From: Vyacheslav Dubeyko <slava@xxxxxxxxxxx>
Subject: [RFC][PATCH 01/15] nilfs2: provide initial description of xattrs support
This patch adds initial description of xattrs support
architecture.
Signed-off-by: Vyacheslav Dubeyko <slava@xxxxxxxxxxx>
CC: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxxxxxx>
---
fs/nilfs2/xafile.c | 116 ++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 116 insertions(+)
create mode 100644 fs/nilfs2/xafile.c
diff --git a/fs/nilfs2/xafile.c b/fs/nilfs2/xafile.c
new file mode 100644
index 0000000..469b256
--- /dev/null
+++ b/fs/nilfs2/xafile.c
@@ -0,0 +1,116 @@
+/*
+ * xafile.c - NILFS extended attributes file (xafile) implementation.
+ *
+ * Copyright (C) 2005-2013 Nippon Telegraph and Telephone Corporation.
+ * Copyright (C) 2013 Vyacheslav Dubeyko <slava@xxxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * Written by Vyacheslav Dubeyko <slava@xxxxxxxxxxx>
+ */
+
+/*
+ * ----------------------------------------------------------------
+ * HOW DOES NILFS2 KEEP XATTRs?
+ * ----------------------------------------------------------------
+ *
+ * Extended file attributes (xattrs) is a file system feature that
+ * enables users to associate computer files with metadata not
+ * interpreted by the filesystem. Any regular file or directory may
+ * have extended attributes consisting of a name and associated data.
+ * The name must be a null-terminated string prefixed by a namespace
+ * identifier and a dot character. Currently, four namespaces exist:
+ * user, trusted, security and system. The user namespace has no
+ * restrictions with regard to naming or contents. The system namespace
+ * is primarily used by the kernel for access control lists.
+ * The security namespace is used by SELinux
+ *
+ * There are two possible places of storing xattrs:
+ * (1) inline xattr node (xanode) [OPTIONAL];
+ * (2) xattr file (xafile).
+ *
+ * The xafile is metadata file. It contains group descriptors block,
+ * bitmap blocks and xanodes areas. The xafile is simply aggregate of
+ * xanodes that can provide a xanode of some size (1KB, 2 KB or 4 KB)
+ * by identification number (ID) or, in other words, by index.
+ *
+ * ----------------------------------------------------------------
+ * XAFILE STRUCTURE
+ * ----------------------------------------------------------------
+ *
+ * The xafile is metadata file that, basically, has such structure:
+ *
+ * +0 +1 +2
+ * +------------------+--------+------------------------------+
+ * + Group descriptor | Bitmap | Xanodes area |
+ * + block | block | |
+ * +------------------+--------+------------------------------+
+ *
+ * Thereby, xafile is simply aggregate of xanodes that can provide
+ * xanode of some size (1KB, 2 KB or 4 KB) by identification
+ * number (ID).
+ *
+ * NILFS2 volume contains xafile if s_feature_compat_ro field of
+ * superblock contains NILFS_FEATURE_COMPAT_RO_XAFILE flag.
+ * The s_xafile_xanode_size field of superblock keeps value of
+ * xanode's size in bytes which is determined during mkfs phase.
+ *
+ * The xanodes area haven't any predetermined order of xanodes.
+ * Simply speaking, this area is aggregation of xanodes of different trees.
+ * Every tree begins from a xanode that keeps knowledge about other xanodes
+ * in a tree. The i_xattr field of on-disk inode can keep
+ * ID of xanode.
+ *
+ * The xanode has such structure:
+ *
+ * ----------------
+ * | HEADER |
+ * ----------------
+ * | INDEX KEYS |
+ * ----------------
+ * | LEAF KEYS |
+ * ----------------
+ * | |
+ * | FREE AREA |
+ * | |
+ * ----------------
+ * | ENTRIES |
+ * ----------------
+ *
+ * The xanode begins with header. Initially, empty xanode
+ * contains reserved place for 4 index keys. It is located leaf keys
+ * after space that is reserved for index keys. Leaf keys grow
+ * towards entries. The chain of keys is ended by termination marker
+ * ("end key"). Entries begin from xanode end and its grow from the
+ * end towards "end key".
+ *
+ * ----------------------------------------------------------------
+ * XANODE ENTRY STRUCTURE
+ * ----------------------------------------------------------------
+ *
+ * The entry of xanode has purpose to store name and
+ * binary value pair.
+ *
+ * Structure of xanode's entry in common case:
+ *
+ * ----------------------------------------
+ * | name[variable_length] | binary value |
+ * ----------------------------------------
+ * |<---------- entry_size ----------->|
+ *
+ * The name hash defines length of the name. The length
+ * of binary value is defined as difference between
+ * entry_size and name_len.
+ */
--
1.7.9.5
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