Re: [PATCH v15 Kernel 2/7] vfio iommu: Remove atomicity of ref_count of pinned pages

[Kirti Wankhede <kwankhede@nvidia.com>]

On 3/24/2020 2:00 AM, Auger Eric wrote:
> Hi Kirti,
> 
> On 3/19/20 9:16 PM, Kirti Wankhede wrote:
>> vfio_pfn.ref_count is always updated by holding iommu->lock, using atomic
>> variable is overkill.
>>
>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>> Reviewed-by: Neo Jia <cjia@nvidia.com>
> Reviewed-by: Eric Auger <eric.auger@redhat.com>
> 

Thanks.

Kirti.

> Thanks
> 
> Eric
>> ---
>>   drivers/vfio/vfio_iommu_type1.c | 9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 9fdfae1cb17a..70aeab921d0f 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -112,7 +112,7 @@ struct vfio_pfn {
>>   	struct rb_node		node;
>>   	dma_addr_t		iova;		/* Device address */
>>   	unsigned long		pfn;		/* Host pfn */
>> -	atomic_t		ref_count;
>> +	unsigned int		ref_count;
>>   };
>>   
>>   struct vfio_regions {
>> @@ -233,7 +233,7 @@ static int vfio_add_to_pfn_list(struct vfio_dma *dma, dma_addr_t iova,
>>   
>>   	vpfn->iova = iova;
>>   	vpfn->pfn = pfn;
>> -	atomic_set(&vpfn->ref_count, 1);
>> +	vpfn->ref_count = 1;
>>   	vfio_link_pfn(dma, vpfn);
>>   	return 0;
>>   }
>> @@ -251,7 +251,7 @@ static struct vfio_pfn *vfio_iova_get_vfio_pfn(struct vfio_dma *dma,
>>   	struct vfio_pfn *vpfn = vfio_find_vpfn(dma, iova);
>>   
>>   	if (vpfn)
>> -		atomic_inc(&vpfn->ref_count);
>> +		vpfn->ref_count++;
>>   	return vpfn;
>>   }
>>   
>> @@ -259,7 +259,8 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
>>   {
>>   	int ret = 0;
>>   
>> -	if (atomic_dec_and_test(&vpfn->ref_count)) {
>> +	vpfn->ref_count--;
>> +	if (!vpfn->ref_count) {
>>   		ret = put_pfn(vpfn->pfn, dma->prot);
>>   		vfio_remove_from_pfn_list(dma, vpfn);
>>   	}
>>
>