netdev - Re: [PATCH v4 bpf-next 2/3] veth: rework veth_xdp_rcv

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Date:   Thu, 10 Mar 2022 12:43:14 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Toke Høiland-Jørgensen <toke@...hat.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, brouer@...hat.com,
        pabeni@...hat.com, echaudro@...hat.com, toshiaki.makita1@...il.com,
        andrii@...nel.org
Subject: Re: [PATCH v4 bpf-next 2/3] veth: rework veth_xdp_rcv_skb in order
 to accept non-linear skb

> Lorenzo Bianconi <lorenzo@...nel.org> writes:
> 
> > Introduce veth_convert_xdp_buff_from_skb routine in order to
> > convert a non-linear skb into a xdp buffer. If the received skb
> > is cloned or shared, veth_convert_xdp_buff_from_skb will copy it
> > in a new skb composed by order-0 pages for the linear and the
> > fragmented area. Moreover veth_convert_xdp_buff_from_skb guarantees
> > we have enough headroom for xdp.
> > This is a preliminary patch to allow attaching xdp programs with frags
> > support on veth devices.
> >
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> 
> It's cool that we can do this! A few comments below:

Hi Toke,

thx for the review :)

[...]

> > +static int veth_convert_xdp_buff_from_skb(struct veth_rq *rq,
> > +					  struct xdp_buff *xdp,
> > +					  struct sk_buff **pskb)
> > +{
> 
> nit: It's not really "converting" and skb into an xdp_buff, since the
> xdp_buff lives on the stack; so maybe 'veth_init_xdp_buff_from_skb()'?

I kept the previous naming convention used for xdp_convert_frame_to_buff()
(my goal would be to move it in xdp.c and reuse this routine for the
generic-xdp use case) but I am fine with veth_init_xdp_buff_from_skb().

> 
> > +	struct sk_buff *skb = *pskb;
> > +	u32 frame_sz;
> >  
> >  	if (skb_shared(skb) || skb_head_is_locked(skb) ||
> > -	    skb_is_nonlinear(skb) || headroom < XDP_PACKET_HEADROOM) {
> > +	    skb_shinfo(skb)->nr_frags) {
> 
> So this always clones the skb if it has frags? Is that really needed?

if we look at skb_cow_data(), paged area is always considered not writable

> 
> Also, there's a lot of memory allocation and copying going on here; have
> you measured the performance?

even in the previous implementation we always reallocate the skb if the
conditions above are verified so I do not expect any difference in the single
buffer use-case but I will run some performance tests.

> 

[...]

> > +
> > +	if (xdp_buff_has_frags(&xdp))
> > +		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > +	else
> > +		skb->data_len = 0;
> 
> We can remove entire frags using xdp_adjust_tail, right? Will that get
> propagated in the right way to the skb frags due to the dual use of
> skb_shared_info, or?

bpf_xdp_frags_shrink_tail() can remove entire frags and it will modify metadata
contained in the skb_shared_info (e.g. nr_frags or the frag size of
the given page). We should consider the data_len field in this case. Agree?

Regards,
Lorenzo

> 

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