Re: [PATCH net 2/2] page_pool: fix IOMMU crash when driver has already unbound

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/9/20 13:29, Ilias Apalodimas wrote:
> Hi Jesper,
> 
> On Fri, 20 Sept 2024 at 00:04, Jesper Dangaard Brouer <hawk@kernel.org> wrote:
>>
>>
>>
>> On 19/09/2024 13.15, Yunsheng Lin wrote:
>>> On 2024/9/19 17:42, Jesper Dangaard Brouer wrote:
>>>>
>>>> On 18/09/2024 19.06, Ilias Apalodimas wrote:
>>>>>> In order not to do the dma unmmapping after driver has already
>>>>>> unbound and stall the unloading of the networking driver, add
>>>>>> the pool->items array to record all the pages including the ones
>>>>>> which are handed over to network stack, so the page_pool can
>>>>>> do the dma unmmapping for those pages when page_pool_destroy()
>>>>>> is called.
>>>>>
>>>>> So, I was thinking of a very similar idea. But what do you mean by
>>>>> "all"? The pages that are still in caches (slow or fast) of the pool
>>>>> will be unmapped during page_pool_destroy().
>>>>
>>>> I really dislike this idea of having to keep track of all outstanding pages.
>>>>
>>>> I liked Jakub's idea of keeping the netdev around for longer.
>>>>
>>>> This is all related to destroying the struct device that have points to
>>>> the DMA engine, right?
>>>
>>> Yes, the problem seems to be that when device_del() is called, there is
>>> no guarantee hw behind the 'struct device ' will be usable even if we
>>> call get_device() on it.
>>>
>>>>
>>>> Why don't we add an API that allow netdev to "give" struct device to
>>>> page_pool.  And then the page_poll will take over when we can safely
>>>> free the stuct device?
>>>
>>> By 'allow netdev to "give" struct device to page_pool', does it mean
>>> page_pool become the driver for the device?
>>> If yes, it seems that is similar to jakub's idea, as both seems to stall
>>> the calling of device_del() by not returning when the driver unloading.
>>
>> Yes, this is what I mean. (That is why I mentioned Jakub's idea).

I am not sure what dose the API that allows netdev to "give" struct device
to page_pool look like or how to implement the API yet, but the obvious way
to stall the calling of device_del() is to wait for the inflight page to
come back in page_pool_destroy(), which seems the same as the jakub's
way from the viewpoint of user, and jakub's way seems more elegant than
waiting in page_pool_destroy().

> 
> Keeping track of inflight packets that need to be unmapped is
> certainly more complex. Delaying the netdevice destruction certainly
> solves the problem but there's a huge cost IMHO. Those devices might
> stay there forever and we have zero guarantees that the network stack
> will eventually release (and unmap) those packets. What happens in
> that case? The user basically has to reboot the entire machine, just
> because he tries to bring an interface down and up again.

Yes.
The problem seems to be how long page_pool is allowed to stall the driver
unloading? Does the driver unload stalling affect some feature like device
hotplug?
As the problem in [1], the stall might be forever due to caching in the
network stack as discussed in [2], and there might be some other caching
we don't know yet.

The stalling log in [1] is caused by the caching in skb_attempt_defer_free(),
we may argue that a timeout is needed for those kind of caching, but Eric
seemed to think otherwise in commit log of [3]:
"As Eric pointed out/predicted there's no guarantee that
applications will read / close their sockets so a page pool page
may be stuck in a socket (but not leaked) forever."

1. https://lore.kernel.org/netdev/20240814075603.05f8b0f5@kernel.org/T/#me2f2c89fbeb7f92a27d54a85aab5527efedfe260
2. https://lore.kernel.org/netdev/20240814075603.05f8b0f5@kernel.org/T/#m2687f25537395401cd6a810ac14e0e0d9addf97e
3. https://lore.kernel.org/netdev/ZWfuyc13oEkp583C@makrotopia.org/T/

> 
> Thanks
> /Ilias
>>
>>
>>> If no, it seems that the problem is still existed when the driver for
>>> the device has unbound after device_del() is called.