Re: [PATCH net-next] flow_dissector: save computed hash in __skb_get_hash_symmetric_net()

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Nov 25, 2025 at 11:19:27AM -0700, Jon Kohler wrote:
> tun.c changed from skb_get_hash() to __skb_get_hash_symmetric() on
> commit feec084a7cf4 ("tun: use symmetric hash"), which exposes an
> overhead for OVS datapath, where ovs_dp_process_packet() has to
> calculate the hash again because __skb_get_hash_symmetric() does not
> retain the hash that it calculates.

This seems to be intentional according to commit eb70db875671 ("packet:
Use symmetric hash for PACKET_FANOUT_HASH."): "This hash does not get
installed into and override the normal skb hash, so this change has no
effect whatsoever on the rest of the stack."

> 
> Save the computed hash in __skb_get_hash_symmetric_net() so that the
> calcuation work does not go to waste.
> 
> Fixes: feec084a7cf4 ("tun: use symmetric hash")
> Signed-off-by: Jon Kohler <jon@nutanix.com>
> ---
>  include/linux/skbuff.h    | 4 ++--
>  net/core/flow_dissector.c | 7 +++++--
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index ff90281ddf90..f58afa49a50e 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1568,9 +1568,9 @@ __skb_set_sw_hash(struct sk_buff *skb, __u32 hash, bool is_l4)
>  	__skb_set_hash(skb, hash, true, is_l4);
>  }
>  
> -u32 __skb_get_hash_symmetric_net(const struct net *net, const struct sk_buff *skb);
> +u32 __skb_get_hash_symmetric_net(const struct net *net, struct sk_buff *skb);
>  
> -static inline u32 __skb_get_hash_symmetric(const struct sk_buff *skb)
> +static inline u32 __skb_get_hash_symmetric(struct sk_buff *skb)
>  {
>  	return __skb_get_hash_symmetric_net(NULL, skb);
>  }
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 1b61bb25ba0e..4a74dcc4799c 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -1864,9 +1864,10 @@ EXPORT_SYMBOL(make_flow_keys_digest);
>  
>  static struct flow_dissector flow_keys_dissector_symmetric __read_mostly;
>  
> -u32 __skb_get_hash_symmetric_net(const struct net *net, const struct sk_buff *skb)
> +u32 __skb_get_hash_symmetric_net(const struct net *net, struct sk_buff *skb)
>  {
>  	struct flow_keys keys;
> +	u32 flow_hash;
>  
>  	__flow_hash_secret_init();
>  
> @@ -1874,7 +1875,9 @@ u32 __skb_get_hash_symmetric_net(const struct net *net, const struct sk_buff *sk
>  	__skb_flow_dissect(net, skb, &flow_keys_dissector_symmetric,
>  			   &keys, NULL, 0, 0, 0, 0);
>  
> -	return __flow_hash_from_keys(&keys, &hashrnd);
> +	flow_hash = __flow_hash_from_keys(&keys, &hashrnd);
> +	__skb_set_sw_hash(skb, flow_hash, flow_keys_have_l4(&keys));
> +	return flow_hash;
>  }
>  EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric_net);
>  
> -- 
> 2.43.0
>