linux-kernel - Re: [PATCH v2 2/2] iommu/sva: Remove mm parameter from SVA bind API

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Date:   Thu, 15 Apr 2021 14:18:45 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Raj Ashok <ashok.raj@...el.com>, vkoul@...nel.org,
        Jason Gunthorpe <jgg@...dia.com>, zhangfei.gao@...aro.org,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v2 2/2] iommu/sva: Remove mm parameter from SVA bind API

Hi Christoph,

On Thu, 15 Apr 2021 07:44:59 +0100, Christoph Hellwig <hch@...radead.org>
wrote:

> >   *
> >   * Returns 0 on success and < 0 on error.
> > @@ -28,6 +28,9 @@ int iommu_sva_alloc_pasid(struct mm_struct *mm,
> > ioasid_t min, ioasid_t max) int ret = 0;
> >  	ioasid_t pasid;
> >  
> > +	if (mm != current->mm)
> > +		return -EINVAL;
> > +  
> 
> Why not remove the parameter entirely?
It was removed in my v1 but thought it would be cleaner if we treat
iommu_sva_alloc_pasid() as a leaf function of iommu_sva_bind_device(). Then
we don't have to do get_task_mm() every time. But to your point below, it
is better to get low-level driver handle it.
> 
> > @@ -2989,8 +2990,11 @@ iommu_sva_bind_device(struct device *dev, struct
> > mm_struct *mm, unsigned int fla return ERR_PTR(-ENODEV);
> >  
> >  	/* Supervisor SVA does not need the current mm */
> > -	if ((flags & IOMMU_SVA_BIND_SUPERVISOR) && mm)
> > -		return ERR_PTR(-EINVAL);
> > +	if (!(flags & IOMMU_SVA_BIND_SUPERVISOR)) {
> > +		mm = get_task_mm(current);
> > +		if (!mm)
> > +			return ERR_PTR(-EINVAL);
> > +	}  
> 
> I don't see why we need the reference.  I think we should just stop
> passing the mm to ->sva_bind and let the low-level driver deal with
> any reference to current->mm where needed.
The mm users reference is just for precaution, in case low level driver use
kthread etc.
I agree it is cleaner to just remove mm here, let the low-level driver deal
with it.
Let me give it a spin.

Thanks,

Jacob