linux-kernel - Re: [PATCH] vfio/type1: Limit DMA mappings per container

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Date:   Mon, 1 Apr 2019 22:34:13 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        eric.auger@...hat.com, cohuck@...hat.com
Subject: Re: [PATCH] vfio/type1: Limit DMA mappings per container

On Tue, 2 Apr 2019 10:41:15 +0800
Peter Xu <peterx@...hat.com> wrote:

> On Mon, Apr 01, 2019 at 02:16:52PM -0600, Alex Williamson wrote:
> 
> [...]
> 
> > @@ -1081,8 +1088,14 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
> >  		goto out_unlock;
> >  	}
> >  
> > +	if (!atomic_add_unless(&iommu->dma_avail, -1, 0)) {
> > +		ret = -ENOSPC;
> > +		goto out_unlock;
> > +	}
> > +
> >  	dma = kzalloc(sizeof(*dma), GFP_KERNEL);
> >  	if (!dma) {
> > +		atomic_inc(&iommu->dma_avail);  
> 
> This should be the only special path to revert the change.  Not sure
> whether this can be avoided by simply using atomic_read() or even
> READ_ONCE() (I feel like we don't need atomic ops with dma_avail
> because we've had the mutex but it of course it doesn't hurt...) to
> replace atomic_add_unless() above to check against zero then we do
> +1/-1 in vfio_[un]link_dma() only.  But AFAICT this patch is correct.

Thanks for the review, you're right, we're only twiddling this atomic
while holding the iommu->lock mutex, so it appears unnecessary.  Since
we're within the mutex, I think we don't even need a READ_ONCE.  We can
simple test it before alloc and decrement after.  Am I missing something
that would specifically require READ_ONCE within our mutex critical
section?  Thanks,

Alex