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

[Ilias Apalodimas]

Hi Paolo,

Thanks for taking the time.

On Tue, 1 Oct 2024 at 16:32, Paolo Abeni <pabeni@xxxxxxxxxx> 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.

Perhaps, and you are right we need to measure performance before
pulling it but...

>
> 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...

It's not. The problem happens when an SKB has been scheduled for
recycling and has already been mapped via page_pool. If the driver
disappears in the meantime, page_pool will free all the packets it
holds in its private rings (both slow and fast), but is not in control
of the SKB anymore. So any packets coming back for recycling *after*
that point cannot unmap memory properly.

As discussed this can either lead to memory corruption and resource
leaking, or worse as seen in the bug report panics. I am fine with
this going into -next, but it really is a bugfix, although I am not
100% sure that the Fixes: tag in the current patch is correct.

Thanks
/Ilias
>
> /P
>