[PATCH 0/4] io_uring: use ITER_UBUF

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

ITER_UBUF is a more efficient representation when using single vector
buffers, providing small optimizations in the fast path. Most of this
series came from Jens; I just ported them forward to the current release
and tested against various filesystems and devices.

Usage for this new iter type has been extensively exercised via
read/write syscall interface for some time now, so I don't expect
surprises from supporting this with io_uring. There are, however, a
couple difference between the two interfaces:

  1. io_uring will always prefer using the _iter versions of read/write
     callbacks if file_operations implement both, where as the generic
     syscalls will use .read/.write (if implemented) for non-vectored IO.
 
  2. io_uring will use the ITER_UBUF representation for single vector
     readv/writev, but the generic syscalls currently uses ITER_IOVEC for
     these.

That should mean, then, the only potential areas for problem are for
file_operations that implement both .read/.read_iter or
.write/.write_iter. Fortunately there are very few that do that, and I
found only two of them that won't readily work: qib_file_ops, and
snd_pcm_f_ops. The former is already broken with io_uring before this
series, and the latter's vectored read/write only works with ITER_IOVEC,
so that will break, but I don't think anyone is using io_uring to talk
to a sound card driver.

Jens Axboe (3):
  iov: add import_ubuf()
  io_uring: switch network send/recv to ITER_UBUF
  io_uring: use ubuf for single range imports for read/write

Keith Busch (1):
  iov_iter: move iter_ubuf check inside restore WARN

 include/linux/uio.h |  1 +
 io_uring/net.c      | 13 ++++---------
 io_uring/rw.c       |  9 ++++++---
 lib/iov_iter.c      | 15 +++++++++++++--
 4 files changed, 24 insertions(+), 14 deletions(-)

-- 
2.30.2