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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 2 Apr 2019 13:18:02 +0800
Peter Xu <peterx@redhat.com> wrote:

> On Mon, Apr 01, 2019 at 10:34:13PM -0600, Alex Williamson wrote:
> > 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,  
> 
> I don't know very clear on this and I'd be glad to learn about that.
> My understanding is that [READ|WRITE]_ONCE() is the same as volatile
> mem operation and will make sure we don't keep variables in the
> registers.  So if the mutex semantics can support that (say, a "*addr
> = val" following with a mutex_unlock will make sure "val" will
> definitely land into memory of "&addr") then I do think it's fine even
> without it (which corresponds to WRITE_ONCE(&addr, val) in this case).

The READ/WRITE_ONCE macros add memory barriers, but we have the mutex
for protecting concurrent access to the data.  I don't see that there's
anything special about a counter on the iommu object that needs special
attention vs any other elements that might get modified in these
sections.  Thanks,

Alex