Pull io_uring fixes from Jens Axboe:
 "This is mostly io_uring fixes/tweaks. Most of these were actually done
  in time for the last -rc, but I wanted to ensure that everything
  tested out great before including them. The code delta looks larger
  than it really is, as it's mostly just comment additions/changes.

  Outside of the comment additions/changes, this is mostly removal of
  unnecessary barriers. In all, this pull request contains:

   - Tweak to how we handle errors at submission time. We now post a
     completion event if the error occurs on behalf of an sqe, instead
     of returning it through the system call. If the error happens
     outside of a specific sqe, we return the error through the system
     call. This makes it nicer to use and makes the "normal" use case
     behave the same as the offload cases. (me)

   - Fix for a missing req reference drop from async context (me)

   - If an sqe is submitted with RWF_NOWAIT, don't punt it to async
     context. Return -EAGAIN directly, instead of using it as a hint to
     do async punt. (Stefan)

   - Fix notes on barriers (Stefan)

   - Remove unnecessary barriers (Stefan)

   - Fix potential double free of memory in setup error (Mark)

   - Further improve sq poll CPU validation (Mark)

   - Fix page allocation warning and leak on buffer registration error
     (Mark)

   - Fix iov_iter_type() for new no-ref flag (Ming)

   - Fix a case where dio doesn't honor bio no-page-ref (Ming)"

* tag 'for-linus-20190502' of git://git.kernel.dk/linux-block:
  io_uring: avoid page allocation warnings
  iov_iter: fix iov_iter_type
  block: fix handling for BIO_NO_PAGE_REF
  io_uring: drop req submit reference always in async punt
  io_uring: free allocated io_memory once
  io_uring: fix SQPOLL cpu validation
  io_uring: have submission side sqe errors post a cqe
  io_uring: remove unnecessary barrier after unsetting IORING_SQ_NEED_WAKEUP
  io_uring: remove unnecessary barrier after incrementing dropped counter
  io_uring: remove unnecessary barrier before reading SQ tail
  io_uring: remove unnecessary barrier after updating SQ head
  io_uring: remove unnecessary barrier before reading cq head
  io_uring: remove unnecessary barrier before wq_has_sleeper
  io_uring: fix notes on barriers
  io_uring: fix handling SQEs requesting NOWAIT