netdev - [PATCH v3 50/55] kcm: Use sendmsg(MSG_SPLICE

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Date:   Fri, 31 Mar 2023 17:09:09 +0100
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Chuck Lever III <chuck.lever@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        netdev@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Tom Herbert <tom@...bertland.com>,
        Tom Herbert <tom@...ntonium.net>
Subject: [PATCH v3 50/55] kcm: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage

When transmitting data, call down into TCP using a single sendmsg with
MSG_SPLICE_PAGES to indicate that content should be spliced rather than
performing several sendmsg and sendpage calls to transmit header, data
pages and trailer.

Signed-off-by: David Howells <dhowells@...hat.com>
cc: Tom Herbert <tom@...bertland.com>
cc: Tom Herbert <tom@...ntonium.net>
cc: "David S. Miller" <davem@...emloft.net>
cc: Eric Dumazet <edumazet@...gle.com>
cc: Jakub Kicinski <kuba@...nel.org>
cc: Paolo Abeni <pabeni@...hat.com>
cc: Jens Axboe <axboe@...nel.dk>
cc: Matthew Wilcox <willy@...radead.org>
cc: netdev@...r.kernel.org
---
 net/kcm/kcmsock.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index d77d28fbf389..9c9d379aafb1 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -641,6 +641,10 @@ static int kcm_write_msgs(struct kcm_sock *kcm)
 
 		for (fragidx = 0; fragidx < skb_shinfo(skb)->nr_frags;
 		     fragidx++) {
+			struct bio_vec bvec;
+			struct msghdr msg = {
+				.msg_flags = MSG_DONTWAIT | MSG_SPLICE_PAGES,
+			};
 			skb_frag_t *frag;
 
 			frag_offset = 0;
@@ -651,11 +655,12 @@ static int kcm_write_msgs(struct kcm_sock *kcm)
 				goto out;
 			}
 
-			ret = kernel_sendpage(psock->sk->sk_socket,
-					      skb_frag_page(frag),
-					      skb_frag_off(frag) + frag_offset,
-					      skb_frag_size(frag) - frag_offset,
-					      MSG_DONTWAIT);
+			bvec_set_page(&bvec,
+				      skb_frag_page(frag),
+				      skb_frag_size(frag) - frag_offset,
+				      skb_frag_off(frag) + frag_offset);
+			iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, bvec.bv_len);
+			ret = sock_sendmsg(psock->sk->sk_socket, &msg);
 			if (ret <= 0) {
 				if (ret == -EAGAIN) {
 					/* Save state to try again when there's