Re: [PATCH net-next] gso: Support frag_list splitting with head_frag

[David Miller <davem@davemloft.net>]

From: <ilant@mellanox.com>
Date: Sat, 8 Apr 2017 02:07:08 +0300

> From: Ilan Tayari <ilant@mellanox.com>
> 
> A driver may use build_skb() for received packets.
> These SKBs then have a head_frag.
> 
> Since commit d7e8883cfcf4 ("net: make GRO aware of
> skb->head_frag"), GRO may build frag_list SKBs out of
> head_frag received SKBs.
> In such a case, the chained SKBs end up with a head_frag.
> 
> Commit 07b26c9454a2 ("gso: Support partial splitting at
> the frag_list pointer") adds partial segmentation of frag_list
> SKB chains into individual SKBs.
> However, this is not done if the chained SKBs have any
> linear part, because the device may not be able to DMA
> the private linear buffer.
> 
> A chained frag_list SKB with head_frag is wrongfully
> detected in this case as having a private linear part
> and thus falls back to software GSO, while in fact the
> linear part is backed by a DMA page just like any other frag.
> 
> This causes low performance when forwarding those packets
> that were built with build_skb()
> 
> Allow partial segmentation at the frag_list pointer for
> chained SKBs with head_frag.
> 
> Note that such SKBs can only be created by GRO, when applied
> to received packets with head_frag.
> Also note that this change only affects the data path that
> performs the partial segmentation at frag_list pointer, and
> not any of the other more common data paths.
> 
> Signed-off-by: Ilan Tayari <ilant@mellanox.com>

Applied, thank you for the detailed commit message.