net-zerocopy: Fast return if inq < PAGE_SIZE

Sometimes, we may call tcp receive zerocopy when inq is 0,
or inq < PAGE_SIZE, in which case we cannot remap pages. In this case,
simply return the appropriate hint for regular copying without taking
mmap_sem.
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

@@ -1904,6 +1904,14 @@ static int tcp_zerocopy_receive(struct sock *sk,
 
 	sock_rps_record_flow(sk);
 
+	if (inq < PAGE_SIZE) {
+		zc->length = 0;
+		zc->recv_skip_hint = inq;
+		if (!inq && sock_flag(sk, SOCK_DONE))
+			return -EIO;
+		return 0;
+	}
+
 	mmap_read_lock(current->mm);
 
 	vma = find_vma(current->mm, address);