On Sep 16, Jakub Kicinski wrote:
> On Fri, 10 Sep 2021 18:14:16 +0200 Lorenzo Bianconi wrote:
> > From: Eelco Chaudron <echaudro@xxxxxxxxxx>
> >
> > 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.
> >
> > Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
> > Co-developed-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> > Signed-off-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> > Signed-off-by: Eelco Chaudron <echaudro@xxxxxxxxxx>
>
> > +static inline unsigned int xdp_get_frag_tailroom(const skb_frag_t *frag)
> > +{
> > + struct page *page = skb_frag_page(frag);
> > +
> > + return page_size(page) - skb_frag_size(frag) - skb_frag_off(frag);
> > +}
>
> How do we deal with NICs which can pack multiple skbs into a page frag?
> skb_shared_info field to mark the end of last fragment? Just want to make
> sure there is a path to supporting such designs.
I guess here, intead of using page_size(page) we can rely on xdp_buff->frame_sz
or even on skb_shared_info()->xdp_frag_truesize (assuming all fragments from a
given hw have the same truesize, but I think this is something we can rely on)
static inline unsigned int xdp_get_frag_tailroom(struct xdp_buff *xdp,
const skb_frag_t *frag)
{
return xdp->frame_sz - skb_frag_size(frag) - skb_frag_off(frag);
}
what do you think?
>
> > +static int bpf_xdp_mb_adjust_tail(struct xdp_buff *xdp, int offset)
> > +{
> > + struct skb_shared_info *sinfo;
> > +
> > + sinfo = xdp_get_shared_info_from_buff(xdp);
> > + if (offset >= 0) {
> > + skb_frag_t *frag = &sinfo->frags[sinfo->nr_frags - 1];
> > + int size;
> > +
> > + if (unlikely(offset > xdp_get_frag_tailroom(frag)))
> > + return -EINVAL;
> > +
> > + size = skb_frag_size(frag);
> > + memset(skb_frag_address(frag) + size, 0, offset);
> > + skb_frag_size_set(frag, size + offset);
> > + sinfo->xdp_frags_size += offset;
> > + } else {
> > + int i, n_frags_free = 0, len_free = 0, tlen_free = 0;
> > +
> > + offset = abs(offset);
> > + if (unlikely(offset > ((int)(xdp->data_end - xdp->data) +
> > + sinfo->xdp_frags_size - ETH_HLEN)))
> > + return -EINVAL;
> > +
> > + for (i = sinfo->nr_frags - 1; i >= 0 && offset > 0; i--) {
> > + skb_frag_t *frag = &sinfo->frags[i];
> > + int size = skb_frag_size(frag);
> > + int shrink = min_t(int, offset, size);
> > +
> > + len_free += shrink;
> > + offset -= shrink;
> > +
> > + if (unlikely(size == shrink)) {
> > + struct page *page = skb_frag_page(frag);
> > +
> > + __xdp_return(page_address(page), &xdp->rxq->mem,
> > + false, NULL);
> > + tlen_free += page_size(page);
> > + n_frags_free++;
> > + } else {
> > + skb_frag_size_set(frag, size - shrink);
> > + break;
> > + }
> > + }
> > + sinfo->nr_frags -= n_frags_free;
> > + sinfo->xdp_frags_size -= len_free;
> > + sinfo->xdp_frags_truesize -= tlen_free;
> > +
> > + if (unlikely(offset > 0)) {
> > + xdp_buff_clear_mb(xdp);
> > + xdp->data_end -= offset;
> > + }
> > + }
> > +
> > + return 0;
> > +}
>
> nit: most of this function is indented, situation is ripe for splitting
> it into two
sure, I will fix it.
Regards,
Lorenzo
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