On Mon, Aug 16 2010 at 12:52pm -0400,
Tejun Heo [off-list ref] wrote:

From: Tejun Heo <redacted>

This patch converts dm to support REQ_FLUSH/FUA instead of now
deprecated REQ_HARDBARRIER.

What tree does this patch apply to?  I know it doesn't apply to
v2.6.36-rc1, e.g.: http://git.kernel.org/linus/708e929513502fb0

For bio-based dm,

...

* -EOPNOTSUPP retry logic dropped.

That logic wasn't just about retries (at least not in the latest
kernel).  With commit 708e929513502fb0 the -EOPNOTSUPP checking also
serves to optimize the barrier+discard case (when discards aren't
supported).

For request-based dm,

* Nothing much changes.  It just needs to handle FLUSH requests as
  before.  It would be beneficial to advertise FUA capability so that
  it can propagate FUA flags down to member request_queues instead of
  sequencing it as WRITE + FLUSH at the top queue.

Can you expand on that TODO a bit?  What is the mechanism to propagate
FUA down to a DM device's members?  I'm only aware of propagating member
devices' features up to the top-level DM device's request-queue (not the
opposite).

Are you saying that establishing the FUA capability on the top-level DM
device's request_queue is sufficient?  If so then why not make the
change?

Lightly tested linear, stripe, raid1, snap and crypt targets.  Please
proceed with caution as I'm not familiar with the code base.

This is concerning... if we're to offer more comprehensive review I
think we need more detail on what guided your changes rather than
details of what the resulting changes are.

Mike