netdev - Re: [PATCH net RFC] net: Clear IFF_TX_SKB_SHARING for all Ethernet devices using skb

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Message-ID: <20201023103110.3017f961@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net>
Date:   Fri, 23 Oct 2020 10:31:10 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xie He <xie.he.0141@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>
Subject: Re: [PATCH net RFC] net: Clear IFF_TX_SKB_SHARING for all Ethernet
 devices using skb_padto

On Thu, 22 Oct 2020 19:25:40 -0700 Xie He wrote:
> On Thu, Oct 22, 2020 at 6:56 PM Xie He <xie.he.0141@...il.com> wrote:
> > My patch isn't complete. Because there are so many drivers with this
> > problem, I feel it's hard to solve them all at once. So I only grepped
> > "skb_padto" under "drivers/net/ethernet". There are other drivers
> > under "ethernet" using "skb_pad", "skb_put_padto" or "eth_skb_pad".
> > There are also (fake) Ethernet drivers under "drivers/net/wireless". I
> > feel it'd take a long time and also be error-prone to solve them all,
> > so I feel it'd be the best if there are other solutions.  
> 
> BTW, I also see some Ethernet drivers calling skb_push to prepend
> strange headers to the skbs. For example,
> 
> drivers/net/ethernet/mellanox/mlxsw/switchx2.c prepends a header of
> MLXSW_TXHDR_LEN (16).
> 
> We can't send shared skbs to these drivers either because they modify the skbs.
> 
> It seems to me that many drivers have always assumed that they can
> modify the skb whenever needed. They've never considered there might
> be shared skbs. I guess adding IFF_TX_SKB_SHARING to ether_setup was a
> bad idea. It not only made the code less clean, but also didn't agree
> with the actual situations of the drivers.

Indeed. If we remove IFF_TX_SKB_SHARING from ether_setup we may need to
add the flag to the drivers that used to work, otherwise people using
pktgen will see a regression.