aufs5.7-20200622.patch

diff -Naur null/Documentation/ABI/testing/debugfs-aufs linux-5.7/Documentation/ABI/testing/debugfs-aufs
--- /dev/null
+++ linux-5.7/Documentation/ABI/testing/debugfs-aufs	2020-08-22 11:57:41.465116229 +0200
@@ -0,0 +1,55 @@
+What:		/debug/aufs/si_<id>/
+Date:		March 2009
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		Under /debug/aufs, a directory named si_<id> is created
+		per aufs mount, where <id> is a unique id generated
+		internally.
+
+What:		/debug/aufs/si_<id>/plink
+Date:		Apr 2013
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		It has three lines and shows the information about the
+		pseudo-link. The first line is a single number
+		representing a number of buckets. The second line is a
+		number of pseudo-links per buckets (separated by a
+		blank). The last line is a single number representing a
+		total number of psedo-links.
+		When the aufs mount option 'noplink' is specified, it
+		will show "1\n0\n0\n".
+
+What:		/debug/aufs/si_<id>/xib
+Date:		March 2009
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		It shows the consumed blocks by xib (External Inode Number
+		Bitmap), its block size and file size.
+		When the aufs mount option 'noxino' is specified, it
+		will be empty. About XINO files, see the aufs manual.
+
+What:		/debug/aufs/si_<id>/xi0, xi1 ... xiN and xiN-N
+Date:		March 2009
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		It shows the consumed blocks by xino (External Inode Number
+		Translation Table), its link count, block size and file
+		size.
+		Due to the file size limit, there may exist multiple
+		xino files per branch.  In this case, "-N" is added to
+		the filename and it corresponds to the index of the
+		internal xino array.  "-0" is omitted.
+		When the aufs mount option 'noxino' is specified, Those
+		entries won't exist.  About XINO files, see the aufs
+		manual.
+
+What:		/debug/aufs/si_<id>/xigen
+Date:		March 2009
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		It shows the consumed blocks by xigen (External Inode
+		Generation Table), its block size and file size.
+		If CONFIG_AUFS_EXPORT is disabled, this entry will not
+		be created.
+		When the aufs mount option 'noxino' is specified, it
+		will be empty. About XINO files, see the aufs manual.
diff -Naur null/Documentation/ABI/testing/sysfs-aufs linux-5.7/Documentation/ABI/testing/sysfs-aufs
--- /dev/null
+++ linux-5.7/Documentation/ABI/testing/sysfs-aufs	2020-08-22 11:57:41.465116229 +0200
@@ -0,0 +1,31 @@
+What:		/sys/fs/aufs/si_<id>/
+Date:		March 2009
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		Under /sys/fs/aufs, a directory named si_<id> is created
+		per aufs mount, where <id> is a unique id generated
+		internally.
+
+What:		/sys/fs/aufs/si_<id>/br0, br1 ... brN
+Date:		March 2009
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		It shows the abolute path of a member directory (which
+		is called branch) in aufs, and its permission.
+
+What:		/sys/fs/aufs/si_<id>/brid0, brid1 ... bridN
+Date:		July 2013
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		It shows the id of a member directory (which is called
+		branch) in aufs.
+
+What:		/sys/fs/aufs/si_<id>/xi_path
+Date:		March 2009
+Contact:	J. R. Okajima <hooanon05g@gmail.com>
+Description:
+		It shows the abolute path of XINO (External Inode Number
+		Bitmap, Translation Table and Generation Table) file
+		even if it is the default path.
+		When the aufs mount option 'noxino' is specified, it
+		will be empty. About XINO files, see the aufs manual.
diff -Naur null/Documentation/filesystems/aufs/README linux-5.7/Documentation/filesystems/aufs/README
--- /dev/null
+++ linux-5.7/Documentation/filesystems/aufs/README	2020-08-22 11:57:41.465116229 +0200
@@ -0,0 +1,401 @@
+
+Aufs5 -- advanced multi layered unification filesystem version 5.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 became totally different from
+Unionfs while keeping the basic features.
+Later, Unionfs Version 2.x series began taking some of the same
+approaches to aufs1's.
+Unionfs was being developed by Professor Erez Zadok at Stony Brook
+University and his team.
+
+Aufs5 supports linux-v5.0 and later, If you want older kernel version
+support,
+- for linux-v4.x series, try aufs4-linux.git or aufs4-standalone.git
+- for linux-v3.x series, try aufs3-linux.git or aufs3-standalone.git
+- for linux-v2.6.16 and later, try aufs2-2.6.git, aufs2-standalone.git
+  or 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 aufs5.
+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 aufs5, aufs5-linux.git,
+aufs5-standalone.git, and aufs-util.git. Note that there is no "5" in
+"aufs-util.git."
+While the aufs-util is always necessary, you need either of aufs5-linux
+or aufs5-standalone.
+
+The aufs5-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 aufs5 as an external kernel module.
