[PATCH 03/21] Unionfs: Documentation updates for branch-management

[Josef 'Jeff' Sipek]

From: Erez Zadok <ezk@xxxxxxxxxxxxx>

Describe dynamic branch-management introduced by subsequent patches.

Signed-off-by: Erez Zadok <ezk@xxxxxxxxxxxxx>
Signed-off-by: Josef 'Jeff' Sipek <jsipek@xxxxxxxxxxxxx>
---
 Documentation/filesystems/unionfs/concepts.txt |    5 ++
 Documentation/filesystems/unionfs/issues.txt   |   22 +++++++-
 Documentation/filesystems/unionfs/usage.txt    |   61 +++++++++++++++++++++++-
 3 files changed, 84 insertions(+), 4 deletions(-)

diff --git a/Documentation/filesystems/unionfs/concepts.txt b/Documentation/filesystems/unionfs/concepts.txt
index d417576..83d45b9 100644
--- a/Documentation/filesystems/unionfs/concepts.txt
+++ b/Documentation/filesystems/unionfs/concepts.txt
@@ -1,3 +1,6 @@
+Unionfs 2.0 CONCEPTS:
+=====================
+
 This file describes the concepts needed by a namespace unification file
 system.
 
@@ -68,3 +71,5 @@ though to this copy. The copy must be made on a higher priority branch so
 that lookup and readdir return this newer "version" of the file rather than
 the original (see duplicate elimination).
 
+
+For more information, see <http://unionfs.filesystems.org/>.
diff --git a/Documentation/filesystems/unionfs/issues.txt b/Documentation/filesystems/unionfs/issues.txt
index b070175..a434fee 100644
--- a/Documentation/filesystems/unionfs/issues.txt
+++ b/Documentation/filesystems/unionfs/issues.txt
@@ -1,5 +1,5 @@
-KNOWN Unionfs ISSUES:
-=====================
+KNOWN Unionfs 2.0 ISSUES:
+=========================
 
 1. The NFS server returns -EACCES for read-only exports, instead of -EROFS.
    This means we can't reliably detect a read-only NFS export.
@@ -14,10 +14,26 @@ KNOWN Unionfs ISSUES:
    introducing small VFS/MM changes that would allow a file system to handle
    cache coherency correctly.
 
+   Unionfs 2.0 has a temporary workaround for this.  You can force Unionfs
+   to increase the superblock generation number, and hence purge all cached
+   Unionfs objects, which would then  be re-gotten from the lower branches.
+   This should ensure cache consistency.  To increase the generation number,
+   executed the command:
+
+	mount -t unionfs -o remount,incgen none MOUNTPOINT
+
+   Note that the older way of incrementing the generation number using an
+   ioctl, is no longer supported in Unionfs 2.0.  Ioctls in general are not
+   encouraged.  Plus, an ioctl is per-file concept, whereas the generation
+   number is a per-file-system concept.  Worse, such an ioctl requires an
+   open file, which then has to be invalidated by the very nature of the
+   generation number increase (read: the old generation increase ioctl was
+   pretty racy).
+
 3. Unionfs should not use lookup_one_len() on the underlying f/s as it
    confuses NFS.  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.
 
 
-For more information, see <unionfs.filesystems.org>.
+For more information, see <http://unionfs.filesystems.org/>.
diff --git a/Documentation/filesystems/unionfs/usage.txt b/Documentation/filesystems/unionfs/usage.txt
index 14e0856..13fbcea 100644
--- a/Documentation/filesystems/unionfs/usage.txt
+++ b/Documentation/filesystems/unionfs/usage.txt
@@ -28,4 +28,63 @@ Example:
 	dirs=/writable_branch=rw:/read-only_branch=ro
 
 
-For more information, see unionfs.filesystems.org.
+DYNAMIC BRANCH MANAGEMENT AND REMOUNTS
+======================================
+
+You can remount a union and change its overall mode, or reconfigure the
+branches, as follows.
+
+To downgrade a union from read-write to read-only:
+
+# mount -t unionfs -o remount,ro none MOUNTPOINT
+
+To upgrade a union from read-only to read-write:
+
+# mount -t unionfs -o remount,rw none MOUNTPOINT
+
+To delete a branch /foo, regardless where it is in the current union:
+
+# mount -t unionfs -o del=/foo none MOUNTPOINT
+
+To insert (add) a branch /foo before /bar:
+
+# mount -t unionfs -o remount,add=/bar:/foo none MOUNTPOINT
+
+To insert (add) a branch /foo (with the "rw" mode flag) before /bar:
+
+# mount -t unionfs -o remount,add=/bar:/foo=rw none MOUNTPOINT
+
+To insert (add) a branch /foo (in "rw" mode) at the very beginning (i.e., a
+new highest-priority branch), you can use the above syntax, or use a short
+hand version as follows:
+
+# mount -t unionfs -o remount,add=/foo none MOUNTPOINT
+
+To append a branch to the very end (new lowest-priority branch):
+
+# mount -t unionfs -o remount,add=:/foo none MOUNTPOINT
+
+To append a branch to the very end (new lowest-priority branch), in
+read-only mode:
+
+# mount -t unionfs -o remount,add=:/foo:ro none MOUNTPOINT
+
+Finally, to change the mode of one existing branch, say /foo, from read-only
+to read-write, and change /bar from read-write to read-only:
+
+# mount -t unionfs -o remount,mode=/foo=rw,mode=/bar=ro none MOUNTPOINT
+
+
+CACHE CONSISTENCY
+=================
+
+If you modify any file on any of the lower branches directly, while there is
+a Unionfs 2.0 mounted above any of those branches, you should tell Unionfs
+to purge its caches and re-get the objects.  To do that, you have to
+incremenet the generation number of the superblock using the following
+command:
+
+# mount -t unionfs -o remount,remount,incgen none MOUNTPOINT
+
+
+For more information, see <http://unionfs.filesystems.org/>.
-- 
1.5.0.3.268.g3dda

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