Re: [RFC PATCHES 1/2] iommu: Add RCU-protected page free support

[Jason Gunthorpe]

On Thu, Jun 09, 2022 at 03:08:10PM +0800, Lu Baolu wrote:
> The IOMMU page tables are updated using iommu_map/unmap() interfaces.
> Currently, there is no mandatory requirement for drivers to use locks
> to ensure concurrent updates to page tables, because it's assumed that
> overlapping IOVA ranges do not have concurrent updates. Therefore the
> IOMMU drivers only need to take care of concurrent updates to level
> page table entries.
> 
> But enabling new features challenges this assumption. For example, the
> hardware assisted dirty page tracking feature requires scanning page
> tables in interfaces other than mapping and unmapping. This might result
> in a use-after-free scenario in which a level page table has been freed
> by the unmap() interface, while another thread is scanning the next level
> page table.
> 
> This adds RCU-protected page free support so that the pages are really
> freed and reused after a RCU grace period. Hence, the page tables are
> safe for scanning within a rcu_read_lock critical region. Considering
> that scanning the page table is a rare case, this also adds a domain
> flag and the RCU-protected page free is only used when this flat is set.
> 
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> ---
>  include/linux/iommu.h |  9 +++++++++
>  drivers/iommu/iommu.c | 23 +++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 5e1afe169549..6f68eabb8567 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -95,6 +95,7 @@ struct iommu_domain {
>  	void *handler_token;
>  	struct iommu_domain_geometry geometry;
>  	struct iommu_dma_cookie *iova_cookie;
> +	unsigned long concurrent_traversal:1;
>  };
>  
>  static inline bool iommu_is_dma_domain(struct iommu_domain *domain)
> @@ -657,6 +658,12 @@ static inline void dev_iommu_priv_set(struct device *dev, void *priv)
>  	dev->iommu->priv = priv;
>  }
>  
> +static inline void domain_set_concurrent_traversal(struct iommu_domain *domain,
> +						   bool value)
> +{
> +	domain->concurrent_traversal = value;
> +}

?? If you want it to be a driver opt in I would just add a flags to
the domain ops. "DOMAIN_FLAG_RCU_FREE_PAGES"

> +void iommu_free_pgtbl_pages(struct iommu_domain *domain,
> +			    struct list_head *pages)
> +{
> +	struct page *page, *next;
> +
> +	if (!domain->concurrent_traversal) {
> +		put_pages_list(pages);
> +		return;
> +	}
> +
> +	list_for_each_entry_safe(page, next, pages, lru) {
> +		list_del(&page->lru);
> +		call_rcu(&page->rcu_head, pgtble_page_free_rcu);
> +	}

It seems OK, but I wonder if there is benifit to using
put_pages_list() from the rcu callback

Jason