[PATCH v3 15/55] ip, udp: Support MSG_SPLICE_PAGES

[David Howells]

Make IP/UDP sendmsg() support MSG_SPLICE_PAGES.  This causes pages to be
spliced from the source iterator.

This allows ->sendpage() to be replaced by something that can handle
multiple multipage folios in a single transaction.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
cc: Eric Dumazet <edumazet@xxxxxxxxxx>
cc: Jakub Kicinski <kuba@xxxxxxxxxx>
cc: Paolo Abeni <pabeni@xxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: netdev@xxxxxxxxxxxxxxx
---
 net/ipv4/ip_output.c | 102 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 99 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 4e4e308c3230..e2eaba817c1f 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -956,6 +956,79 @@ csum_page(struct page *page, int offset, int copy)
 	return csum;
 }
 
+/*
+ * Allocate a packet for MSG_SPLICE_PAGES.
+ */
+static int __ip_splice_alloc(struct sock *sk, struct sk_buff **pskb,
+			     unsigned int fragheaderlen, unsigned int maxfraglen,
+			     unsigned int hh_len)
+{
+	struct sk_buff *skb_prev = *pskb, *skb;
+	unsigned int fraggap = skb_prev->len - maxfraglen;
+	unsigned int alloclen = fragheaderlen + hh_len + fraggap + 15;
+
+	skb = sock_wmalloc(sk, alloclen, 1, sk->sk_allocation);
+	if (unlikely(!skb))
+		return -ENOBUFS;
+
+	/* Fill in the control structures */
+	skb->ip_summed = CHECKSUM_NONE;
+	skb->csum = 0;
+	skb_reserve(skb, hh_len);
+
+	/* Find where to start putting bytes. */
+	skb_put(skb, fragheaderlen + fraggap);
+	skb_reset_network_header(skb);
+	skb->transport_header = skb->network_header + fragheaderlen;
+	if (fraggap) {
+		skb->csum = skb_copy_and_csum_bits(skb_prev, maxfraglen,
+						   skb_transport_header(skb),
+						   fraggap);
+		skb_prev->csum = csum_sub(skb_prev->csum, skb->csum);
+		pskb_trim_unique(skb_prev, maxfraglen);
+	}
+
+	/* Put the packet on the pending queue. */
+	__skb_queue_tail(&sk->sk_write_queue, skb);
+	*pskb = skb;
+	return 0;
+}
+
+/*
+ * Add (or copy) data pages for MSG_SPLICE_PAGES.
+ */
+static int __ip_splice_pages(struct sock *sk, struct sk_buff *skb,
+			     void *from, int *pcopy)
+{
+	struct msghdr *msg = from;
+	struct page *page = NULL, **pages = &page;
+	ssize_t copy = *pcopy;
+	size_t off;
+	int err;
+
+	copy = iov_iter_extract_pages(&msg->msg_iter, &pages, copy, 1, 0, &off);
+	if (copy <= 0)
+		return copy ?: -EIO;
+
+	err = skb_append_pagefrags(skb, page, off, copy);
+	if (err < 0) {
+		iov_iter_revert(&msg->msg_iter, copy);
+		return err;
+	}
+
+	if (skb->ip_summed == CHECKSUM_NONE) {
+		__wsum csum;
+
+		csum = csum_page(page, off, copy);
+		skb->csum = csum_block_add(skb->csum, csum, skb->len);
+	}
+
+	skb_len_add(skb, copy);
+	refcount_add(copy, &sk->sk_wmem_alloc);
+	*pcopy = copy;
+	return 0;
+}
+
 static int __ip_append_data(struct sock *sk,
 			    struct flowi4 *fl4,
 			    struct sk_buff_head *queue,
@@ -977,7 +1050,7 @@ static int __ip_append_data(struct sock *sk,
 	int err;
 	int offset = 0;
 	bool zc = false;
-	unsigned int maxfraglen, fragheaderlen, maxnonfragsize;
+	unsigned int maxfraglen, fragheaderlen, maxnonfragsize, initial_length;
 	int csummode = CHECKSUM_NONE;
 	struct rtable *rt = (struct rtable *)cork->dst;
 	unsigned int wmem_alloc_delta = 0;
@@ -1017,6 +1090,7 @@ static int __ip_append_data(struct sock *sk,
 	    (!exthdrlen || (rt->dst.dev->features & NETIF_F_HW_ESP_TX_CSUM)))
 		csummode = CHECKSUM_PARTIAL;
 
+	initial_length = length;
 	if ((flags & MSG_ZEROCOPY) && length) {
 		struct msghdr *msg = from;
 
@@ -1047,6 +1121,14 @@ static int __ip_append_data(struct sock *sk,
 				skb_zcopy_set(skb, uarg, &extra_uref);
 			}
 		}
+	} else if ((flags & MSG_SPLICE_PAGES) && length) {
+		if (inet->hdrincl)
+			return -EPERM;
+		if (rt->dst.dev->features & NETIF_F_SG)
+			/* We need an empty buffer to attach stuff to */
+			initial_length = transhdrlen;
+		else
+			flags &= ~MSG_SPLICE_PAGES;
 	}
 
 	cork->length += length;
@@ -1074,6 +1156,16 @@ static int __ip_append_data(struct sock *sk,
 			unsigned int alloclen, alloc_extra;
 			unsigned int pagedlen;
 			struct sk_buff *skb_prev;
+
+			if (unlikely(flags & MSG_SPLICE_PAGES)) {
+				err = __ip_splice_alloc(sk, &skb, fragheaderlen,
+							maxfraglen, hh_len);
+				if (err < 0)
+					goto error;
+				continue;
+			}
+			initial_length = length;
+
 alloc_new_skb:
 			skb_prev = skb;
 			if (skb_prev)
@@ -1085,7 +1177,7 @@ static int __ip_append_data(struct sock *sk,
 			 * If remaining data exceeds the mtu,
 			 * we know we need more fragment(s).
 			 */
-			datalen = length + fraggap;
+			datalen = initial_length + fraggap;
 			if (datalen > mtu - fragheaderlen)
 				datalen = maxfraglen - fragheaderlen;
 			fraglen = datalen + fragheaderlen;
@@ -1099,7 +1191,7 @@ static int __ip_append_data(struct sock *sk,
 			 * because we have no idea what fragment will be
 			 * the last.
 			 */
-			if (datalen == length + fraggap)
+			if (datalen == initial_length + fraggap)
 				alloc_extra += rt->dst.trailer_len;
 
 			if ((flags & MSG_MORE) &&
@@ -1206,6 +1298,10 @@ static int __ip_append_data(struct sock *sk,
 				err = -EFAULT;
 				goto error;
 			}
+		} else if (flags & MSG_SPLICE_PAGES) {
+			err = __ip_splice_pages(sk, skb, from, &copy);
+			if (err < 0)
+				goto error;
 		} else if (!zc) {
 			int i = skb_shinfo(skb)->nr_frags;