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+
+Aufs4 -- advanced multi layered unification filesystem version 4.x
+http://aufs.sf.net
+Junjiro R. Okajima
+
+
+0. Introduction
+----------------------------------------
+In the early days, aufs was entirely re-designed and re-implemented
+Unionfs Version 1.x series. Adding many original ideas, approaches,
+improvements and implementations, it becomes totally different from
+Unionfs while keeping the basic features.
+Recently, Unionfs Version 2.x series begin taking some of the same
+approaches to aufs1's.
+Unionfs is being developed by Professor Erez Zadok at Stony Brook
+University and his team.
+
+Aufs4 supports linux-4.0 and later, and for linux-3.x series try aufs3.
+If you want older kernel version support, try aufs2-2.6.git or
+aufs2-standalone.git repository, aufs1 from CVS on SourceForge.
+
+Note: it becomes clear that "Aufs was rejected. Let's give it up."
+      According to Christoph Hellwig, linux rejects all union-type
+      filesystems but UnionMount.
+<http://marc.info/?l=linux-kernel&m=123938533724484&w=2>
+
+PS. Al Viro seems have a plan to merge aufs as well as overlayfs and
+    UnionMount, and he pointed out an issue around a directory mutex
+    lock and aufs addressed it. But it is still unsure whether aufs will
+    be merged (or any other union solution).
+<http://marc.info/?l=linux-kernel&m=136312705029295&w=1>
+
+
+1. Features
+----------------------------------------
+- unite several directories into a single virtual filesystem. The member
+  directory is called as a branch.
+- you can specify the permission flags to the branch, which are 'readonly',
+  'readwrite' and 'whiteout-able.'
+- by upper writable branch, internal copyup and whiteout, files/dirs on
+  readonly branch are modifiable logically.
+- dynamic branch manipulation, add, del.
+- etc...
+
+Also there are many enhancements in aufs, such as:
+- test only the highest one for the directory permission (dirperm1)
+- copyup on open (coo=)
+- 'move' policy for copy-up between two writable branches, after
+  checking free space.
+- xattr, acl
+- readdir(3) in userspace.
+- keep inode number by external inode number table
+- keep the timestamps of file/dir in internal copyup operation
+- seekable directory, supporting NFS readdir.
+- whiteout is hardlinked in order to reduce the consumption of inodes
+  on branch
+- do not copyup, nor create a whiteout when it is unnecessary
+- revert a single systemcall when an error occurs in aufs
+- remount interface instead of ioctl
+- maintain /etc/mtab by an external command, /sbin/mount.aufs.
+- loopback mounted filesystem as a branch
+- kernel thread for removing the dir who has a plenty of whiteouts
+- support copyup sparse file (a file which has a 'hole' in it)
+- default permission flags for branches
+- selectable permission flags for ro branch, whether whiteout can
+  exist or not
+- export via NFS.
+- support <sysfs>/fs/aufs and <debugfs>/aufs.
+- support multiple writable branches, some policies to select one
+  among multiple writable branches.
+- a new semantics for link(2) and rename(2) to support multiple
+  writable branches.
+- no glibc changes are required.
+- pseudo hardlink (hardlink over branches)
+- allow a direct access manually to a file on branch, e.g. bypassing aufs.
+  including NFS or remote filesystem branch.
+- userspace wrapper for pathconf(3)/fpathconf(3) with _PC_LINK_MAX.
+- and more...
+
+Currently these features are dropped temporary from aufs4.
+See design/08plan.txt in detail.
+- nested mount, i.e. aufs as readonly no-whiteout branch of another aufs
+  (robr)
+- statistics of aufs thread (/sys/fs/aufs/stat)
+
+Features or just an idea in the future (see also design/*.txt),
+- reorder the branch index without del/re-add.
+- permanent xino files for NFSD
+- an option for refreshing the opened files after add/del branches
+- light version, without branch manipulation. (unnecessary?)
+- copyup in userspace
+- inotify in userspace
+- readv/writev
+
+
+2. Download
+----------------------------------------
+There are three GIT trees for aufs4, aufs4-linux.git,
+aufs4-standalone.git, and aufs-util.git. Note that there is no "4" in
+"aufs-util.git."
+While the aufs-util is always necessary, you need either of aufs4-linux
+or aufs4-standalone.
+
+The aufs4-linux tree includes the whole linux mainline GIT tree,
+git://git.kernel.org/.../torvalds/linux.git.
+And you cannot select CONFIG_AUFS_FS=m for this version, eg. you cannot
+build aufs4 as an external kernel module.
+Several extra patches are not included in this tree. Only
+aufs4-standalone tree contains them. They are described in the later
+section "Configuration and Compilation."
+
+On the other hand, the aufs4-standalone tree has only aufs source files
+and necessary patches, and you can select CONFIG_AUFS_FS=m.
+But you need to apply all aufs patches manually.
+
+You will find GIT branches whose name is in form of "aufs4.x" where "x"
+represents the linux kernel version, "linux-4.x". For instance,
+"aufs4.0" is for linux-4.0. For latest "linux-4.x-rcN", use
+"aufs4.x-rcN" branch.
+
+o aufs4-linux tree
+$ git clone --reference /your/linux/git/tree \
+	git://github.com/sfjro/aufs4-linux.git aufs4-linux.git
+- if you don't have linux GIT tree, then remove "--reference ..."
+$ cd aufs4-linux.git
+$ git checkout origin/aufs4.0
+
+Or You may want to directly git-pull aufs into your linux GIT tree, and
+leave the patch-work to GIT.
+$ cd /your/linux/git/tree
+$ git remote add aufs4 git://github.com/sfjro/aufs4-linux.git
+$ git fetch aufs4
+$ git checkout -b my4.0 v4.0
+$ (add your local change...)
+$ git pull aufs4 aufs4.0
+- now you have v4.0 + your_changes + aufs4.0 in you my4.0 branch.
+- you may need to solve some conflicts between your_changes and
+  aufs4.0. in this case, git-rerere is recommended so that you can
+  solve the similar conflicts automatically when you upgrade to 4.1 or
+  later in the future.
+
+o aufs4-standalone tree
+$ git clone git://github.com/sfjro/aufs4-standalone.git aufs4-standalone.git
+$ cd aufs4-standalone.git
+$ git checkout origin/aufs4.0
+
+o aufs-util tree
+$ git clone git://git.code.sf.net/p/aufs/aufs-util aufs-util.git
+- note that the public aufs-util.git is on SourceForge instead of
+  GitHUB.
+$ cd aufs-util.git
+$ git checkout origin/aufs4.0
+
+Note: The 4.x-rcN branch is to be used with `rc' kernel versions ONLY.
+The minor version number, 'x' in '4.x', of aufs may not always
+follow the minor version number of the kernel.
+Because changes in the kernel that cause the use of a new
+minor version number do not always require changes to aufs-util.
+
+Since aufs-util has its own minor version number, you may not be
+able to find a GIT branch in aufs-util for your kernel's
+exact minor version number.
+In this case, you should git-checkout the branch for the
+nearest lower number.
+
+For (an unreleased) example:
+If you are using "linux-4.10" and the "aufs4.10" branch
+does not exist in aufs-util repository, then "aufs4.9", "aufs4.8"
+or something numerically smaller is the branch for your kernel.
+
+Also you can view all branches by
+	$ git branch -a
+
+
+3. Configuration and Compilation
+----------------------------------------
+Make sure you have git-checkout'ed the correct branch.
+
+For aufs4-linux tree,
+- enable CONFIG_AUFS_FS.
+- set other aufs configurations if necessary.
+
+For aufs4-standalone tree,
+There are several ways to build.
+
+1.
+- apply ./aufs4-kbuild.patch to your kernel source files.
+- apply ./aufs4-base.patch too.
+- apply ./aufs4-mmap.patch too.
+- apply ./aufs4-standalone.patch too, if you have a plan to set
+  CONFIG_AUFS_FS=m. otherwise you don't need ./aufs4-standalone.patch.
+- copy ./{Documentation,fs,include/uapi/linux/aufs_type.h} files to your
+  kernel source tree. Never copy $PWD/include/uapi/linux/Kbuild.
+- enable CONFIG_AUFS_FS, you can select either
+  =m or =y.
