Re: [PATCH 7/7] io_uring/net: add provided buffer and bundle support to send zc

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/23/24 17:07, Jens Axboe wrote:
> Provided buffers inform the kernel which buffer group ID to pick a
> buffer from for transfer. Normally that buffer contains the usual
> addr + length information, as well as a buffer ID that is passed back
> at completion time to inform the application of which buffer was used
> for the transfer.
> 
> However, if registered and provided buffers are combined, then the
> provided buffer must instead tell the kernel which registered buffer
> index should be used, and the length/offset within that buffer. Rather
> than store the addr + length, the application must instead store this
> information instead.
> 
> If provided buffers are used with send zc, then those buffers must be
> an index into a registered buffer. Change the mapping type to use
> KBUF_MODE_BVEC, which tells the kbuf handlers to turn the mappings
> into bio_vecs rather than iovecs. Then all that is needed is to
> setup our iov_iterator to use iov_iter_bvec().
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
...
> diff --git a/io_uring/net.h b/io_uring/net.h
> index 52bfee05f06a..e052762cf85d 100644
> --- a/io_uring/net.h
> +++ b/io_uring/net.h
> @@ -5,9 +5,15 @@
>   
>   struct io_async_msghdr {
>   #if defined(CONFIG_NET)
> -	struct iovec			fast_iov;
> +	union {
> +		struct iovec		fast_iov;
> +		struct bio_vec		fast_bvec;
> +	};
>   	/* points to an allocated iov, if NULL we use fast_iov instead */
> -	struct iovec			*free_iov;
> +	union {
> +		struct iovec		*free_iov;
> +		struct bio_vec		*free_bvec;

I'd rather not do it like that, aliasing with reusing memory and
counting the number is a recipe for disaster when scattered across
code. E.g. seems you change all(?) iovec allocations to allocate
based on the size of the larger structure.

Counting bytes as in my series is less fragile, otherwise it needs
a new structure and a set of helpers that can be kept together.


-- 
Pavel Begunkov