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

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

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.

> Nak.

Right. Your patch doesn't explicitly mention fixing the issues I tracked
down, but it does seem to fix them. It would make sens to add this fact
to the commit log.

--Imre

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