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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