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Subject[PATCH 05/42] Unionfs: documentation for any known issues
Signed-off-by: Erez Zadok <>
Documentation/filesystems/unionfs/issues.txt | 24 ++++++++++++++++++++++++
1 files changed, 24 insertions(+), 0 deletions(-)
create mode 100644 Documentation/filesystems/unionfs/issues.txt

diff --git a/Documentation/filesystems/unionfs/issues.txt b/Documentation/filesystems/unionfs/issues.txt
new file mode 100644
index 0000000..9db1d70
--- /dev/null
+++ b/Documentation/filesystems/unionfs/issues.txt
@@ -0,0 +1,24 @@
+KNOWN Unionfs 2.1 ISSUES:
+1. Unionfs should not use lookup_one_len() on the underlying f/s as it
+ confuses NFSv4. Currently, unionfs_lookup() passes lookup intents to the
+ lower file-system, this eliminates part of the problem. The remaining
+ calls to lookup_one_len may need to be changed to pass an intent. We are
+ currently introducing VFS changes to fs/namei.c's do_path_lookup() to
+ allow proper file lookup and opening in stackable file systems.
+2. Lockdep (a debugging feature) isn't aware of stacking, and so it
+ incorrectly complains about locking problems. The problem boils down to
+ this: Lockdep considers all objects of a certain type to be in the same
+ class, for example, all inodes. Lockdep doesn't like to see a lock held
+ on two inodes within the same task, and warns that it could lead to a
+ deadlock. However, stackable file systems do precisely that: they lock
+ an upper object, and then a lower object, in a strict order to avoid
+ locking problems; in addition, Unionfs, as a fan-out file system, may
+ have to lock several lower inodes. We are currently looking into Lockdep
+ to see how to make it aware of stackable file systems. In the meantime,
+ if you get any warnings from Lockdep, you can safely ignore them (or feel
+ free to report them to the Unionfs maintainers, just to be sure).
+For more information, see <>.

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