Unionfs: document behavior when the lower topology changes

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>

diff --git a/Documentation/filesystems/unionfs/concepts.txt b/Documentation/filesystems/unionfs/concepts.txt
index bed69bdc87d7165179cbc53f3a4a092d08be983b..8d9a1c5c475f08376ef79d7d560496ad936d4c1d 100644 (file)
--- a/Documentation/filesystems/unionfs/concepts.txt
+++ b/Documentation/filesystems/unionfs/concepts.txt
@@ -210,4 +210,17 @@ there's a lot of concurrent activity on both the upper and lower objects,
 for the same file(s).  Lastly, this delayed time attribute detection is
 similar to how NFS clients operate (e.g., acregmin).
 
+Finally, there is no way currently in Linux to prevent lower directories
+from being moved around (i.e., topology changes); there's no way to prevent
+modifications to directory sub-trees of whole file systems which are mounted
+read-write.  It is therefore possible for in-flight operations in unionfs to
+take place, while a lower directory is being moved around.  Therefore, if
+you try to, say, create a new file in a directory through unionfs, while the
+directory is being moved around directly, then the new file may get created
+in the new location where that directory was moved to.  This is a somewhat
+similar behaviour in NFS: an NFS client could be creating a new file while
+th NFS server is moving th directory around; the file will get successfully
+created in the new location.  (The one exception in unionfs is that if the
+branch is marked read-only by unionfs, then a copyup will take place.)
+
 For more information, see <http://unionfs.filesystems.org/>.