Subject: Re: [PATCH net-next-2.6] net: pskb_expand_head() optimization

From: Jarek Poplawski <[email-masked]>
Date: Sat, 11 Sep 2010 14:31:40 +0200

On Fri, Sep 10, 2010 at 12:54:49PM -0700, David Miller wrote:
> From: Eric Dumazet <[email-masked]>
> Date: Tue, 07 Sep 2010 11:37:28 +0200
> 
> > Le mardi 07 septembre 2010 ? 09:16 +0000, Jarek Poplawski a écrit :
> >> On 2010-09-07 07:02, Eric Dumazet wrote:
> > 
> >> > 
> >> > I understand what you want to do, but problem is we need to perform a
> >> > CAS2 operation : atomically changes two values (dataref and frag_list)
> >> 
> >> Alas I can't understand why do you think these clone and atomic tests
> >> in skb_release_data() don't protect skb_shinfo(skb)->frag_list enough.
> >> 
> > 
> > It was early in the morning, before a cup of tea.
> > 
> > David only had to set frag_list in the new shinfo, not the old.
> 
> Ok, how does this look?
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 752c197..aaa9750 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -327,6 +327,32 @@ static void skb_clone_fraglist(struct sk_buff *skb)
>  		skb_get(list);
>  }
>  
> +static struct sk_buff *skb_copy_fraglist(struct sk_buff *parent,
> +					 gfp_t gfp_mask)
> +{
> +	struct sk_buff *first_skb = NULL;
> +	struct sk_buff *prev_skb = NULL;
> +	struct sk_buff *skb;
> +
> +	skb_walk_frags(parent, skb) {
> +		struct sk_buff *nskb = pskb_copy(skb, gfp_mask);
> +
> +		if (!nskb)
> +			goto fail;
> +		if (!first_skb)
> +			first_skb = skb;

Probably here and below: "= nskb"

> +		else
> +			prev_skb->next = skb;
> +		prev_skb = skb;
> +	}
> +
> +	return first_skb;
> +
> +fail:

With "if (first_skb)" here it would look better to me even if it
currently doesn't matter.

Otherwise seems OK, but I still would like to know the scenario
demanding this change.

Jarek P.

> +	skb_drop_list(&first_skb);
> +	return NULL;
> +}
> +
>  static void skb_release_data(struct sk_buff *skb)
>  {
>  	if (!skb->cloned ||
> @@ -775,11 +801,12 @@ EXPORT_SYMBOL(pskb_copy);
>  int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  		     gfp_t gfp_mask)
>  {
> -	int i;
> -	u8 *data;
>  	int size = nhead + (skb_end_pointer(skb) - skb->head) + ntail;
> -	long off;
> +	struct skb_shared_info *new_shinfo;
>  	bool fastpath;
> +	u8 *data;
> +	long off;
> +	int i;
>  
>  	BUG_ON(nhead < 0);
>  
> @@ -797,8 +824,8 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  	 */
>  	memcpy(data + nhead, skb->head, skb_tail_pointer(skb) - skb->head);
>  
> -	memcpy((struct skb_shared_info *)(data + size),
> -	       skb_shinfo(skb),
> +	new_shinfo = (struct skb_shared_info *)(data + size);
> +	memcpy(new_shinfo, skb_shinfo(skb),
>  	       offsetof(struct skb_shared_info, frags[skb_shinfo(skb)->nr_frags]));
>  
>  	/* Check if we can avoid taking references on fragments if we own
> @@ -815,14 +842,20 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  	if (fastpath) {
>  		kfree(skb->head);
>  	} else {
> +		if (skb_has_frag_list(skb)) {
> +			struct sk_buff *new_list;
> +
> +			new_list = skb_copy_fraglist(skb, gfp_mask);
> +			if (!new_list)
> +				goto free_data;
> +			new_shinfo->frag_list = new_list;
> +		}
>  		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
>  			get_page(skb_shinfo(skb)->frags[i].page);
>  
> -		if (skb_has_frag_list(skb))
> -			skb_clone_fraglist(skb);
> -
>  		skb_release_data(skb);
>  	}
> +
>  	off = (data + nhead) - skb->head;
>  
>  	skb->head     = data;
> @@ -848,6 +881,8 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  	atomic_set(&skb_shinfo(skb)->dataref, 1);
>  	return 0;
>  
> +free_data:
> +	kfree(data);
>  nodata:
>  	return -ENOMEM;
>  }