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

From: Maciej Fijalkowski <[email-masked]>
Date: Tue, 22 Oct 2024 15:00:52 +0200

On Mon, Oct 21, 2024 at 04:10:30PM +0200, Alexander Lobakin wrote:
> From: Maciej Fijalkowski <[email-masked]>
> 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? :>

Right, although impact there was a bit bigger.
Typos are minor, so:

Reviewed-by: Maciej Fijalkowski <[email-masked]>

unless others have some opinion on union being introduced here.

> 
> > 
> >> +	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