Subject: Re: [PATCH v5 net-next 3/3] xdp: add multi-buff support for xdp running in generic mode

From: Lorenzo Bianconi <[email-masked]>
Date: Thu, 21 Dec 2023 09:23:14 +0100

> 
> 
> On 14/12/2023 15.29, Lorenzo Bianconi wrote:
> > Similar to native xdp, do not always linearize the skb in
> > netif_receive_generic_xdp routine but create a non-linear xdp_buff to be
> > processed by the eBPF program. This allow to add  multi-buffer support
> > for xdp running in generic mode.
> > 
> > Signed-off-by: Lorenzo Bianconi <[email-masked]>
> > ---
> >   net/core/dev.c | 153 +++++++++++++++++++++++++++++++++++++++++++------
> >   1 file changed, 134 insertions(+), 19 deletions(-)
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index d7857de03dba..47164acc3268 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4854,6 +4854,12 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> >   	xdp_init_buff(xdp, frame_sz, &rxqueue->xdp_rxq);
> >   	xdp_prepare_buff(xdp, hard_start, skb_headroom(skb) - mac_len,
> >   			 skb_headlen(skb) + mac_len, true);
> > +	if (skb_is_nonlinear(skb)) {
> > +		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
> > +		xdp_buff_set_frags_flag(xdp);
> > +	} else {
> > +		xdp_buff_clear_frags_flag(xdp);
> > +	}
> >   	orig_data_end = xdp->data_end;
> >   	orig_data = xdp->data;
> > @@ -4883,6 +4889,14 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> >   		skb->len += off; /* positive on grow, negative on shrink */
> >   	}
> > +	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> > +	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> > +	 */
> > +	if (xdp_buff_has_frags(xdp))
> > +		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > +	else
> > +		skb->data_len = 0;
> > +
> >   	/* check if XDP changed eth hdr such SKB needs update */
> >   	eth = (struct ethhdr *)xdp->data;
> >   	if ((orig_eth_type != eth->h_proto) ||
> > @@ -4916,12 +4930,118 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> >   	return act;
> >   }
> > +static int netif_skb_segment_for_xdp(struct sk_buff **pskb,
> 
> This function "...segment_for_xdp" always reallocate SKB and copies all
> bits over.
> Should it have been named "skb_realloc_for_xdp" ?

Hi Jesper,

ack, naming is always hard :)

> 
> I was really hopeing we can find a design to avoid doing this realloc.
> 
> If the BPF-prog doesn't write into any of the fragments, then we can
> avoid this realloc (+copy) dance. We designed XDP multi-buff to have
> exactly the same layout+location as SKB in skb_shared_info, exactly to
> avoid having to reallocated.

I 100% agree with you, but we will need a similar copy fallback approach
anyway, right? It is just a matter to understand if we should implement it
with page_pool or page_frag_cache (or something different).

> 
> More comments inline below...
> 
> > +				     struct bpf_prog *prog)
> > +{
> > +#if IS_ENABLED(CONFIG_PAGE_POOL)
> > +	struct softnet_data *sd = this_cpu_ptr(&softnet_data);
> > +	u32 size, truesize, len, max_head_size, off;
> > +	struct sk_buff *skb = *pskb, *nskb;
> > +	int err, i, head_off;
> > +	void *data;
> > +
> > +	/* XDP does not support fraglist so we need to linearize
> > +	 * the skb.
> > +	 */
> > +	if (skb_has_frag_list(skb) || !prog->aux->xdp_has_frags)
> > +		return -EOPNOTSUPP;
> > +
> > +	max_head_size = SKB_WITH_OVERHEAD(PAGE_SIZE - XDP_PACKET_HEADROOM);
> > +	if (skb->len > max_head_size + MAX_SKB_FRAGS * PAGE_SIZE)
> > +		return -ENOMEM;
> > +
> > +	size = min_t(u32, skb->len, max_head_size);
> > +	truesize = SKB_HEAD_ALIGN(size) + XDP_PACKET_HEADROOM;
> > +	data = page_pool_dev_alloc_va(sd->page_pool, &truesize);
> > +	if (!data)
> > +		return -ENOMEM;
> > +
> > +	nskb = napi_build_skb(data, truesize);
> > +	if (!nskb) {
> > +		page_pool_free_va(sd->page_pool, data, true);
> > +		return -ENOMEM;
> > +	}
> > +
> > +	skb_reserve(nskb, XDP_PACKET_HEADROOM);
> > +	skb_copy_header(nskb, skb);
> > +	skb_mark_for_recycle(nskb);
> > +
> > +	err = skb_copy_bits(skb, 0, nskb->data, size);
> 
> This will likely copy part of the "frags" into the SKB "head" area.
> 
> Especially for netstack generated TCP packets, this will change the
> segmentation layout significantly.  I wonder what (performance) effects
> this will have on further handling of these SKBs.

Do you think it can be a problem? I think in this way we can reduce the
number of allocated page. Moreover, what about the case when skb head is
bigger than a single page? Do you think we should use bigger allocation order?

