[PATCH v5 06/10] io_uring/rw: add support to send metadata along with read/write

[Keith Busch]

On Tue, Oct 29, 2024 at 09:53:58PM +0530, Anuj Gupta wrote:
> This patch adds the capability of sending metadata along with read/write.
> A new meta_type field is introduced in SQE which indicates the type of
> metadata being passed. This meta is represented by a newly introduced
> 'struct io_uring_meta_pi' which specifies information such as flags,buffer
> length,seed and apptag. Application sets up a SQE128 ring, prepares
> io_uring_meta_pi within the second SQE.
> The patch processes the user-passed information to prepare uio_meta
> descriptor and passes it down using kiocb->private.
> 
> Meta exchange is supported only for direct IO.
> Also vectored read/write operations with meta are not supported
> currently.

It looks like it is reasonable to add support for fixed buffers too.
There would be implications for subsequent patches, mostly patch 10, but
it looks like we can do that.

Anyway, this patch mostly looks okay to me. I don't know about the whole
"meta_type" thing. My understanding from Pavel was wanting a way to
chain command specific extra options. For example, userspace metadata
and write hints, and this doesn't look like it can be extended to do
that.