net-zerocopy: Refactor frag-is-remappable test.

Refactor frag-is-remappable test for tcp receive zerocopy. This is
part of a patch set that introduces short-circuited hybrid copies
for small receive operations, which results in roughly 33% fewer
syscalls for small RPC scenarios.

Signed-off-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp.c

@@ -1780,6 +1780,26 @@ static skb_frag_t *skb_advance_to_frag(struct sk_buff *skb, u32 offset_skb,
 	return frag;
 }
 
+static bool can_map_frag(const skb_frag_t *frag)
+{
+	return skb_frag_size(frag) == PAGE_SIZE && !skb_frag_off(frag);
+}
+
+static int find_next_mappable_frag(const skb_frag_t *frag,
+				   int remaining_in_skb)
+{
+	int offset = 0;
+
+	if (likely(can_map_frag(frag)))
+		return 0;
+
+	while (offset < remaining_in_skb && !can_map_frag(frag)) {
+		offset += skb_frag_size(frag);
+		++frag;
+	}
+	return offset;
+}
+
 static int tcp_copy_straggler_data(struct tcp_zerocopy_receive *zc,
 				   struct sk_buff *skb, u32 copylen,
 				   u32 *offset, u32 *seq)
@@ -1905,6 +1925,8 @@ static int tcp_zerocopy_receive(struct sock *sk,
 	ret = 0;
 	curr_addr = address;
 	while (length + PAGE_SIZE <= zc->length) {
+		int mappable_offset;
+
 		if (zc->recv_skip_hint < PAGE_SIZE) {
 			u32 offset_frag;
 
@@ -1932,15 +1954,11 @@ static int tcp_zerocopy_receive(struct sock *sk,
 			if (!frags || offset_frag)
 				break;
 		}
-		if (skb_frag_size(frags) != PAGE_SIZE || skb_frag_off(frags)) {
-			int remaining = zc->recv_skip_hint;
 
-			while (remaining && (skb_frag_size(frags) != PAGE_SIZE ||
-					     skb_frag_off(frags))) {
-				remaining -= skb_frag_size(frags);
-				frags++;
-			}
-			zc->recv_skip_hint -= remaining;
+		mappable_offset = find_next_mappable_frag(frags,
+							  zc->recv_skip_hint);
+		if (mappable_offset) {
+			zc->recv_skip_hint = mappable_offset;
 			break;
 		}
 		pages[pg_idx] = skb_frag_page(frags);