tcp: adjust rcv zerocopy hints based on frag sizes

When SKBs are coalesced, we can have SKBs with different
frag sizes. Some with PAGE_SIZE and some not with PAGE_SIZE.
Since recv_skip_hint is always set to the full SKB size,
it can overestimate the amount that should be read using
normal read for coalesced packets.

Change the recv_skip_hint so that it only includes the first
frags that are not of PAGE_SIZE.

Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1805,8 +1805,17 @@ static int tcp_zerocopy_receive(struct sock *sk,
 				frags++;
 			}
 		}
-		if (frags->size != PAGE_SIZE || frags->page_offset)
+		if (frags->size != PAGE_SIZE || frags->page_offset) {
+			int remaining = zc->recv_skip_hint;
+
+			while (remaining && (frags->size != PAGE_SIZE ||
+					     frags->page_offset)) {
+				remaining -= frags->size;
+				frags++;
+			}
+			zc->recv_skip_hint -= remaining;
 			break;
+		}
 		ret = vm_insert_page(vma, address + length,
 				     skb_frag_page(frags));
 		if (ret)