On Thu, Sep 18, 2014 at 06:46:21AM +0000, Dexuan Cui wrote:

I've got the answer:
ext2.ko itself does support fsfreeze, but typical linux distros don't supply
ext2.ko at all now -- instead, they usually supply ext3.ko and have ext4 builtin.

So when I 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 --
but, why did ext4 choose this behavior for ext2?

It wasn't a deliberate design choice.  It was just that when
no-journal mode was added to ext4, freeeze support was never
implemented, and up until now, no one had asked for it.  We can add it
to ext4; thanks for calling it to our attention.

Cheers,

					- Ted