netdev - Re: [PATCH 2/2 bpf v2] xsk: avoid data corruption on cq descriptor number

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Message-ID: <b21cf80c-5d69-4914-aa45-00f9527f3436@suse.de>
Date: Wed, 29 Oct 2025 08:51:58 +0100
From: Fernando Fernandez Mancera <fmancera@...e.de>
To: Jakub Kicinski <kuba@...nel.org>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, magnus.karlsson@...el.com,
 maciej.fijalkowski@...el.com, sdf@...ichev.me, kerneljasonxing@...il.com,
 fw@...len.de
Subject: Re: [PATCH 2/2 bpf v2] xsk: avoid data corruption on cq descriptor
 number

On 10/29/25 12:01 AM, Jakub Kicinski wrote:
> On Tue, 28 Oct 2025 19:30:32 +0100 Fernando Fernandez Mancera wrote:
>> Since commit 30f241fcf52a ("xsk: Fix immature cq descriptor
>> production"), the descriptor number is stored in skb control block and
>> xsk_cq_submit_addr_locked() relies on it to put the umem addrs onto
>> pool's completion queue.
> 
> Looking at the past discussion it sounds like you want to optimize
> the single descriptor case? Can you not use a magic pointer for that?
> 
> 	#define XSK_DESTRUCT_SINGLE_BUF	(void *)1
> 	destructor_arg = XSK_DESTRUCT_SINGLE_BUF
> 
> Let's target this fix at net, please, I think the complexity here is
> all in skbs paths.

I might be missing something here but if the destructor_arg pointer is 
used to do this, where should we store the umem address associated with 
it? In the proposed approach the skb extension should not be increased 
for non-fragmented traffic as there is only a single descriptor and 
therefore we can store the umem address in destructor_arg directly.

The size of the skb extension will only increase for fragmented traffic 
(multiple descriptors).. but sure, if there is a fallback to the 
slowpath, it will burden a bit the performance. Although, for that to 
happen the must have tried to use AF_XDP family initially.. AFAICS, the 
size of skb extension is only increased when skb_ext_add() is called.

Thanks,
Fernando.