netdev - Re: [PATCH net-next 02/15] ipv6: add dev->gso_ipv6_max

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Message-ID: <90c19324a093536f1e0e2e3de3a36df4207a28d3.camel@redhat.com>
Date:   Thu, 03 Feb 2022 09:57:11 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH net-next 02/15] ipv6: add dev->gso_ipv6_max_size

Hello,

On Wed, 2022-02-02 at 17:51 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> This enable TCP stack to build TSO packets bigger than
> 64KB if the driver is LSOv2 compatible.
> 
> This patch introduces new variable gso_ipv6_max_size
> that is modifiable through ip link.
> 
> ip link set dev eth0 gso_ipv6_max_size 185000
> 
> User input is capped by driver limit.
> 
> Signed-off-by: Coco Li <lixiaoyan@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/netdevice.h          | 12 ++++++++++++
>  include/uapi/linux/if_link.h       |  1 +
>  net/core/dev.c                     |  1 +
>  net/core/rtnetlink.c               | 15 +++++++++++++++
>  net/core/sock.c                    |  6 ++++++
>  tools/include/uapi/linux/if_link.h |  1 +
>  6 files changed, 36 insertions(+)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index b1f68df2b37bc4b623f61cc2c6f0c02ba2afbe02..2a563869ba44f7d48095d36b1395e3fbd8cfff87 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1949,6 +1949,7 @@ enum netdev_ml_priv_type {
>   *	@linkwatch_dev_tracker:	refcount tracker used by linkwatch.
>   *	@watchdog_dev_tracker:	refcount tracker used by watchdog.
>   *	@tso_ipv6_max_size:	Maximum size of IPv6 TSO packets (driver/NIC limit)
> + *	@gso_ipv6_max_size:	Maximum size of IPv6 GSO packets (user/admin limit)
>   *
>   *	FIXME: cleanup struct net_device such that network protocol info
>   *	moves out.
> @@ -2284,6 +2285,7 @@ struct net_device {
>  	netdevice_tracker	linkwatch_dev_tracker;
>  	netdevice_tracker	watchdog_dev_tracker;
>  	unsigned int		tso_ipv6_max_size;
> +	unsigned int		gso_ipv6_max_size;
>  };
>  #define to_net_dev(d) container_of(d, struct net_device, dev)
>  
> @@ -4804,6 +4806,10 @@ static inline void netif_set_gso_max_size(struct net_device *dev,
>  {
>  	/* dev->gso_max_size is read locklessly from sk_setup_caps() */
>  	WRITE_ONCE(dev->gso_max_size, size);
> +
> +	/* legacy drivers want to lower gso_max_size, regardless of family. */
> +	size = min(size, dev->gso_ipv6_max_size);
> +	WRITE_ONCE(dev->gso_ipv6_max_size, size);
>  }
>  
>  static inline void netif_set_gso_max_segs(struct net_device *dev,
> @@ -4827,6 +4833,12 @@ static inline void netif_set_tso_ipv6_max_size(struct net_device *dev,
>  	dev->tso_ipv6_max_size = size;
>  }
>  
> +static inline void netif_set_gso_ipv6_max_size(struct net_device *dev,
> +					       unsigned int size)
> +{
> +	size = min(size, dev->tso_ipv6_max_size);
> +	WRITE_ONCE(dev->gso_ipv6_max_size, size);

Dumb questions on my side: should the above be limited to
tso_ipv6_max_size ? or increasing gso_ipv6_max_size helps even if the
egress NIC does not support LSOv2?

Should gso_ipv6_max_size be capped to some reasonable value (well lower
than 4G), to avoid the stack building very complex skbs?

Thanks!

Paolo