net-zerocopy: Set zerocopy hint when data is copied

Set zerocopy hint, event when falling back to copy, so that the
pending data can be efficiently received using zerocopy when
possible.
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

@@ -1800,6 +1800,43 @@ static int find_next_mappable_frag(const skb_frag_t *frag,
 	return offset;
 }
 
+static void tcp_zerocopy_set_hint_for_skb(struct sock *sk,
+					  struct tcp_zerocopy_receive *zc,
+					  struct sk_buff *skb, u32 offset)
+{
+	u32 frag_offset, partial_frag_remainder = 0;
+	int mappable_offset;
+	skb_frag_t *frag;
+
+	/* worst case: skip to next skb. try to improve on this case below */
+	zc->recv_skip_hint = skb->len - offset;
+
+	/* Find the frag containing this offset (and how far into that frag) */
+	frag = skb_advance_to_frag(skb, offset, &frag_offset);
+	if (!frag)
+		return;
+
+	if (frag_offset) {
+		struct skb_shared_info *info = skb_shinfo(skb);
+
+		/* We read part of the last frag, must recvmsg() rest of skb. */
+		if (frag == &info->frags[info->nr_frags - 1])
+			return;
+
+		/* Else, we must at least read the remainder in this frag. */
+		partial_frag_remainder = skb_frag_size(frag) - frag_offset;
+		zc->recv_skip_hint -= partial_frag_remainder;
+		++frag;
+	}
+
+	/* partial_frag_remainder: If part way through a frag, must read rest.
+	 * mappable_offset: Bytes till next mappable frag, *not* counting bytes
+	 * in partial_frag_remainder.
+	 */
+	mappable_offset = find_next_mappable_frag(frag, zc->recv_skip_hint);
+	zc->recv_skip_hint = mappable_offset + partial_frag_remainder;
+}
+
 static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
 			      int nonblock, int flags,
 			      struct scm_timestamping_internal *tss,
@@ -1830,6 +1867,14 @@ static int receive_fallback_to_copy(struct sock *sk,
 		return err;
 
 	zc->copybuf_len = err;
+	if (likely(zc->copybuf_len)) {
+		struct sk_buff *skb;
+		u32 offset;
+
+		skb = tcp_recv_skb(sk, tcp_sk(sk)->copied_seq, &offset);
+		if (skb)
+			tcp_zerocopy_set_hint_for_skb(sk, zc, skb, offset);
+	}
 	return 0;
 }