Re: [PATCH net-next 1/3] net: ipv6: ioam6_iptunnel: mitigate 2-realloc issue

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Justin Iurman <justin.iurman@uliege.be>
Date: Fri, 25 Oct 2024 15:37:25 +0200

> This patch mitigates the two-reallocations issue with ioam6_iptunnel by
> providing the dst_entry (in the cache) to the first call to
> skb_cow_head(). As a result, the very first iteration would still
> trigger two reallocations (i.e., empty cache), while next iterations
> would only trigger a single reallocation.

[...]

>  static int ioam6_do_inline(struct net *net, struct sk_buff *skb,
> -			   struct ioam6_lwt_encap *tuninfo)
> +			   struct ioam6_lwt_encap *tuninfo,
> +			   struct dst_entry *dst)
>  {
>  	struct ipv6hdr *oldhdr, *hdr;
>  	int hdrlen, err;
>  
>  	hdrlen = (tuninfo->eh.hdrlen + 1) << 3;
>  
> -	err = skb_cow_head(skb, hdrlen + skb->mac_len);
> +	err = skb_cow_head(skb, hdrlen + (!dst ? skb->mac_len
> +					       : LL_RESERVED_SPACE(dst->dev)));

You use this pattern a lot throughout the series. I believe you should
make a static inline or a macro from it.

static inline u32 some_name(const *dst, const *skb)
{
	return dst ? LL_RESERVED_SPACE(dst->dev) : skb->mac_len;
}

BTW why do you check for `!dst`, not `dst`? Does changing this affects
performance?


>  	if (unlikely(err))
>  		return err;

Thanks,
Olek