[PATCH 0/3] Consistently look up fixed buffers before going async

[Caleb Sander Mateos <csander@purestorage.com>]

To use ublk zero copy, an application submits a sequence of io_uring
operations:
(1) Register a ublk request's buffer into the fixed buffer table
(2) Use the fixed buffer in some I/O operation
(3) Unregister the buffer from the fixed buffer table

The ordering of these operations is critical; if the fixed buffer lookup
occurs before the register or after the unregister operation, the I/O
will fail with EFAULT or even corrupt a different ublk request's buffer.
It is possible to guarantee the correct order by linking the operations,
but that adds overhead and doesn't allow multiple I/O operations to
execute in parallel using the same ublk request's buffer. Ideally, the
application could just submit the register, I/O, and unregister SQEs in
the desired order without links and io_uring would ensure the ordering.
This mostly works, leveraging the fact that each io_uring SQE is prepped
and issued non-blocking in order (barring link, drain, and force-async
flags). But it requires the fixed buffer lookup to occur during the
initial non-blocking issue.

This patch series fixes the 2 gaps where the initial issue can return
EAGAIN before looking up the fixed buffer:
- IORING_OP_SEND_ZC using IORING_RECVSEND_POLL_FIRST
- IORING_OP_URING_CMD, of which NVMe passthru is currently the only
  fixed buffer user. blk_mq_alloc_request() can return EAGAIN before
  io_uring_cmd_import_fixed() is called to look up the fixed buffer.

Caleb Sander Mateos (3):
  io_uring/net: only import send_zc buffer once
  io_uring/net: import send_zc fixed buffer before going async
  io_uring/uring_cmd: import fixed buffer before going async

 drivers/nvme/host/ioctl.c    | 10 ++++------
 include/linux/io_uring/cmd.h |  6 ++----
 io_uring/net.c               | 13 ++++++++-----
 io_uring/rsrc.c              |  6 ++++++
 io_uring/rsrc.h              |  2 ++
 io_uring/uring_cmd.c         | 10 +++++++---
 6 files changed, 29 insertions(+), 18 deletions(-)

-- 
2.45.2