On Wed, Dec 09, 2020 at 08:36:45AM +0000, Christoph Hellwig wrote:

This is making the iter type even more of a mess than it already is.
I think we at least need placeholders for 0/1 here and an explicit
flags namespace, preferably after the types.

Then again I'd much prefer if we didn't even add the flag or at best
just add it for a short-term transition and move everyone over to the
new scheme.  Otherwise the amount of different interfaces and supporting
code keeps exploding.

Yes.  The only problem I see is how to describe the rules - "bdev-backed
iterators need the bvec array to stay allocated until IO completes"?
And one way or another, that needs to be documented - D/f/porting with
"mandatory" for priority.