On Mon, Aug 29, 2016 at 08:57:41AM -0400, Theodore Ts'o wrote:

It's been on my todo list.  The only reason why I haven't done it yet
is because I knew you were working on a solution, and I didn't want to
do things one way for buffered I/O, and a different way for Direct
I/O, and disentangling the DIO code and the different assumptions of
how different file systems interact with the DIO code is a *mess*.

It is.  I have an almost working iomap direct I/O implementation, which
simplifies a lot of this.  Partially because it expects sane locking
(a shared lock should be held over read, fortunately we now have i_rwsem
for that) and allows less opt-in/out behavior, and partially just
because we have so much better infrastructure now (iomap, bio chaining,
iov_iters, ...).

The iomap version is still work in progress, but I'm about to post
a cut down version for block devices which reduces I/O latency by
20%.