[PATCH] xfs: preserve inode versioning across remounts

[Eric Sandeen <sandeen@redhat.com>]

The MS_I_VERSION mount flag is exposed via the VFS, as documented
in the mount manpages etc; see the iversion and noiversion mount
options in mount(8).

As a result, mount -o remount looks for this option in /proc/mounts
and will only send the I_VERSION flag back in during remount it it
is present.  Since it's not there, a remount will /remove/ the
I_VERSION flag at the vfs level, and iversion functionality is lost.

xfs v5 superblocks intend to always have i_version enabled; it is
set as a default at mount time, but is lost during remount for the
reasons above.

The generic fix would be to expose this documented option in 
/proc/mounts, but since that was rejected, fix it up again in the
xfs remount path instead, so that at least xfs won't suffer from
this misbehavior.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
--

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 379cbff438bc..9239985571af 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1714,6 +1714,10 @@ xfs_fc_reconfigure(
 	int			flags = fc->sb_flags;
 	int			error;
 
+	/* version 5 superblocks always support version counters. */
+	if (XFS_SB_VERSION_NUM(&mp->m_sb) == XFS_SB_VERSION_5)
+		fc->sb_flags |= SB_I_VERSION;
+
 	error = xfs_fc_validate_params(new_mp);
 	if (error)
 		return error;