[Intel-gfx] [PATCH] drm/i915: avoid leaking DMA mappings

[Daniel Vetter]

On Mon, Jul 06, 2015 at 03:57:44PM +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.

How does this happen? Essentially list_empty(obj->vmas) ==
!dma_mapping_exists should hold for objects of the 3rd type. I don't
understand how this is broken in the current code. There was definitely
versions of the ppgtt code where this wasn't working properly, but I
thought we've fixed that up again.

> > 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.

I do kinda like the distinction between just grabbing the backing storage
and making it accessible to the hw. Small one, but I think it does help if
we keep these two maps separate. Now the function names otoh are
super-confusing, that I agree with.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch