linux-kernel - Re: [PATCH v2 net-next] net: core: use listified Rx for GRO_NORMAL in napi_gro

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Date:   Wed, 24 Jun 2020 15:28:02 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Alexander Lobakin <alobakin@...nk.ru>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Edward Cree <ecree@...arflare.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Petr Machata <petrm@...lanox.com>,
        Sabrina Dubroca <sd@...asysnail.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net-next] net: core: use listified Rx for GRO_NORMAL
 in napi_gro_receive()

On Wed, Jun 24, 2020 at 03:06:10PM -0600, Jason A. Donenfeld wrote:
> Hi Alexander,
> 
> This patch introduced a behavior change around GRO_DROP:
> 
> napi_skb_finish used to sometimes return GRO_DROP:
> 
> > -static gro_result_t napi_skb_finish(gro_result_t ret, struct sk_buff *skb)
> > +static gro_result_t napi_skb_finish(struct napi_struct *napi,
> > +				    struct sk_buff *skb,
> > +				    gro_result_t ret)
> >  {
> >  	switch (ret) {
> >  	case GRO_NORMAL:
> > -		if (netif_receive_skb_internal(skb))
> > -			ret = GRO_DROP;
> > +		gro_normal_one(napi, skb);
> >
> 
> But under your change, gro_normal_one and the various calls that makes
> never propagates its return value, and so GRO_DROP is never returned to
> the caller, even if something drops it.
> 
> Was this intentional? Or should I start looking into how to restore it?
> 
> Thanks,
> Jason

For some context, I'm consequently mulling over this change in my code,
since checking for GRO_DROP now constitutes dead code:

diff --git a/drivers/net/wireguard/receive.c b/drivers/net/wireguard/receive.c
index 91438144e4f7..9b2ab6fc91cd 100644
--- a/drivers/net/wireguard/receive.c
+++ b/drivers/net/wireguard/receive.c
@@ -414,14 +414,8 @@ static void wg_packet_consume_data_done(struct wg_peer *peer,
 	if (unlikely(routed_peer != peer))
 		goto dishonest_packet_peer;

-	if (unlikely(napi_gro_receive(&peer->napi, skb) == GRO_DROP)) {
-		++dev->stats.rx_dropped;
-		net_dbg_ratelimited("%s: Failed to give packet to userspace from peer %llu (%pISpfsc)\n",
-				    dev->name, peer->internal_id,
-				    &peer->endpoint.addr);
-	} else {
-		update_rx_stats(peer, message_data_len(len_before_trim));
-	}
+	napi_gro_receive(&peer->napi, skb);
+	update_rx_stats(peer, message_data_len(len_before_trim));
 	return;

 dishonest_packet_peer: