rxrpc: Implement path-MTU probing using padded PING ACKs (RFC8899)

Implement path-MTU probing (along the lines of RFC8899) by padding some of
the PING ACKs we send.  PING ACKs get their own individual responses quite
apart from the acking of data (though, as ACKs, they fulfil that role
also).

The probing concentrates on packet sizes that correspond how many
subpackets can be stuffed inside a jumbo packet as jumbo DATA packets are
just aggregations of individual DATA packets and can be split easily for
retransmission purposes.

If we want to perform probing, we advertise this by setting the maximum
number of jumbo subpackets to 0 in the ack trailer when we send an ACK and
see if the peer is also advertising the service.  This is interpreted by
non-supporting Rx stacks as an indication that jumbo packets aren't
supported.

The MTU sizes advertised in the ACK trailer AF_RXRPC transmits are pegged
at a maximum of 1444 unless pmtud is supported by both sides.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20241204074710.990092-10-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 124 insertions, 0 deletions

diff --git a/include/trace/events/rxrpc.h b/include/trace/events/rxrpc.h
index d86b5f07d292..9dcadad88e76 100644
--- a/include/trace/events/rxrpc.h
+++ b/include/trace/events/rxrpc.h
@@ -364,6 +364,7 @@
 	EM(rxrpc_propose_ack_ping_for_lost_ack,	"LostAck") \
 	EM(rxrpc_propose_ack_ping_for_lost_reply, "LostRpl") \
 	EM(rxrpc_propose_ack_ping_for_0_retrans, "0-Retrn") \
+	EM(rxrpc_propose_ack_ping_for_mtu_probe, "MTUProb") \
 	EM(rxrpc_propose_ack_ping_for_old_rtt,	"OldRtt ") \
 	EM(rxrpc_propose_ack_ping_for_params,	"Params ") \
 	EM(rxrpc_propose_ack_ping_for_rtt,	"Rtt    ") \
@@ -478,6 +479,11 @@
 	EM(rxrpc_txbuf_see_send_more,		"SEE SEND+  ")	\
 	E_(rxrpc_txbuf_see_unacked,		"SEE UNACKED")
 
+#define rxrpc_pmtud_reduce_traces \
+	EM(rxrpc_pmtud_reduce_ack,		"Ack  ")	\
+	EM(rxrpc_pmtud_reduce_icmp,		"Icmp ")	\
+	E_(rxrpc_pmtud_reduce_route,		"Route")
+
 /*
  * Generate enums for tracing information.
  */
@@ -498,6 +504,7 @@ enum rxrpc_congest_change	{ rxrpc_congest_changes } __mode(byte);
 enum rxrpc_conn_trace		{ rxrpc_conn_traces } __mode(byte);
 enum rxrpc_local_trace		{ rxrpc_local_traces } __mode(byte);
 enum rxrpc_peer_trace		{ rxrpc_peer_traces } __mode(byte);
+enum rxrpc_pmtud_reduce_trace	{ rxrpc_pmtud_reduce_traces } __mode(byte);
 enum rxrpc_propose_ack_outcome	{ rxrpc_propose_ack_outcomes } __mode(byte);
 enum rxrpc_propose_ack_trace	{ rxrpc_propose_ack_traces } __mode(byte);
 enum rxrpc_receive_trace	{ rxrpc_receive_traces } __mode(byte);
@@ -534,6 +541,7 @@ rxrpc_congest_changes;
 rxrpc_congest_modes;
 rxrpc_conn_traces;
 rxrpc_local_traces;
+rxrpc_pmtud_reduce_traces;
 rxrpc_propose_ack_traces;
 rxrpc_receive_traces;
 rxrpc_recvmsg_traces;
@@ -2040,6 +2048,122 @@ TRACE_EVENT(rxrpc_sack,
 		      __entry->sack)
 	    );
 
