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

[Luke Hsiao]

From: Luke Hsiao <lukehs...@google.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 <arjun...@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soh...@google.com>
Acked-by: Eric Dumazet <eduma...@google.com>
Reviewed-by: Jens Axboe <ax...@kernel.dk>
Signed-off-by: Luke Hsiao <lukehs...@google.com>
---
 fs/io_uring.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index dc506b75659c..fd5353e31a2c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -79,6 +79,7 @@
 #include <linux/splice.h>
 #include <linux/task_work.h>
 #include <linux/pagemap.h>
+#include <linux/socket.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/io_uring.h>
@@ -4902,7 +4903,8 @@ static __poll_t __io_arm_poll_handler(struct io_kiocb 
*req,
        return mask;
 }
 
-static bool io_arm_poll_handler(struct io_kiocb *req)
+static bool io_arm_poll_handler(struct io_kiocb *req,
+                               const struct io_uring_sqe *sqe)
 {
        const struct io_op_def *def = &io_op_defs[req->opcode];
        struct io_ring_ctx *ctx = req->ctx;
@@ -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;
+
        mask |= POLLERR | POLLPRI;
 
        ipt.pt._qproc = io_async_queue_proc;
@@ -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)) {
 punt:
                        ret = io_prep_work_files(req);
                        if (unlikely(ret))
-- 
2.28.0.297.g1956fa8f8d-goog