Re: [PATCH] drm/i915: avoid leaking DMA mappings

[Imre Deak <imre.deak@intel.com>]

On ma, 2015-07-06 at 16:28 +0100, Chris Wilson wrote:
> On Mon, Jul 06, 2015 at 06:11:40PM +0300, Imre Deak wrote:
> > On ma, 2015-07-06 at 15:57 +0100, Chris Wilson wrote:
> > > On Mon, Jul 06, 2015 at 05:50:37PM +0300, Imre Deak wrote:
> > > > We have 3 types of DMA mappings for GEM objects:
> > > > 1. physically contiguous for stolen and for objects needing contiguous
> > > >    memory
> > > > 2. DMA-buf mappings imported via a DMA-buf attach operation
> > > > 3. SG DMA mappings for shmem backed and userptr objects
> > > > 
> > > > For 1. and 2. the lifetime of the DMA mapping matches the lifetime of the
> > > > corresponding backing pages and so in practice we create/release the
> > > > mapping in the object's get_pages/put_pages callback.
> > > > 
> > > > For 3. the lifetime of the mapping matches that of any existing GPU binding
> > > > of the object, so we'll create the mapping when the object is bound to
> > > > the first vma and release the mapping when the object is unbound from its
> > > > last vma.
> > > > 
> > > > Since the object can be bound to multiple vmas, we can end up creating a
> > > > new DMA mapping in the 3. case even if the object already had one. This
> > > > is not allowed by the DMA API and can lead to leaked mapping data and
> > > > IOMMU memory space starvation in certain cases. For example HW IOMMU
> > > > drivers (intel_iommu) allocate a new range from their memory space
> > > > whenever a mapping is created, silently overriding a pre-existing
> > > > mapping.
> > > > 
> > > > Fix this by adding new callbacks to create/release the DMA mapping. This
> > > > way we can use the has_dma_mapping flag for objects of the 3. case also
> > > > (so far the flag was only used for the 1. and 2. case) and skip creating
> > > > a new mapping if one exists already.
> > > > 
> > > > Note that I also thought about simply creating/releasing the mapping
> > > > when get_pages/put_pages is called. However since creating a DMA mapping
> > > > may have associated resources (at least in case of HW IOMMU) it does
> > > > make sense to release these resources as early as possible. We can
> > > > release the DMA mapping as soon as the object is unbound from the last
> > > > vma, before we drop the backing pages, hence it's worth keeping the two
> > > > operations separate.
> > > > 
> > > > I noticed this issue by enabling DMA debugging, which got disabled after
> > > > a while due to its internal mapping tables getting full. It also reported
> > > > errors in connection to random other drivers that did a DMA mapping for
> > > > an address that was previously mapped by i915 but was never released.
> > > > Besides these diagnostic messages and the memory space starvation
> > > > problem for IOMMUs, I'm not aware of this causing a real issue.
> > > 
> > > Nope, it is much much simpler. Since we only do the dma prepare/finish
> > > from inside get_pages/put_pages, we can put the calls there. The only
> > > caveat there is userptr worker, but that can be easily fixed up.
> > > 
> > > http://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=nightly&id=f55727d7d6f76aeee687c1f2d31411662ff03b6f
> > 
> > Yes, that's what I meant by creating/releasing the mapping in the
> > get_pages/put_pages callbacks. It does have the disadvantage of keeping
> > on to IOMMU mapping resources longer than it's needed as I described
> > above.
> 
> I don't think that is a disadvantage though. You haven't introduced a
> dma shrinker which is what you need to handle a limited resource. So
> it's a moot point as we don't handle the allocation failure smartly. By
> moving the failure into get pages, at least it is tractable.

That's true, but we could do this in the future, if we had the new
callbacks.


> -Chris
> 


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