rxrpc: Send jumbo DATA packets

Send jumbo DATA packets if the path-MTU probing using padded PING ACK
packets shows up sufficient capacity to do so.  This allows larger chunks
of data to be sent without reducing the retryability as the subpackets in a
jumbo packet can also be retransmitted individually.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/rxrpc/ar-internal.h

@@ -710,6 +710,7 @@ struct rxrpc_call {
 	u16			tx_backoff;	/* Delay to insert due to Tx failure (ms) */
 	u8			tx_winsize;	/* Maximum size of Tx window */
 #define RXRPC_TX_MAX_WINDOW	128
+	u8			tx_jumbo_max;	/* Maximum subpkts peer will accept */
 	ktime_t			tx_last_sent;	/* Last time a transmission occurred */
 
 	/* Received data tracking */

net/rxrpc/call_event.c

@@ -288,7 +288,7 @@ static void rxrpc_transmit_fresh_data(struct rxrpc_call *call)
 		struct rxrpc_txqueue *tq;
 		struct rxrpc_txbuf *txb;
 		rxrpc_seq_t send_top, seq;
-		int limit = min(space, 1);
+		int limit = min(space, max(call->peer->pmtud_jumbo, 1));
 
 		/* Order send_top before the contents of the new txbufs and
 		 * txqueue pointers

net/rxrpc/call_object.c

@@ -155,6 +155,7 @@ struct rxrpc_call *rxrpc_alloc_call(struct rxrpc_sock *rx, gfp_t gfp,
 	refcount_set(&call->ref, 1);
 	call->debug_id		= debug_id;
 	call->tx_total_len	= -1;
+	call->tx_jumbo_max	= 1;
 	call->next_rx_timo	= 20 * HZ;
 	call->next_req_timo	= 1 * HZ;
 	call->ackr_window	= 1;

net/rxrpc/input.c

@@ -796,8 +796,11 @@ static void rxrpc_input_ack_trailer(struct rxrpc_call *call, struct sk_buff *skb
 		peer->ackr_adv_pmtud = true;
 	} else {
 		peer->ackr_adv_pmtud = false;
+		capacity = clamp(capacity, 1, jumbo_max);
 	}
 
+	call->tx_jumbo_max = capacity;
+
 	if (wake)
 		wake_up(&call->waitq);
 }