Re: [PATCH v3 bpf 2/4] xsk: fix usage of multi-buffer BPF helpers for ZC XDP

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Tue, Jan 02, 2024 at 02:58:00PM -0800, Martin KaFai Lau wrote:
> On 12/21/23 5:26 AM, Maciej Fijalkowski wrote:
> > This comes from __xdp_return() call with xdp_buff argument passed as
> > NULL which is supposed to be consumed by xsk_buff_free() call.
> > 
> > To address this properly, in ZC case, a node that represents the frag
> > being removed has to be pulled out of xskb_list. Introduce
> > appriopriate xsk helpers to do such node operation and use them
> > accordingly within bpf_xdp_adjust_tail().
> 
> [ ... ]
> 
> > +static inline struct xdp_buff *xsk_buff_get_tail(struct xdp_buff *first)
> > +{
> > +	struct xdp_buff_xsk *xskb = container_of(first, struct xdp_buff_xsk, xdp);
> > +	struct xdp_buff_xsk *frag;
> > +
> > +	frag = list_last_entry(&xskb->pool->xskb_list, struct xdp_buff_xsk,
> > +			       xskb_list_node);
> > +	return &frag->xdp;
> > +}
> > +
> 
> [ ... ]
> 
> > +static void __shrink_data(struct xdp_buff *xdp, struct xdp_mem_info *mem_info,
> > +			  skb_frag_t *frag, int shrink)
> > +{
> > +	if (mem_info->type == MEM_TYPE_XSK_BUFF_POOL) {
> > +		struct xdp_buff *tail = xsk_buff_get_tail(xdp);
> > +
> > +		if (tail)
> > +			tail->data_end -= shrink;
> > +	}
> > +	skb_frag_size_sub(frag, shrink);
> > +}
> > +
> > +static bool shrink_data(struct xdp_buff *xdp, skb_frag_t *frag, int shrink)
> > +{
> > +	struct xdp_mem_info *mem_info = &xdp->rxq->mem;
> > +
> > +	if (skb_frag_size(frag) == shrink) {
> > +		struct page *page = skb_frag_page(frag);
> > +		struct xdp_buff *zc_frag = NULL;
> > +
> > +		if (mem_info->type == MEM_TYPE_XSK_BUFF_POOL) {
> > +			zc_frag = xsk_buff_get_tail(xdp);
> > +
> > +			if (zc_frag) {
> 
> Based on the xsk_buff_get_tail(), would zc_frag ever be NULL?

Hey Martin thanks for taking a look, I had to do this in order to satisfy
!CONFIG_XDP_SOCKETS builds :/

> 
> > +				xdp_buff_clear_frags_flag(zc_frag);
> > +				xsk_buff_del_tail(zc_frag);
> > +			}
> > +		}
> > +
> > +		__xdp_return(page_address(page), mem_info, false, zc_frag);
> 
> and iiuc, this patch is fixing a bug when zc_frag is NULL and
> MEM_TYPE_XSK_BUFF_POOL.

Generally I don't see the need for xdp_return_buff() (which calls in the
end __xdp_return() being discussed) to handle MEM_TYPE_XSK_BUFF_POOL, this
could be refactored later and then probably this fix would look different,
but this is out of the scope now.

> 
> > +		return true;
> > +	}
> > +	__shrink_data(xdp, mem_info, frag, shrink);
> > +	return false;
> > +}
> > +
> 
>