Comment 5 for bug 1362574

ext2.ko itself does support fsfreeze, but typical linux distros, like Ubuntu, don't supply ext2.ko at all now -- instead, they usually supply ext3.ko and have ext4 built-in.

So when we mount an ext2 partition, actually the kernel is registering the ext4 driver as an ext2 driver and in this case the ext2's s_op->freeze_fs is NULL -- so fsfreeze on ext3 can get -EOPNOTSUPP.

The discussion is at http://marc.info/?t=141102289600007&r=1&w=2

The ext4 community has posted the patches to support ext2 fsfreeze when registering ext4 as ext2:
http://marc.info/?l=linux-ext4&m=141107200212887&w=2
http://marc.info/?l=linux-ext4&m=141107200312889&w=2
http://marc.info/?l=linux-ext4&m=141107481213962&w=2