ovl: document copying layers restrictions with inodes index

[platform/kernel/linux-rpi.git] / Documentation / filesystems / overlayfs.txt

diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 634d03e..36f528a 100644 (file)
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -21,12 +21,19 @@ from accessing the corresponding object from the original filesystem.
 This is most obvious from the 'st_dev' field returned by stat(2).
 
 While directories will report an st_dev from the overlay-filesystem,
-all non-directory objects will report an st_dev from the lower or
+non-directory objects may report an st_dev from the lower filesystem or
 upper filesystem that is providing the object.  Similarly st_ino will
 only be unique when combined with st_dev, and both of these can change
 over the lifetime of a non-directory object.  Many applications and
 tools ignore these values and will not be affected.
 
+In the special case of all overlay layers on the same underlying
+filesystem, all objects will report an st_dev from the overlay
+filesystem and st_ino from the underlying filesystem.  This will
+make the overlay mount more compliant with filesystem scanners and
+overlay objects will be distinguishable from the corresponding
+objects in the original filesystem.
+
 Upper and Lower
 ---------------
 
@@ -194,6 +201,40 @@ rightmost one and going left.  In the above example lower1 will be the
 top, lower2 the middle and lower3 the bottom layer.
 
 
+Sharing and copying layers
+--------------------------
+
+Lower layers may be shared among several overlay mounts and that is indeed
+a very common practice.  An overlay mount may use the same lower layer
+path as another overlay mount and it may use a lower layer path that is
+beneath or above the path of another overlay lower layer path.
+
+Using an upper layer path and/or a workdir path that are already used by
+another overlay mount is not allowed and will fail with EBUSY.  Using
+partially overlapping paths is not allowed but will not fail with EBUSY.
+
+Mounting an overlay using an upper layer path, where the upper layer path
+was previously used by another mounted overlay in combination with a
+different lower layer path, is allowed, unless the "inodes index" feature
+is enabled.
+
+With the "inodes index" feature, on the first time mount, an NFS file
+handle of the lower layer root directory, along with the UUID of the lower
+filesystem, are encoded and stored in the "trusted.overlay.origin" extended
+attribute on the upper layer root directory.  On subsequent mount attempts,
+the lower root directory file handle and lower filesystem UUID are compared
+to the stored origin in upper root directory.  On failure to verify the
+lower root origin, mount will fail with ESTALE.  An overlayfs mount with
+"inodes index" enabled will fail with EOPNOTSUPP if the lower filesystem
+does not support NFS export, lower filesystem does not have a valid UUID or
+if the upper filesystem does not support extended attributes.
+
+It is quite a common practice to copy overlay layers to a different
+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.
+
+
 Non-standard behavior
 ---------------------