netdev - Re: [PATCH bpf-next 1/3] xsk: avoid overwriting skb fields for multi-buffer traffic

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Message-ID: <aNLAJJoWLibivhXR@boxer>
Date: Tue, 23 Sep 2025 17:43:32 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Stanislav Fomichev <stfomichev@...il.com>
CC: <bpf@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
	<andrii@...nel.org>, <netdev@...r.kernel.org>, <magnus.karlsson@...el.com>,
	<kerneljasonxing@...il.com>
Subject: Re: [PATCH bpf-next 1/3] xsk: avoid overwriting skb fields for
 multi-buffer traffic

On Mon, Sep 22, 2025 at 10:25:32AM -0700, Stanislav Fomichev wrote:
> On 09/22, Maciej Fijalkowski wrote:
> > We are unnecessarily setting a bunch of skb fields per each processed
> > descriptor, which is redundant for fragmented frames.
> > 
> > Let us set these respective members for first fragment only.
> > 
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> > ---
> >  net/xdp/xsk.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > index 72e34bd2d925..72194f0a3fc0 100644
> > --- a/net/xdp/xsk.c
> > +++ b/net/xdp/xsk.c
> > @@ -758,6 +758,10 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> >  				goto free_err;
> >  
> >  			xsk_set_destructor_arg(skb, desc->addr);
> > +			skb->dev = dev;
> > +			skb->priority = READ_ONCE(xs->sk.sk_priority);
> > +			skb->mark = READ_ONCE(xs->sk.sk_mark);
> > +			skb->destructor = xsk_destruct_skb;
> >  		} else {
> >  			int nr_frags = skb_shinfo(skb)->nr_frags;
> >  			struct xsk_addr_node *xsk_addr;
> > @@ -826,14 +830,10 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> >  
> >  			if (meta->flags & XDP_TXMD_FLAGS_LAUNCH_TIME)
> >  				skb->skb_mstamp_ns = meta->request.launch_time;
> > +			xsk_tx_metadata_to_compl(meta, &skb_shinfo(skb)->xsk_meta);
> >  		}
> >  	}
> >  
> > -	skb->dev = dev;
> > -	skb->priority = READ_ONCE(xs->sk.sk_priority);
> > -	skb->mark = READ_ONCE(xs->sk.sk_mark);
> > -	skb->destructor = xsk_destruct_skb;
> > -	xsk_tx_metadata_to_compl(meta, &skb_shinfo(skb)->xsk_meta);
> >  	xsk_inc_num_desc(skb);
> 
> What about IFF_TX_SKB_NO_LINEAR case? I'm not super familiar with
> it, but I don't see priority/mark being set over there after this change.

The thing is I tricked myself with running xskxceiver against the changes
and not seeing issues :< so IFF_TX_SKB_NO_LINEAR case needs a test
coverage pretty badly I'd say...