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

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

On Sun, 29 Sept 2024 at 05:44, Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> On 2024/9/28 15:34, Ilias Apalodimas wrote:
>
> ...
>
> >
> > Yes, that wasn't very clear indeed, apologies for any confusion. I was
> > trying to ask on a linked list that only lives in struct page_pool.
> > But I now realize this was a bad idea since the lookup would be way
> > slower.
> >
> >> If I understand question correctly, the single/doubly linked list
> >> is more costly than array as the page_pool case as my understanding.
> >>
> >> For single linked list, it doesn't allow deleting a specific entry but
> >> only support deleting the first entry and all the entries. It does support
> >> lockless operation using llist, but have limitation as below:
> >> https://elixir.bootlin.com/linux/v6.7-rc8/source/include/linux/llist.h#L13
> >>
> >> For doubly linked list, it needs two pointer to support deleting a specific
> >> entry and it does not support lockless operation.
> >
> > I didn't look at the patch too carefully at first. Looking a bit
> > closer now, the array is indeed better, since the lookup is faster.
> > You just need the stored index in struct page to find the page we need
> > to unmap. Do you remember if we can reduce the atomic pp_ref_count to
> > 32bits? If so we can reuse that space for the index. Looking at it
>
> For 64 bits system, yes, we can reuse that.
> But for 32 bits system, we may have only 16 bits for each of them, and it
> seems that there is no atomic operation for variable that is less than 32
> bits.
>
> > requires a bit more work in netmem, but that's mostly swapping all the
> > atomic64 calls to atomic ones.
> >
> >>
> >> For pool->items, as the alloc side is protected by NAPI context, and the
> >> free side use item->pp_idx to ensure there is only one producer for each
> >> item, which means for each item in pool->items, there is only one consumer
> >> and one producer, which seems much like the case when the page is not
> >> recyclable in __page_pool_put_page, we don't need a lock protection when
> >> calling page_pool_return_page(), the 'struct page' is also one consumer
> >> and one producer as the pool->items[item->pp_idx] does:
> >> https://elixir.bootlin.com/linux/v6.7-rc8/source/net/core/page_pool.c#L645
> >>
> >> We only need a lock protection when page_pool_destroy() is called to
> >> check if there is inflight page to be unmapped as a consumer, and the
> >> __page_pool_put_page() may also called to unmapped the inflight page as
> >> another consumer,
> >
> > Thanks for the explanation. On the locking side, page_pool_destroy is
> > called once from the driver and then it's either the workqueue for
> > inflight packets or an SKB that got freed and tried to recycle right?
> > But do we still need to do all the unmapping etc from the delayed
> > work? Since the new function will unmap all packets in
> > page_pool_destroy, we can just skip unmapping when the delayed work
> > runs
>
> Yes, the pool->dma_map is clear in page_pool_item_uninit() after it does
> the unmapping for all inflight pages with the protection of pool->destroy_lock,
> so that the unmapping is skipped in page_pool_return_page() when those inflight
> pages are returned back to page_pool.

Ah yes, the entire destruction path is protected which seems correct.
Instead of that WARN_ONCE in page_pool_item_uninit() can we instead
check the number of inflight packets vs what we just unmapped? IOW
check 'mask' against what page_pool_inflight() gives you and warn if
those aren't equal.


Thanks
/Ilias
>
> >