Re: [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