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

[Kirti Wankhede <kwankhede@xxxxxxxxxx>]

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@xxxxxxxxxx>
> > Reviewed-by: Neo Jia <cjia@xxxxxxxxxx>
> Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx>

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);
> >   	}