The upcoming patch will introduced backlog processing for MPTCP
socket, and we want to leverage coalescing in such data path.

Factor out the relevant bits not touching memory accounting to
deal with such use-case.

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 909c611d5b528..bd83abefe4965 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -142,22 +142,34 @@ static void mptcp_drop(struct sock *sk, struct sk_buff *skb)
 	__kfree_skb(skb);
 }
 
-static bool mptcp_try_coalesce(struct sock *sk, struct sk_buff *to,
-			       struct sk_buff *from)
+static int __mptcp_try_coalesce(struct sock *sk, struct sk_buff *to,
+				struct sk_buff *from, bool *fragstolen)
 {
-	bool fragstolen;
+	int limit = READ_ONCE(sk->sk_rcvbuf);
 	int delta;
 
 	if (unlikely(MPTCP_SKB_CB(to)->cant_coalesce) ||
 	    MPTCP_SKB_CB(from)->offset ||
-	    ((to->len + from->len) > (sk->sk_rcvbuf >> 3)) ||
-	    !skb_try_coalesce(to, from, &fragstolen, &delta))
-		return false;
+	    ((to->len + from->len) > (limit >> 3)) ||
+	    !skb_try_coalesce(to, from, fragstolen, &delta))
+		return 0;
 
 	pr_debug("colesced seq %llx into %llx new len %d new end seq %llx\n",
 		 MPTCP_SKB_CB(from)->map_seq, MPTCP_SKB_CB(to)->map_seq,
 		 to->len, MPTCP_SKB_CB(from)->end_seq);
 	MPTCP_SKB_CB(to)->end_seq = MPTCP_SKB_CB(from)->end_seq;
+	return delta;
+}
+
+static bool mptcp_try_coalesce(struct sock *sk, struct sk_buff *to,
+			       struct sk_buff *from)
+{
+	bool fragstolen;
+	int delta;
+
+	delta = __mptcp_try_coalesce(sk, to, from, &fragstolen);
+	if (!delta)
+		return false;
 
 	/* note the fwd memory can reach a negative value after accounting
 	 * for the delta, but the later skb free will restore a non
-- 
2.51.0