netdev - Re: [PATCH net-next v2 2/2] io_uring: ignore POLLIN for recvmsg on MSG

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Date:   Fri, 21 Aug 2020 20:08:55 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Luke Hsiao <luke.w.hsiao@...il.com>,
        David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        Luke Hsiao <lukehsiao@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v2 2/2] io_uring: ignore POLLIN for recvmsg on
 MSG_ERRQUEUE

On 8/21/20 8:04 PM, Luke Hsiao wrote:
> From: Luke Hsiao <lukehsiao@...gle.com>
> 
> Currently, io_uring's recvmsg subscribes to both POLLERR and POLLIN. In
> the context of TCP tx zero-copy, this is inefficient since we are only
> reading the error queue and not using recvmsg to read POLLIN responses.
> 
> This patch was tested by using a simple sending program to call recvmsg
> using io_uring with MSG_ERRQUEUE set and verifying with printks that the
> POLLIN is correctly unset when the msg flags are MSG_ERRQUEUE.

Sorry, one more minor thing to fix up:

> @@ -4932,6 +4934,11 @@ static bool io_arm_poll_handler(struct io_kiocb *req)
>  		mask |= POLLIN | POLLRDNORM;
>  	if (def->pollout)
>  		mask |= POLLOUT | POLLWRNORM;
> +
> +	/* If reading from MSG_ERRQUEUE using recvmsg, ignore POLLIN */
> +	if (req->opcode == IORING_OP_RECVMSG && (sqe->msg_flags & MSG_ERRQUEUE))
> +		mask &= ~POLLIN;
> +

Don't pass in the sqe here, but use req->sr_msg.msg_flags for this check. This
is actually really important, as you don't want to re-read anything from the
sqe.

I'm actually surprised this one got past Jann :-)

> @@ -6146,7 +6153,7 @@ static void __io_queue_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
>  	 * doesn't support non-blocking read/write attempts
>  	 */
>  	if (ret == -EAGAIN && !(req->flags & REQ_F_NOWAIT)) {
> -		if (!io_arm_poll_handler(req)) {
> +		if (!io_arm_poll_handler(req, sqe)) {

Also means you can drop this part.

-- 
Jens Axboe