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

[Ilias Apalodimas]

On Wed, 2 Oct 2024 at 09:46, Ilias Apalodimas
<ilias.apalodimas@xxxxxxxxxx> wrote:
>
> 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,

Apologies, this wasn't correct. It's the device that has to disappear
not the driver

> 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
> >