Re: [RFC PATCH v7 11/19] Use callback to deal with skb_release_data() specially.

Eric Dumazet
2010-06-05T14:56:13Z

Le samedi 05 juin 2010 à 18:14 +0800, xiaohui.xin@intel.com a écrit :
> From: Xin Xiaohui <xiaohui.xin@intel.com>
>
> If buffer is external, then use the callback to destruct
> buffers.
>
> Signed-off-by: Xin Xiaohui <xiaohui.xin@intel.com>
> Signed-off-by: Zhao Yu <yzhao81new@gmail.com>
> Reviewed-by: Jeff Dike <jdike@linux.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