+- and build your kernel as usual.
+- install the built kernel.
+  Note: Since linux-3.9, every filesystem module requires an alias
+  "fs-<fsname>". You should make sure that "fs-aufs" is listed in your
+  modules.aliases file if you set CONFIG_AUFS_FS=m.
+- install the header files too by "make headers_install" to the
+  directory where you specify. By default, it is $PWD/usr.
+  "make help" shows a brief note for headers_install.
+- and reboot your system.
+
+2.
+- module only (CONFIG_AUFS_FS=m).
+- apply ./aufs4-base.patch to your kernel source files.
+- apply ./aufs4-mmap.patch too.
+- apply ./aufs4-standalone.patch too.
+- build your kernel, don't forget "make headers_install", and reboot.
+- edit ./config.mk and set other aufs configurations if necessary.
+  Note: You should read $PWD/fs/aufs/Kconfig carefully which describes
+  every aufs configurations.
+- build the module by simple "make".
+  Note: Since linux-3.9, every filesystem module requires an alias
+  "fs-<fsname>". You should make sure that "fs-aufs" is listed in your
+  modules.aliases file.
+- you can specify ${KDIR} make variable which points to your kernel
+  source tree.
+- install the files
+  + run "make install" to install the aufs module, or copy the built
+    $PWD/aufs.ko to /lib/modules/... and run depmod -a (or reboot simply).
+  + run "make install_headers" (instead of headers_install) to install
+    the modified aufs header file (you can specify DESTDIR which is
+    available in aufs standalone version's Makefile only), or copy
+    $PWD/usr/include/linux/aufs_type.h to /usr/include/linux or wherever
+    you like manually. By default, the target directory is $PWD/usr.
+- no need to apply aufs4-kbuild.patch, nor copying source files to your
+  kernel source tree.
+
+Note: The header file aufs_type.h is necessary to build aufs-util
+      as well as "make headers_install" in the kernel source tree.
+      headers_install is subject to be forgotten, but it is essentially
+      necessary, not only for building aufs-util.
+      You may not meet problems without headers_install in some older
+      version though.
+
+And then,
+- read README in aufs-util, build and install it
+- note that your distribution may contain an obsoleted version of
+  aufs_type.h in /usr/include/linux or something. When you build aufs
+  utilities, make sure that your compiler refers the correct aufs header
+  file which is built by "make headers_install."
+- if you want to use readdir(3) in userspace or pathconf(3) wrapper,
+  then run "make install_ulib" too. And refer to the aufs manual in
+  detail.
+
+There several other patches in aufs4-standalone.git. They are all
+optional. When you meet some problems, they will help you.
+- aufs4-loopback.patch
+  Supports a nested loopback mount in a branch-fs. This patch is
+  unnecessary until aufs produces a message like "you may want to try
+  another patch for loopback file".
+- vfs-ino.patch
+  Modifies a system global kernel internal function get_next_ino() in
+  order to stop assigning 0 for an inode-number. Not directly related to
+  aufs, but recommended generally.
+- tmpfs-idr.patch
+  Keeps the tmpfs inode number as the lowest value. Effective to reduce
+  the size of aufs XINO files for tmpfs branch. Also it prevents the
+  duplication of inode number, which is important for backup tools and
+  other utilities. When you find aufs XINO files for tmpfs branch
+  growing too much, try this patch.
+- lockdep-debug.patch
+  Because aufs is not only an ordinary filesystem (callee of VFS), but
+  also a caller of VFS functions for branch filesystems, subclassing of
+  the internal locks for LOCKDEP is necessary. LOCKDEP is a debugging
+  feature of linux kernel. If you enable CONFIG_LOCKDEP, then you will
+  need to apply this debug patch to expand several constant values.
+  If don't know what LOCKDEP, then you don't have apply this patch.
+
+
+4. Usage
+----------------------------------------
+At first, make sure aufs-util are installed, and please read the aufs
+manual, aufs.5 in aufs-util.git tree.
+$ man -l aufs.5
+
+And then,
+$ mkdir /tmp/rw /tmp/aufs
+# mount -t aufs -o br=/tmp/rw:${HOME} none /tmp/aufs
+
+Here is another example. The result is equivalent.
+# mount -t aufs -o br=/tmp/rw=rw:${HOME}=ro none /tmp/aufs
+  Or
+# mount -t aufs -o br:/tmp/rw none /tmp/aufs
+# mount -o remount,append:${HOME} /tmp/aufs
+
+Then, you can see whole tree of your home dir through /tmp/aufs. If
+you modify a file under /tmp/aufs, the one on your home directory is
+not affected, instead the same named file will be newly created under
+/tmp/rw. And all of your modification to a file will be applied to
+the one under /tmp/rw. This is called the file based Copy on Write
+(COW) method.
+Aufs mount options are described in aufs.5.
+If you run chroot or something and make your aufs as a root directory,
+then you need to customize the shutdown script. See the aufs manual in
+detail.
+
+Additionally, there are some sample usages of aufs which are a
+diskless system with network booting, and LiveCD over NFS.
+See sample dir in CVS tree on SourceForge.
+
+
+5. Contact
+----------------------------------------
+When you have any problems or strange behaviour in aufs, please let me
+know with:
+- /proc/mounts (instead of the output of mount(8))
+- /sys/module/aufs/*
+- /sys/fs/aufs/* (if you have them)
+- /debug/aufs/* (if you have them)
+- linux kernel version
+  if your kernel is not plain, for example modified by distributor,
+  the url where i can download its source is necessary too.
+- aufs version which was printed at loading the module or booting the
+  system, instead of the date you downloaded.
+- configuration (define/undefine CONFIG_AUFS_xxx)
+- kernel configuration or /proc/config.gz (if you have it)
+- behaviour which you think to be incorrect
+- actual operation, reproducible one is better
+- mailto: aufs-users at lists.sourceforge.net
+
+Usually, I don't watch the Public Areas(Bugs, Support Requests, Patches,
+and Feature Requests) on SourceForge. Please join and write to
+aufs-users ML.
+
+
+6. Acknowledgements
+----------------------------------------
+Thanks to everyone who have tried and are using aufs, whoever
+have reported a bug or any feedback.
+
+Especially donators:
+Tomas Matejicek(slax.org) made a donation (much more than once).
+	Since Apr 2010, Tomas M (the author of Slax and Linux Live
+	scripts) is making "doubling" donations.
+	Unfortunately I cannot list all of the donators, but I really
+	appreciate.
+	It ends Aug 2010, but the ordinary donation URL is still available.
+	<http://sourceforge.net/donate/index.php?group_id=167503>
+Dai Itasaka made a donation (2007/8).
+Chuck Smith made a donation (2008/4, 10 and 12).
+Henk Schoneveld made a donation (2008/9).
+Chih-Wei Huang, ASUS, CTC donated Eee PC 4G (2008/10).
+Francois Dupoux made a donation (2008/11).
+Bruno Cesar Ribas and Luis Carlos Erpen de Bona, C3SL serves public
+	aufs2 GIT tree (2009/2).
+William Grant made a donation (2009/3).
+Patrick Lane made a donation (2009/4).
+The Mail Archive (mail-archive.com) made donations (2009/5).
+Nippy Networks (Ed Wildgoose) made a donation (2009/7).
+New Dream Network, LLC (www.dreamhost.com) made a donation (2009/11).
+Pavel Pronskiy made a donation (2011/2).
+Iridium and Inmarsat satellite phone retailer (www.mailasail.com), Nippy
+	Networks (Ed Wildgoose) made a donation for hardware (2011/3).
+Max Lekomcev (DOM-TV project) made a donation (2011/7, 12, 2012/3, 6 and
+11).
+Sam Liddicott made a donation (2011/9).
+Era Scarecrow made a donation (2013/4).
+Bor Ratajc made a donation (2013/4).
+Alessandro Gorreta made a donation (2013/4).
+POIRETTE Marc made a donation (2013/4).
+Alessandro Gorreta made a donation (2013/4).
+lauri kasvandik made a donation (2013/5).
+"pemasu from Finland" made a donation (2013/7).
+The Parted Magic Project made a donation (2013/9 and 11).
+Pavel Barta made a donation (2013/10).
