[PATCH] xfs: Don't keep silent if sunit/swidth can not be changed via mount

[Jeff Liu <jeff.liu@oracle.com>]

From: Jie Liu <jeff.liu@oracle.com>

As per the mount man page, sunit and swidth can be changed via
mount options.  For XFS, on the face of it, those options seems
works if the specified alignments is properly, e.g.
# mount -o sunit=4096,swidth=8192 /dev/sdb1 /mnt
# mount | grep sdb1
/dev/sdb1 on /mnt type xfs (rw,sunit=4096,swidth=8192)

However, neither sunit nor swidth is shown from the xfs_info output.
# xfs_info /mnt
meta-data=/dev/sdb1    isize=256    agcount=4, agsize=262144 blks
         =             sectsz=512   attr=2
data     =             bsize=4096   blocks=1048576, imaxpct=25
         =             sunit=0      swidth=0 blks
		       ^^^^^^^^^^^^^^^^^^^^^^^^^^
naming   =version 2    bsize=4096   ascii-ci=0
log      =internal     bsize=4096   blocks=2560, version=2
         =             sectsz=512   sunit=0 blks, lazy-count=1
realtime =none         extsz=4096   blocks=0, rtextents=0

The reason is that the alignment can only be changed if the relevant
super block is already configured with alignments, otherwise, the
given value will be silently ignored, so it's better to tell user
that the alignment-changing can not take affect in one way or another.

With this fix, the attempt to mount a storage without strip alignment
setup on super block will failed if XFS_MOUNT_RETERR is enabled, or
just ignore the given alignment and drop a warning to indicate the
cause in syslog.

# mount -o sunit=4096,swidth=8192 /dev/sdb1 /mnt
mount: wrong fs type, bad option, bad superblock on /dev/sdb1,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

# dmesg|tail
.......
XFS (sdb1): can not change alignment: no data alignment on superblock

Signed-off-by: Jie Liu <jeff.liu@oracle.com>

---
 fs/xfs/xfs_mount.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 3806088..bc7fdd4 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -924,6 +924,13 @@ xfs_update_alignment(xfs_mount_t *mp)
 				sbp->sb_width = mp->m_swidth;
 				mp->m_update_flags |= XFS_SB_WIDTH;
 			}
+		} else {
+			xfs_warn(mp, "can not change alignment: "
+				"no data alignment on superblock");
+			if (mp->m_flags & XFS_MOUNT_RETERR)
+				return XFS_ERROR(EINVAL);
+			mp->m_dalign = 0;
+			mp->m_swidth = 0;
 		}
 	} else if ((mp->m_flags & XFS_MOUNT_NOALIGN) != XFS_MOUNT_NOALIGN &&
 		    xfs_sb_version_hasdalign(&mp->m_sb)) {
-- 
1.7.9.5

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