netdev - Re: [PATCH 02/11] net: add function to allocate skbuff head without data area

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Date:	Mon, 20 Aug 2012 08:41:40 +0200 (MEST)
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Florian.Westphal@...hos.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 02/11] net: add function to allocate skbuff head without
 data area

On Mon, 20 Aug 2012, Eric Dumazet wrote:

> On Mon, 2012-08-20 at 08:18 +0200, Patrick McHardy wrote:
>> Add a function to allocate a skbuff head without any data. This will be
>> used by memory mapped netlink to attached data from the mmaped area to
>> the skb.
>>
>> Additionally change skb_release_all() to check whether the skb has a
>> data area to allow the skb destructor to clear the data pointer in
>> case only a head has been allocated.
>
>      * ...
>
>>
>> +struct sk_buff *__alloc_skb_head(gfp_t gfp_mask, int node)
>> +{
>> +	struct sk_buff *skb;
>> +
>> +	/* Get the HEAD */
>> +	skb = kmem_cache_alloc_node(skbuff_head_cache,
>> +				    gfp_mask & ~__GFP_DMA, node);
>> +	if (!skb)
>> +		goto out;
>> +	prefetchw(skb);
>> +
>
> This prefetch is probably not worth it, since you write on skb on the
> following memset(). Cpu wont have enough 'time' to take benefit from
> this hint.

Right, I remember we talked about that before and I already removed it.
Must have forgotten to actually commit that change, thanks!

>
>> +	/*
>> +	 * Only clear those fields we need to clear, not those that we will
>> +	 * actually initialise below. Hence, don't put any more fields after
>> +	 * the tail pointer in struct sk_buff!
>> +	 */
>> +	memset(skb, 0, offsetof(struct sk_buff, tail));
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