netdev - Re: [PATCH net-next 4/6] kcm: Kernel Connection Multiplexor module

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Date:	Fri, 20 Nov 2015 15:10:54 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, kernel-team@...com,
	davewatson@...com
Subject: Re: [PATCH net-next 4/6] kcm: Kernel Connection Multiplexor module

On Fri, Nov 20, 2015 at 01:21:58PM -0800, Tom Herbert wrote:
> +
> +	while (eaten < orig_len) {
> +		/* Always clone since we will consume something */
> +		skb = skb_clone(orig_skb, GFP_ATOMIC);
> +		if (!skb) {
> +			desc->error = -ENOMEM;
> +			break;
> +		}
...
> +		if (!pskb_pull(skb, offset + eaten)) {

pskb_pull after clone == pskb_expand_head
meaning that we'll be linearizing things all the time.
can we try skb_share_check() first ?
the packet shouldn't be shared at this point, so it might save a lot.

> +			kfree_skb(skb);
> +			desc->error = -ENOMEM;
> +			break;
> +		}
...
> +		/* Need to trim, should be rare? */
> +		err = pskb_trim(skb, orig_len - eaten);
...
> +		if (!rxm->full_len) {
> +			ssize_t len;
> +
> +			len = KCM_RUN_FILTER(psock->bpf_prog, head);

bpf can read packet beyond linear header just fine.
It's slower than reads from header, since extra calls invovled,
but if that becomes a bottle neck we can optimize the way we do JIT there.

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