> 
> 
> 
> > +	if (err) {
> > +		consume_skb(nskb);
> > +		return err;
> > +	}
> > +	skb_put(nskb, size);
> > +
> > +	head_off = skb_headroom(nskb) - skb_headroom(skb);
> > +	skb_headers_offset_update(nskb, head_off);
> > +
> > +	off = size;
> > +	len = skb->len - off;
> > +	for (i = 0; i < MAX_SKB_FRAGS && off < skb->len; i++) {
> > +		struct page *page;
> > +		u32 page_off;
> > +
> > +		size = min_t(u32, len, PAGE_SIZE);
> > +		truesize = size;
> > +
> > +		page = page_pool_dev_alloc(sd->page_pool, &page_off,
> > +					   &truesize);
> > +		if (!data) {
> > +			consume_skb(nskb);
> > +			return -ENOMEM;
> > +		}
> > +
> > +		skb_add_rx_frag(nskb, i, page, page_off, size, truesize);
> > +		err = skb_copy_bits(skb, off, page_address(page) + page_off,
> > +				    size);
> 
> I think it is correct, but we can easily endup with the new SKB (nskb)
> having a different nskb->nr_frags.

see above

> 
> 
> > +		if (err) {
> > +			consume_skb(nskb);
> > +			return err;
> > +		}
> > +
> > +		len -= size;
> > +		off += size;
> > +	}
> > +
> > +	consume_skb(skb);
> > +	*pskb = nskb;
> > +
> > +	return 0;
> > +#else
> > +	return -EOPNOTSUPP;
> > +#endif
> > +}
> > +
> > +static int netif_skb_check_for_xdp(struct sk_buff **pskb,
> > +				   struct bpf_prog *prog)
> > +{
> > +	struct sk_buff *skb = *pskb;
> > +	int err, hroom, troom;
> > +
> > +	if (!netif_skb_segment_for_xdp(pskb, prog))
> > +		return 0;
> 
> IMHO the code call logic, does not make it easy to add cases where we
> can avoid the realloc.  With this patch, it feels like the realloc+copy
> code path is the "main" code path for XDP-generic.

ack, I think it would depend about the logic to avoid realloc+copy. We could
revaluate it when we have this logic in place. What do you think?

> 
> Our goal should be to avoid realloc.
> 
> My goal for XDP multi-buff was/is that it can co-exist with GSO/GRO
> packets.  This patchset is a step in the direction of enabling GRO on
> devices with XDP (generic) loaded.  And I was really excited about this,
> but the overhead is going to be massive compared to normal GRO (without
> realloc+copy) that XDP end-users are going to be disappointed.
> 
> 
> > +
> > +	/* In case we have to go down the path and also linearize,
> > +	 * then lets do the pskb_expand_head() work just once here.
> > +	 */
> > +	hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
> > +	troom = skb->tail + skb->data_len - skb->end;
> > +	err = pskb_expand_head(skb,
> > +			       hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
> > +			       troom > 0 ? troom + 128 : 0, GFP_ATOMIC);
> > +	if (err)
> > +		return err;
> > +
> > +	return skb_linearize(skb);
> > +}
> > +
> >   static u32 netif_receive_generic_xdp(struct sk_buff **pskb,
> >   				     struct xdp_buff *xdp,
> >   				     struct bpf_prog *xdp_prog)
> >   {
> >   	struct sk_buff *skb = *pskb;
> > -	u32 act = XDP_DROP;
> > +	u32 mac_len, act = XDP_DROP;
> >   	/* Reinjected packets coming from act_mirred or similar should
> >   	 * not get XDP generic processing.
> > @@ -4929,41 +5049,36 @@ static u32 netif_receive_generic_xdp(struct sk_buff **pskb,
> >   	if (skb_is_redirected(skb))
> >   		return XDP_PASS;
> > -	/* XDP packets must be linear and must have sufficient headroom
> > -	 * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
> > -	 * native XDP provides, thus we need to do it here as well.
> > +	/* XDP packets must have sufficient headroom of XDP_PACKET_HEADROOM
> > +	 * bytes. This is the guarantee that also native XDP provides,
> > +	 * thus we need to do it here as well.
> 
> Some "native" XDP provider only have 192 bytes as HEADROOM and XDP code
> can this not being static (256 bytes).  So, perhaps it is time to allow
> XDP generic to only require 192 bytes?

ack, agree. I think this can be added a separated patch.

Regards,
Lorenzo

> 
> >   	 */
> > +	mac_len = skb->data - skb_mac_header(skb);
> > +	__skb_push(skb, mac_len);
> > +
> >   	if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
> >   	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> > -		int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
> > -		int troom = skb->tail + skb->data_len - skb->end;
> > -
> > -		/* In case we have to go down the path and also linearize,
> > -		 * then lets do the pskb_expand_head() work just once here.
> > -		 */
> > -		if (pskb_expand_head(skb,
> > -				     hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
> > -				     troom > 0 ? troom + 128 : 0, GFP_ATOMIC))
> > -			goto do_drop;
> > -		if (skb_linearize(skb))
> > +		if (netif_skb_check_for_xdp(pskb, xdp_prog))
> >   			goto do_drop;
> >   	}
> > -	act = bpf_prog_run_generic_xdp(skb, xdp, xdp_prog);
> > +	__skb_pull(*pskb, mac_len);
> > +
> > +	act = bpf_prog_run_generic_xdp(*pskb, xdp, xdp_prog);
> >   	switch (act) {
> >   	case XDP_REDIRECT:
> >   	case XDP_TX:
> >   	case XDP_PASS:
> >   		break;
> >   	default:
> > -		bpf_warn_invalid_xdp_action(skb->dev, xdp_prog, act);
> > +		bpf_warn_invalid_xdp_action((*pskb)->dev, xdp_prog, act);
> >   		fallthrough;
> >   	case XDP_ABORTED:
> > -		trace_xdp_exception(skb->dev, xdp_prog, act);
> > +		trace_xdp_exception((*pskb)->dev, xdp_prog, act);
> >   		fallthrough;
> >   	case XDP_DROP:
> >   	do_drop:
> > -		kfree_skb(skb);
> > +		kfree_skb(*pskb);
> >   		break;
> >   	}