netdev - Re: [PATCH net] net: ipv6: ip6_output: alloc skb with tailroom

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Date:   Wed, 6 Jun 2018 16:26:19 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     aring@...atatu.com, Network Development <netdev@...r.kernel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        david.palma@...u.no, rabinarayans0828@...il.com,
        Jamal Hadi Salim <jhs@...atatu.com>, stefan@....samsung.com,
        linux-wpan@...r.kernel.org, kernel@...atatu.com
Subject: Re: [PATCH net] net: ipv6: ip6_output: alloc skb with tailroom

On Wed, Jun 6, 2018 at 2:11 PM, David Miller <davem@...emloft.net> wrote:
> From: Alexander Aring <aring@...atatu.com>
> Date: Wed, 6 Jun 2018 14:09:20 -0400
>
>> okay, then you want to have this patch for net-next? As an optimization?
>>
>> Of course, when it's open again.
>
> Like you, I have questions about where this adjustment is applied and
> why.  So I'm not sure yet.
>
> For example, only IPV6 really takes it into consideration and as you
> saw only really for the fragmentation path and not the normal output
> path.
>
> This needs more consideration and investigation.

This is the unconditional skb_put in ieee802154_tx. In many cases
there is some tailroom due to SKB_DATA_ALIGN in __alloc_skb,
so it may take a specific case to not have even 2 bytes of tailroom
available.