FAILED: patch "[PATCH] mptcp: schedule rtx timer only after pushing data" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 2ea6190f42d0416a4310e60a7fcb0b49fcbbd4fb
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025122952-supervise-regulate-a26b@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 2ea6190f42d0416a4310e60a7fcb0b49fcbbd4fb Mon Sep 17 00:00:00 2001
From: Paolo Abeni <[email protected]>
Date: Fri, 5 Dec 2025 19:55:16 +0100
Subject: [PATCH] mptcp: schedule rtx timer only after pushing data

The MPTCP protocol usually schedule the retransmission timer only when there is some chances for such retransmissions to happen.

With a notable exception: __mptcp_push_pending() currently schedule such timer unconditionally, potentially leading to unnecessary rtx timer expiration.

The issue is present since the blamed commit below but become easily reproducible after commit 27b0e701d387 ("mptcp: drop bogus optimization in __mptcp_check_push()")

index e212c1374bd0..d8a7f7029164 100644
@@ -1623,7 +1623,7 @@ void __mptcp_push_pending(struct sock *sk, unsigned int flags)
 	struct mptcp_sendmsg_info info = {
 				.flags = flags,
 	};
-	bool do_check_data_fin = false;
+	bool copied = false;
 	int push_count = 1;
 
 	while (mptcp_send_head(sk) && (push_count > 0)) {

@@ -1665,7 +1665,7 @@ void __mptcp_push_pending(struct sock *sk, unsigned int flags)
 						push_count--;
 					continue;
 				}
-				do_check_data_fin = true;
+				copied = true;
 			}
 		}
 	}

@@ -1674,11 +1674,14 @@ void __mptcp_push_pending(struct sock *sk, unsigned int flags)
 	if (ssk)
 		mptcp_push_release(ssk, &info);
 
-	/* ensure the rtx timer is running */
-	if (!mptcp_rtx_timer_pending(sk))
-		mptcp_reset_rtx_timer(sk);
-	if (do_check_data_fin)
+	/* Avoid scheduling the rtx timer if no data has been pushed; the timer
+	 * will be updated on positive acks by __mptcp_cleanup_una().
+	 */
+	if (copied) {
+		if (!mptcp_rtx_timer_pending(sk))
+			mptcp_reset_rtx_timer(sk);
 		mptcp_check_send_data_fin(sk);
+	}
 }
 
 static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk, bool first)