[PATCH 0/6] io_uring/nvme: support fixed buffer for metadata

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

io_uring NVMe passthrough supports using a "fixed" (registered) buffer
for data, but not metadata. On high-IOPS workloads, the pinning and
unpinning overhead for the metadata pages is significant and could be
avoided if fixed metadata buffers were supported.

This patch series adds the necessary plumbing to allow NVMe passthrough
commands to use fixed buffers for their metadata. The metadata and data
fixed buffer indices can be specified (or omitted) independently.
Supporting separate fixed buffers is important as metadata buffers are
often stored in separate memory from the corresponding data buffers. For
ublk zero-copy I/Os, sharing a buffer index would be impossible as the
data buffer is a kernel registered buffer while the metadata buffer is a
userspace registered buffer.

My main question is which layer the metadata fixed buffer should belong
to: core io_uring, io_uring_cmd, or NVMe passthrough? In this initial
implementation, the io_uring_cmd layer stores the metadata buffer node
and the NVMe passthrough layer defines the UAPI.

The main argument for moving the implementation to a more generic layer
would be to reuse it for other io_uring request types. For example,
IORING_RW_ATTR_FLAG_PI could also benefit from a fixed metadata buffer
option, though this series doesn't implement it yet.

On the other hand, core io_uring_sqe and io_kiocb space is very limited,
so it may be undesirable to dedicate it for a somewhat niche use case.
The metadata buffer node storage could be pushed to the NVMe passthrough
layer after my in-flight series [1] to reclaim nvme_uring_cmd_pdu space.
However, managing request-scoped resources from a ->uring_cmd()
implementation is a pain, as the same request can call ->uring_cmd()
multiple times and may or may not complete when ->uring_cmd() returns,
depending on the ->uring_cmd() return value. io_req_uring_cleanup(), in
contrast, provides a single cleanup path for all uring_cmds.

[1]: https://lore.kernel.org/io-uring/20260909155848.2069290-1-csander@purestorage.com/T/

Caleb Sander Mateos (6):
  bio-integrity: remove dead bio_integrity_copy_user() error path
  nvme/ioctl: remove struct nvme_uring_data
  blk-integrity: pass iov_iter to blk_rq_integrity_map_user()
  nvme/ioctl: pass iov_iter to nvme_map_user_request()
  io_uring/cmd: support fixed buffer for metadata
  nvme/ioctl: support fixed buffer for metadata

 block/bio-integrity.c           | 51 ++++++++++--------
 block/blk-integrity.c           |  7 +--
 drivers/nvme/host/ioctl.c       | 91 +++++++++++++++++++--------------
 include/linux/bio-integrity.h   |  1 +
 include/linux/blk-integrity.h   |  6 +--
 include/linux/io_uring/cmd.h    | 12 ++++-
 include/uapi/linux/nvme_ioctl.h |  5 +-
 io_uring/rsrc.c                 | 28 ++++------
 io_uring/rsrc.h                 | 17 ++++++
 io_uring/uring_cmd.c            | 30 ++++++++++-
 10 files changed, 160 insertions(+), 88 deletions(-)

-- 
2.55.0