netdev - Re: [PATCH v3 bpf-next] net: veth: alloc skb in bulk for ndo_xdp

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Date:   Thu, 4 Feb 2021 01:14:56 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, toshiaki.makita1@...il.com,
        lorenzo.bianconi@...hat.com, brouer@...hat.com, toke@...hat.com
Subject: Re: [PATCH v3 bpf-next] net: veth: alloc skb in bulk for ndo_xdp_xmit

On 1/29/21 11:04 PM, Lorenzo Bianconi wrote:
> Split ndo_xdp_xmit and ndo_start_xmit use cases in veth_xdp_rcv routine
> in order to alloc skbs in bulk for XDP_PASS verdict.
> Introduce xdp_alloc_skb_bulk utility routine to alloc skb bulk list.
> The proposed approach has been tested in the following scenario:
[...]
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 0d2630a35c3e..05354976c1fc 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -514,6 +514,17 @@ void xdp_warn(const char *msg, const char *func, const int line)
>   };
>   EXPORT_SYMBOL_GPL(xdp_warn);
>   
> +int xdp_alloc_skb_bulk(void **skbs, int n_skb, gfp_t gfp)
> +{
> +	n_skb = kmem_cache_alloc_bulk(skbuff_head_cache, gfp,
> +				      n_skb, skbs);

Applied, but one question I was wondering about when reading the kmem_cache_alloc_bulk()
code was whether it would be safer to simply test for kmem_cache_alloc_bulk() != n_skb
given it could potentially in future also alloc less objs than requested, but I presume
if such extension would get implemented then call-sites might need to indicate 'best
effort' somehow via flag instead (to handle < n_skb case). Either way all current callers
assume for != 0 that everything went well, so lgtm.

> +	if (unlikely(!n_skb))
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(xdp_alloc_skb_bulk);
> +
>   struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>   					   struct sk_buff *skb,
>   					   struct net_device *dev)
>