Re: [PATCH RESEND bpf-next 14/15] net, xdp: allow metadata > 32

[Alexander Lobakin]

From: Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx>
Date: Tue, 16 May 2023 17:35:27 +0200

> 
> 
> On 16/05/2023 14.37, Alexander Lobakin wrote:
>> From: Larysa Zaremba<larysa.zaremba@xxxxxxxxx>
>> Date: Mon, 15 May 2023 19:08:39 +0200
>>
>>> On Mon, May 15, 2023 at 06:17:02PM +0200, Jesper Dangaard Brouer wrote:
>>>>
>>>> On 12/05/2023 17.26, Larysa Zaremba wrote:
>>>>> From: Aleksander Lobakin<aleksander.lobakin@xxxxxxxxx>
>>>>>
>>>>> When using XDP hints, metadata sometimes has to be much bigger
>>>>> than 32 bytes. Relax the restriction, allow metadata larger than 32
>>>>> bytes
>>>>> and make __skb_metadata_differs() work with bigger lengths.
>>>>>
>>>>> Now size of metadata is only limited by the fact it is stored as u8
>>>>> in skb_shared_info, so maximum possible value is 255.
>>>>
>>>> I'm confused, IIRC the metadata area isn't stored "in skb_shared_info".
>>>> The maximum possible size is limited by the XDP headroom, which is also
>>>> shared/limited with/by xdp_frame.  I must be reading the sentence
>>>> wrong,
>>>> somehow.
>>
>> skb_shared_info::meta_size  is u8. Since metadata gets carried from
>> xdp_buff to skb, this check is needed (it's compile-time constant
>> anyway).
>> Check for headroom is done separately already (two sentences below).
>>
> 
> Damn, argh, for SKBs the "meta_len" is stored in skb_shared_info, which
> is located on another cacheline.
> That is a sure way to KILL performance! :-(

Have you read the code? I use type_max(typeof_member(shinfo, meta_len)),
what performance are you talking about?

The whole xdp_metalen_invalid() gets expanded into:

	return (metalen % 4) || metalen > 255;

at compile-time. All those typeof shenanigans are only to not open-code
meta_len's type/size/max.

> 
> But only use for SKBs that gets created from xdp with metadata, right?
> 
> 
> 
>>> It's not 'metadata is stored as u8', it's 'metadata size is stored as
>>> u8' :)
>>> Maybe I should rephrase it better in v2.
> 
> Yes, a rephrase will be good.
> 
> --Jesper
> 
> 
> 
> static inline u8 skb_metadata_len(const struct sk_buff *skb)
> {
>     return skb_shinfo(skb)->meta_len;
> }
> 

Thanks,
Olek