[PATCH net v2 2/2] page_pool: fix IOMMU crash when driver has already unbound

[Yunsheng Lin]

On 10/1/2024 9:32 PM, Paolo Abeni wrote:
> On 9/25/24 09:57, Yunsheng Lin wrote:
>> Networking driver with page_pool support may hand over page
>> still with dma mapping to network stack and try to reuse that
>> page after network stack is done with it and passes it back
>> to page_pool to avoid the penalty of dma mapping/unmapping.
>> With all the caching in the network stack, some pages may be
>> held in the network stack without returning to the page_pool
>> soon enough, and with VF disable causing the driver unbound,
>> the page_pool does not stop the driver from doing it's
>> unbounding work, instead page_pool uses workqueue to check
>> if there is some pages coming back from the network stack
>> periodically, if there is any, it will do the dma unmmapping
>> related cleanup work.
>>
>> As mentioned in [1], attempting DMA unmaps after the driver
>> has already unbound may leak resources or at worst corrupt
>> memory. Fundamentally, the page pool code cannot allow DMA
>> mappings to outlive the driver they belong to.
>>
>> Currently it seems there are at least two cases that the page
>> is not released fast enough causing dma unmmapping done after
>> driver has already unbound:
>> 1. ipv4 packet defragmentation timeout: this seems to cause
>>     delay up to 30 secs.
>> 2. skb_defer_free_flush(): this may cause infinite delay if
>>     there is no triggering for net_rx_action().
>>
>> 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. As the pool->items need to be large enough to avoid
>> performance degradation, add a 'item_full' stat to indicate the
>> allocation failure due to unavailability of pool->items.
> 
> This looks really invasive, with room for potentially large performance 
> regressions or worse. At very least it does not look suitable for net.

I am open to targetting this to net-next, it can be backported when some
testing is done through one or two kernel versions and there is still
some interest to backport it too.

Or if there is some non-invasive way to fix this.

> 
> Is the problem only tied to VFs drivers? It's a pity all the page_pool 
> users will have to pay a bill for it...

I am afraid it is not only tied to VFs drivers, as:
attempting DMA unmaps after the driver has already unbound may leak
resources or at worst corrupt memory.

Unloading PFs driver might cause the above problems too, I guess the
probability of crashing is low for the PF as PF can not be disable
unless it can be hot-unplug'ed, but the probability of leaking resources
behind the dma mapping might be similar.

> 
> /P
> 
>