+``count`` entries in the dx\_root/dx\_node structures are adjusted as
+necessary to fit the dx\_tail into the block. If there is no space for
+the dx\_tail, the user is notified to run e2fsck -D to rebuild the
+directory index (which will ensure that there's space for the checksum.
+The dx\_tail structure is 8 bytes long and looks like this:
+
+.. list-table::
+   :widths: 1 1 1 77
+   :header-rows: 1
+
+   * - Offset
+     - Type
+     - Name
+     - Description
+   * - 0x0
+     - u32
+     - dt\_reserved
+     - Zero.
+   * - 0x4
+     - \_\_le32
+     - dt\_checksum
+     - Checksum of the htree directory block.
+
+The checksum is calculated against the FS UUID, the htree index header
+(dx\_root or dx\_node), all of the htree indices (dx\_entry) that are in
+use, and the tail block (dx\_tail).

Documentation/filesystems/ext4/ondisk/dynamic.rst

@@ -8,3 +8,4 @@ allocated to files.
 
 .. include:: inodes.rst
 .. include:: ifork.rst
+.. include:: directory.rst