+Nikolay Pertsev made a donation (2014/5).
+James B made a donation (2014/7 and 2015/7).
+Stefano Di Biase made a donation (2014/8).
+Daniel Epellei made a donation (2015/1).
+OmegaPhil made a donation (2016/1).
+
+Thank you very much.
+Donations are always, including future donations, very important and
+helpful for me to keep on developing aufs.
+
+
+7.
+----------------------------------------
+If you are an experienced user, no explanation is needed. Aufs is
+just a linux filesystem.
+
+
+Enjoy!
+
+# Local variables: ;
+# mode: text;
+# End: ;
diff --git a/Documentation/filesystems/aufs/design/01intro.txt b/Documentation/filesystems/aufs/design/01intro.txt
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+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+Introduction
+----------------------------------------
+
+aufs [ei ju: ef es] | [a u f s]
+1. abbrev. for "advanced multi-layered unification filesystem".
+2. abbrev. for "another unionfs".
+3. abbrev. for "auf das" in German which means "on the" in English.
+   Ex. "Butter aufs Brot"(G) means "butter onto bread"(E).
+   But "Filesystem aufs Filesystem" is hard to understand.
+
+AUFS is a filesystem with features:
+- multi layered stackable unification filesystem, the member directory
+  is called as a branch.
+- branch permission and attribute, 'readonly', 'real-readonly',
+  'readwrite', 'whiteout-able', 'link-able whiteout', etc. and their
+  combination.
+- internal "file copy-on-write".
+- logical deletion, whiteout.
+- dynamic branch manipulation, adding, deleting and changing permission.
+- allow bypassing aufs, user's direct branch access.
+- external inode number translation table and bitmap which maintains the
+  persistent aufs inode number.
+- seekable directory, including NFS readdir.
+- file mapping, mmap and sharing pages.
+- pseudo-link, hardlink over branches.
+- loopback mounted filesystem as a branch.
+- several policies to select one among multiple writable branches.
+- revert a single systemcall when an error occurs in aufs.
+- and more...
+
+
+Multi Layered Stackable Unification Filesystem
+----------------------------------------------------------------------
+Most people already knows what it is.
+It is a filesystem which unifies several directories and provides a
+merged single directory. When users access a file, the access will be
+passed/re-directed/converted (sorry, I am not sure which English word is
+correct) to the real file on the member filesystem. The member
+filesystem is called 'lower filesystem' or 'branch' and has a mode
+'readonly' and 'readwrite.' And the deletion for a file on the lower
+readonly branch is handled by creating 'whiteout' on the upper writable
+branch.
+
+On LKML, there have been discussions about UnionMount (Jan Blunck,
+Bharata B Rao and Valerie Aurora) and Unionfs (Erez Zadok). They took
+different approaches to implement the merged-view.
+The former tries putting it into VFS, and the latter implements as a
+separate filesystem.
+(If I misunderstand about these implementations, please let me know and
+I shall correct it. Because it is a long time ago when I read their
+source files last time).
+
+UnionMount's approach will be able to small, but may be hard to share
+branches between several UnionMount since the whiteout in it is
+implemented in the inode on branch filesystem and always
+shared. According to Bharata's post, readdir does not seems to be
+finished yet.
+There are several missing features known in this implementations such as
+- for users, the inode number may change silently. eg. copy-up.
+- link(2) may break by copy-up.
+- read(2) may get an obsoleted filedata (fstat(2) too).
+- fcntl(F_SETLK) may be broken by copy-up.
+- unnecessary copy-up may happen, for example mmap(MAP_PRIVATE) after
+  open(O_RDWR).
+
+In linux-3.18, "overlay" filesystem (formerly known as "overlayfs") was
+merged into mainline. This is another implementation of UnionMount as a
+separated filesystem. All the limitations and known problems which
+UnionMount are equally inherited to "overlay" filesystem.
+
+Unionfs has a longer history. When I started implementing a stackable
+filesystem (Aug 2005), it already existed. It has virtual super_block,
+inode, dentry and file objects and they have an array pointing lower
+same kind objects. After contributing many patches for Unionfs, I
+re-started my project AUFS (Jun 2006).
+
+In AUFS, the structure of filesystem resembles to Unionfs, but I
+implemented my own ideas, approaches and enhancements and it became
+totally different one.
+
+Comparing DM snapshot and fs based implementation
+- the number of bytes to be copied between devices is much smaller.
+- the type of filesystem must be one and only.
+- the fs must be writable, no readonly fs, even for the lower original
+  device. so the compression fs will not be usable. but if we use
+  loopback mount, we may address this issue.
+  for instance,
+	mount /cdrom/squashfs.img /sq
+	losetup /sq/ext2.img
+	losetup /somewhere/cow
+	dmsetup "snapshot /dev/loop0 /dev/loop1 ..."
+- it will be difficult (or needs more operations) to extract the
+  difference between the original device and COW.
+- DM snapshot-merge may help a lot when users try merging. in the
+  fs-layer union, users will use rsync(1).
+
+You may want to read my old paper "Filesystems in LiveCD"
+(http://aufs.sourceforge.net/aufs2/report/sq/sq.pdf).
+
+
+Several characters/aspects/persona of aufs
+----------------------------------------------------------------------
+
+Aufs has several characters, aspects or persona.
+1. a filesystem, callee of VFS helper
+2. sub-VFS, caller of VFS helper for branches
+3. a virtual filesystem which maintains persistent inode number
+4. reader/writer of files on branches such like an application
+
+1. Callee of VFS Helper
+As an ordinary linux filesystem, aufs is a callee of VFS. For instance,
+unlink(2) from an application reaches sys_unlink() kernel function and
+then vfs_unlink() is called. vfs_unlink() is one of VFS helper and it
+calls filesystem specific unlink operation. Actually aufs implements the
+unlink operation but it behaves like a redirector.
+
+2. Caller of VFS Helper for Branches
+aufs_unlink() passes the unlink request to the branch filesystem as if
+it were called from VFS. So the called unlink operation of the branch
+filesystem acts as usual. As a caller of VFS helper, aufs should handle
+every necessary pre/post operation for the branch filesystem.
+- acquire the lock for the parent dir on a branch
+- lookup in a branch
+- revalidate dentry on a branch
+- mnt_want_write() for a branch
+- vfs_unlink() for a branch
+- mnt_drop_write() for a branch
+- release the lock on a branch
+
+3. Persistent Inode Number
+One of the most important issue for a filesystem is to maintain inode
+numbers. This is particularly important to support exporting a
+filesystem via NFS. Aufs is a virtual filesystem which doesn't have a
+backend block device for its own. But some storage is necessary to
+keep and maintain the inode numbers. It may be a large space and may not
+suit to keep in memory. Aufs rents some space from its first writable
+branch filesystem (by default) and creates file(s) on it. These files
+are created by aufs internally and removed soon (currently) keeping
+opened.
+Note: Because these files are removed, they are totally gone after
+      unmounting aufs. It means the inode numbers are not persistent
+      across unmount or reboot. I have a plan to make them really
+      persistent which will be important for aufs on NFS server.
+
+4. Read/Write Files Internally (copy-on-write)
+Because a branch can be readonly, when you write a file on it, aufs will
+"copy-up" it to the upper writable branch internally. And then write the
+originally requested thing to the file. Generally kernel doesn't
+open/read/write file actively. In aufs, even a single write may cause a
+internal "file copy". This behaviour is very similar to cp(1) command.
+
+Some people may think it is better to pass such work to user space
+helper, instead of doing in kernel space. Actually I am still thinking
+about it. But currently I have implemented it in kernel space.
diff --git a/Documentation/filesystems/aufs/design/02struct.txt b/Documentation/filesystems/aufs/design/02struct.txt
new file mode 100644
index 000000000..783328a75
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+++ b/Documentation/filesystems/aufs/design/02struct.txt
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+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+Basic Aufs Internal Structure
+
+Superblock/Inode/Dentry/File Objects
+----------------------------------------------------------------------
+As like an ordinary filesystem, aufs has its own
+superblock/inode/dentry/file objects. All these objects have a
+dynamically allocated array and store the same kind of pointers to the
+lower filesystem, branch.
