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

Maciej Fijalkowski
2024-10-22T13:02:35Z

On Mon, Oct 21, 2024 at 04:10:30PM +0200, Alexander Lobakin wrote:
> From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> 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 <maciej.fijalkowski@intel.com>

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