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

[Baolu Lu]

On 2022/6/10 01:06, Raj, Ashok wrote:
> 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.
>
> The last part doesn't read well..
> s/updates to level page table entries/ updates to page-table entries at the
> same level
>
> > 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.
>
> s/flat/flag

Above updated. Thank you!

> > 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;
>
> Does this need to be a bitfield? Even though you are needing just one bit
> now, you can probably make have maskbits?

As discussed in another reply, I am about to drop the driver opt-in and
wrapper the helper around the iommu_iotlb_gather.

> >   };
> >   
> >   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;
> > +}
> > +
> >   int iommu_probe_device(struct device *dev);
> >   void iommu_release_device(struct device *dev);
> >   
> > @@ -677,6 +684,8 @@ int iommu_group_claim_dma_owner(struct iommu_group *group, void *owner);
> >   void iommu_group_release_dma_owner(struct iommu_group *group);
> >   bool iommu_group_dma_owner_claimed(struct iommu_group *group);
> >   
> > +void iommu_free_pgtbl_pages(struct iommu_domain *domain,
> > +			    struct list_head *pages);
> >   #else /* CONFIG_IOMMU_API */
> >   
> >   struct iommu_ops {};
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 847ad47a2dfd..ceeb97ebe3e2 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -3252,3 +3252,26 @@ bool iommu_group_dma_owner_claimed(struct iommu_group *group)
> >   	return user;
> >   }
> >   EXPORT_SYMBOL_GPL(iommu_group_dma_owner_claimed);
> > +
> > +static void pgtble_page_free_rcu(struct rcu_head *rcu)
>
> maybe the names can be consistent? pgtble_ vs pgtbl below.
>
> vote to drop the 'e' :-)

Updated.

> > +{
> > +	struct page *page = container_of(rcu, struct page, rcu_head);
> > +
> > +	__free_pages(page, 0);
> > +}
> > +
> > +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);
> > +	}
> > +}
> > --
> > 2.25.1

Best regards,
baolu