Subject: [PATCH RFC net-next 07/11] udp: zerocopy

From: Willem de Bruijn <[email-masked]>
Date: Tue, 17 Apr 2018 16:00:57 -0400

From: Willem de Bruijn <[email-masked]>

Extend zerocopy to udp sockets. Allow setting sockopt SO_ZEROCOPY and
interpret flag MSG_ZEROCOPY.

This patch was previously part of the zerocopy RFC patchsets. Zerocopy
is not effective at small MTU. With segmentation offload building
larger datagram, the benefit of page flipping outweights the cost of
generating a completion notification.

The datagram implementation has initial refcnt of 0, instead of 1 for
TCP. The tcp_sendmsg_locked function has to hold a reference on
uarg independent those held by the skbs it generates, because the
skbs can be sent and freed in the function main loop. This is not
needed for other sockets.

Benefit depends on the ratio of cycles spent copying. For an initial
benchmark with udp gso that spends 13% of (systemwide) cycles in
copy_user_fast_string, cycle savings of zerocopy are proportionate:

    udp gso
     2139 MB/s 36282 msg/s 36282 calls/s
        11,204,374,561      cycles

    udp gso zerocopy
     2394 MB/s 40608 msg/s 40608 calls/s
        11,205,017,927      cycles

This is likely not the best demonstrator benchmark.

Signed-off-by: Willem de Bruijn <[email-masked]>
---
 include/linux/skbuff.h |  1 +
 net/core/skbuff.c      | 12 +++++++++++-
 net/core/sock.c        |  5 ++++-
 net/ipv4/ip_output.c   | 24 ++++++++++++++++++++++--
 net/ipv6/ip6_output.c  | 23 ++++++++++++++++++++++-
 5 files changed, 60 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6850643508c1..756206bc8eca 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -483,6 +483,7 @@ void sock_zerocopy_put_abort(struct ubuf_info *uarg);
 
 void sock_zerocopy_callback(struct ubuf_info *uarg, bool success);
 
+int skb_zerocopy_iter_dgram(struct sk_buff *skb, struct msghdr *msg, int len);
 int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
 			     struct msghdr *msg, int len,
 			     struct ubuf_info *uarg);
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 3eed21f64e0b..92df6db2c851 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -959,7 +959,7 @@ struct ubuf_info *sock_zerocopy_alloc(struct sock *sk, size_t size)
 	uarg->len = 1;
 	uarg->bytelen = size;
 	uarg->zerocopy = 1;
-	refcount_set(&uarg->refcnt, 1);
+	refcount_set(&uarg->refcnt, sk->sk_type == SOCK_STREAM ? 1 : 0);
 	sock_hold(sk);
 
 	return uarg;
@@ -1099,6 +1099,10 @@ void sock_zerocopy_put_abort(struct ubuf_info *uarg)
 		atomic_dec(&sk->sk_zckey);
 		uarg->len--;
 
+		/* datagram does not hold an extra ref for the syscall itself */
+		if (sk->sk_type != SOCK_STREAM && !refcount_read(&uarg->refcnt))
+			refcount_set(&uarg->refcnt, 1);
+
 		sock_zerocopy_put(uarg);
 	}
 }
@@ -1107,6 +1111,12 @@ EXPORT_SYMBOL_GPL(sock_zerocopy_put_abort);
 extern int __zerocopy_sg_from_iter(struct sock *sk, struct sk_buff *skb,
 				   struct iov_iter *from, size_t length);
 
+int skb_zerocopy_iter_dgram(struct sk_buff *skb, struct msghdr *msg, int len)
+{
+	return __zerocopy_sg_from_iter(skb->sk, skb, &msg->msg_iter, len);
+}
+EXPORT_SYMBOL_GPL(skb_zerocopy_iter_dgram);
+
 int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
 			     struct msghdr *msg, int len,
 			     struct ubuf_info *uarg)
diff --git a/net/core/sock.c b/net/core/sock.c
index b2c3db169ca1..1480d7d92294 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1053,7 +1053,10 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 
 	case SO_ZEROCOPY:
 		if (sk->sk_family == PF_INET || sk->sk_family == PF_INET6) {
-			if (sk->sk_protocol != IPPROTO_TCP)
+			if (sk->sk_type == SOCK_RAW)
+				ret = -ENOTSUPP;
+			if (sk->sk_protocol != IPPROTO_TCP &&
+			    sk->sk_protocol != IPPROTO_UDP)
 				ret = -ENOTSUPP;
 		} else if (sk->sk_family != PF_RDS) {
 			ret = -ENOTSUPP;
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 9ccd6c28e420..b5986ff1250e 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -864,8 +864,8 @@ static int __ip_append_data(struct sock *sk,
 			    unsigned int flags)
 {
 	struct inet_sock *inet = inet_sk(sk);
+	struct ubuf_info *uarg = NULL;
 	struct sk_buff *skb;
-
 	struct ip_options *opt = cork->opt;
 	int hh_len;
 	int exthdrlen;
@@ -913,6 +913,20 @@ static int __ip_append_data(struct sock *sk,
 	    !exthdrlen)
 		csummode = CHECKSUM_PARTIAL;
 
