[PATCH] net: Set trailer iff skb1 is the last one

[PATCH] net: Set trailer iff skb1 is the last one

[Miaohe Lin]

Set trailer iff skb1 is the skbuff where the tailbits space begins.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 net/core/skbuff.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 0b24aed04060..18ed56316e56 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4488,8 +4488,9 @@ int skb_cow_data(struct sk_buff *skb, int tailbits, struct sk_buff **trailer)
 			skb1 = skb2;
 		}
 		elt++;
-		*trailer = skb1;
 		skb_p = &skb1->next;
+		if (!*skb_p)
+			*trailer = skb1;
 	}
 
 	return elt;
-- 
2.19.1

Re: [PATCH] net: Set trailer iff skb1 is the last one

[Eric Dumazet]

On Thu, Aug 27, 2020 at 4:31 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> Set trailer iff skb1 is the skbuff where the tailbits space begins.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  net/core/skbuff.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 0b24aed04060..18ed56316e56 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4488,8 +4488,9 @@ int skb_cow_data(struct sk_buff *skb, int tailbits, struct sk_buff **trailer)
>                         skb1 = skb2;
>                 }
>                 elt++;
> -               *trailer = skb1;
>                 skb_p = &skb1->next;
> +               if (!*skb_p)
> +                       *trailer = skb1;
>

Why is adding a conditional test going to help ?

cpu will have hard time predicting this one, I doubt this kind of
change is a win.