Subject: [PATCH v2 net] splice: do not checksum AF_UNIX sockets

From: Frederik Deweerdt <[email-masked]>
Date: Mon, 9 Dec 2024 21:06:48 -0800

When `skb_splice_from_iter` was introduced, it inadvertently added
checksumming for AF_UNIX sockets. This resulted in significant
slowdowns, for example when using sendfile over unix sockets.

Using the test code in [1] in my test setup (2G single core qemu),
the client receives a 1000M file in:
- without the patch: 1482ms (+/- 36ms)
- with the patch: 652.5ms (+/- 22.9ms)

This commit addresses the issue by marking checksumming as unnecessary in
`unix_stream_sendmsg`

Cc: [email-masked]
Signed-off-by: Frederik Deweerdt <[email-masked]>
Fixes: 2e910b95329c ("net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES")
---
 net/unix/af_unix.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 001ccc55ef0f..6b1762300443 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2313,6 +2313,7 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 		fds_sent = true;
 
 		if (unlikely(msg->msg_flags & MSG_SPLICE_PAGES)) {
+			skb->ip_summed = CHECKSUM_UNNECESSARY;
 			err = skb_splice_from_iter(skb, &msg->msg_iter, size,
 						   sk->sk_allocation);
 			if (err < 0) {
-- 
2.44.1