netdev - [PATCH net-next v3 2/2] io_uring: ignore POLLIN for recvmsg on MSG

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Date:   Fri, 21 Aug 2020 21:41:05 -0700
From:   Luke Hsiao <luke.w.hsiao@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        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: [PATCH net-next v3 2/2] io_uring: ignore POLLIN for recvmsg on MSG_ERRQUEUE

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.

Signed-off-by: Arjun Roy <arjunroy@...gle.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Acked-by: Eric Dumazet <edumazet@...gle.com>
Reviewed-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Luke Hsiao <lukehsiao@...gle.com>
---
 fs/io_uring.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index dc506b75659c..1aa2191ea683 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4932,6 +4932,12 @@ 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) &&
+	    (req->sr_msg.msg_flags & MSG_ERRQUEUE))
+		mask &= ~POLLIN;
+
 	mask |= POLLERR | POLLPRI;
 
 	ipt.pt._qproc = io_async_queue_proc;
-- 
2.28.0.297.g1956fa8f8d-goog