Re: [PATCH net-next v2 3/3] page_pool: Track DMA-mapped pages and unmap them when destroying the pool

[Mina Almasry <almasrymina@google.com>]

On Wed, Mar 26, 2025 at 11:22 AM Saeed Mahameed <saeedm@nvidia.com> wrote:
>
> On 25 Mar 16:45, Toke Høiland-Jørgensen wrote:
> >When enabling DMA mapping in page_pool, pages are kept DMA mapped until
> >they are released from the pool, to avoid the overhead of re-mapping the
> >pages every time they are used. This causes resource leaks and/or
> >crashes when there are pages still outstanding while the device is torn
> >down, because page_pool will attempt an unmap through a non-existent DMA
> >device on the subsequent page return.
> >
>
> Why dynamically track when it is guaranteed the page_pool consumer (driver)
> will return all outstanding pages before disabling the DMA device.
> When a page pool is destroyed by the driver, just mark it as "DMA-inactive",
> and on page_pool_return_page() if DMA-inactive don't recycle those pages
> and immediately DMA unmap and release them.

That doesn't work, AFAIU. DMA unmaping after page_pool_destroy has
been called in what's causing the very bug this series is trying to
fix. What happens is:

1. Driver calls page_pool_destroy,
2. Driver removes the net_device (and I guess the associated iommu
structs go away with it).
3. Page-pool tries to unmap after page_pool_destroy is called, trying
to fetch iommu resources that have been freed due to the netdevice
gone away = bad stuff.

(but maybe I misunderstood your suggestion)

-- 
Thanks,
Mina