> On Thu, 4 Nov 2021 18:35:32 +0100 Lorenzo Bianconi wrote:
> > This change adds support for tail growing and shrinking for XDP multi-buff.
> >
> > When called on a multi-buffer packet with a grow request, it will always
> > work on the last fragment of the packet. So the maximum grow size is the
> > last fragments tailroom, i.e. no new buffer will be allocated.
> >
> > When shrinking, it will work from the last fragment, all the way down to
> > the base buffer depending on the shrinking size. It's important to mention
> > that once you shrink down the fragment(s) are freed, so you can not grow
> > again to the original size.
>
> > +static int bpf_xdp_mb_increase_tail(struct xdp_buff *xdp, int offset)
> > +{
> > + struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> > + skb_frag_t *frag = &sinfo->frags[sinfo->nr_frags - 1];
> > + int size, tailroom;
> > +
> > + tailroom = xdp->frame_sz - skb_frag_size(frag) - skb_frag_off(frag);
>
> I know I complained about this before but the assumption that we can
> use all the space up to xdp->frame_sz makes me uneasy.
>
> Drivers may not expect the idea that core may decide to extend the
> last frag.. I don't think the skb path would ever do this.
>
> How do you feel about any of these options:
> - dropping this part for now (return an error for increase)
> - making this an rxq flag or reading the "reserved frag size"
> from rxq (so that drivers explicitly opt-in)
> - adding a test that can be run on real NICs
> ?
I think this has been added to be symmetric with bpf_xdp_adjust_tail().
I do think there is a real use-case for it so far so I am fine to just
support the shrink part.
@Eelco, Jesper, Toke: any comments on it?
>
> > +static int bpf_xdp_mb_shrink_tail(struct xdp_buff *xdp, int offset)
> > +{
> > + struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> > + int i, n_frags_free = 0, len_free = 0, tlen_free = 0;
> > +
> > + if (unlikely(offset > ((int)xdp_get_buff_len(xdp) - ETH_HLEN)))
>
> nit: outer parens unnecessary
ack, I will fix it.
>
> > + return -EINVAL;
>
>
> > @@ -371,6 +371,7 @@ static void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct,
> > break;
> > }
> > }
> > +EXPORT_SYMBOL_GPL(__xdp_return);
>
> Why the export?
ack, I will remove it
Regards,
Lorenzo
Attachment:
signature.asc
Description: PGP signature
