Re: [PATCH net-next v2 11/18] xdp: add generic xdp_buff_add_frag()

[Alexander Lobakin]

From: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>
Date: Thu, 17 Oct 2024 14:26:48 +0200

> On Tue, Oct 15, 2024 at 04:53:43PM +0200, Alexander Lobakin wrote:
>> The code piece which would attach a frag to &xdp_buff is almost
>> identical across the drivers supporting XDP multi-buffer on Rx.
>> Make it a generic elegant onelner.
> 
> oneliner
> 
>> Also, I see lots of drivers calculating frags_truesize as
>> `xdp->frame_sz * nr_frags`. I can't say this is fully correct, since
>> frags might be backed by chunks of different sizes, especially with
>> stuff like the header split. Even page_pool_alloc() can give you two
>> different truesizes on two subsequent requests to allocate the same
>> buffer size. Add a field to &skb_shared_info (unionized as there's no
>> free slot currently on x6_64) to track the "true" truesize. It can be
> 
> x86_64

What a shame from these two typos >_<

> 
>> used later when updating an skb.

[...]

>> +
>> +	prev = &sinfo->frags[nr_frags - 1];
>> +	if (try_coalesce && page == skb_frag_page(prev) &&
>> +	    offset == skb_frag_off(prev) + skb_frag_size(prev))
>> +		skb_frag_size_add(prev, size);
>> +	else
>> +fill:
>> +		__skb_fill_page_desc_noacc(sinfo, nr_frags++, page,
>> +					   offset, size);
>> +
>> +	sinfo->nr_frags = nr_frags;
> 
> is it really necessary to work on local nr_frags instead of directly
> update it from sinfo?

I think you remember the difference when you started to work on ntu and
ntc locally instead of accessing the ring struct all the time? :>

> 
>> +	sinfo->xdp_frags_size += size;
>> +	sinfo->xdp_frags_truesize += truesize;
>> +
>> +	return true;
>> +}

[...]

>> @@ -230,7 +312,13 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
>>  			   unsigned int size, unsigned int truesize,
>>  			   bool pfmemalloc)
>>  {
>> -	skb_shinfo(skb)->nr_frags = nr_frags;
>> +	struct skb_shared_info *sinfo = skb_shinfo(skb);
>> +
>> +	sinfo->nr_frags = nr_frags;
>> +	/* ``destructor_arg`` is unionized with ``xdp_frags_{,true}size``,
>> +	 * reset it after that these fields aren't used anymore.
>> +	 */
>> +	sinfo->destructor_arg = NULL;
> 
> wouldn't clearing size and truesize from union be more obvious?

But here we actually need to reset the destructor arg pointer.
size/truesize are not needed at this point anymore, but the arg can be
used/tested later, so I thought clearing it here is more clear to the
readers?

> OTOH it's one write vs two :)

Sometimes the compiler can optimize two subsequent writes (e.g. to addr
and addr + 4) into one bigger, but I wouldn't rely on it (that's why in
patch #18 I intensively use casts to u64).

> 
>>  
>>  	skb->len += size;
>>  	skb->data_len += size;
>> -- 
>> 2.46.2

Thanks,
Olek