netdev - Re: [PATCH v5 bpf 1/2] xdp: introduce xdp_convert_skb_to

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Message-ID: <20251030082519.5db297f3@kernel.org>
Date: Thu, 30 Oct 2025 08:25:19 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: <bpf@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
 <hawk@...nel.org>, <netdev@...r.kernel.org>, <magnus.karlsson@...el.com>,
 <aleksander.lobakin@...el.com>, <ilias.apalodimas@...aro.org>,
 <toke@...hat.com>, <lorenzo@...nel.org>,
 <syzbot+ff145014d6b0ce64a173@...kaller.appspotmail.com>, Ihor Solodrai
 <ihor.solodrai@...ux.dev>, Octavian Purdila <tavip@...gle.com>
Subject: Re: [PATCH v5 bpf 1/2] xdp: introduce xdp_convert_skb_to_buff()

On Thu, 30 Oct 2025 13:14:16 +0100 Maciej Fijalkowski wrote:
> On Wed, Oct 29, 2025 at 04:50:20PM -0700, Jakub Kicinski wrote:
> > On Wed, 29 Oct 2025 23:13:14 +0100 Maciej Fijalkowski wrote:  
> > > +	xdp->rxq->mem.type = skb->pp_recycle ? MEM_TYPE_PAGE_POOL :
> > > +					       MEM_TYPE_PAGE_SHARED;  
> > 
> > You really need to stop sending patches before I had a chance 
> > to reply :/ And this is wrong.  
> 
> Why do you say so?
> 
> netif_receive_generic_xdp()
> 	netif_skb_check_for_xdp()
> 	skb_cow_data_for_xdp() failed
> 		go through skb linearize path
> 			returned skb data is backed by kmalloc, not page_pool,
> 			means mem type for this particular xdp_buff has to be
> 			MEM_TYPE_PAGE_SHARED
> 
> Are we on the same page now?

No, I think I already covered this, maybe you disagreed and I missed it.

The mem_type set here is expected to be used only for freeing pages. 
XDP can only free fagments (when pkt is trimmed), it cannot free the
head from under the skb. So only fragments matter here, we can ignore
the head.