mptcp: pm: don't try to create sf if alloc failed

It sounds better to avoid wasting cycles and / or put extreme memory
pressure on the system by trying to create new subflows if it was not
possible to add a new item in the announce list.

While at it, a warning is now printed if the entry was already in the
list as it should not happen with the in-kernel path-manager. With this
PM, mptcp_pm_alloc_anno_list() should only fail in case of memory
pressure.

Fixes: b6c08380 ("mptcp: remove addr and subflow in PM netlink")
Cc: stable@vger.kernel.org
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240731-upstream-net-20240731-mptcp-endp-subflow-signal-v1-4-c8a9b036493b@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_netlink.c

@@ -348,7 +348,7 @@ bool mptcp_pm_alloc_anno_list(struct mptcp_sock *msk,
 	add_entry = mptcp_lookup_anno_list_by_saddr(msk, addr);
 
 	if (add_entry) {
-		if (mptcp_pm_is_kernel(msk))
+		if (WARN_ON_ONCE(mptcp_pm_is_kernel(msk)))
 			return false;
 
 		sk_reset_timer(sk, &add_entry->add_timer,
@@ -555,8 +555,6 @@ static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 
 	/* check first for announce */
 	if (msk->pm.add_addr_signaled < add_addr_signal_max) {
-		local = select_signal_address(pernet, msk);
-
 		/* due to racing events on both ends we can reach here while
 		 * previous add address is still running: if we invoke now
 		 * mptcp_pm_announce_addr(), that will fail and the
@@ -567,11 +565,15 @@ static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 		if (msk->pm.addr_signal & BIT(MPTCP_ADD_ADDR_SIGNAL))
 			return;
 
+		local = select_signal_address(pernet, msk);
 		if (!local)
 			goto subflow;
 
+		/* If the alloc fails, we are on memory pressure, not worth
+		 * continuing, and trying to create subflows.
+		 */
 		if (!mptcp_pm_alloc_anno_list(msk, &local->addr))
-			goto subflow;
+			return;
 
 		__clear_bit(local->addr.id, msk->pm.id_avail_bitmap);
 		msk->pm.add_addr_signaled++;