netdev - Re: [RFC PATCH v7 11/19] Use callback to deal with skb_release

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Date:	Sat, 05 Jun 2010 16:56:13 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	xiaohui.xin@...el.com
Cc:	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mst@...hat.com, mingo@...e.hu,
	davem@...emloft.net, herbert@...dor.apana.org.au,
	jdike@...ux.intel.com
Subject: Re: [RFC PATCH v7 11/19] Use callback to deal with
 skb_release_data() specially.

Le samedi 05 juin 2010 à 18:14 +0800, xiaohui.xin@...el.com a écrit :
> From: Xin Xiaohui <xiaohui.xin@...el.com>
> 
> If buffer is external, then use the callback to destruct
> buffers.
> 
> Signed-off-by: Xin Xiaohui <xiaohui.xin@...el.com>
> Signed-off-by: Zhao Yu <yzhao81new@...il.com>
> Reviewed-by: Jeff Dike <jdike@...ux.intel.com>
> ---
>  net/core/skbuff.c |   11 +++++++++++
>  1 files changed, 11 insertions(+), 0 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 37587f0..418457c 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -385,6 +385,11 @@ static void skb_clone_fraglist(struct sk_buff *skb)
>  
>  static void skb_release_data(struct sk_buff *skb)
>  {
> +	/* check if the skb has external buffers, we have use destructor_arg
> +	 * here to indicate
> +	 */
> +	struct skb_external_page *ext_page = skb_shinfo(skb)->destructor_arg;
> +

Oh well. This is v7 of your series, and nobody complained yet ?

This is a new cache miss on a _critical_ path.


>  	if (!skb->cloned ||
>  	    !atomic_sub_return(skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1,
>  			       &skb_shinfo(skb)->dataref)) {
> @@ -397,6 +402,12 @@ static void skb_release_data(struct sk_buff *skb)
>  		if (skb_has_frags(skb))
>  			skb_drop_fraglist(skb);
>  
> +		/* if the skb has external buffers, use destructor here,
> +		 * since after that skb->head will be kfree, in case skb->head
> +		 * from external buffer cannot use kfree to destroy.
> +		 */

Why not deferring here the access to skb_shinfo(skb)->destructor_arg ?

> +		if (dev_is_mpassthru(skb->dev) && ext_page && ext_page->dtor)
> +			ext_page->dtor(ext_page);
>  		kfree(skb->head);
>  	}
>  }

if (dev_is_mpassthru(skb->dev)) {
	struct skb_external_page *ext_page =
				skb_shinfo(skb)->destructor_arg;
	if (ext_page && ext_page->dtor)
		ext_page->dtor(ext_page);
}

destructor_arg should me moved before frags[] if you really want to use it.

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index bf243fc..b136d90 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -202,10 +202,11 @@ struct skb_shared_info {
         */
        atomic_t        dataref;
 
-       skb_frag_t      frags[MAX_SKB_FRAGS];
        /* Intermediate layers must ensure that destructor_arg
         * remains valid until skb destructor */
        void *          destructor_arg;
+
+       skb_frag_t      frags[MAX_SKB_FRAGS];
 };
 
 /* We divide dataref into two halves.  The higher 16 bits hold references



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