Subject: Re: [PATCH net-next v2 1/6] skbuff: Add skb_list_linearize()

From: Alexander Duyck <[email-masked]>
Date: Thu, 29 Jan 2015 19:28:50 -0800

On 01/29/2015 03:29 PM, Pravin B Shelar wrote:
> similar to skb_linearize(), this API takes skb list as arg and
> linearize it into one big skb. STT driver patch will use this.
>
> Signed-off-by: Pravin B Shelar <[email-masked]>
> ---
> Fixed according to comments from Eric.
> ---
>  include/linux/skbuff.h |  2 ++
>  net/core/skbuff.c      | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 85ab7d7..c9194c1 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2532,6 +2532,8 @@ static inline int skb_linearize(struct sk_buff *skb)
>  	return skb_is_nonlinear(skb) ? __skb_linearize(skb) : 0;
>  }
>  
> +int skb_list_linearize(struct sk_buff *head, gfp_t gfp_mask);
> +
>  /**
>   * skb_has_shared_frag - can any frag be overwritten
>   * @skb: buffer to test
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 56db472..d6358a7 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2329,6 +2329,40 @@ void skb_copy_and_csum_dev(const struct sk_buff *skb, u8 *to)
>  }
>  EXPORT_SYMBOL(skb_copy_and_csum_dev);
>  
> +int skb_list_linearize(struct sk_buff *head, gfp_t gfp_mask)
> +{
> +	struct sk_buff *skb;
> +	int tlen = 0;
> +	int err;
> +
> +	err = skb_linearize(head);
> +	if (err)
> +		return err;

What is the point in linearizing the head if you are just going to
reallocated it pskb_expand_head anyway?  It seems like you should
probably just be calling __pskb_pull_tail after your call
pskb_expand_head below.

> +	skb = head->next;
> +	while (skb) {
> +		tlen += skb->len;
> +		skb = skb->next;
> +	}
> +	err = pskb_expand_head(head, 0, tlen, gfp_mask);
> +	if (err)
> +		return err;
> +
> +	skb = head->next;
> +	while (skb) {
> +		err = skb_copy_bits(skb, 0, skb_tail_pointer(head), skb->len);
> +		if (err)
> +			return err;
> +		head->tail += skb->len;
> +		skb = skb->next;
> +	}
> +	kfree_skb_list(head->next);
> +	head->next = NULL;
> +	head->len += tlen;
> +	return 0;
> +}
> +EXPORT_SYMBOL(skb_list_linearize);
> +
>  /**
>   *	skb_dequeue - remove from the head of the queue
>   *	@list: list to dequeue from

I'm not completely convinced this is even necessary.  Seems like you are
wasting cycles copying the frames around when you could probably just
pull the header of the first frame and then use page frages to fill in
the rest.

- Alex