Re: [PATCH v4 net-next 1/9] i40e: don't reserve excessive XDP_PACKET_HEADROOM on XSK Rx to skb

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 08/12/2021 15.06, Alexander Lobakin wrote:
> {__,}napi_alloc_skb() allocates and reserves additional NET_SKB_PAD
> + NET_IP_ALIGN for any skb.
> OTOH, i40e_construct_skb_zc() currently allocates and reserves
> additional `xdp->data - xdp->data_hard_start`, which is
> XDP_PACKET_HEADROOM for XSK frames.
> There's no need for that at all as the frame is post-XDP and will
> go only to the networking stack core.

I disagree with this assumption, that headroom is not needed by netstack.
Why "no need for that at all" for netstack?

Having headroom is important for netstack in general.  When packet will 
grow we avoid realloc of SKB.  Use-case could also be cpumap or veth 
redirect, or XDP-generic, that expect this headroom.


> Pass the size of the actual data only to __napi_alloc_skb() and
> don't reserve anything. This will give enough headroom for stack
> processing.
> 
> Fixes: 0a714186d3c0 ("i40e: add AF_XDP zero-copy Rx support")
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
> ---
>   drivers/net/ethernet/intel/i40e/i40e_xsk.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> index f08d19b8c554..9564906b7da8 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> @@ -245,13 +245,11 @@ static struct sk_buff *i40e_construct_skb_zc(struct i40e_ring *rx_ring,
>   	struct sk_buff *skb;
>   
>   	/* allocate a skb to store the frags */
> -	skb = __napi_alloc_skb(&rx_ring->q_vector->napi,
> -			       xdp->data_end - xdp->data_hard_start,
> +	skb = __napi_alloc_skb(&rx_ring->q_vector->napi, datasize,
>   			       GFP_ATOMIC | __GFP_NOWARN);
>   	if (unlikely(!skb))
>   		goto out;
>   
> -	skb_reserve(skb, xdp->data - xdp->data_hard_start);
>   	memcpy(__skb_put(skb, datasize), xdp->data, datasize);
>   	if (metasize)
>   		skb_metadata_set(skb, metasize);
>