On Thu, Aug 17, 2023 at 2:05 PM Alexander Larsson <alexl@xxxxxxxxxx> wrote: > > Signed-off-by: Alexander Larsson <alexl@xxxxxxxxxx> > --- > Documentation/filesystems/overlayfs.rst | 22 ++++++++++++++++++++++ > 1 file changed, 22 insertions(+) > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > index 35853906accb..e38b2f5fadaf 100644 > --- a/Documentation/filesystems/overlayfs.rst > +++ b/Documentation/filesystems/overlayfs.rst > @@ -492,6 +492,28 @@ directory tree on the same or different underlying filesystem, and even > to a different machine. With the "inodes index" feature, trying to mount > the copied layers will fail the verification of the lower root file handle. > > +Nesting overlayfs mounts > +------------------------ > + > +It is possible to use a lower directory that is stored on an overlayfs > +mount. For regular files this does not need any special care. However, files > +that have overlayfs attributes, such as whiteouts or `overlay.*` xattrs will > +be interpreted by the underlying overlayfs mount and stripped out. In order to > +allow the second overlayfs mount to see the attributes they must be escaped. > + > +Overlayfs specific xattrs are escaped by using a special prefix of > +`overlay.overlay.`. So, a file with a `trusted.overlay.overlay.metacopy` xattr > +in the lower dir will be exposed as a regular file with a > +`trusted.overlay.metacopy` xattr in the overlayfs mount. This can be nested > +by repeating the prefix multiple time, as each instance only removes one > +prefix. > + > +Whiteouts files marked with a `overlay.nowhiteout` xattr will cause overlayfs > +not to treat them as a whiteout. Since this xattr is then stripped out, the > +next layer will instead apply the whiteout. > + > +Files created via overlayfs will automatically be created with the right > +escapes in the upper directory. > I was wondering why you used `` around xattr names. Is that intentional? I am not an expert in rst format, but other places in this doc use "" around xattr names and it does not look like the formatting of `` is handled well by github at least: https://github.com/alexlarsson/linux/blob/ovl-nesting/Documentation/filesystems/overlayfs.rst Thanks, Amir.