+For example, when you build a union with one readwrite branch and one
+readonly, mounted /au, /rw and /ro respectively.
+- /au = /rw + /ro
+- /ro/fileA exists but /rw/fileA
+
+Aufs lookup operation finds /ro/fileA and gets dentry for that. These
+pointers are stored in a aufs dentry. The array in aufs dentry will be,
+- [0] = NULL (because /rw/fileA doesn't exist)
+- [1] = /ro/fileA
+
+This style of an array is essentially same to the aufs
+superblock/inode/dentry/file objects.
+
+Because aufs supports manipulating branches, ie. add/delete/change
+branches dynamically, these objects has its own generation. When
+branches are changed, the generation in aufs superblock is
+incremented. And a generation in other object are compared when it is
+accessed. When a generation in other objects are obsoleted, aufs
+refreshes the internal array.
+
+
+Superblock
+----------------------------------------------------------------------
+Additionally aufs superblock has some data for policies to select one
+among multiple writable branches, XIB files, pseudo-links and kobject.
+See below in detail.
+About the policies which supports copy-down a directory, see
+wbr_policy.txt too.
+
+
+Branch and XINO(External Inode Number Translation Table)
+----------------------------------------------------------------------
+Every branch has its own xino (external inode number translation table)
+file. The xino file is created and unlinked by aufs internally. When two
+members of a union exist on the same filesystem, they share the single
+xino file.
+The struct of a xino file is simple, just a sequence of aufs inode
+numbers which is indexed by the lower inode number.
+In the above sample, assume the inode number of /ro/fileA is i111 and
+aufs assigns the inode number i999 for fileA. Then aufs writes 999 as
+4(8) bytes at 111 * 4(8) bytes offset in the xino file.
+
+When the inode numbers are not contiguous, the xino file will be sparse
+which has a hole in it and doesn't consume as much disk space as it
+might appear. If your branch filesystem consumes disk space for such
+holes, then you should specify 'xino=' option at mounting aufs.
+
+Aufs has a mount option to free the disk blocks for such holes in XINO
+files on tmpfs or ramdisk. But it is not so effective actually. If you
+meet a problem of disk shortage due to XINO files, then you should try
+"tmpfs-ino.patch" (and "vfs-ino.patch" too) in aufs4-standalone.git.
+The patch localizes the assignment inumbers per tmpfs-mount and avoid
+the holes in XINO files.
+
+Also a writable branch has three kinds of "whiteout bases". All these
+are existed when the branch is joined to aufs, and their names are
+whiteout-ed doubly, so that users will never see their names in aufs
+hierarchy.
+1. a regular file which will be hardlinked to all whiteouts.
+2. a directory to store a pseudo-link.
+3. a directory to store an "orphan"-ed file temporary.
+
+1. Whiteout Base
+   When you remove a file on a readonly branch, aufs handles it as a
+   logical deletion and creates a whiteout on the upper writable branch
+   as a hardlink of this file in order not to consume inode on the
+   writable branch.
+2. Pseudo-link Dir
+   See below, Pseudo-link.
+3. Step-Parent Dir
+   When "fileC" exists on the lower readonly branch only and it is
+   opened and removed with its parent dir, and then user writes
+   something into it, then aufs copies-up fileC to this
+   directory. Because there is no other dir to store fileC. After
+   creating a file under this dir, the file is unlinked.
+
+Because aufs supports manipulating branches, ie. add/delete/change
+dynamically, a branch has its own id. When the branch order changes,
+aufs finds the new index by searching the branch id.
+
+
+Pseudo-link
+----------------------------------------------------------------------
+Assume "fileA" exists on the lower readonly branch only and it is
+hardlinked to "fileB" on the branch. When you write something to fileA,
+aufs copies-up it to the upper writable branch. Additionally aufs
+creates a hardlink under the Pseudo-link Directory of the writable
+branch. The inode of a pseudo-link is kept in aufs super_block as a
+simple list. If fileB is read after unlinking fileA, aufs returns
+filedata from the pseudo-link instead of the lower readonly
+branch. Because the pseudo-link is based upon the inode, to keep the
+inode number by xino (see above) is essentially necessary.
+
+All the hardlinks under the Pseudo-link Directory of the writable branch
+should be restored in a proper location later. Aufs provides a utility
+to do this. The userspace helpers executed at remounting and unmounting
+aufs by default.
+During this utility is running, it puts aufs into the pseudo-link
+maintenance mode. In this mode, only the process which began the
+maintenance mode (and its child processes) is allowed to operate in
+aufs. Some other processes which are not related to the pseudo-link will
+be allowed to run too, but the rest have to return an error or wait
+until the maintenance mode ends. If a process already acquires an inode
+mutex (in VFS), it has to return an error.
+
+
+XIB(external inode number bitmap)
+----------------------------------------------------------------------
+Addition to the xino file per a branch, aufs has an external inode number
+bitmap in a superblock object. It is also an internal file such like a
+xino file.
+It is a simple bitmap to mark whether the aufs inode number is in-use or
+not.
+To reduce the file I/O, aufs prepares a single memory page to cache xib.
+
+As well as XINO files, aufs has a feature to truncate/refresh XIB to
+reduce the number of consumed disk blocks for these files.
+
+
+Virtual or Vertical Dir, and Readdir in Userspace
+----------------------------------------------------------------------
+In order to support multiple layers (branches), aufs readdir operation
+constructs a virtual dir block on memory. For readdir, aufs calls
+vfs_readdir() internally for each dir on branches, merges their entries
+with eliminating the whiteout-ed ones, and sets it to file (dir)
+object. So the file object has its entry list until it is closed. The
+entry list will be updated when the file position is zero and becomes
+obsoleted. This decision is made in aufs automatically.
+
+The dynamically allocated memory block for the name of entries has a
+unit of 512 bytes (by default) and stores the names contiguously (no
+padding). Another block for each entry is handled by kmem_cache too.
+During building dir blocks, aufs creates hash list and judging whether
+the entry is whiteouted by its upper branch or already listed.
+The merged result is cached in the corresponding inode object and
+maintained by a customizable life-time option.
+
+Some people may call it can be a security hole or invite DoS attack
+since the opened and once readdir-ed dir (file object) holds its entry
+list and becomes a pressure for system memory. But I'd say it is similar
+to files under /proc or /sys. The virtual files in them also holds a
+memory page (generally) while they are opened. When an idea to reduce
+memory for them is introduced, it will be applied to aufs too.
+For those who really hate this situation, I've developed readdir(3)
+library which operates this merging in userspace. You just need to set
+LD_PRELOAD environment variable, and aufs will not consume no memory in
+kernel space for readdir(3).
+
+
+Workqueue
+----------------------------------------------------------------------
+Aufs sometimes requires privilege access to a branch. For instance,
+in copy-up/down operation. When a user process is going to make changes
+to a file which exists in the lower readonly branch only, and the mode
+of one of ancestor directories may not be writable by a user
+process. Here aufs copy-up the file with its ancestors and they may
+require privilege to set its owner/group/mode/etc.
+This is a typical case of a application character of aufs (see
+Introduction).
+
+Aufs uses workqueue synchronously for this case. It creates its own
+workqueue. The workqueue is a kernel thread and has privilege. Aufs
+passes the request to call mkdir or write (for example), and wait for
+its completion. This approach solves a problem of a signal handler
+simply.
+If aufs didn't adopt the workqueue and changed the privilege of the
+process, then the process may receive the unexpected SIGXFSZ or other
+signals.
+
+Also aufs uses the system global workqueue ("events" kernel thread) too
+for asynchronous tasks, such like handling inotify/fsnotify, re-creating a
+whiteout base and etc. This is unrelated to a privilege.
+Most of aufs operation tries acquiring a rw_semaphore for aufs
+superblock at the beginning, at the same time waits for the completion
+of all queued asynchronous tasks.
+
+
+Whiteout
+----------------------------------------------------------------------
+The whiteout in aufs is very similar to Unionfs's. That is represented
+by its filename. UnionMount takes an approach of a file mode, but I am
+afraid several utilities (find(1) or something) will have to support it.
+
+Basically the whiteout represents "logical deletion" which stops aufs to
+lookup further, but also it represents "dir is opaque" which also stop
+further lookup.
