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

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Date: Tue, 9 Jan 2024 10:47:40 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Kevin Tian <kevin.tian@...el.com>,
 Jean-Philippe Brucker <jean-philippe@...aro.org>,
 Nicolin Chen <nicolinc@...dia.com>, Yi Liu <yi.l.liu@...el.com>,
 Jacob Pan <jacob.jun.pan@...ux.intel.com>,
 Longfang Liu <liulongfang@...wei.com>, Yan Zhao <yan.y.zhao@...el.com>,
 iommu@...ts.linux.dev, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 12/14] iommu: Use refcount for fault data access

On 1/6/24 12:09 AM, Jason Gunthorpe wrote:
> 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@...dia.com>
>> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>> Tested-by: Yan Zhao <yan.y.zhao@...el.com>
>> ---
>>   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()

Removed.

> 
>> @@ -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.

Done.

> 
> With those:
> 
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> 
> Jason

Thank you very much!

Best regards,
baolu