Enforcing exclusive ownership on upper/work dirs caused a docker
regression: https://github.com/moby/moby/issues/34672.

Euan spotted the regression and pointed to the offending commit.
Vivek has brought the regression to my attention and provided this
reproducer:

Terminal 1:

  mount -t overlay -o workdir=work,lowerdir=lower,upperdir=upper none
        merged/

Terminal 2:

  unshare -m

Terminal 1:

  umount merged
  mount -t overlay -o workdir=work,lowerdir=lower,upperdir=upper none
        merged/
  mount: /root/overlay-testing/merged: none already mounted or mount point
         busy

To fix the regression, I replaced the error with an alarming warning.
With index feature enabled, mount does fail, but logs a suggestion to
override exclusive dir protection by disabling index.
Note that index=off mount does take the inuse locks, so a concurrent
index=off will issue the warning and a concurrent index=on mount will fail.

Documentation was updated to reflect this change.

Fixes: 2cac0c00 ("ovl: get exclusive ownership on upper/work dirs")
Cc: <stable@vger.kernel.org> # v4.13
Reported-by: NEuan Kemp <euank@euank.com>
Reported-by: NVivek Goyal <vgoyal@redhat.com>
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

The inodes index feature introduces a behavior change - on mount,
upper root origin file handle is verified to match the lower root dir.
This implies that copied layers cannot be mounted with the inodes index
feature enabled, without explicitly removing the upper dir origin xattr
and the index dir.

The inodes index feature is required to support:
- Prevent breaking hardlinks on copy up
- NFS export support (upcoming)
- Overlayfs snapshots (POC)
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

 - Fix broken long line block quote
 - Fix missing newline before bullets list
 - Use correct numbered list syntax
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

Current code returns EXDEV when a directory would need to be copied up to
move.  We could copy up the directory tree in this case, but there's
another, simpler solution: point to old lower directory from moved upper
directory.

This is achieved with a "trusted.overlay.redirect" xattr storing the path
relative to the root of the overlay.  After such attribute has been set,
the directory can be moved without further actions required.

This is a backward incompatible feature, old kernels won't be able to
correctly mount an overlay containing redirected directories.
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

The quirk for file locks and leases no longer applies.

Add missing info about renaming directory residing on lower layer.
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

Some of the documented quirks no longer apply.
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

Two "fixme" items are actually fixed now.
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

I'm getting a surprising large number of questions about overlayfs sent
to me personally, rather than to a relevant mailing list.

So remove my email address from the documentation, and add a note
about looking in the MAINTAINERS file.
Signed-off-by: NNeilBrown <neilb@suse.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Reported-by: Fabian Sturm <fabian.sturm@aduu.de> 
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

Reported-by: NSedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

Allow "lowerdir=" option to contain multiple lower directories separated by
a colon (e.g. "lowerdir=/bin:/usr/bin").  Colon characters in filenames can
be escaped with a backslash.
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

Some distributions carry an "old" format of overlayfs while mainline has a
"new" format.

The distros will possibly want to keep the old overlayfs alongside the new
for compatibility reasons.

To make it possible to differentiate the two versions change the name of
the new one from "overlayfs" to "overlay".
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>
Reported-by: NSerge Hallyn <serge.hallyn@ubuntu.com>
Cc: Andy Whitcroft <apw@canonical.com>

Document the overlay filesystem.
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>