+
+In aufs, rmdir(2) and rename(2) for dir uses whiteout alternatively.
+In order to make several functions in a single systemcall to be
+revertible, aufs adopts an approach to rename a directory to a temporary
+unique whiteouted name.
+For example, in rename(2) dir where the target dir already existed, aufs
+renames the target dir to a temporary unique whiteouted name before the
+actual rename on a branch, and then handles other actions (make it opaque,
+update the attributes, etc). If an error happens in these actions, aufs
+simply renames the whiteouted name back and returns an error. If all are
+succeeded, aufs registers a function to remove the whiteouted unique
+temporary name completely and asynchronously to the system global
+workqueue.
+
+
+Copy-up
+----------------------------------------------------------------------
+It is a well-known feature or concept.
+When user modifies a file on a readonly branch, aufs operate "copy-up"
+internally and makes change to the new file on the upper writable branch.
+When the trigger systemcall does not update the timestamps of the parent
+dir, aufs reverts it after copy-up.
+
+
+Move-down (aufs3.9 and later)
+----------------------------------------------------------------------
+"Copy-up" is one of the essential feature in aufs. It copies a file from
+the lower readonly branch to the upper writable branch when a user
+changes something about the file.
+"Move-down" is an opposite action of copy-up. Basically this action is
+ran manually instead of automatically and internally.
+For desgin and implementation, aufs has to consider these issues.
+- whiteout for the file may exist on the lower branch.
+- ancestor directories may not exist on the lower branch.
+- diropq for the ancestor directories may exist on the upper branch.
+- free space on the lower branch will reduce.
+- another access to the file may happen during moving-down, including
+  UDBA (see "Revalidate Dentry and UDBA").
+- the file should not be hard-linked nor pseudo-linked. they should be
+  handled by auplink utility later.
+
+Sometimes users want to move-down a file from the upper writable branch
+to the lower readonly or writable branch. For instance,
+- the free space of the upper writable branch is going to run out.
+- create a new intermediate branch between the upper and lower branch.
+- etc.
+
+For this purpose, use "aumvdown" command in aufs-util.git.
diff --git a/Documentation/filesystems/aufs/design/03atomic_open.txt b/Documentation/filesystems/aufs/design/03atomic_open.txt
new file mode 100644
index 000000000..741ad6d66
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/03atomic_open.txt
@@ -0,0 +1,72 @@
+
+# Copyright (C) 2015-2016 Junjiro R. Okajima
+
+Support for a branch who has its ->atomic_open()
+----------------------------------------------------------------------
+The filesystems who implement its ->atomic_open() are not majority. For
+example NFSv4 does, and aufs should call NFSv4 ->atomic_open,
+particularly for open(O_CREAT|O_EXCL, 0400) case. Other than
+->atomic_open(), NFSv4 returns an error for this open(2). While I am not
+sure whether all filesystems who have ->atomic_open() behave like this,
+but NFSv4 surely returns the error.
+
+In order to support ->atomic_open() for aufs, there are a few
+approaches.
+
+A. Introduce aufs_atomic_open()
+   - calls one of VFS:do_last(), lookup_open() or atomic_open() for
+     branch fs.
+B. Introduce aufs_atomic_open() calling create, open and chmod. this is
+   an aufs user Pip Cet's approach
+   - calls aufs_create(), VFS finish_open() and notify_change().
+   - pass fake-mode to finish_open(), and then correct the mode by
+     notify_change().
+C. Extend aufs_open() to call branch fs's ->atomic_open()
+   - no aufs_atomic_open().
+   - aufs_lookup() registers the TID to an aufs internal object.
+   - aufs_create() does nothing when the matching TID is registered, but
+     registers the mode.
+   - aufs_open() calls branch fs's ->atomic_open() when the matching
+     TID is registered.
+D. Extend aufs_open() to re-try branch fs's ->open() with superuser's
+   credential
+   - no aufs_atomic_open().
+   - aufs_create() registers the TID to an internal object. this info
+     represents "this process created this file just now."
+   - when aufs gets EACCES from branch fs's ->open(), then confirm the
+     registered TID and re-try open() with superuser's credential.
+
+Pros and cons for each approach.
+
+A.
+   - straightforward but highly depends upon VFS internal.
+   - the atomic behavaiour is kept.
+   - some of parameters such as nameidata are hard to reproduce for
+     branch fs.
+   - large overhead.
+B.
+   - easy to implement.
+   - the atomic behavaiour is lost.
+C.
+   - the atomic behavaiour is kept.
+   - dirty and tricky.
+   - VFS checks whether the file is created correctly after calling
+     ->create(), which means this approach doesn't work.
+D.
+   - easy to implement.
+   - the atomic behavaiour is lost.
+   - to open a file with superuser's credential and give it to a user
+     process is a bad idea, since the file object keeps the credential
+     in it. It may affect LSM or something. This approach doesn't work
+     either.
+
+The approach A is ideal, but it hard to implement. So here is a
+variation of A, which is to be implemented.
+
+A-1. Introduce aufs_atomic_open()
+     - calls branch fs ->atomic_open() if exists. otherwise calls
+       vfs_create() and finish_open().
+     - the demerit is that the several checks after branch fs
+       ->atomic_open() are lost. in the ordinary case, the checks are
+       done by VFS:do_last(), lookup_open() and atomic_open(). some can
+       be implemented in aufs, but not all I am afraid.
diff --git a/Documentation/filesystems/aufs/design/03lookup.txt b/Documentation/filesystems/aufs/design/03lookup.txt
new file mode 100644
index 000000000..5b6b000b5
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/03lookup.txt
@@ -0,0 +1,100 @@
+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+Lookup in a Branch
+----------------------------------------------------------------------
+Since aufs has a character of sub-VFS (see Introduction), it operates
+lookup for branches as VFS does. It may be a heavy work. But almost all
+lookup operation in aufs is the simplest case, ie. lookup only an entry
+directly connected to its parent. Digging down the directory hierarchy
+is unnecessary. VFS has a function lookup_one_len() for that use, and
+aufs calls it.
+
+When a branch is a remote filesystem, aufs basically relies upon its
+->d_revalidate(), also aufs forces the hardest revalidate tests for
+them.
+For d_revalidate, aufs implements three levels of revalidate tests. See
+"Revalidate Dentry and UDBA" in detail.
+
+
+Test Only the Highest One for the Directory Permission (dirperm1 option)
+----------------------------------------------------------------------
+Let's try case study.
+- aufs has two branches, upper readwrite and lower readonly.
+  /au = /rw + /ro
+- "dirA" exists under /ro, but /rw. and its mode is 0700.
+- user invoked "chmod a+rx /au/dirA"
+- the internal copy-up is activated and "/rw/dirA" is created and its
+  permission bits are set to world readable.
+- then "/au/dirA" becomes world readable?
+
+In this case, /ro/dirA is still 0700 since it exists in readonly branch,
+or it may be a natively readonly filesystem. If aufs respects the lower
+branch, it should not respond readdir request from other users. But user
+allowed it by chmod. Should really aufs rejects showing the entries
+under /ro/dirA?
+
+To be honest, I don't have a good solution for this case. So aufs
+implements 'dirperm1' and 'nodirperm1' mount options, and leave it to
+users.
+When dirperm1 is specified, aufs checks only the highest one for the
+directory permission, and shows the entries. Otherwise, as usual, checks
+every dir existing on all branches and rejects the request.
+
+As a side effect, dirperm1 option improves the performance of aufs
+because the number of permission check is reduced when the number of
+branch is many.
+
+
+Revalidate Dentry and UDBA (User's Direct Branch Access)
+----------------------------------------------------------------------
+Generally VFS helpers re-validate a dentry as a part of lookup.
+0. digging down the directory hierarchy.
+1. lock the parent dir by its i_mutex.
+2. lookup the final (child) entry.
+3. revalidate it.
+4. call the actual operation (create, unlink, etc.)
+5. unlock the parent dir
+
+If the filesystem implements its ->d_revalidate() (step 3), then it is
+called. Actually aufs implements it and checks the dentry on a branch is
+still valid.
+But it is not enough. Because aufs has to release the lock for the
+parent dir on a branch at the end of ->lookup() (step 2) and
+->d_revalidate() (step 3) while the i_mutex of the aufs dir is still
+held by VFS.
