squashfs: update documentation to include description of xattr layout
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
diff --git a/Documentation/filesystems/squashfs.txt b/Documentation/filesystems/squashfs.txt
index b324c03..203f720 100644
--- a/Documentation/filesystems/squashfs.txt
+++ b/Documentation/filesystems/squashfs.txt
@@ -38,7 +38,8 @@
Real inode numbers: yes no
32-bit uids/gids: yes no
File creation time: yes no
-Xattr and ACL support: no no
+Xattr support: yes no
+ACL support: no no
Squashfs compresses data, inodes and directories. In addition, inode and
directory data are highly compacted, and packed on byte boundaries. Each
@@ -58,7 +59,7 @@
3. SQUASHFS FILESYSTEM DESIGN
-----------------------------
-A squashfs filesystem consists of seven parts, packed together on a byte
+A squashfs filesystem consists of a maximum of eight parts, packed together on a byte
alignment:
---------------
@@ -80,6 +81,9 @@
|---------------|
| uid/gid |
| lookup table |
+ |---------------|
+ | xattr |
+ | table |
---------------
Compressed data blocks are written to the filesystem as files are read from
@@ -192,6 +196,26 @@
used to locate these. This second index table for speed of access (and because
it is small) is read at mount time and cached in memory.
+3.7 Xattr table
+---------------
+
+The xattr table contains extended attributes for each inode. The xattrs
+for each inode are stored in a list, each list entry containing a type,
+name and value field. The type field encodes the xattr prefix
+("user.", "trusted." etc) and it also encodes how the name/value fields
+should be interpreted. Currently the type indicates whether the value
+is stored inline (in which case the value field contains the xattr value),
+or if it is stored out of line (in which case the value field stores a
+reference to where the actual value is stored). This allows large values
+to be stored out of line improving scanning and lookup performance and it
+also allows values to be de-duplicated, the value being stored once, and
+all other occurences holding an out of line reference to that value.
+
+The xattr lists are packed into compressed 8K metadata blocks.
+To reduce overhead in inodes, rather than storing the on-disk
+location of the xattr list inside each inode, a 32-bit xattr id
+is stored. This xattr id is mapped into the location of the xattr
+list using a second xattr id lookup table.
4. TODOS AND OUTSTANDING ISSUES
-------------------------------
@@ -199,9 +223,7 @@
4.1 Todo list
-------------
-Implement Xattr and ACL support. The Squashfs 4.0 filesystem layout has hooks
-for these but the code has not been written. Once the code has been written
-the existing layout should not require modification.
+Implement ACL support.
4.2 Squashfs internal cache
---------------------------