Merge tag 'io_uring-6.15-20250403' of git://git.kernel.dk/linux

Pull more io_uring updates from Jens Axboe:
 "Set of fixes/updates for io_uring that should go into this release.

  The ublk bits could've gone via either tree - usually I put them in
  block, but they got a bit mixed this series with the zero-copy
  supported that ended up dipping into both trees.

  This contains:

   - Fix for sendmsg zc, include in pinned pages accounting like we do
     for the other zc types

   - Series for ublk fixing request aborting, doing various little
     cleanups, fixing some zc issues, and adding queue_rqs support

   - Another ublk series doing some code cleanups

   - Series cleaning up the io_uring send path, mostly in preparation
     for registered buffers

   - Series doing little MSG_RING cleanups

   - Fix for the newly added zc rx, fixing len being 0 for the last
     invocation of the callback

   - Add vectored registered buffer support for ublk. With that, then
     ublk also supports this feature in the kernel revision where it
     could generically introduced for rw/net

   - A bunch of selftest additions for ublk. This is the majority of the
     diffstat

   - Silence a KCSAN data race warning for io-wq

   - Various little cleanups and fixes"

* tag 'io_uring-6.15-20250403' of git://git.kernel.dk/linux: (44 commits)
  io_uring: always do atomic put from iowq
  selftests: ublk: enable zero copy for stripe target
  io_uring: support vectored kernel fixed buffer
  block: add for_each_mp_bvec()
  io_uring: add validate_fixed_range() for validate fixed buffer
  selftests: ublk: kublk: fix an error log line
  selftests: ublk: kublk: use ioctl-encoded opcodes
  io_uring/zcrx: return early from io_zcrx_recv_skb if readlen is 0
  io_uring/net: avoid import_ubuf for regvec send
  io_uring/rsrc: check size when importing reg buffer
  io_uring: cleanup {g,s]etsockopt sqe reading
  io_uring: hide caches sqes from drivers
  io_uring: make zcrx depend on CONFIG_IO_URING
  io_uring: add req flag invariant build assertion
  Documentation: ublk: remove dead footnote
  selftests: ublk: specify io_cmd_buf pointer type
  ublk: specify io_cmd_buf pointer type
  io_uring: don't pass ctx to tw add remote helper
  io_uring/msg: initialise msg request opcode
  io_uring/msg: rename io_double_lock_ctx()
  ...

Documentation/block/ublk.rst

@@ -309,18 +309,35 @@ with specified IO tag in the command data:
   ``UBLK_IO_COMMIT_AND_FETCH_REQ`` to the server, ublkdrv needs to copy
   the server buffer (pages) read to the IO request pages.
 
-Future development
-==================
-
 Zero copy
 ---------
 
-Zero copy is a generic requirement for nbd, fuse or similar drivers. A
-problem [#xiaoguang]_ Xiaoguang mentioned is that pages mapped to userspace
-can't be remapped any more in kernel with existing mm interfaces. This can
-occurs when destining direct IO to ``/dev/ublkb*``. Also, he reported that
-big requests (IO size >= 256 KB) may benefit a lot from zero copy.
-
+ublk zero copy relies on io_uring's fixed kernel buffer, which provides
+two APIs: `io_buffer_register_bvec()` and `io_buffer_unregister_bvec`.
+
+ublk adds IO command of `UBLK_IO_REGISTER_IO_BUF` to call
+`io_buffer_register_bvec()` for ublk server to register client request
+buffer into io_uring buffer table, then ublk server can submit io_uring
+IOs with the registered buffer index. IO command of `UBLK_IO_UNREGISTER_IO_BUF`
+calls `io_buffer_unregister_bvec()` to unregister the buffer, which is
+guaranteed to be live between calling `io_buffer_register_bvec()` and
+`io_buffer_unregister_bvec()`. Any io_uring operation which supports this
+kind of kernel buffer will grab one reference of the buffer until the
+operation is completed.
+
+ublk server implementing zero copy or user copy has to be CAP_SYS_ADMIN and
+be trusted, because it is ublk server's responsibility to make sure IO buffer
+filled with data for handling read command, and ublk server has to return
+correct result to ublk driver when handling READ command, and the result
+has to match with how many bytes filled to the IO buffer. Otherwise,
+uninitialized kernel IO buffer will be exposed to client application.
+
+ublk server needs to align the parameter of `struct ublk_param_dma_align`
+with backend for zero copy to work correctly.
+
+For reaching best IO performance, ublk server should align its segment
+parameter of `struct ublk_param_segment` with backend for avoiding
+unnecessary IO split, which usually hurts io_uring performance.
 
 References
 ==========
@@ -332,5 +349,3 @@ References
 .. [#userspace_nbdublk] https://gitlab.com/rwmjones/libnbd/-/tree/nbdublk
 
 .. [#userspace_readme] https://github.com/ming1/ubdsrv/blob/master/README
-
-.. [#xiaoguang] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/