Re: [PATCH 4/4] io_uring/uring_cmd: implement ->sqe_copy() to avoid unnecessary copies

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

On Fri, Jun 6, 2025 at 2:56 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> uring_cmd currently copies the full SQE at prep time, just in case it
> needs it to be stable. However, for inline completions or requests that
> get queued up on the device side, there's no need to ever copy the SQE.
> This is particularly important, as various use cases of uring_cmd will
> be using 128b sized SQEs.
>
> Opt in to using ->sqe_copy() to let the core of io_uring decide when to
> copy SQEs, rather than to it upfront unconditionally.
>
> This provides two checks to see if ioucmd->sqe is still valid:
>
> 1) IO_URING_F_INLINE must be set, indicating the ->sqe_copy() call is
>    happening inline from the syscal submitting the IO. As long as we're
>    in that context, the SQE cannot have been reused.
>
> 2) If the SQE being passed in is NULL, then we're off the task_work
>    submission path. This check could be combined with IO_URING_F_INLINE,
>    but it'd require an additional branch-and-check in SQE queueing.
>
> If either of these aren't true and the SQE hasn't been copied already,
> then fail the request with -EFAULT and trigger a WARN_ON_ONCE() to
> indicate that there's a bug to figure out. With that, it should not be
> possible to ever reuse an SQE outside of the direct syscall path.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

With the parts about IO_URING_F_INLINE and NULL SQE removed from the
commit message,

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>