Re: [PATCH v19 bpf-next 12/23] bpf: add multi-buff support to the bpf_xdp_adjust_tail() API

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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> Lorenzo Bianconi wrote:
> > > Lorenzo Bianconi wrote:
> > > > From: Eelco Chaudron <echaudro@redhat.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@kernel.org>
> > > > Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > > > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > > > Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> > > > ---
> 
> [...]
> 
> pasting full function here to help following along.
> 
> +
> +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)) {
> +			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);
> +			break;
> +		}
> +	}
> +	sinfo->nr_frags -= n_frags_free;
> +	sinfo->xdp_frags_size -= len_free;
> +
> +	if (unlikely(offset > 0)) {
> +		xdp_buff_clear_mb(xdp);
> +		xdp->data_end -= offset;
> +	}
> +
> +	return 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).
> 
> But still missing if we need to clear the mb bit or not when we shrink down
> to a single frag. I think its fine, but worth double checking. As an example
> consider I shrink 2k from a 3k pkt with two frags, one full 2k and another
> 1k extra,
> 
> On the first run through,
> 
>  i = 1;
>  offset = 2k
> 
> +	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);
> 
> shrink = 1k; // min_t(int, offset, size) -> size
> 
> +
> +		len_free += shrink;
> +		offset -= shrink;
> 
> offset = 1k
> 
> +		if (unlikely(size == shrink)) {
> +			struct page *page = skb_frag_page(frag);
> +
> +			__xdp_return(page_address(page), &xdp->rxq->mem,
> +				     false, NULL);
> +			n_frags_free++;
> 
> Will free the frag
> 
> Then next run through
> 
> i = 0;
> offset = 1k;
> 
> +		skb_frag_t *frag = &sinfo->frags[i];
> +		int size = skb_frag_size(frag);
> +		int shrink = min_t(int, offset, size);
> 
> shrink = 1k; // min_t(int, offset, size) -> offset
> 
> +
> +		len_free += shrink;
> +		offset -= shrink;
> 
> offset = 0k
> 
> +
> +		if (unlikely(size == shrink)) { ...
> +		} else {
> +			skb_frag_size_set(frag, size - shrink);
> +			break;
> +		}
> 
> Then later there is the check 'if (unlikely(offset > 0) { ...}', but that
> wont hit this case and we shrunk it back to a single frag. Did we want
> to clear the mb in this case? I'm not seeing how it harms things to have
> the mb bit set just trying to follow code here.

If I followed correctly your example, we will have sinfo->nr_frags = 1 at the
end of the processing (since the first fragment has 2k size), right?
If so mb bit must be set to 1. Am I missing something?
Re-looking at the code I guess we should clear mb bit using sinfo->nr_frags
instead:

	if (!sinfo->nr_frags)
		xdp_buff_clear_mb(xdp);

Agree?

Regards,
Lorenzo

> 
> Would offset > 0 indicate we weren't able to shrink the xdp buff enough
> for some reason. Need some coffee perhaps.
> 
> Thanks,
> John
> 

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