linux-kernel - Re: [PATCH net-next 1/3] net: dsa: don't pass cloned skb's to drivers xmit function

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Message-ID: <20201017004816.q4l6cypw4fd4vu5f@skbuf>
Date:   Sat, 17 Oct 2020 03:48:16 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Christian Eggers <ceggers@...i.de>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        "David S . Miller" <davem@...emloft.net>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net: dsa: don't pass cloned skb's to
 drivers xmit function

On Fri, Oct 16, 2020 at 10:02:24PM +0200, Christian Eggers wrote:
> Ensure that the skb is not cloned and has enough tail room for the tail
> tag. This code will be removed from the drivers in the next commits.
> 
> Signed-off-by: Christian Eggers <ceggers@...i.de>
> ---

Does 1588 work for you using this change, or you haven't finished
implementing it yet? If you haven't, I would suggest finishing that
part first.

The post-reallocation skb looks nothing like the one before.

Before:
skb len=68 headroom=2 headlen=68 tailroom=186
mac=(2,14) net=(16,-1) trans=-1
shinfo(txflags=1 nr_frags=0 gso(size=0 type=0 segs=0))
csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
hash(0x9d6927ec sw=1 l4=0) proto=0x88f7 pkttype=0 iif=0
dev name=swp2 feat=0x0x0002000000005020
sk family=17 type=3 proto=0

After:
skb len=68 headroom=2 headlen=68 tailroom=186
mac=(2,16) net=(18,-17) trans=1
shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
hash(0x0 sw=0 l4=0) proto=0x0000 pkttype=0 iif=0

Notice how you've changed shinfo(txflags), among other things.

Which proves that you can't just copy&paste whatever you found in
tag_trailer.c.

I am not yet sure whether there is any helper that can be used instead
of this crazy open-coding. Right now, not having tested anything yet, my
candidates of choice would be pskb_expand_head or __pskb_pull_tail. You
should probably also try to cater here for the potential reallocation
done in the skb_cow_head() of non-tail taggers. Which would lean the
balance towards pskb_expand_head(), I believe.

Also, if the result is going to be longer than ~20 lines of code, I
strongly suggest moving the reallocation to a separate function so you
don't clutter dsa_slave_xmit.

Also, please don't redeclare struct sk_buff *nskb, you don't need to.