linux-stable/Documentation/filesystems/ext4/ifork.rst

.. SPDX-License-Identifier: GPL-2.0

The Contents of inode.i_block
------------------------------

Depending on the type of file an inode describes, the 60 bytes of
storage in ``inode.i_block`` can be used in different ways. In general,
regular files and directories will use it for file block indexing
information, and special files will use it for special purposes.

Symbolic Links
~~~~~~~~~~~~~~

The target of a symbolic link will be stored in this field if the target
string is less than 60 bytes long. Otherwise, either extents or block
maps will be used to allocate data blocks to store the link target.

Direct/Indirect Block Addressing
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In ext2/3, file block numbers were mapped to logical block numbers by
means of an (up to) three level 1-1 block map. To find the logical block
that stores a particular file block, the code would navigate through
this increasingly complicated structure. Notice that there is neither a
magic number nor a checksum to provide any level of confidence that the
block isn't full of garbage.

.. ifconfig:: builder != 'latex'

   .. include:: blockmap.rst

.. ifconfig:: builder == 'latex'

   [Table omitted because LaTeX doesn't support nested tables.]

Note that with this block mapping scheme, it is necessary to fill out a
lot of mapping data even for a large contiguous file! This inefficiency
led to the creation of the extent mapping scheme, discussed below.

Notice also that a file using this mapping scheme cannot be placed
higher than 2^32 blocks.

Extent Tree
~~~~~~~~~~~

In ext4, the file to logical block map has been replaced with an extent
tree. Under the old scheme, allocating a contiguous run of 1,000 blocks
requires an indirect block to map all 1,000 entries; with extents, the
mapping is reduced to a single ``struct ext4_extent`` with
``ee_len = 1000``. If flex_bg is enabled, it is possible to allocate
very large files with a single extent, at a considerable reduction in
metadata block use, and some improvement in disk efficiency. The inode
must have the extents flag (0x80000) flag set for this feature to be in
use.

Extents are arranged as a tree. Each node of the tree begins with a
``struct ext4_extent_header``. If the node is an interior node
(``eh.eh_depth`` > 0), the header is followed by ``eh.eh_entries``
instances of ``struct ext4_extent_idx``; each of these index entries
points to a block containing more nodes in the extent tree. If the node
is a leaf node (``eh.eh_depth == 0``), then the header is followed by
``eh.eh_entries`` instances of ``struct ext4_extent``; these instances
point to the file's data blocks. The root node of the extent tree is
stored in ``inode.i_block``, which allows for the first four extents to
be recorded without the use of extra metadata blocks.

The extent tree header is recorded in ``struct ext4_extent_header``,
which is 12 bytes long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - __le16
     - eh_magic
     - Magic number, 0xF30A.
   * - 0x2
     - __le16
     - eh_entries
     - Number of valid entries following the header.
   * - 0x4
     - __le16
     - eh_max
     - Maximum number of entries that could follow the header.
   * - 0x6
     - __le16
     - eh_depth
     - Depth of this extent node in the extent tree. 0 = this extent node
       points to data blocks; otherwise, this extent node points to other
       extent nodes. The extent tree can be at most 5 levels deep: a logical
       block number can be at most ``2^32``, and the smallest ``n`` that
       satisfies ``4*(((blocksize - 12)/12)^n) >= 2^32`` is 5.
   * - 0x8
     - __le32
     - eh_generation
     - Generation of the tree. (Used by Lustre, but not standard ext4).

Internal nodes of the extent tree, also known as index nodes, are
recorded as ``struct ext4_extent_idx``, and are 12 bytes long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - __le32
     - ei_block
     - This index node covers file blocks from 'block' onward.
   * - 0x4
     - __le32
     - ei_leaf_lo
     - Lower 32-bits of the block number of the extent node that is the next
       level lower in the tree. The tree node pointed to can be either another
       internal node or a leaf node, described below.
   * - 0x8
     - __le16
     - ei_leaf_hi
     - Upper 16-bits of the previous field.
   * - 0xA
     - __u16
     - ei_unused
     -

Leaf nodes of the extent tree are recorded as ``struct ext4_extent``,
and are also 12 bytes long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - __le32
     - ee_block
     - First file block number that this extent covers.
   * - 0x4
     - __le16
     - ee_len
     - Number of blocks covered by extent. If the value of this field is <=
       32768, the extent is initialized. If the value of the field is > 32768,
       the extent is uninitialized and the actual extent length is ``ee_len`` -
       32768. Therefore, the maximum length of a initialized extent is 32768
       blocks, and the maximum length of an uninitialized extent is 32767.
   * - 0x6
     - __le16
     - ee_start_hi
     - Upper 16-bits of the block number to which this extent points.
   * - 0x8
     - __le32
     - ee_start_lo
     - Lower 32-bits of the block number to which this extent points.

Prior to the introduction of metadata checksums, the extent header +
extent entries always left at least 4 bytes of unallocated space at the
end of each extent tree data block (because (2^x % 12) >= 4). Therefore,
the 32-bit checksum is inserted into this space. The 4 extents in the
inode do not need checksumming, since the inode is already checksummed.
The checksum is calculated against the FS UUID, the inode number, the
inode generation, and the entire extent block leading up to (but not
including) the checksum itself.

``struct ext4_extent_tail`` is 4 bytes long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - __le32
     - eb_checksum
     - Checksum of the extent block, crc32c(uuid+inum+igeneration+extentblock)

Inline Data
~~~~~~~~~~~

If the inline data feature is enabled for the filesystem and the flag is
set for the inode, it is possible that the first 60 bytes of the file
data are stored here.