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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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