On Tue, Oct 14, 2014 at 03:27:11PM +1100, Dave Chinner wrote:

Broken usage, IMO. If you are going to use the block layer ioctls to
manipulate data int eh block device, you should be using direct Io
for all your data IO to the block device. Otherwise, coherency
problems occur....

I'd say BLKZEROOUT semantics are broken.  Having an ioctl exposed
that maniulates on-disk data without cache coherence is a nightmare
that people trip over easily.  Even experienced people like Darrick.