Messages in this thread | | | From | Xie He <> | Date | Thu, 22 Oct 2020 12:59:45 -0700 | Subject | Re: [PATCH net RFC] net: Clear IFF_TX_SKB_SHARING for all Ethernet devices using skb_padto |
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On Thu, Oct 22, 2020 at 8:22 AM Jakub Kicinski <kuba@kernel.org> wrote: > > Are most of these drivers using skb_padto()? Is that the reason they > can't be sharing the SKB?
Yes, I think if a driver calls skb_pad / skb_padto / skb_put_padto / eth_skb_pad, the driver can't accept shared skbs because it may modify the skbs.
> I think the IFF_TX_SKB_SHARING flag is only used by pktgen, so perhaps > we can make sure pktgen doesn't generate skbs < dev->min_mtu, and then > the drivers won't pad?
Yes, I see a lot of drivers just want to pad the skb to ETH_ZLEN, or just call eth_skb_pad. In this case, requiring the shared skb to be at least dev->min_mtu long can solve the problem for these drivers.
But I also see some drivers that want to pad the skb to a strange length, and don't set their special min_mtu to match this length. For example:
drivers/net/ethernet/packetengines/yellowfin.c wants to pad the skb to a dynamically calculated value.
drivers/net/ethernet/ti/cpsw.c, cpsw_new.c and tlan.c want to pad the skb to macro defined values.
drivers/net/ethernet/intel/iavf/iavf_txrx.c wants to pad the skb to IAVF_MIN_TX_LEN (17).
drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c wants to pad the skb to 17.
Another solution I can think of is to add a "skb_shared" check to "__skb_pad", so that if __skb_pad encounters a shared skb, it just returns an error. The driver would think this is a memory allocation failure. This way we can ensure shared skbs are not modified.
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