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Subject[PATCH 03/17] Unionfs: document behavior when the lower topology changes
Signed-off-by: Erez Zadok <>
Documentation/filesystems/unionfs/concepts.txt | 13 +++++++++++++
1 files changed, 13 insertions(+), 0 deletions(-)
diff --git a/Documentation/filesystems/unionfs/concepts.txt b/Documentation/filesystems/unionfs/concepts.txt
index bed69bd..8d9a1c5 100644
--- 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 <>.

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