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Message-ID: <Ya4nI6DKPmGOpfMf@lore-desk>
Date: Mon, 6 Dec 2021 16:07:15 +0100
From: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
dsahern@...nel.org, brouer@...hat.com, echaudro@...hat.com,
jasowang@...hat.com, alexander.duyck@...il.com, saeed@...nel.org,
maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
tirthendu.sarkar@...el.com, toke@...hat.com
Subject: Re: [PATCH v19 bpf-next 12/23] bpf: add multi-buff support to the
bpf_xdp_adjust_tail() API
> Lorenzo Bianconi wrote:
> > From: Eelco Chaudron <echaudro@...hat.com>
> >
> > 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 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.
> > A XDP mb capable driver is expected to set frag_size in xdp_rxq_info data
> > structure to notify the XDP core the fragment size. frag_size set to 0 is
> > interpreted by the XDP core as tail growing is not allowed.
> > Introduce __xdp_rxq_info_reg utility routine to initialize frag_size field.
> >
> > 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: Jakub Kicinski <kuba@...nel.org>
> > Co-developed-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
> > ---
> > drivers/net/ethernet/marvell/mvneta.c | 3 +-
> > include/net/xdp.h | 16 ++++++-
> > net/core/filter.c | 67 +++++++++++++++++++++++++++
> > net/core/xdp.c | 12 +++--
> > 4 files changed, 90 insertions(+), 8 deletions(-)
>
> Some nits and one questiopn about offset > 0 on shrink.
Hi John,
thx for the review.
>
> > void xdp_rxq_info_unreg(struct xdp_rxq_info *xdp_rxq);
> > void xdp_rxq_info_unused(struct xdp_rxq_info *xdp_rxq);
> > bool xdp_rxq_info_is_reg(struct xdp_rxq_info *xdp_rxq);
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index b9bfe6fac6df..ace67957e685 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -3831,11 +3831,78 @@ static const struct bpf_func_proto bpf_xdp_adjust_head_proto = {
> > .arg2_type = ARG_ANYTHING,
> > };
> >
> > +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];
> > + struct xdp_rxq_info *rxq = xdp->rxq;
> > + int size, tailroom;
>
> These could be 'unsized int'.
ack, I will fix it.
>
> > +
> > + if (!rxq->frag_size || rxq->frag_size > xdp->frame_sz)
> > + return -EOPNOTSUPP;
> > +
> > + tailroom = rxq->frag_size - skb_frag_size(frag) - skb_frag_off(frag);
> > + if (unlikely(offset > tailroom))
> > + return -EINVAL;
> > +
> > + size = skb_frag_size(frag);
> > + memset(skb_frag_address(frag) + size, 0, offset);
> > + skb_frag_size_set(frag, size + offset);
>
> Could probably make this a helper skb_frag_grow() or something in
> skbuff.h we have sub, add, put_zero, etc. there.
I guess we can just use skb_frag_size_add() here.
>
> > + sinfo->xdp_frags_size += offset;
> > +
> > + return 0;
> > +}
> > +
> > +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;
> > +
> > + if (unlikely(offset > (int)xdp_get_buff_len(xdp) - 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)) {
>
> not so sure about the unlikely.
I will let Eelco comment on it since he is the author of the patch.
>
> > + struct page *page = skb_frag_page(frag);
> > +
> > + __xdp_return(page_address(page), &xdp->rxq->mem,
> > + false, NULL);
> > + n_frags_free++;
> > + } else {
> > + skb_frag_size_set(frag, size - shrink);
>
> skb_frag_size_sub() maybe, but you need to pull out size anyways
> so its not a big deal to me.
ack, I agree to use skb_frag_size_sub().
>
> > + break;
> > + }
> > + }
> > + sinfo->nr_frags -= n_frags_free;
> > + sinfo->xdp_frags_size -= len_free;
> > +
> > + if (unlikely(offset > 0)) {
>
> hmm whats the case for offset to != 0? Seems with initial unlikely
> check and shrinking while walking backwards through the frags it
> should be zero? Maybe a comment would help?
Looking at the code, offset can be > 0 here whenever we reduce the mb frame to
a legacy frame (so whenever offset will move the boundary into the linear
area).
Regards,
Lorenzo
>
> > + xdp_buff_clear_mb(xdp);
> > + xdp->data_end -= offset;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
> > {
> > void *data_hard_end = xdp_data_hard_end(xdp); /* use xdp->frame_sz */
> > void *data_end = xdp->data_end + offset;
> >
> > + if (unlikely(xdp_buff_is_mb(xdp))) { /* xdp multi-buffer */
> > + if (offset < 0)
> > + return bpf_xdp_mb_shrink_tail(xdp, -offset);
> > +
> > + return bpf_xdp_mb_increase_tail(xdp, offset);
> > + }
> > +
> > /* Notice that xdp_data_hard_end have reserved some tailroom */
> > if (unlikely(data_end > data_hard_end))
> > return -EINVAL;
>
> [...]
>
> Thanks,
> John
>
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