I've noticed that when the MMP check fails that the message reported does not show the name of the host where the filesystem is mounted, the documents I've seen on this imply that it it supposed to and the output makes it look like it is but the field says "Linux" instead of a hostname.

Example logs from both 2.1.1 and 1.8.6, both using the rpms provided by whamcloud.
2.1.1:
LDISKFS-fs warning (device dm-6): ldiskfs_multi_mount_protect: MMP failure info: last update time: 1331808934, last update node: Linux, last update device: dm-6
1.8.6:
LDISKFS-fs warning (device sfa0565): ldiskfs_multi_mount_protect: MMP failure info: last update time: 1331809026, last update node: Linux, last update device: sfa0472