Subject: Re: [PATCH net v4 2/2] xsk: Fix zero-copy AF_XDP fragment drop

From: Jakub Kicinski <[email-masked]>
Date: Thu, 19 Feb 2026 14:55:29 -0800

On Tue, 17 Feb 2026 21:08:51 +0000 Nikhil P. Rao wrote:
> AF_XDP should ensure that only a complete packet is sent to application.
> In the zero-copy case, if the Rx queue gets full as fragments are being
> enqueued, the remaining fragments are dropped.
> 
> For the multi-buffer case, add a check to ensure that the Rx queue has
> enough space for all fragments of a packet before starting to enqueue
> them.
> 
> Fixes: 24ea50127ecf ("xsk: support mbuf on ZC RX")
> Acked-by: Maciej Fijalkowski <[email-masked]>
> Signed-off-by: Nikhil P. Rao <[email-masked]>
> ---
>  net/xdp/xsk.c | 23 +++++++++++++++--------
>  1 file changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index f2ec4f78bbb6..f7f816a5cb80 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -167,25 +167,32 @@ static int xsk_rcv_zc(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
>  	struct xdp_buff_xsk *pos, *tmp;
>  	struct list_head *xskb_list;
>  	u32 contd = 0;
> +	u32 num_desc;
>  	int err;
>  
> -	if (frags)
> +	if (frags) {
> +		num_desc = xdp_get_shared_info_from_buff(xdp)->nr_frags + 1;
>  		contd = XDP_PKT_CONTD;

[1]

> +	} else {
> +		err = __xsk_rcv_zc(xs, xskb, len, contd);
> +		if (err)
> +			goto err;
> +		return 0;
> +	}
>  
> -	err = __xsk_rcv_zc(xs, xskb, len, contd);
> -	if (err)
> +	if (xskq_prod_nb_free(xs->rx, num_desc) < num_desc) {

We can pull this check into the branch at [1]
It will let us preserve the existing flow.

Either that or handle the non-frag case fully upfront:

if (likely(!frags)) {
	err = __xsk_rcv_zc(xs, xskb, len, 0);
	if (err)
		goto err;
	return 0;
}

As is you have a weird mix of the two.

> +		xs->rx_queue_full++;
> +		err = -ENOBUFS;
>  		goto err;
> -	if (likely(!frags))
> -		return 0;
> +	}
>  
> +	__xsk_rcv_zc(xs, xskb, len, contd);

Personal preference perhaps but removing error checking always
gives me pause. Maybe:

	bool frag_fail;

	frag_fail = __xsk_rcv_zc(xs, xskb, len, contd);
	list_for_each...
		...
		frag_fail |= __xsk_rcv_zc(xs, xskb, len, contd);
	DEBUG_NET_WARN_ON_ONCE(frag_fail);

?

>  	xskb_list = &xskb->pool->xskb_list;
>  	list_for_each_entry_safe(pos, tmp, xskb_list, list_node) {
>  		if (list_is_singular(xskb_list))
>  			contd = 0;
>  		len = pos->xdp.data_end - pos->xdp.data;
> -		err = __xsk_rcv_zc(xs, pos, len, contd);
> -		if (err)
> -			goto err;
> +		__xsk_rcv_zc(xs, pos, len, contd);
>  		list_del_init(&pos->list_node);
>  	}
>  
-- 
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