Re: [PATCH net-next v3 3/3] page_pool: fix IOMMU crash when driver has already unbound

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/10/29 21:58, Toke Høiland-Jørgensen wrote:
> Yunsheng Lin <linyunsheng@huawei.com> writes:
> 
>>>> I would prefer the waiting too if simple waiting fixed the test cases that
>>>> Youglong and Haiqing were reporting and I did not look into the rabbit hole
>>>> of possible caching in networking.
>>>>
>>>> As mentioned in commit log and [1]:
>>>> 1. ipv4 packet defragmentation timeout: this seems to cause delay up to 30
>>>>    secs, which was reported by Haiqing.
>>>> 2. skb_defer_free_flush(): this may cause infinite delay if there is no
>>>>    triggering for net_rx_action(), which was reported by Yonglong.
>>>>
>>>> For case 1, is it really ok to stall the driver unbound up to 30 secs for the
>>>> default setting of defragmentation timeout?
>>>>
>>>> For case 2, it is possible to add timeout for those kind of caching like the
>>>> defragmentation timeout too, but as mentioned in [2], it seems to be a normal
>>>> thing for this kind of caching in networking:
>>>
>>> Both 1 and 2 seem to be cases where the netdev teardown code can just
>>> make sure to kick the respective queues and make sure there's nothing
>>> outstanding (for (1), walk the defrag cache and clear out anything
>>> related to the netdev going away, for (2) make sure to kick
>>> net_rx_action() as part of the teardown).
>>
>> It would be good to be more specific about the 'kick' here, does it mean
>> taking the lock and doing one of below action for each cache instance?
>> 1. flush all the cache of each cache instance.
>> 2. scan for the page_pool owned page and do the finegrained flushing.
> 
> Depends on the context. The page pool is attached to a device, so it
> should be possible to walk the skb frags queue and just remove any skbs
> that refer to that netdevice, or something like that.

I am not sure if netdevice is still the same when passing through all sorts
of software netdevice, checking if it is the page_pool owned page seems safer?

The scaning/flushing seems complicated and hard to get it right if it is
depending on internal detail of other subsystem's cache implementation.

> 
> As for the lack of net_rx_action(), this is related to the deferred
> freeing of skbs, so it seems like just calling skb_defer_free_flush() on
> teardown could be an option.

That was my initial thinking about the above case too if we know which percpu
sd to be passed to skb_defer_free_flush() or which cpu to trigger its
net_rx_action().
But it seems hard to tell which cpu napi is running in before napi is disabled,
which means skb_defer_free_flush() might need to be called for every cpu with
softirq disabled, as skb_defer_free_flush() calls napi_consume_skb() with
budget being 1 or call kick_defer_list_purge() for each CPU.

> 
>>>> "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."
>>>
>>> As for this one, I would put that in the "well, let's see if this
>>> becomes a problem in practice" bucket.
>>
>> As the commit log in [2], it seems it is already happening.
>>
>> Those cache are mostly per-cpu and per-socket, and there may be hundreds of
>> CPUs and thousands of sockets in one system, are you really sure we need
>> to take the lock of each cache instance, which may be thousands of them,
>> and do the flushing/scaning of memory used in networking, which may be as
>> large as '24 GiB' mentioned by Jesper?
> 
> Well, as above, the two issues you mentioned are per-netns (or possibly
> per-CPU), so those seem to be manageable to do on device teardown if the
> wait is really a problem.

As above, I am not sure if it is still the same netns if the skb is passing
through all sorts of software netdevice?

> 
> But, well, I'm not sure it is? You seem to be taking it as axiomatic
> that the wait in itself is bad. Why? It's just a bit memory being held
> on to while it is still in use, and so what?

Actually, I thought about adding some sort of timeout or kicking based on
jakub's waiting patch too.

But after looking at more caching in the networking, waiting and kicking/flushing
seems harder than recording the inflight pages, mainly because kicking/flushing
need very subsystem using page_pool owned page to provide a kicking/flushing
mechanism for it to work, not to mention how much time does it take to do all
the kicking/flushing.

It seems rdma subsystem uses a similar mechanism:
https://lwn.net/Articles/989087/

> 
> -Toke
> 
> 
>