+If the file on a branch is changed directly, eg. bypassing aufs, after
+aufs released the lock, then the subsequent operation may cause
+something unpleasant result.
+
+This situation is a result of VFS architecture, ->lookup() and
+->d_revalidate() is separated. But I never say it is wrong. It is a good
+design from VFS's point of view. It is just not suitable for sub-VFS
+character in aufs.
+
+Aufs supports such case by three level of revalidation which is
+selectable by user.
+1. Simple Revalidate
+   Addition to the native flow in VFS's, confirm the child-parent
+   relationship on the branch just after locking the parent dir on the
+   branch in the "actual operation" (step 4). When this validation
+   fails, aufs returns EBUSY. ->d_revalidate() (step 3) in aufs still
+   checks the validation of the dentry on branches.
+2. Monitor Changes Internally by Inotify/Fsnotify
+   Addition to above, in the "actual operation" (step 4) aufs re-lookup
+   the dentry on the branch, and returns EBUSY if it finds different
+   dentry.
+   Additionally, aufs sets the inotify/fsnotify watch for every dir on branches
+   during it is in cache. When the event is notified, aufs registers a
+   function to kernel 'events' thread by schedule_work(). And the
+   function sets some special status to the cached aufs dentry and inode
+   private data. If they are not cached, then aufs has nothing to
+   do. When the same file is accessed through aufs (step 0-3) later,
+   aufs will detect the status and refresh all necessary data.
+   In this mode, aufs has to ignore the event which is fired by aufs
+   itself.
+3. No Extra Validation
+   This is the simplest test and doesn't add any additional revalidation
+   test, and skip the revalidation in step 4. It is useful and improves
+   aufs performance when system surely hide the aufs branches from user,
+   by over-mounting something (or another method).
diff --git a/Documentation/filesystems/aufs/design/04branch.txt b/Documentation/filesystems/aufs/design/04branch.txt
new file mode 100644
index 000000000..e68f4d3df
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/04branch.txt
@@ -0,0 +1,61 @@
+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+Branch Manipulation
+
+Since aufs supports dynamic branch manipulation, ie. add/remove a branch
+and changing its permission/attribute, there are a lot of works to do.
+
+
+Add a Branch
+----------------------------------------------------------------------
+o Confirm the adding dir exists outside of aufs, including loopback
+  mount, and its various attributes.
+o Initialize the xino file and whiteout bases if necessary.
+  See struct.txt.
+
+o Check the owner/group/mode of the directory
+  When the owner/group/mode of the adding directory differs from the
+  existing branch, aufs issues a warning because it may impose a
+  security risk.
+  For example, when a upper writable branch has a world writable empty
+  top directory, a malicious user can create any files on the writable
+  branch directly, like copy-up and modify manually. If something like
+  /etc/{passwd,shadow} exists on the lower readonly branch but the upper
+  writable branch, and the writable branch is world-writable, then a
+  malicious guy may create /etc/passwd on the writable branch directly
+  and the infected file will be valid in aufs.
+  I am afraid it can be a security issue, but aufs can do nothing except
+  producing a warning.
+
+
+Delete a Branch
+----------------------------------------------------------------------
+o Confirm the deleting branch is not busy
+  To be general, there is one merit to adopt "remount" interface to
+  manipulate branches. It is to discard caches. At deleting a branch,
+  aufs checks the still cached (and connected) dentries and inodes. If
+  there are any, then they are all in-use. An inode without its
+  corresponding dentry can be alive alone (for example, inotify/fsnotify case).
+
+  For the cached one, aufs checks whether the same named entry exists on
+  other branches.
+  If the cached one is a directory, because aufs provides a merged view
+  to users, as long as one dir is left on any branch aufs can show the
+  dir to users. In this case, the branch can be removed from aufs.
+  Otherwise aufs rejects deleting the branch.
+
+  If any file on the deleting branch is opened by aufs, then aufs
+  rejects deleting.
+
+
+Modify the Permission of a Branch
+----------------------------------------------------------------------
+o Re-initialize or remove the xino file and whiteout bases if necessary.
+  See struct.txt.
+
+o rw --> ro: Confirm the modifying branch is not busy
+  Aufs rejects the request if any of these conditions are true.
+  - a file on the branch is mmap-ed.
+  - a regular file on the branch is opened for write and there is no
+    same named entry on the upper branch.
diff --git a/Documentation/filesystems/aufs/design/05wbr_policy.txt b/Documentation/filesystems/aufs/design/05wbr_policy.txt
new file mode 100644
index 000000000..1726d5d06
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/05wbr_policy.txt
@@ -0,0 +1,51 @@
+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+Policies to Select One among Multiple Writable Branches
+----------------------------------------------------------------------
+When the number of writable branch is more than one, aufs has to decide
+the target branch for file creation or copy-up. By default, the highest
+writable branch which has the parent (or ancestor) dir of the target
+file is chosen (top-down-parent policy).
+By user's request, aufs implements some other policies to select the
+writable branch, for file creation several policies, round-robin,
+most-free-space, and other policies. For copy-up, top-down-parent,
+bottom-up-parent, bottom-up and others.
+
+As expected, the round-robin policy selects the branch in circular. When
+you have two writable branches and creates 10 new files, 5 files will be
+created for each branch. mkdir(2) systemcall is an exception. When you
+create 10 new directories, all will be created on the same branch.
+And the most-free-space policy selects the one which has most free
+space among the writable branches. The amount of free space will be
+checked by aufs internally, and users can specify its time interval.
+
+The policies for copy-up is more simple,
+top-down-parent is equivalent to the same named on in create policy,
+bottom-up-parent selects the writable branch where the parent dir
+exists and the nearest upper one from the copyup-source,
+bottom-up selects the nearest upper writable branch from the
+copyup-source, regardless the existence of the parent dir.
+
+There are some rules or exceptions to apply these policies.
+- If there is a readonly branch above the policy-selected branch and
+  the parent dir is marked as opaque (a variation of whiteout), or the
+  target (creating) file is whiteout-ed on the upper readonly branch,
+  then the result of the policy is ignored and the target file will be
+  created on the nearest upper writable branch than the readonly branch.
+- If there is a writable branch above the policy-selected branch and
+  the parent dir is marked as opaque or the target file is whiteouted
+  on the branch, then the result of the policy is ignored and the target
+  file will be created on the highest one among the upper writable
+  branches who has diropq or whiteout. In case of whiteout, aufs removes
+  it as usual.
+- link(2) and rename(2) systemcalls are exceptions in every policy.
+  They try selecting the branch where the source exists as possible
+  since copyup a large file will take long time. If it can't be,
+  ie. the branch where the source exists is readonly, then they will
+  follow the copyup policy.
+- There is an exception for rename(2) when the target exists.
+  If the rename target exists, aufs compares the index of the branches
+  where the source and the target exists and selects the higher
+  one. If the selected branch is readonly, then aufs follows the
+  copyup policy.
diff --git a/Documentation/filesystems/aufs/design/06fhsm.txt b/Documentation/filesystems/aufs/design/06fhsm.txt
new file mode 100644
index 000000000..84b46dc5b
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/06fhsm.txt
@@ -0,0 +1,105 @@
+
+# Copyright (C) 2011-2016 Junjiro R. Okajima
+
+File-based Hierarchical Storage Management (FHSM)
+----------------------------------------------------------------------
+Hierarchical Storage Management (or HSM) is a well-known feature in the
+storage world. Aufs provides this feature as file-based with multiple
+writable branches, based upon the principle of "Colder, the Lower".
+Here the word "colder" means that the less used files, and "lower" means
+that the position in the order of the stacked branches vertically.
+These multiple writable branches are prioritized, ie. the topmost one
+should be the fastest drive and be used heavily.
+
+o Characters in aufs FHSM story
+- aufs itself and a new branch attribute.
+- a new ioctl interface to move-down and to establish a connection with
+  the daemon ("move-down" is a converse of "copy-up").
+- userspace tool and daemon.
+
+The userspace daemon establishes a connection with aufs and waits for
+the notification. The notified information is very similar to struct
+statfs containing the number of consumed blocks and inodes.
