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Date:   Thu, 12 May 2022 18:54:46 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Oleksij Rempel <o.rempel@...gutronix.de>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     Devid Antonio Filoni <devid.filoni@...uetechnologies.com>,
        kernel@...gutronix.de, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH v2] can: skb: add extended skb support

Hi Oleksij,

On 12.05.22 14:59, Oleksij Rempel wrote:
> Add CAN specific skb extension support and add first currently needed
> local_origin variable.
> 
> On the CAN stack we push same skb data in different direction depending
> on the interface type:
> - to the HW egress and at same time back to the stack as echo
> - over virtual vcan/vxcan interfaces as egress on one side and ingress on other
>    side of the vxcan tunnel.
> We can't use skb->sk as marker of the origin, because not all packets
> not all packets with local_origin are assigned to some socket. Some of
> them are generate from the kernel, for example like J1939 control messages.
> So, to properly detect flow direction is is better to store this information
> as part of the SKB.
> 
> The advantage of using skb_ext is that it is options and extendable
> without affecting other skb users. It can be shared between cloned skbs and
> duplicated only if needed.
> 
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> Cc: Devid Antonio Filoni <devid.filoni@...uetechnologies.com>
> ---
> changes v2:
> - migrate it to SKB_EXT

The use of SKB_EXT seems to be very costly to just store a boolean value.

What I could see from some of the other SKB_EXT users this extension 
(which performs alloc & COW) is used in special circumstances.

With your suggestion this additional effort is needed for every CAN 
related skb.

So at least for this use-case extending struct can_skb_priv seems to be 
more efficient.

https://elixir.bootlin.com/linux/latest/source/include/linux/can/skb.h#L44

We might get into problems with PF_PACKET sockets when extending the 
can_skb_priv length beyond HH_DATA_MOD, see:

https://elixir.bootlin.com/linux/latest/source/include/linux/can/skb.h#L99

But for now I'm not sure that SKB_EXT isn't too heavy to store that 
single flag.

