Re: [RFC PATCH v4 13/16] io_uring: add io_recvzc request

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

On 3/13/24 20:25, Jens Axboe wrote:
> On 3/12/24 3:44 PM, David Wei wrote:
>> Add an io_uring opcode OP_RECV_ZC for doing ZC reads from a socket that
>> is set up for ZC Rx. The request reads skbs from a socket. Completions
>> are posted into the main CQ for each page frag read.
>>
>> Big CQEs (CQE32) is required as the OP_RECV_ZC specific metadata (ZC
>> region, offset, len) are stored in the extended 16 bytes as a
>> struct io_uring_rbuf_cqe.
>>
>> For now there is no limit as to how much work each OP_RECV_ZC request
>> does. It will attempt to drain a socket of all available data.
>>
>> Multishot requests are also supported. The first time an io_recvzc
>> request completes, EAGAIN is returned which arms an async poll. Then, in
>> subsequent runs in task work, IOU_ISSUE_SKIP_COMPLETE is returned to
>> continue async polling.
> 
> I'd probably drop that last paragraph, this is how all multishot
> requests work and is implementation detail that need not go in the
> commit message. Probably suffices just to say it supports multishot.
> 
>> @@ -695,7 +701,7 @@ static inline bool io_recv_finish(struct io_kiocb *req, int *ret,
>>   	unsigned int cflags;
>>   
>>   	cflags = io_put_kbuf(req, issue_flags);
>> -	if (msg->msg_inq && msg->msg_inq != -1)
>> +	if (msg && msg->msg_inq && msg->msg_inq != -1)
>>   		cflags |= IORING_CQE_F_SOCK_NONEMPTY;
>>   
>>   	if (!(req->flags & REQ_F_APOLL_MULTISHOT)) {
>> @@ -723,7 +729,7 @@ static inline bool io_recv_finish(struct io_kiocb *req, int *ret,
>>   			goto enobufs;
>>   
>>   		/* Known not-empty or unknown state, retry */
>> -		if (cflags & IORING_CQE_F_SOCK_NONEMPTY || msg->msg_inq == -1) {
>> +		if (cflags & IORING_CQE_F_SOCK_NONEMPTY || (msg && msg->msg_inq == -1)) {
>>   			if (sr->nr_multishot_loops++ < MULTISHOT_MAX_RETRY)
>>   				return false;
>>   			/* mshot retries exceeded, force a requeue */
> 
> Maybe refactor this a bit so that you don't need to add these NULL
> checks? That seems pretty fragile, hard to read, and should be doable
> without extra checks.

That chunk can be completely thrown away, we're not using
io_recv_finish() here anymore


>> @@ -1053,6 +1058,85 @@ struct io_zc_rx_ifq *io_zc_verify_sock(struct io_kiocb *req,
>>   	return ifq;
>>   }
>>   
>> +int io_recvzc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>> +{
>> +	struct io_recvzc *zc = io_kiocb_to_cmd(req, struct io_recvzc);
>> +
>> +	/* non-iopoll defer_taskrun only */
>> +	if (!req->ctx->task_complete)
>> +		return -EINVAL;
> 
> What's the reasoning behind this?

CQ locking, see the comment a couple lines below


>> +	struct io_recvzc *zc = io_kiocb_to_cmd(req, struct io_recvzc);
>> +	struct io_zc_rx_ifq *ifq;
>> +	struct socket *sock;
>> +	int ret;
>> +
>> +	/*
>> +	 * We're posting CQEs deeper in the stack, and to avoid taking CQ locks
>> +	 * we serialise by having only the master thread modifying the CQ with
>> +	 * DEFER_TASkRUN checked earlier and forbidding executing it from io-wq.
>> +	 * That's similar to io_check_multishot() for multishot CQEs.
>> +	 */

This one ^^, though it doesn't read well, I should reword it for
next time.

>> +	if (issue_flags & IO_URING_F_IOWQ)
>> +		return -EAGAIN;
>> +	if (WARN_ON_ONCE(!(issue_flags & IO_URING_F_NONBLOCK)))
>> +		return -EAGAIN;
> 
> If rebased on the current tree, does this go away?

It's just a little behind not to have that change

-- 
Pavel Begunkov