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Date:   Mon, 16 Mar 2020 16:03:12 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Joe Stringer <joe@...d.net.nz>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        <daniel@...earbox.net>, <ast@...nel.org>, <eric.dumazet@...il.com>,
        <lmb@...udflare.com>
Subject: Re: [PATCH bpf-next 4/7] dst: Prefetch established socket
 destinations

On Thu, Mar 12, 2020 at 04:36:45PM -0700, Joe Stringer wrote:
> Enhance the dst_sk_prefetch logic to temporarily store the socket
> receive destination, to save the route lookup later on. The dst
> reference is kept alive by the caller's socket reference.
> 
> Suggested-by: Daniel Borkmann <daniel@...earbox.net>
> Signed-off-by: Joe Stringer <joe@...d.net.nz>
> ---
>  include/net/dst_metadata.h |  2 +-
>  net/core/dst.c             | 20 +++++++++++++++++---
>  net/core/filter.c          |  2 +-
>  3 files changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
> index 31574c553a07..4f16322b08d5 100644
> --- a/include/net/dst_metadata.h
> +++ b/include/net/dst_metadata.h
> @@ -230,7 +230,7 @@ static inline bool skb_dst_is_sk_prefetch(const struct sk_buff *skb)
>  	return dst_is_sk_prefetch(skb_dst(skb));
>  }
>  
> -void dst_sk_prefetch_store(struct sk_buff *skb);
> +void dst_sk_prefetch_store(struct sk_buff *skb, struct sock *sk);
>  void dst_sk_prefetch_fetch(struct sk_buff *skb);
>  
>  /**
> diff --git a/net/core/dst.c b/net/core/dst.c
> index cf1a1d5b6b0a..5068d127d9c2 100644
> --- a/net/core/dst.c
> +++ b/net/core/dst.c
> @@ -346,11 +346,25 @@ EXPORT_SYMBOL(dst_sk_prefetch);
>  
>  DEFINE_PER_CPU(unsigned long, dst_sk_prefetch_dst);
>  
> -void dst_sk_prefetch_store(struct sk_buff *skb)
> +void dst_sk_prefetch_store(struct sk_buff *skb, struct sock *sk)
>  {
> -	unsigned long refdst;
> +	unsigned long refdst = 0L;
> +
> +	WARN_ON(!rcu_read_lock_held() &&
> +		!rcu_read_lock_bh_held());
> +	if (sk_fullsock(sk)) {
> +		struct dst_entry *dst = READ_ONCE(sk->sk_rx_dst);
> +
> +		if (dst)
> +			dst = dst_check(dst, 0);
v6 requires a cookie.  tcp_v6_early_demux() could be a good example.

> +		if (dst)
> +			refdst = (unsigned long)dst | SKB_DST_NOREF;
> +	}
> +	if (!refdst)
> +		refdst = skb->_skb_refdst;
> +	if (skb->_skb_refdst != refdst)
> +		skb_dst_drop(skb);
>  
> -	refdst = skb->_skb_refdst;
>  	__this_cpu_write(dst_sk_prefetch_dst, refdst);
>  	skb_dst_set_noref(skb, (struct dst_entry *)&dst_sk_prefetch.dst);
>  }

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