linux-kernel - Re: [PATCH net-next] net: core: allow fast GRO for skbs with Ethernet header in head

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Message-ID: <097eb720466a7c429c8fd91c792e7cd5@dlink.ru>
Date:   Fri, 15 Nov 2019 10:36:08 +0300
From:   Alexander Lobakin <alobakin@...nk.ru>
To:     David Miller <davem@...emloft.net>
Cc:     ecree@...arflare.com, jiri@...lanox.com, edumazet@...gle.com,
        idosch@...lanox.com, pabeni@...hat.com, petrm@...lanox.com,
        sd@...asysnail.net, f.fainelli@...il.com,
        jaswinder.singh@...aro.org, manishc@...vell.com,
        GR-Linux-NIC-Dev@...vell.com, johannes.berg@...el.com,
        emmanuel.grumbach@...el.com, luciano.coelho@...el.com,
        linuxwifi@...el.com, kvalo@...eaurora.org, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: core: allow fast GRO for skbs with Ethernet
 header in head

Hi Dave,

David Miller wrote 15.11.2019 04:25:

> From: Alexander Lobakin <alobakin@...nk.ru>
> Date: Tue, 12 Nov 2019 15:28:43 +0300
> 
>> Commit 78d3fd0b7de8 ("gro: Only use skb_gro_header for completely
>> non-linear packets") back in May'09 (2.6.31-rc1) has changed the
>> original condition '!skb_headlen(skb)' to the current
>> 'skb_mac_header(skb) == skb_tail_pointer(skb)' in gro_reset_offset()
>> saying: "Since the drivers that need this optimisation all provide
>> completely non-linear packets".
> 
> Please reference the appropriate SHA1-ID both here in this paragraph 
> and
> also in an appropriate Fixes: tag.

Sorry for confusing. The SHA1-ID from commit message is correct
actually. At the moment of 2.6.31 we used skb->mac_header and skb->tail
pointers directly, so the original condition was
'skb->mac_header == skb->tail'.
Commit ced14f6804a9 ("net: Correct comparisons and calculations using
skb->tail and skb-transport_header") has changed this condition to
the referred 'skb_mac_header(skb) == skb_tail_pointer(skb)' without
any functional changes.
I didn't add the "Fixes:" tag because at the moment of 2.6.31 it was
a needed change, but it became obsolete later, so now we can revert
it back to speed up skbs with only Ethernet header in head.
Please let me know if I must send v2 of this patch with corrected
description before getting any further reviews.

Thanks.

> If this goes so far back that it is before GIT, then you need to 
> provide
> a reference to the patch posting via lore.kernel.org or similar because
> it is absolutely essentialy for people reviewing this patch to be able
> to do some digging into why the condition is code the way that it is
> currently.
> 
> Thank you.

Regards,
ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