btrfs: Add "barrier" option to support "-o remount,barrier"

Btrfs can be remounted without barrier, but there is no "barrier" option
so nobody can remount btrfs back with barrier on. Only umount and
mount again can re-enable barrier.(Quite awkward)

Also the mount options in the document is also changed slightly for the
further pairing options changes.

Reported-by: Daniel Blueman <daniel@quora.org>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Mike Fleetwood <mike.fleetwood@googlemail.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/Documentation/filesystems/btrfs.txt b/Documentation/filesystems/btrfs.txt
index 5dd282dda55c..ce487a2a88f8 100644
--- a/Documentation/filesystems/btrfs.txt
+++ b/Documentation/filesystems/btrfs.txt
@@ -38,7 +38,7 @@ Mount Options
 =============
 
 When mounting a btrfs filesystem, the following option are accepted.
-Unless otherwise specified, all options default to off.
+Options with (*) are default options and will not show in the mount options.
 
   alloc_start=<bytes>
         Debugging option to force all block allocations above a certain
@@ -138,12 +138,13 @@ Unless otherwise specified, all options default to off.
         Disable support for Posix Access Control Lists (ACLs).  See the
         acl(5) manual page for more information about ACLs.
 
+  barrier(*)
   nobarrier
-        Disables the use of block layer write barriers.  Write barriers ensure
-        that certain IOs make it through the device cache and are on persistent
-        storage.  If used on a device with a volatile (non-battery-backed)
-        write-back cache, this option will lead to filesystem corruption on a
-        system crash or power loss.
+        Enable/disable the use of block layer write barriers.  Write barriers
+        ensure that certain IOs make it through the device cache and are on
+        persistent storage. If disabled on a device with a volatile
+        (non-battery-backed) write-back cache, nobarrier option will lead to
+        filesystem corruption on a system crash or power loss.
 
   nodatacow
         Disable data copy-on-write for newly created files.  Implies nodatasum,