netdev - Re: [PATCH 2/3] skb: Drop "fastpath" variable for skb_cloned check in pskb_expand

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Date:	Sat, 05 May 2012 07:37:24 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	jeffrey.t.kirsher@...el.com
Subject: Re: [PATCH 2/3] skb: Drop "fastpath" variable for skb_cloned check
 in pskb_expand_head

On Fri, 2012-05-04 at 17:26 -0700, Alexander Duyck wrote:
> Since there is now only one spot that actually uses "fastpath" there isn't
> much point in carrying it.  Instead we can just use a check for skb_cloned
> to verify if we can perform the fast-path free for the head or not.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---
> 
>  net/core/skbuff.c |   22 ++++++++--------------
>  1 files changed, 8 insertions(+), 14 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 4d085d4..17e4b1e 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -932,7 +932,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  	u8 *data;
>  	int size = nhead + (skb_end_pointer(skb) - skb->head) + ntail;
>  	long off;
> -	bool fastpath;
>  
>  	BUG_ON(nhead < 0);
>  
> @@ -941,16 +940,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  
>  	size = SKB_DATA_ALIGN(size);
>  
> -	/* Check if we can avoid taking references on fragments if we own
> -	 * the last reference on skb->head. (see skb_release_data())
> -	 */
> -	if (!skb->cloned)
> -		fastpath = true;
> -	else {
> -		int delta = skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1;
> -		fastpath = atomic_read(&skb_shinfo(skb)->dataref) == delta;
> -	}
> -
>  	data = kmalloc(size + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)),
>  		       gfp_mask);
>  	if (!data)
> @@ -966,9 +955,12 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  	       skb_shinfo(skb),
>  	       offsetof(struct skb_shared_info, frags[skb_shinfo(skb)->nr_frags]));
>  
> -	if (fastpath) {
> -		skb_free_head(skb);
> -	} else {
> +	/*
> +	 * if shinfo is shared we must drop the old head gracefully, but if it
> +	 * is not we can just drop the old head and let the existing refcount
> +	 * be since all we did is relocate the values
> +	 */
> +	if (skb_cloned(skb)) {
>  		/* copy this zero copy skb frags */
>  		if (skb_shinfo(skb)->tx_flags & SKBTX_DEV_ZEROCOPY) {
>  			if (skb_copy_ubufs(skb, gfp_mask))
> @@ -981,6 +973,8 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>  			skb_clone_fraglist(skb);
>  
>  		skb_release_data(skb);
> +	} else {
> +		skb_free_head(skb);
>  	}
>  	off = (data + nhead) - skb->head;
>  
> 

Excellent

Acked-by: Eric Dumazet <edumazet@...gle.com>



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