Re: [PATCH v9 12/14] iommu: Use refcount for fault data access

[Jason Gunthorpe]

On Wed, Dec 20, 2023 at 09:23:30AM +0800, Lu Baolu wrote:
> The per-device fault data structure stores information about faults
> occurring on a device. Its lifetime spans from IOPF enablement to
> disablement. Multiple paths, including IOPF reporting, handling, and
> responding, may access it concurrently.
> 
> Previously, a mutex protected the fault data from use after free. But
> this is not performance friendly due to the critical nature of IOPF
> handling paths.
> 
> Refine this with a refcount-based approach. The fault data pointer is
> obtained within an RCU read region with a refcount. The fault data
> pointer is returned for usage only when the pointer is valid and a
> refcount is successfully obtained. The fault data is freed with
> kfree_rcu(), ensuring data is only freed after all RCU critical regions
> complete.
> 
> An iopf handling work starts once an iopf group is created. The handling
> work continues until iommu_page_response() is called to respond to the
> iopf and the iopf group is freed. During this time, the device fault
> parameter should always be available. Add a pointer to the device fault
> parameter in the iopf_group structure and hold the reference until the
> iopf_group is freed.
> 
> Make iommu_page_response() static as it is only used in io-pgfault.c.
> 
> Co-developed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Tested-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> ---
>  include/linux/iommu.h      |  17 +++--
>  drivers/iommu/io-pgfault.c | 129 +++++++++++++++++++++++--------------
>  drivers/iommu/iommu-sva.c  |   2 +-
>  3 files changed, 90 insertions(+), 58 deletions(-)

This looks basically Ok

> +/* Caller must hold a reference of the fault parameter. */
> +static void iopf_put_dev_fault_param(struct iommu_fault_param *fault_param)
> +{
> +	if (refcount_dec_and_test(&fault_param->users))
> +		kfree_rcu(fault_param, rcu);
> +}

[..]

> @@ -402,10 +429,12 @@ int iopf_queue_add_device(struct iopf_queue *queue, struct device *dev)
>  	INIT_LIST_HEAD(&fault_param->faults);
>  	INIT_LIST_HEAD(&fault_param->partial);
>  	fault_param->dev = dev;
> +	refcount_set(&fault_param->users, 1);
> +	init_rcu_head(&fault_param->rcu);

No need to do init_rcu_head() when only using it for calling
kfree_rcu()

> @@ -454,8 +485,10 @@ int iopf_queue_remove_device(struct iopf_queue *queue, struct device *dev)
>  	list_for_each_entry_safe(iopf, next, &fault_param->partial, list)
>  		kfree(iopf);
>  
> -	param->fault_param = NULL;
> -	kfree(fault_param);
> +	/* dec the ref owned by iopf_queue_add_device() */
> +	rcu_assign_pointer(param->fault_param, NULL);
> +	if (refcount_dec_and_test(&fault_param->users))
> +		kfree_rcu(fault_param, rcu);

Why open code iopf_put_dev_fault_param()? Just call it.

With those:

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Jason