+When the consumed blocks/inodes of a branch exceeds the user-specified
+upper watermark, the daemon activates its move-down process until the
+consumed blocks/inodes reaches the user-specified lower watermark.
+
+The actual move-down is done by aufs based upon the request from
+user-space since we need to maintain the inode number and the internal
+pointer arrays in aufs.
+
+Currently aufs FHSM handles the regular files only. Additionally they
+must not be hard-linked nor pseudo-linked.
+
+
+o Cowork of aufs and the user-space daemon
+  During the userspace daemon established the connection, aufs sends a
+  small notification to it whenever aufs writes something into the
+  writable branch. But it may cost high since aufs issues statfs(2)
+  internally. So user can specify a new option to cache the
+  info. Actually the notification is controlled by these factors.
+  + the specified cache time.
+  + classified as "force" by aufs internally.
+  Until the specified time expires, aufs doesn't send the info
+  except the forced cases. When aufs decide forcing, the info is always
+  notified to userspace.
+  For example, the number of free inodes is generally large enough and
+  the shortage of it happens rarely. So aufs doesn't force the
+  notification when creating a new file, directory and others. This is
+  the typical case which aufs doesn't force.
+  When aufs writes the actual filedata and the files consumes any of new
+  blocks, the aufs forces notifying.
+
+
+o Interfaces in aufs
+- New branch attribute.
+  + fhsm
+    Specifies that the branch is managed by FHSM feature. In other word,
+    participant in the FHSM.
+    When nofhsm is set to the branch, it will not be the source/target
+    branch of the move-down operation. This attribute is set
+    independently from coo and moo attributes, and if you want full
+    FHSM, you should specify them as well.
+- New mount option.
+  + fhsm_sec
+    Specifies a second to suppress many less important info to be
+    notified.
+- New ioctl.
+  + AUFS_CTL_FHSM_FD
+    create a new file descriptor which userspace can read the notification
+    (a subset of struct statfs) from aufs.
+- Module parameter 'brs'
+  It has to be set to 1. Otherwise the new mount option 'fhsm' will not
+  be set.
+- mount helpers /sbin/mount.aufs and /sbin/umount.aufs
+  When there are two or more branches with fhsm attributes,
+  /sbin/mount.aufs invokes the user-space daemon and /sbin/umount.aufs
+  terminates it. As a result of remounting and branch-manipulation, the
+  number of branches with fhsm attribute can be one. In this case,
+  /sbin/mount.aufs will terminate the user-space daemon.
+
+
+Finally the operation is done as these steps in kernel-space.
+- make sure that,
+  + no one else is using the file.
+  + the file is not hard-linked.
+  + the file is not pseudo-linked.
+  + the file is a regular file.
+  + the parent dir is not opaqued.
+- find the target writable branch.
+- make sure the file is not whiteout-ed by the upper (than the target)
+  branch.
+- make the parent dir on the target branch.
+- mutex lock the inode on the branch.
+- unlink the whiteout on the target branch (if exists).
+- lookup and create the whiteout-ed temporary name on the target branch.
+- copy the file as the whiteout-ed temporary name on the target branch.
+- rename the whiteout-ed temporary name to the original name.
+- unlink the file on the source branch.
+- maintain the internal pointer array and the external inode number
+  table (XINO).
+- maintain the timestamps and other attributes of the parent dir and the
+  file.
+
+And of course, in every step, an error may happen. So the operation
+should restore the original file state after an error happens.
diff --git a/Documentation/filesystems/aufs/design/06mmap.txt b/Documentation/filesystems/aufs/design/06mmap.txt
new file mode 100644
index 000000000..991c0b1fa
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/06mmap.txt
@@ -0,0 +1,59 @@
+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+mmap(2) -- File Memory Mapping
+----------------------------------------------------------------------
+In aufs, the file-mapped pages are handled by a branch fs directly, no
+interaction with aufs. It means aufs_mmap() calls the branch fs's
+->mmap().
+This approach is simple and good, but there is one problem.
+Under /proc, several entries show the mmapped files by its path (with
+device and inode number), and the printed path will be the path on the
+branch fs's instead of virtual aufs's.
+This is not a problem in most cases, but some utilities lsof(1) (and its
+user) may expect the path on aufs.
+
+To address this issue, aufs adds a new member called vm_prfile in struct
+vm_area_struct (and struct vm_region). The original vm_file points to
+the file on the branch fs in order to handle everything correctly as
+usual. The new vm_prfile points to a virtual file in aufs, and the
+show-functions in procfs refers to vm_prfile if it is set.
+Also we need to maintain several other places where touching vm_file
+such like
+- fork()/clone() copies vma and the reference count of vm_file is
+  incremented.
+- merging vma maintains the ref count too.
+
+This is not a good approach. It just fakes the printed path. But it
+leaves all behaviour around f_mapping unchanged. This is surely an
+advantage.
+Actually aufs had adopted another complicated approach which calls
+generic_file_mmap() and handles struct vm_operations_struct. In this
+approach, aufs met a hard problem and I could not solve it without
+switching the approach.
+
+There may be one more another approach which is
+- bind-mount the branch-root onto the aufs-root internally
+- grab the new vfsmount (ie. struct mount)
+- lazy-umount the branch-root internally
+- in open(2) the aufs-file, open the branch-file with the hidden
+  vfsmount (instead of the original branch's vfsmount)
+- ideally this "bind-mount and lazy-umount" should be done atomically,
+  but it may be possible from userspace by the mount helper.
+
+Adding the internal hidden vfsmount and using it in opening a file, the
+file path under /proc will be printed correctly. This approach looks
+smarter, but is not possible I am afraid.
+- aufs-root may be bind-mount later. when it happens, another hidden
+  vfsmount will be required.
+- it is hard to get the chance to bind-mount and lazy-umount
+  + in kernel-space, FS can have vfsmount in open(2) via
+    file->f_path, and aufs can know its vfsmount. But several locks are
+    already acquired, and if aufs tries to bind-mount and lazy-umount
+    here, then it may cause a deadlock.
+  + in user-space, bind-mount doesn't invoke the mount helper.
+- since /proc shows dev and ino, aufs has to give vma these info. it
+  means a new member vm_prinode will be necessary. this is essentially
+  equivalent to vm_prfile described above.
+
+I have to give up this "looks-smater" approach.
diff --git a/Documentation/filesystems/aufs/design/06xattr.txt b/Documentation/filesystems/aufs/design/06xattr.txt
new file mode 100644
index 000000000..7bfa94f7b
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/06xattr.txt
@@ -0,0 +1,81 @@
+
+# Copyright (C) 2014-2016 Junjiro R. Okajima
+
+Listing XATTR/EA and getting the value
+----------------------------------------------------------------------
+For the inode standard attributes (owner, group, timestamps, etc.), aufs
+shows the values from the topmost existing file. This behaviour is good
+for the non-dir entries since the bahaviour exactly matches the shown
+information. But for the directories, aufs considers all the same named
+entries on the lower branches. Which means, if one of the lower entry
+rejects readdir call, then aufs returns an error even if the topmost
+entry allows it. This behaviour is necessary to respect the branch fs's
+security, but can make users confused since the user-visible standard
+attributes don't match the behaviour.
+To address this issue, aufs has a mount option called dirperm1 which
+checks the permission for the topmost entry only, and ignores the lower
+entry's permission.
+
+A similar issue can happen around XATTR.
+getxattr(2) and listxattr(2) families behave as if dirperm1 option is
+always set. Otherwise these very unpleasant situation would happen.
+- listxattr(2) may return the duplicated entries.
+- users may not be able to remove or reset the XATTR forever,
+
+
+XATTR/EA support in the internal (copy,move)-(up,down)
+----------------------------------------------------------------------
+Generally the extended attributes of inode are categorized as these.
+- "security" for LSM and capability.
+- "system" for posix ACL, 'acl' mount option is required for the branch
+  fs generally.
+- "trusted" for userspace, CAP_SYS_ADMIN is required.
+- "user" for userspace, 'user_xattr' mount option is required for the
+  branch fs generally.
+
+Moreover there are some other categories. Aufs handles these rather
+unpopular categories as the ordinary ones, ie. there is no special
+condition nor exception.
