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

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Message-ID: <b7974772-2e34-44df-924f-702e96ac20d3@suse.de>
Date: Thu, 30 Oct 2025 09:38:02 +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 10/30/25 12:22 AM, Jakub Kicinski wrote:
> On Wed, 29 Oct 2025 08:51:58 +0100 Fernando Fernandez Mancera wrote:
>> 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.
> 
> I see. Pointers are always aligned to 8B, you can stash the "pointer
> type" there. If the bottom bit is 1 it's a umem and the skb was
> single-chunk. If it's non-0 then it's a full kmalloc'ed struct.
> 

That is a good point. Pointer tagging might be a good solution here. 
Thanks, let me try that.

>> 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.
> 
> To be clear by adding an skb extension you are de-facto allocating
> a bit in the skb struct. Just one of the bits of the active_extensions
> field instead of a separate bitfield. If you can depend on the socket
> association instead this is quite wasteful.
>