Best regards,
Oliver


> 
>   drivers/net/can/vxcan.c |  4 ++++
>   include/linux/can/skb.h |  4 ++++
>   include/linux/skbuff.h  |  3 +++
>   net/can/Kconfig         |  1 +
>   net/can/af_can.c        |  5 +++++
>   net/can/raw.c           | 10 ++++++++--
>   net/core/skbuff.c       |  7 +++++++
>   7 files changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/can/vxcan.c b/drivers/net/can/vxcan.c
> index 577a80300514..93701a698008 100644
> --- a/drivers/net/can/vxcan.c
> +++ b/drivers/net/can/vxcan.c
> @@ -39,6 +39,7 @@ static netdev_tx_t vxcan_xmit(struct sk_buff *oskb, struct net_device *dev)
>   	struct net_device *peer;
>   	struct canfd_frame *cfd = (struct canfd_frame *)oskb->data;
>   	struct net_device_stats *peerstats, *srcstats = &dev->stats;
> +	struct can_skb_ext *can_ext;
>   	struct sk_buff *skb;
>   	u8 len;
>   
> @@ -66,6 +67,9 @@ static netdev_tx_t vxcan_xmit(struct sk_buff *oskb, struct net_device *dev)
>   	skb->pkt_type   = PACKET_BROADCAST;
>   	skb->dev        = peer;
>   	skb->ip_summed  = CHECKSUM_UNNECESSARY;
> +	can_ext = skb_ext_add(skb, SKB_EXT_CAN);
> +	if (can_ext)
> +		can_ext->local_origin = false;
>   
>   	len = cfd->can_id & CAN_RTR_FLAG ? 0 : cfd->len;
>   	if (netif_rx(skb) == NET_RX_SUCCESS) {
> diff --git a/include/linux/can/skb.h b/include/linux/can/skb.h
> index fdb22b00674a..401b08890d74 100644
> --- a/include/linux/can/skb.h
> +++ b/include/linux/can/skb.h
> @@ -55,6 +55,10 @@ struct can_skb_priv {
>   	struct can_frame cf[];
>   };
>   
> +struct can_skb_ext {
> +	bool local_origin;
> +};
> +
>   static inline struct can_skb_priv *can_skb_prv(struct sk_buff *skb)
>   {
>   	return (struct can_skb_priv *)(skb->head);
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 3270cb72e4d8..d39e70e5f7f2 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -4563,6 +4563,9 @@ enum skb_ext_id {
>   #endif
>   #if IS_ENABLED(CONFIG_MCTP_FLOWS)
>   	SKB_EXT_MCTP,
> +#endif
> +#if IS_ENABLED(CONFIG_CAN)
> +	SKB_EXT_CAN,
>   #endif
>   	SKB_EXT_NUM, /* must be last */
>   };
> diff --git a/net/can/Kconfig b/net/can/Kconfig
> index a9ac5ffab286..eb826e3771fe 100644
> --- a/net/can/Kconfig
> +++ b/net/can/Kconfig
> @@ -5,6 +5,7 @@
>   
>   menuconfig CAN
>   	tristate "CAN bus subsystem support"
> +	select SKB_EXTENSIONS
>   	help
>   	  Controller Area Network (CAN) is a slow (up to 1Mbit/s) serial
>   	  communications protocol. Development of the CAN bus started in
> diff --git a/net/can/af_can.c b/net/can/af_can.c
> index 1fb49d51b25d..329c540d3ddf 100644
> --- a/net/can/af_can.c
> +++ b/net/can/af_can.c
> @@ -201,6 +201,7 @@ int can_send(struct sk_buff *skb, int loop)
>   	struct sk_buff *newskb = NULL;
>   	struct canfd_frame *cfd = (struct canfd_frame *)skb->data;
>   	struct can_pkg_stats *pkg_stats = dev_net(skb->dev)->can.pkg_stats;
> +	struct can_skb_ext *can_ext;
>   	int err = -EINVAL;
>   
>   	if (skb->len == CAN_MTU) {
> @@ -240,6 +241,10 @@ int can_send(struct sk_buff *skb, int loop)
>   	skb_reset_network_header(skb);
>   	skb_reset_transport_header(skb);
>   
> +	can_ext = skb_ext_add(skb, SKB_EXT_CAN);
> +	if (can_ext)
> +		can_ext->local_origin = true;
> +
>   	if (loop) {
>   		/* local loopback of sent CAN frames */
>   
> diff --git a/net/can/raw.c b/net/can/raw.c
> index b7dbb57557f3..cba18cdf017f 100644
> --- a/net/can/raw.c
> +++ b/net/can/raw.c
> @@ -121,6 +121,7 @@ static void raw_rcv(struct sk_buff *oskb, void *data)
>   {
>   	struct sock *sk = (struct sock *)data;
>   	struct raw_sock *ro = raw_sk(sk);
> +	struct can_skb_ext *can_ext;
>   	struct sockaddr_can *addr;
>   	struct sk_buff *skb;
>   	unsigned int *pflags;
> @@ -173,8 +174,13 @@ static void raw_rcv(struct sk_buff *oskb, void *data)
>   	/* add CAN specific message flags for raw_recvmsg() */
>   	pflags = raw_flags(skb);
>   	*pflags = 0;
> -	if (oskb->sk)
> -		*pflags |= MSG_DONTROUTE;
> +
> +	can_ext = skb_ext_find(oskb, SKB_EXT_CAN);
> +	if (can_ext) {
> +		if (can_ext->local_origin)
> +			*pflags |= MSG_DONTROUTE;
> +	}
> +
>   	if (oskb->sk == sk)
>   		*pflags |= MSG_CONFIRM;
>   
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 475183f37891..5a5409ccb767 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -61,6 +61,7 @@
>   #include <linux/if_vlan.h>
>   #include <linux/mpls.h>
>   #include <linux/kcov.h>
> +#include <linux/can/skb.h>
>   
>   #include <net/protocol.h>
>   #include <net/dst.h>
> @@ -4338,6 +4339,9 @@ static const u8 skb_ext_type_len[] = {
>   #if IS_ENABLED(CONFIG_MCTP_FLOWS)
>   	[SKB_EXT_MCTP] = SKB_EXT_CHUNKSIZEOF(struct mctp_flow),
>   #endif
> +#if IS_ENABLED(CONFIG_CAN)
> +	[SKB_EXT_CAN] = SKB_EXT_CHUNKSIZEOF(struct can_skb_ext),
> +#endif
>   };
>   
>   static __always_inline unsigned int skb_ext_total_length(void)
> @@ -4357,6 +4361,9 @@ static __always_inline unsigned int skb_ext_total_length(void)
>   #endif
>   #if IS_ENABLED(CONFIG_MCTP_FLOWS)
>   		skb_ext_type_len[SKB_EXT_MCTP] +
> +#endif
> +#if IS_ENABLED(CONFIG_CAN)
> +		skb_ext_type_len[SKB_EXT_CAN] +
>   #endif
>   		0;
>   }

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