+
+In copy-up, the support for XATTR on the dst branch may differ from the
+src branch. In this case, the copy-up operation will get an error and
+the original user operation which triggered the copy-up will fail. It
+can happen that even all copy-up will fail.
+When both of src and dst branches support XATTR and if an error occurs
+during copying XATTR, then the copy-up should fail obviously. That is a
+good reason and aufs should return an error to userspace. But when only
+the src branch support that XATTR, aufs should not return an error.
+For example, the src branch supports ACL but the dst branch doesn't
+because the dst branch may natively un-support it or temporary
+un-support it due to "noacl" mount option. Of course, the dst branch fs
+may NOT return an error even if the XATTR is not supported. It is
+totally up to the branch fs.
+
+Anyway when the aufs internal copy-up gets an error from the dst branch
+fs, then aufs tries removing the just copied entry and returns the error
+to the userspace. The worst case of this situation will be all copy-up
+will fail.
+
+For the copy-up operation, there two basic approaches.
+- copy the specified XATTR only (by category above), and return the
+  error unconditionally if it happens.
+- copy all XATTR, and ignore the error on the specified category only.
+
+In order to support XATTR and to implement the correct behaviour, aufs
+chooses the latter approach and introduces some new branch attributes,
+"icexsec", "icexsys", "icextr", "icexusr", and "icexoth".
+They correspond to the XATTR namespaces (see above). Additionally, to be
+convenient, "icex" is also provided which means all "icex*" attributes
+are set (here the word "icex" stands for "ignore copy-error on XATTR").
+
+The meaning of these attributes is to ignore the error from setting
+XATTR on that branch.
+Note that aufs tries copying all XATTR unconditionally, and ignores the
+error from the dst branch according to the specified attributes.
+
+Some XATTR may have its default value. The default value may come from
+the parent dir or the environment. If the default value is set at the
+file creating-time, it will be overwritten by copy-up.
+Some contradiction may happen I am afraid.
+Do we need another attribute to stop copying XATTR? I am unsure. For
+now, aufs implements the branch attributes to ignore the error.
diff --git a/Documentation/filesystems/aufs/design/07export.txt b/Documentation/filesystems/aufs/design/07export.txt
new file mode 100644
index 000000000..c23930b49
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/07export.txt
@@ -0,0 +1,45 @@
+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+Export Aufs via NFS
+----------------------------------------------------------------------
+Here is an approach.
+- like xino/xib, add a new file 'xigen' which stores aufs inode
+  generation.
+- iget_locked(): initialize aufs inode generation for a new inode, and
+  store it in xigen file.
+- destroy_inode(): increment aufs inode generation and store it in xigen
+  file. it is necessary even if it is not unlinked, because any data of
+  inode may be changed by UDBA.
+- encode_fh(): for a root dir, simply return FILEID_ROOT. otherwise
+  build file handle by
+  + branch id (4 bytes)
+  + superblock generation (4 bytes)
+  + inode number (4 or 8 bytes)
+  + parent dir inode number (4 or 8 bytes)
+  + inode generation (4 bytes))
+  + return value of exportfs_encode_fh() for the parent on a branch (4
+    bytes)
+  + file handle for a branch (by exportfs_encode_fh())
+- fh_to_dentry():
+  + find the index of a branch from its id in handle, and check it is
+    still exist in aufs.
+  + 1st level: get the inode number from handle and search it in cache.
+  + 2nd level: if not found in cache, get the parent inode number from
+    the handle and search it in cache. and then open the found parent
+    dir, find the matching inode number by vfs_readdir() and get its
+    name, and call lookup_one_len() for the target dentry.
+  + 3rd level: if the parent dir is not cached, call
+    exportfs_decode_fh() for a branch and get the parent on a branch,
+    build a pathname of it, convert it a pathname in aufs, call
+    path_lookup(). now aufs gets a parent dir dentry, then handle it as
+    the 2nd level.
+  + to open the dir, aufs needs struct vfsmount. aufs keeps vfsmount
+    for every branch, but not itself. to get this, (currently) aufs
+    searches in current->nsproxy->mnt_ns list. it may not be a good
+    idea, but I didn't get other approach.
+  + test the generation of the gotten inode.
+- every inode operation: they may get EBUSY due to UDBA. in this case,
+  convert it into ESTALE for NFSD.
+- readdir(): call lockdep_on/off() because filldir in NFSD calls
+  lookup_one_len(), vfs_getattr(), encode_fh() and others.
diff --git a/Documentation/filesystems/aufs/design/08shwh.txt b/Documentation/filesystems/aufs/design/08shwh.txt
new file mode 100644
index 000000000..ad58ebe15
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/08shwh.txt
@@ -0,0 +1,39 @@
+
+# Copyright (C) 2005-2016 Junjiro R. Okajima
+
+Show Whiteout Mode (shwh)
+----------------------------------------------------------------------
+Generally aufs hides the name of whiteouts. But in some cases, to show
+them is very useful for users. For instance, creating a new middle layer
+(branch) by merging existing layers.
+
+(borrowing aufs1 HOW-TO from a user, Michael Towers)
+When you have three branches,
+- Bottom: 'system', squashfs (underlying base system), read-only
+- Middle: 'mods', squashfs, read-only
+- Top: 'overlay', ram (tmpfs), read-write
+
+The top layer is loaded at boot time and saved at shutdown, to preserve
+the changes made to the system during the session.
+When larger changes have been made, or smaller changes have accumulated,
+the size of the saved top layer data grows. At this point, it would be
+nice to be able to merge the two overlay branches ('mods' and 'overlay')
+and rewrite the 'mods' squashfs, clearing the top layer and thus
+restoring save and load speed.
+
+This merging is simplified by the use of another aufs mount, of just the
+two overlay branches using the 'shwh' option.
+# mount -t aufs -o ro,shwh,br:/livesys/overlay=ro+wh:/livesys/mods=rr+wh \
+	aufs /livesys/merge_union
+
+A merged view of these two branches is then available at
+/livesys/merge_union, and the new feature is that the whiteouts are
+visible!
+Note that in 'shwh' mode the aufs mount must be 'ro', which will disable
+writing to all branches. Also the default mode for all branches is 'ro'.
+It is now possible to save the combined contents of the two overlay
+branches to a new squashfs, e.g.:
+# mksquashfs /livesys/merge_union /path/to/newmods.squash
+
+This new squashfs archive can be stored on the boot device and the
+initramfs will use it to replace the old one at the next boot.
diff --git a/Documentation/filesystems/aufs/design/10dynop.txt b/Documentation/filesystems/aufs/design/10dynop.txt
new file mode 100644
index 000000000..49afc5899
--- /dev/null
+++ b/Documentation/filesystems/aufs/design/10dynop.txt
@@ -0,0 +1,34 @@
+
+# Copyright (C) 2010-2016 Junjiro R. Okajima
+
+Dynamically customizable FS operations
+----------------------------------------------------------------------
+Generally FS operations (struct inode_operations, struct
+address_space_operations, struct file_operations, etc.) are defined as
+"static const", but it never means that FS have only one set of
+operation. Some FS have multiple sets of them. For instance, ext2 has
+three sets, one for XIP, for NOBH, and for normal.
+Since aufs overrides and redirects these operations, sometimes aufs has
+to change its behaviour according to the branch FS type. More importantly
+VFS acts differently if a function (member in the struct) is set or
+not. It means aufs should have several sets of operations and select one
+among them according to the branch FS definition.
+
+In order to solve this problem and not to affect the behaviour of VFS,
+aufs defines these operations dynamically. For instance, aufs defines
+dummy direct_IO function for struct address_space_operations, but it may
+not be set to the address_space_operations actually. When the branch FS
+doesn't have it, aufs doesn't set it to its address_space_operations
+while the function definition itself is still alive. So the behaviour
+itself will not change, and it will return an error when direct_IO is
+not set.
+
+The lifetime of these dynamically generated operation object is
+maintained by aufs branch object. When the branch is removed from aufs,
+the reference counter of the object is decremented. When it reaches
+zero, the dynamically generated operation object will be freed.
+
+This approach is designed to support AIO (io_submit), Direct I/O and
+XIP (DAX) mainly.
+Currently this approach is applied to address_space_operations for
+regular files only.