[PATCH 10/22] docs: add XFS dir/attr btree structure to the DS&A book

Darrick J. Wong
2018-10-04T04:19:36Z

.../filesystems/xfs-data-structures/dabtrees.rst | 221 ++++++++++++++++++++
.../filesystems/xfs-data-structures/globals.rst | 1
2 files changed, 222 insertions(+)
create mode 100644 Documentation/filesystems/xfs-data-structures/dabtrees.rst

diff --git a/Documentation/filesystems/xfs-data-structures/dabtrees.rst b/Documentation/filesystems/xfs-data-structures/dabtrees.rst
new file mode 100644
index 000000000000..9daac6295941
--- /dev/null
+++ b/Documentation/filesystems/xfs-data-structures/dabtrees.rst
@@ -0,0 +1,221 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Variable Length Record B+trees
+------------------------------
+
+Directories and extended attributes are implemented as a simple key-value
+record store inside the blocks pointed to by the data or attribute fork of a
+file. Blocks referenced by either data structure are block offsets of an inode
+fork, not physical blocks.
+
+Directory and attribute data are stored as a linear array of variable-length
+records in the low blocks of a fork. Both data types share the property that
+record keys and record values are both arbitrary and unique sequences of
+bytes. See the respective sections about `directories <#directories>`__ or
+`attributes <#extended-attributes>`__ for more information about the exact
+record formats.
+
+The dir/attr b+tree (or "dabtree"), if present, computes a hash of the record
+key to produce the b+tree key, and b+tree keys are used to index the fork
+block in which the record may be found. Unlike the fixed-length b+trees, the
+variable length b+trees can index the same key multiple times. B+tree
+keypointers and records both take this format:
+
+::
+
+ +---------+--------------+
+ | hashval | before_block |
+ +---------+--------------+
+
+The "before block" is the block offset in the inode fork of the block in which
+we can find the record whose hashed key is "hashval". The hash function is as
+follows:
+
+.. code:: c
+
+ #define rol32(x,y) (((x) << (y)) | ((x) >> (32 - (y))))
+
+ xfs_dahash_t
+ xfs_da_hashname(const uint8_t *name, int namelen)
+ {
+ xfs_dahash_t hash;
+
+ /*
+ * Do four characters at a time as long as we can.
+ */
+ for (hash = 0; namelen >= 4; namelen -= 4, name += 4)
+ hash = (name[0] << 21) ^ (name[1] << 14) ^ (name[2] << 7) ^
+ (name[3] << 0) ^ rol32(hash, 7 * 4);
+
+ /*
+ * Now do the rest of the characters.
+ */
+ switch (namelen) {
+ case 3:
+ return (name[0] << 14) ^ (name[1] << 7) ^ (name[2] << 0) ^
+ rol32(hash, 7 * 3);
+ case 2:
+ return (name[0] << 7) ^ (name[1] << 0) ^ rol32(hash, 7 * 2);
+ case 1:
+ return (name[0] << 0) ^ rol32(hash, 7 * 1);
+ default: /* case 0: */
+ return hash;
+ }
+ }
+
+.. _directory-attribute-block-header:
+
+Block Headers
+~~~~~~~~~~~~~
+
+- Tree nodes, leaf and node `directories <#directories>`__, and leaf and node
+ `extended attributes <#extended-attributes>`__ use the xfs\_da\_blkinfo\_t
+ filesystem block header. The structure appears as follows:
+
+.. code:: c
+
+ typedef struct xfs_da_blkinfo {
+ __be32 forw;
+ __be32 back;
+ __be16 magic;
+ __be16 pad;
+ } xfs_da_blkinfo_t;
+
+**forw**
+ Logical block offset of the previous B+tree block at this level.
+
+**back**
+ Logical block offset of the next B+tree block at this level.
+
+**magic**
+ Magic number for this directory/attribute block.
+
+**pad**
+ Padding to maintain alignment.
+
+- On a v5 filesystem, the leaves use the struct xfs\_da3\_blkinfo\_t
+ filesystem block header. This header is used in the same place as
+ xfs\_da\_blkinfo\_t:
+
+.. code:: c
+
+ struct xfs_da3_blkinfo {
+ /* these values are inside xfs_da_blkinfo */
+ __be32 forw;
+ __be32 back;
+ __be16 magic;
+ __be16 pad;
+
+ __be32 crc;
+ __be64 blkno;
+ __be64 lsn;
+ uuid_t uuid;
+ __be64 owner;
+ };
+
+**forw**
+ Logical block offset of the previous B+tree block at this level.
+
+**back**
+ Logical block offset of the next B+tree block at this level.
+
+**magic**
+ Magic number for this directory/attribute block.
+
+**pad**
+ Padding to maintain alignment.
+
+**crc**
+ Checksum of the directory/attribute block.
+
+**blkno**
+ Block number of this directory/attribute block.
+
+**lsn**
+ Log sequence number of the last write to this block.
+
+**uuid**
+ The UUID of this block, which must match either sb\_uuid or sb\_meta\_uuid
+ depending on which features are set.
+
+**owner**
+ The inode number that this directory/attribute block belongs to.
+
+.. _directory-attribute-internal-node:
+
+Internal Nodes
+~~~~~~~~~~~~~~
+
+The nodes of a dabtree have the following format:
+
+.. code:: c
+
+ typedef struct xfs_da_intnode {
+ struct xfs_da_node_hdr {
+ xfs_da_blkinfo_t info;
+ __uint16_t count;
+ __uint16_t level;
+ } hdr;
+ struct xfs_da_node_entry {
+ xfs_dahash_t hashval;
+ xfs_dablk_t before;
+ } btree[1];
+ } xfs_da_intnode_t;
+
+**info**
+ Directory/attribute block info. The magic number is XFS\_DA\_NODE\_MAGIC
+ (0xfebe).
+
+**count**
+ Number of node entries in this block.
+
+**level**
+ The level of this block in the B+tree. Levels start at 1 for blocks that
+ point to directory or attribute data blocks and increase towards the root.
+
+**hashval**
+ The hash value of a particular record.
+
+**before**
+ The directory/attribute logical block containing all entries up to the
+ corresponding hash value.
+
+ - On a v5 filesystem, the directory/attribute node blocks have the
+ following structure:
+
+.. code:: c
+
+ struct xfs_da3_intnode {
+ struct xfs_da3_node_hdr {
+ struct xfs_da3_blkinfo info;
+ __uint16_t count;
+ __uint16_t level;
+ __uint32_t pad32;
+ } hdr;
+ struct xfs_da_node_entry {
+ xfs_dahash_t hashval;
+ xfs_dablk_t before;
+ } btree[1];
+ };
+
+**info**
+ Directory/attribute block info. The magic number is XFS\_DA3\_NODE\_MAGIC
+ (0x3ebe).
+
+**count**
+ Number of node entries in this block.
+
+**level**
+ The level of this block in the B+tree. Levels start at 1 for blocks that
+ point to directory or attribute data blocks, and increase towards the
+ root.
+
+**pad32**
+ Padding to maintain alignment.
+
+**hashval**
+ The hash value of a particular record.
+
+**before**
+ The directory/attribute logical block containing all entries up to the
+ corresponding hash value.
diff --git a/Documentation/filesystems/xfs-data-structures/globals.rst b/Documentation/filesystems/xfs-data-structures/globals.rst
index 8a2173908b0e..546968699a56 100644
--- a/Documentation/filesystems/xfs-data-structures/globals.rst
+++ b/Documentation/filesystems/xfs-data-structures/globals.rst
@@ -4,3 +4,4 @@ Global Structures
=================

.. include:: btrees.rst
+.. include:: dabtrees.rst