+Several extra patches are not included in this tree. Only
+aufs5-standalone tree contains them. They are described in the later
+section "Configuration and Compilation."
+
+On the other hand, the aufs5-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 "aufs5.x" where "x"
+represents the linux kernel version, "linux-5.x". For instance,
+"aufs5.0" is for linux-5.0. For latest "linux-5.x-rcN", use
+"aufs5.x-rcN" branch.
+
+o aufs5-linux tree
+$ git clone --reference /your/linux/git/tree \
+	git://github.com/sfjro/aufs5-linux.git aufs5-linux.git
+- if you don't have linux GIT tree, then remove "--reference ..."
+$ cd aufs5-linux.git
+$ git checkout origin/aufs5.0
+
+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 aufs5 git://github.com/sfjro/aufs5-linux.git
+$ git fetch aufs5
+$ git checkout -b my5.0 v5.0
+$ (add your local change...)
+$ git pull aufs5 aufs5.0
+- now you have v5.0 + your_changes + aufs5.0 in you my5.0 branch.
+- you may need to solve some conflicts between your_changes and
+  aufs5.0. in this case, git-rerere is recommended so that you can
+  solve the similar conflicts automatically when you upgrade to 5.1 or
+  later in the future.
+
+o aufs5-standalone tree
+$ git clone git://github.com/sfjro/aufs5-standalone.git aufs5-standalone.git
+$ cd aufs5-standalone.git
+$ git checkout origin/aufs5.0
+
+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/aufs5.0
+
+Note: The 5.x-rcN branch is to be used with `rc' kernel versions ONLY.
+The minor version number, 'x' in '5.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-5.10" and the "aufs5.10" branch
+does not exist in aufs-util repository, then "aufs5.9", "aufs5.8"
+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 aufs5-linux tree,
+- enable CONFIG_AUFS_FS.
+- set other aufs configurations if necessary.
+
+For aufs5-standalone tree,
+There are several ways to build.
+
+1.
+- apply ./aufs5-kbuild.patch to your kernel source files.
+- apply ./aufs5-base.patch too.
+- apply ./aufs5-mmap.patch too.
+- apply ./aufs5-standalone.patch too, if you have a plan to set
+  CONFIG_AUFS_FS=m. otherwise you don't need ./aufs5-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.
+- 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 ./aufs5-base.patch to your kernel source files.
+- apply ./aufs5-mmap.patch too.
+- apply ./aufs5-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".
+- 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 aufs5-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 aufs5-standalone.git. They are all
+optional. When you meet some problems, they will help you.
+- aufs5-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".
+- proc_mounts.patch
+  When there are many mountpoints and many mount(2)/umount(2) are
+  running, then /proc/mounts may not show the all mountpoints.  This
+  patch makes /proc/mounts always show the full mountpoints list.
+  If you don't want to apply this patch and meet such problem, then you
+  need to increase the value of 'ProcMounts_Times' make-variable in
+  aufs-util.git as a second best solution.
+- 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 you don't know what LOCKDEP is, 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)
+- LSM (linux security module, if you are using)
+- 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, 2018/4).
+Tomasz Szewczyk made a donation (2016/4).
+James Burry made a donation (2016/12).
+Carsten Rose made a donation (2018/9).
+Porteus Kiosk made a donation (2018/10).
+
+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 -Naur null/Documentation/filesystems/aufs/design/01intro.txt linux-5.7/Documentation/filesystems/aufs/design/01intro.txt
--- /dev/null
+++ linux-5.7/Documentation/filesystems/aufs/design/01intro.txt	2020-08-22 11:57:41.465116229 +0200
@@ -0,0 +1,171 @@
+
+# Copyright (C) 2005-2020 Junjiro R. Okajima
+# 
+# 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, see <http://www.gnu.org/licenses/>.
+
+Introduction
+----------------------------------------
+
+aufs [ei ju: ef es] | /ey-yoo-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.
+4. abbrev. for "African Urban Fashion Show".
+
+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 -Naur null/Documentation/filesystems/aufs/design/02struct.txt linux-5.7/Documentation/filesystems/aufs/design/02struct.txt
--- /dev/null
+++ linux-5.7/Documentation/filesystems/aufs/design/02struct.txt	2020-08-22 11:57:41.465116229 +0200
@@ -0,0 +1,258 @@
+
+# Copyright (C) 2005-2020 Junjiro R. Okajima
+# 
+# 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, see <http://www.gnu.org/licenses/>.
+
+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 -Naur null/Documentation/filesystems/aufs/design/03atomic_open.txt linux-5.7/Documentation/filesystems/aufs/design/03atomic_open.txt
--- /dev/null
+++ linux-5.7/Documentation/filesystems/aufs/design/03atomic_open.txt	2020-08-22 11:57:41.465116229 +0200
@@ -0,0 +1,85 @@
+
+# Copyright (C) 2015-2020 Junjiro R. Okajima
+# 
+# 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, see <http://www.gnu.org/licenses/>.
+
+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.