[linux-2.6-block.git] / Documentation / filesystems / ext4 / eainode.rst

.. SPDX-License-Identifier: GPL-2.0

Large Extended Attribute Values
-------------------------------

To enable ext4 to store extended attribute values that do not fit in the
inode or in the single extended attribute block attached to an inode,
the EA_INODE feature allows us to store the value in the data blocks of
a regular file inode. This “EA inode” is linked only from the extended
attribute name index and must not appear in a directory entry. The
inode's i_atime field is used to store a checksum of the xattr value;
and i_ctime/i_version store a 64-bit reference count, which enables
sharing of large xattr values between multiple owning inodes. For
backward compatibility with older versions of this feature, the
i_mtime/i_generation *may* store a back-reference to the inode number
and i_generation of the **one** owning inode (in cases where the EA
inode is not referenced by multiple inodes) to verify that the EA inode
is the correct one being accessed.
Commit	Line	Data
c09f3bac DW	1	.. SPDX-License-Identifier: GPL-2.0
	2
	3	Large Extended Attribute Values
	4	-------------------------------
	5
	6	To enable ext4 to store extended attribute values that do not fit in the
	7	inode or in the single extended attribute block attached to an inode,
3103084a	8	the EA_INODE feature allows us to store the value in the data blocks of
c09f3bac DW	9	a regular file inode. This “EA inode” is linked only from the extended
c09f3bac DW	10	attribute name index and must not appear in a directory entry. The
3103084a WJ	11	inode's i_atime field is used to store a checksum of the xattr value;
3103084a WJ	12	and i_ctime/i_version store a 64-bit reference count, which enables
c09f3bac DW	13	sharing of large xattr values between multiple owning inodes. For
c09f3bac DW	14	backward compatibility with older versions of this feature, the
3103084a WJ	15	i_mtime/i_generation may store a back-reference to the inode number
3103084a WJ	16	and i_generation of the one owning inode (in cases where the EA
c09f3bac DW	17	inode is not referenced by multiple inodes) to verify that the EA inode
c09f3bac DW	18	is the correct one being accessed.