Re: [RFC PATCH v2 16/48] ip, udp: Support MSG_SPLICE_PAGES

[David Howells <dhowells@redhat.com>]

Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:

> > +	unsigned int maxfraglen, fragheaderlen, maxnonfragsize, xlength;
> 
> Does x here stand for anything?

Yeah... "bad naming".  How about if I call it initial_length?  I'm trying to
avoid allocating bufferage for the data.

> This does add a lot of code to two functions that are already
> unwieldy. It may be unavoidable, but it if can use helpers, that would
> be preferable.

Something like the attached?  (This is on top of patches 16-17, but I would
need to fold it in)

David
---
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index b38dbb2f9c3f..019ed9bb6745 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -956,6 +956,96 @@ 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, size_t *pcopy)
+{
+	struct msghdr *msg = from;
+	struct page *page = NULL, **pages = &page;
+	ssize_t copy = *pcopy;
+	size_t off;
+	bool put = false;
+	int err;
+
+	copy = iov_iter_extract_pages(&msg->msg_iter, &pages, copy, 1, 0, &off);
+	if (copy <= 0)
+		return copy ?: -EIO;
+
+	if (!sendpage_ok(page)) {
+		const void *p = kmap_local_page(page);
+		void *q;
+
+		q = page_frag_memdup(NULL, p + off, copy,
+				     sk->sk_allocation, ULONG_MAX);
+		kunmap_local(p);
+		if (!q)
+			return -ENOMEM;
+		page = virt_to_page(q);
+		off = offset_in_page(q);
+		put = true;
+	}
+
+	err = skb_append_pagefrags(skb, page, off, copy);
+	if (put)
+		put_page(page);
+	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 +1067,7 @@ static int __ip_append_data(struct sock *sk,
 	int err;
 	int offset = 0;
 	bool zc = false;
-	unsigned int maxfraglen, fragheaderlen, maxnonfragsize, xlength;
+	unsigned int maxfraglen, fragheaderlen, maxnonfragsize, initial_length;
 	int csummode = CHECKSUM_NONE;
 	struct rtable *rt = (struct rtable *)cork->dst;
 	unsigned int wmem_alloc_delta = 0;
@@ -1017,7 +1107,7 @@ static int __ip_append_data(struct sock *sk,
 	    (!exthdrlen || (rt->dst.dev->features & NETIF_F_HW_ESP_TX_CSUM)))
 		csummode = CHECKSUM_PARTIAL;
 
-	xlength = length;
+	initial_length = length;
 	if ((flags & MSG_ZEROCOPY) && length) {
 		struct msghdr *msg = from;
 
@@ -1053,7 +1143,7 @@ static int __ip_append_data(struct sock *sk,
 			return -EPERM;
 		if (!(rt->dst.dev->features & NETIF_F_SG))
 			return -EOPNOTSUPP;
-		xlength = transhdrlen; /* We need an empty buffer to attach stuff to */
+		initial_length = transhdrlen; /* We need an empty buffer to attach stuff to */
 	}
 
 	cork->length += length;
@@ -1083,47 +1173,13 @@ static int __ip_append_data(struct sock *sk,
 			struct sk_buff *skb_prev;
 
 			if (unlikely(flags & MSG_SPLICE_PAGES)) {
-				skb_prev = skb;
-				fraggap = skb_prev->len - maxfraglen;
-
-				alloclen = fragheaderlen + hh_len + fraggap + 15;
-				skb = sock_wmalloc(sk, alloclen, 1, sk->sk_allocation);
-				if (unlikely(!skb)) {
-					err = -ENOBUFS;
+				err = __ip_splice_alloc(sk, &skb, fragheaderlen,
+							maxfraglen, hh_len);
+				if (err < 0)
 					goto error;
-				}
-
-				/*
-				 *	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);
 				continue;
 			}
-			xlength = length;
+			initial_length = length;
 
 alloc_new_skb:
 			skb_prev = skb;
@@ -1136,7 +1192,7 @@ static int __ip_append_data(struct sock *sk,
 			 * If remaining data exceeds the mtu,
 			 * we know we need more fragment(s).
 			 */
-			datalen = xlength + fraggap;
+			datalen = initial_length + fraggap;
 			if (datalen > mtu - fragheaderlen)
 				datalen = maxfraglen - fragheaderlen;
 			fraglen = datalen + fragheaderlen;
@@ -1150,7 +1206,7 @@ static int __ip_append_data(struct sock *sk,
 			 * because we have no idea what fragment will be
 			 * the last.
 			 */
-			if (datalen == xlength + fraggap)
+			if (datalen == initial_length + fraggap)
 				alloc_extra += rt->dst.trailer_len;
 
 			if ((flags & MSG_MORE) &&
@@ -1258,48 +1314,9 @@ static int __ip_append_data(struct sock *sk,
 				goto error;
 			}
 		} else if (flags & MSG_SPLICE_PAGES) {
-			struct msghdr *msg = from;
-			struct page *page = NULL, **pages = &page;
-			size_t off;
-			bool put = false;
-
-			copy = iov_iter_extract_pages(&msg->msg_iter, &pages,
-						      copy, 1, 0, &off);
-			if (copy <= 0) {
-				err = copy ?: -EIO;
-				goto error;
-			}
-
-			if (!sendpage_ok(page)) {
-				const void *p = kmap_local_page(page);
-				void *q;
-
-				q = page_frag_memdup(NULL, p + off, copy,
-						     sk->sk_allocation, ULONG_MAX);
-				kunmap_local(p);
-				if (!q) {
-					err = copy ?: -ENOMEM;
-					goto error;
-				}
-				page = virt_to_page(q);
-				off = offset_in_page(q);
-				put = true;
-			}
-
-			err = skb_append_pagefrags(skb, page, off, copy);
-			if (put)
-				put_page(page);
+			err = __ip_splice_pages(sk, skb, from, &copy);
 			if (err < 0)
 				goto error;
-
-			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);
 		} else if (!zc) {
 			int i = skb_shinfo(skb)->nr_frags;