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SubjectRe: [PATCH] vfio/type1: Limit DMA mappings per container
On Tue, 2 Apr 2019 10:41:15 +0800
Peter Xu <peterx@redhat.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

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