+	if (flags & MSG_ZEROCOPY && length) {
+		uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));
+		if (!uarg)
+			return -ENOBUFS;
+
+		if (rt->dst.dev->features & NETIF_F_SG &&
+		    csummode == CHECKSUM_PARTIAL) {
+			paged = true;
+		} else {
+			uarg->zerocopy = 0;
+			skb_zcopy_set(skb, uarg);
+		}
+	}
+
 	cork->length += length;
 
 	/* So, what's going on in the loop below?
@@ -1002,6 +1016,7 @@ static int __ip_append_data(struct sock *sk,
 			cork->tx_flags = 0;
 			skb_shinfo(skb)->tskey = tskey;
 			tskey = 0;
+			skb_zcopy_set(skb, uarg);
 
 			/*
 			 *	Find where to start putting bytes.
@@ -1063,7 +1078,7 @@ static int __ip_append_data(struct sock *sk,
 				err = -EFAULT;
 				goto error;
 			}
-		} else {
+		} else if (!uarg || !uarg->zerocopy) {
 			int i = skb_shinfo(skb)->nr_frags;
 
 			err = -ENOMEM;
@@ -1093,6 +1108,10 @@ static int __ip_append_data(struct sock *sk,
 			skb->data_len += copy;
 			skb->truesize += copy;
 			wmem_alloc_delta += copy;
+		} else {
+			err = skb_zerocopy_iter_dgram(skb, from, copy);
+			if (err)
+				goto error;
 		}
 		offset += copy;
 		length -= copy;
@@ -1105,6 +1124,7 @@ static int __ip_append_data(struct sock *sk,
 error_efault:
 	err = -EFAULT;
 error:
+	sock_zerocopy_put_abort(uarg);
 	cork->length -= length;
 	IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTDISCARDS);
 	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 9fbcec4fb946..9725d283091c 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1270,6 +1270,7 @@ static int __ip6_append_data(struct sock *sk,
 	int csummode = CHECKSUM_NONE;
 	unsigned int maxnonfragsize, headersize;
 	unsigned int wmem_alloc_delta = 0;
+	struct ubuf_info *uarg = NULL;
 	bool paged;
 
 	skb = skb_peek_tail(queue);
@@ -1338,6 +1339,20 @@ static int __ip6_append_data(struct sock *sk,
 			tskey = sk->sk_tskey++;
 	}
 
+	if (flags & MSG_ZEROCOPY && length) {
+		uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));
+		if (!uarg)
+			return -ENOBUFS;
+
+		if ((rt->dst.dev->features & NETIF_F_SG) &&
+		    csummode == CHECKSUM_PARTIAL) {
+			paged = true;
+		} else {
+			uarg->zerocopy = 0;
+			skb_zcopy_set(skb, uarg);
+		}
+	}
+
 	/*
 	 * Let's try using as much space as possible.
 	 * Use MTU if total length of the message fits into the MTU.
@@ -1460,6 +1475,7 @@ static int __ip6_append_data(struct sock *sk,
 			tx_flags = 0;
 			skb_shinfo(skb)->tskey = tskey;
 			tskey = 0;
+			skb_zcopy_set(skb, uarg);
 
 			/*
 			 *	Find where to start putting bytes
@@ -1520,7 +1536,7 @@ static int __ip6_append_data(struct sock *sk,
 				err = -EFAULT;
 				goto error;
 			}
-		} else {
+		} else if (!uarg || !uarg->zerocopy) {
 			int i = skb_shinfo(skb)->nr_frags;
 
 			err = -ENOMEM;
@@ -1550,6 +1566,10 @@ static int __ip6_append_data(struct sock *sk,
 			skb->data_len += copy;
 			skb->truesize += copy;
 			wmem_alloc_delta += copy;
+		} else {
+			err = skb_zerocopy_iter_dgram(skb, from, copy);
+			if (err)
+				goto error;
 		}
 		offset += copy;
 		length -= copy;
@@ -1562,6 +1582,7 @@ static int __ip6_append_data(struct sock *sk,
 error_efault:
 	err = -EFAULT;
 error:
+	sock_zerocopy_put_abort(uarg);
 	cork->length -= length;
 	IP6_INC_STATS(sock_net(sk), rt->rt6i_idev, IPSTATS_MIB_OUTDISCARDS);
 	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);
-- 
2.17.0.484.g0c8726318c-goog