Subject: Re: [PATCH net-next v4 5/6] page_pool: add a lockdep check for recycling in hardirq

From: Alexander Lobakin <[email-masked]>
Date: Tue, 8 Aug 2023 17:06:05 +0200

From: Alexander Duyck <[email-masked]>
Date: Tue, 8 Aug 2023 07:52:32 -0700

> On Tue, Aug 8, 2023 at 6:59 AM Alexander Lobakin
> <[email-masked]> wrote:
>>
>> From: Alexander Duyck <[email-masked]>
>> Date: Tue, 8 Aug 2023 06:45:26 -0700

[...]

>>>>> Secondly rather than returning an error is there any reason why we
>>>>> couldn't just look at not returning page and instead just drop into the
>>>>> release path which wouldn't take the locks in the first place? Either
>>>>
>>>> That is exception path to quickly catch broken drivers and fix them, why
>>>> bother? It's not something we have to live with.
>>>
>>> My concern is that the current "fix" consists of stalling a Tx ring.
>>> We need to have a way to allow forward progress when somebody mixes
>>> xdp_frame and skb traffic as I suspect we will end up with a number of
>>> devices doing this since they cannot handle recycling the pages in
>>> hardirq context.
>>
>> You could've seen that several vendors already disabled recycling XDP
>> buffers when in hardirq (= netpoll) in their drivers. hardirq is in
>> general not for networking-related operations.
> 
> The whole idea behind the netpoll cleanup is to get the Tx buffers out
> of the way so that we can transmit even after the system has crashed.
> The idea isn't to transmit XDP buffers, but to get the buffers out of
> the way in the cases where somebody is combining both xdp_frame and
> sk_buff on the same queue due to a limited number of rings being
> present on the device.

I see now, thanks a lot!

> 
> My concern is that at some point in the near future somebody is going
> to have a system crash and instead of being able to get the crash log
> message out via their netconsole it is going to get cut off because
> the driver stopped cleaning the Tx ring because somebody was also
> using it as an XDP redirect destination.
> 
>>>
>>> The only reason why the skbs don't have the problem is that they are
>>> queued and then cleaned up in the net_tx_action. That is why I wonder
>>> if we shouldn't look at adding some sort of support for doing
>>> something like that with xdp_frame as well. Something like a
>>> dev_kfree_pp_page_any to go along with the dev_kfree_skb_any.
>>
>> I still don't get why we may need to clean XDP buffers in hardirq, maybe
>> someone could give me some links to read why we may need this and how
>> that happens? netpoll is a very specific thing for some debug
>> operations, isn't it? XDP shouldn't in general be enabled when this
>> happens, should it?
> 
> I think I kind of explained it above. It isn't so much about cleaning
> the XDP buffers as getting them off of the ring and out of the way. If
> we block a Tx queue because of an XDP buffer then we cannot use that
> Tx queue. I would be good with us just deferring the cleanup like we
> do with an sk_buff in dev_kfree_skb_irq, the only issue is we don't
> have the ability to put them on a queue since they don't have
> prev/next pointers.
> 
> I suppose an alternative to cleaning them might be to make a mandatory
> requirement that you cannot support netpoll and mix xdp_frame and
> sk_buff on the same queue. If we enforced that then my concern about
> them blocking a queue would be addressed.

I'm leaning more towards this one TBH. I don't feel sole netpoll as
a solid argument for introducing XDP frame deferred queues :s

> 
>> (unrelated: 6:58 AM West Coast, you use to wake up early or traveling?
>>  :D)
> 
> I am usually up pretty early, especially this time of year. Sunrise
> here is 6AM and I am usually up a little before that.. :)

Nice!

Thanks,
Olek