+TRACE_EVENT(rxrpc_pmtud_tx,
+	    TP_PROTO(struct rxrpc_call *call),
+
+	    TP_ARGS(call),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,	peer_debug_id)
+		    __field(unsigned int,	call_debug_id)
+		    __field(rxrpc_serial_t,	ping_serial)
+		    __field(unsigned short,	pmtud_trial)
+		    __field(unsigned short,	pmtud_good)
+		    __field(unsigned short,	pmtud_bad)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->peer_debug_id = call->peer->debug_id;
+		    __entry->call_debug_id = call->debug_id;
+		    __entry->ping_serial = call->conn->pmtud_probe;
+		    __entry->pmtud_trial = call->peer->pmtud_trial;
+		    __entry->pmtud_good = call->peer->pmtud_good;
+		    __entry->pmtud_bad = call->peer->pmtud_bad;
+			   ),
+
+	    TP_printk("P=%08x c=%08x pr=%08x %u-%u-%u",
+		      __entry->peer_debug_id,
+		      __entry->call_debug_id,
+		      __entry->ping_serial,
+		      __entry->pmtud_good,
+		      __entry->pmtud_trial,
+		      __entry->pmtud_bad)
+	    );
+
+TRACE_EVENT(rxrpc_pmtud_rx,
+	    TP_PROTO(struct rxrpc_connection *conn, rxrpc_serial_t resp_serial),
+
+	    TP_ARGS(conn, resp_serial),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,	peer_debug_id)
+		    __field(unsigned int,	call_debug_id)
+		    __field(rxrpc_serial_t,	ping_serial)
+		    __field(rxrpc_serial_t,	resp_serial)
+		    __field(unsigned short,	max_data)
+		    __field(u8,			jumbo_max)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->peer_debug_id = conn->peer->debug_id;
+		    __entry->call_debug_id = conn->pmtud_call;
+		    __entry->ping_serial = conn->pmtud_probe;
+		    __entry->resp_serial = resp_serial;
+		    __entry->max_data = conn->peer->max_data;
+		    __entry->jumbo_max = conn->peer->pmtud_jumbo;
+			   ),
+
+	    TP_printk("P=%08x c=%08x pr=%08x rr=%08x max=%u jm=%u",
+		      __entry->peer_debug_id,
+		      __entry->call_debug_id,
+		      __entry->ping_serial,
+		      __entry->resp_serial,
+		      __entry->max_data,
+		      __entry->jumbo_max)
+	    );
+
+TRACE_EVENT(rxrpc_pmtud_lost,
+	    TP_PROTO(struct rxrpc_connection *conn, rxrpc_serial_t resp_serial),
+
+	    TP_ARGS(conn, resp_serial),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,	peer_debug_id)
+		    __field(unsigned int,	call_debug_id)
+		    __field(rxrpc_serial_t,	ping_serial)
+		    __field(rxrpc_serial_t,	resp_serial)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->peer_debug_id = conn->peer->debug_id;
+		    __entry->call_debug_id = conn->pmtud_call;
+		    __entry->ping_serial = conn->pmtud_probe;
+		    __entry->resp_serial = resp_serial;
+			   ),
+
+	    TP_printk("P=%08x c=%08x pr=%08x rr=%08x",
+		      __entry->peer_debug_id,
+		      __entry->call_debug_id,
+		      __entry->ping_serial,
+		      __entry->resp_serial)
+	    );
+
+TRACE_EVENT(rxrpc_pmtud_reduce,
+	    TP_PROTO(struct rxrpc_peer *peer, rxrpc_serial_t serial,
+		     unsigned int max_data, enum rxrpc_pmtud_reduce_trace reason),
+
+	    TP_ARGS(peer, serial, max_data, reason),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,	peer_debug_id)
+		    __field(rxrpc_serial_t,	serial)
+		    __field(unsigned int,	max_data)
+		    __field(enum rxrpc_pmtud_reduce_trace, reason)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->peer_debug_id = peer->debug_id;
+		    __entry->serial = serial;
+		    __entry->max_data = max_data;
+		    __entry->reason = reason;
+			   ),
+
+	    TP_printk("P=%08x %s r=%08x m=%u",
+		      __entry->peer_debug_id,
+		      __print_symbolic(__entry->reason, rxrpc_pmtud_reduce_traces),
+		      __entry->serial, __entry->max_data)
+	    );
+
 #undef EM
 #undef E_