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 Nak. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
