tcp: factor out tcp_build_frag()

Will be needed by the next patch, as MPTCP needs to handle
directly the error/memory-allocation-needed path.

No functional changes intended.

Additionally let MPTCP code access the tcp_remove_empty_skb()
helper.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/tcp.h

@@ -322,6 +322,7 @@ void tcp_shutdown(struct sock *sk, int how);
 int tcp_v4_early_demux(struct sk_buff *skb);
 int tcp_v4_rcv(struct sk_buff *skb);
 
+void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb);
 int tcp_v4_tw_remember_stamp(struct inet_timewait_sock *tw);
 int tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size);
 int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size);
@@ -329,6 +330,8 @@ int tcp_sendpage(struct sock *sk, struct page *page, int offset, size_t size,
 		 int flags);
 int tcp_sendpage_locked(struct sock *sk, struct page *page, int offset,
 			size_t size, int flags);
+struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
+			       struct page *page, int offset, size_t *size);
 ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
 		 size_t size, int flags);
 int tcp_send_mss(struct sock *sk, int *size_goal, int flags);

net/ipv4/tcp.c

@@ -954,7 +954,7 @@ int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
  * importantly be able to generate EPOLLOUT for Edge Trigger epoll()
  * users.
  */
-static void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb)
+void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb)
 {
 	if (skb && !skb->len) {
 		tcp_unlink_write_queue(skb, sk);
@@ -964,55 +964,24 @@ static void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb)
 	}
 }
 
-ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
-			 size_t size, int flags)
+struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
+			       struct page *page, int offset, size_t *size)
 {
-	struct tcp_sock *tp = tcp_sk(sk);
-	int mss_now, size_goal;
-	int err;
-	ssize_t copied;
-	long timeo = sock_sndtimeo(sk, flags & MSG_DONTWAIT);
-
-	if (IS_ENABLED(CONFIG_DEBUG_VM) &&
-	    WARN_ONCE(!sendpage_ok(page),
-		      "page must not be a Slab one and have page_count > 0"))
-		return -EINVAL;
-
-	/* Wait for a connection to finish. One exception is TCP Fast Open
-	 * (passive side) where data is allowed to be sent before a connection
-	 * is fully established.
-	 */
-	if (((1 << sk->sk_state) & ~(TCPF_ESTABLISHED | TCPF_CLOSE_WAIT)) &&
-	    !tcp_passive_fastopen(sk)) {
-		err = sk_stream_wait_connect(sk, &timeo);
-		if (err != 0)
-			goto out_err;
-	}
-
-	sk_clear_bit(SOCKWQ_ASYNC_NOSPACE, sk);
-
-	mss_now = tcp_send_mss(sk, &size_goal, flags);
-	copied = 0;
-
-	err = -EPIPE;
-	if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))
-		goto out_err;
-
-	while (size > 0) {
 	struct sk_buff *skb = tcp_write_queue_tail(sk);
-		int copy, i;
+	struct tcp_sock *tp = tcp_sk(sk);
 	bool can_coalesce;
+	int copy, i;
 
 	if (!skb || (copy = size_goal - skb->len) <= 0 ||
 	    !tcp_skb_can_collapse_to(skb)) {
 new_segment:
 		if (!sk_stream_memory_free(sk))
-				goto wait_for_space;
+			return NULL;
 
 		skb = sk_stream_alloc_skb(sk, 0, sk->sk_allocation,
 					  tcp_rtx_and_write_queues_empty(sk));
 		if (!skb)
-				goto wait_for_space;
+			return NULL;
 
 #ifdef CONFIG_TLS_DEVICE
 		skb->decrypted = !!(flags & MSG_SENDPAGE_DECRYPTED);
@@ -1021,8 +990,8 @@ ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
 		copy = size_goal;
 	}
 
-		if (copy > size)
-			copy = size;
+	if (copy > *size)
+		copy = *size;
 
 	i = skb_shinfo(skb)->nr_frags;
 	can_coalesce = skb_can_coalesce(skb, i, page, offset);
@@ -1031,7 +1000,7 @@ ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
 		goto new_segment;
 	}
 	if (!sk_wmem_schedule(sk, copy))
-			goto wait_for_space;
+		return NULL;
 
 	if (can_coalesce) {
 		skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
@@ -1053,6 +1022,52 @@ ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
 	TCP_SKB_CB(skb)->end_seq += copy;
 	tcp_skb_pcount_set(skb, 0);
 
+	*size = copy;
+	return skb;
+}
+
+ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
+			 size_t size, int flags)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int mss_now, size_goal;
+	int err;
+	ssize_t copied;
+	long timeo = sock_sndtimeo(sk, flags & MSG_DONTWAIT);
+
+	if (IS_ENABLED(CONFIG_DEBUG_VM) &&
+	    WARN_ONCE(!sendpage_ok(page),
+		      "page must not be a Slab one and have page_count > 0"))
+		return -EINVAL;
+
+	/* Wait for a connection to finish. One exception is TCP Fast Open
+	 * (passive side) where data is allowed to be sent before a connection
+	 * is fully established.
+	 */
+	if (((1 << sk->sk_state) & ~(TCPF_ESTABLISHED | TCPF_CLOSE_WAIT)) &&
+	    !tcp_passive_fastopen(sk)) {
+		err = sk_stream_wait_connect(sk, &timeo);
+		if (err != 0)
+			goto out_err;
+	}
+
+	sk_clear_bit(SOCKWQ_ASYNC_NOSPACE, sk);
+
+	mss_now = tcp_send_mss(sk, &size_goal, flags);
+	copied = 0;
+
+	err = -EPIPE;
+	if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))
+		goto out_err;
+
+	while (size > 0) {
+		struct sk_buff *skb;
+		size_t copy = size;
+
+		skb = tcp_build_frag(sk, size_goal, flags, page, offset, &copy);
+		if (!skb)
+			goto wait_for_space;
+
 		if (!copied)
 			TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_PSH;