On Fri, Nov 21, 2014 at 2:59 PM, Theodore Ts'o [off-list ref] wrote:

In preparation for adding support for the lazytime mount option, we
need to be able to separate out the update_time() and write_time()
inode operations.  Currently, only btrfs and xfs uses update_time().

We needed to preserve update_time() because btrfs wants to have a
special btrfs_root_readonly() check; otherwise we could drop the
update_time() inode operation entirely.

No objections here, I'll give the queue a whirl.  You can add my 
acked-by-or-Ill-fix-whatever-breaks

I look forward to the patch that makes us all lazy by default.

-chris