Messages in this thread | | | Subject | Re: [PATCH] ARM: dma-mapping: add check for coherent DMA memory without struct page | From | Shuah Khan <> | Date | Thu, 13 Apr 2017 17:42:56 -0600 |
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On 04/13/2017 04:10 PM, Russell King - ARM Linux wrote: > On Thu, Apr 13, 2017 at 03:47:56PM -0600, Shuah Khan wrote: >> When coherent DMA memory without struct page is shared, importer >> fails to find the page and runs into kernel page fault when it >> tries to dmabuf_ops_attach/map_sg/map_page the invalid page found >> in the sg_table. >> >> Add a new dma_check_dev_coherent() interface to check if memory is >> from the device coherent area. There is no way to tell where the >> memory returned by dma_alloc_attrs() came from. >> >> arm_dma_get_sgtable() checks for invalid pages, however this check >> could pass even for memory obtained the coherent allocator. Add an >> additional check to call dma_check_dev_coherent() to confirm that it >> is indeed the coherent DMA memory and fail the sgtable creation with >> -EINVAL. > > Sorry, this doesn't make much sense to me. > > pfn_valid(pfn) must *never* return true if 'pfn' does not have a struct > page associated with it. If it returns true (so we allow > arm_dma_get_sgtable() to succeed) then we know we have a valid struct > page in the supplied scatterlist. > >> Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com> >> --- >> arch/arm/mm/dma-mapping.c | 11 ++++++++--- >> drivers/base/dma-coherent.c | 25 +++++++++++++++++++++++++ >> include/linux/dma-mapping.h | 2 ++ >> 3 files changed, 35 insertions(+), 3 deletions(-) >> >> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c >> index 475811f..27c7d9a 100644 >> --- a/arch/arm/mm/dma-mapping.c >> +++ b/arch/arm/mm/dma-mapping.c >> @@ -954,9 +954,14 @@ int arm_dma_get_sgtable(struct device *dev, struct sg_table *sgt, >> struct page *page; >> int ret; >> >> - /* If the PFN is not valid, we do not have a struct page */ >> - if (!pfn_valid(pfn)) >> - return -ENXIO; >> + /* >> + * If the PFN is not valid, we do not have a struct page >> + * As this check can pass even for memory obtained through >> + * the coherent allocator, do an additional check to determine >> + * if this is coherent DMA memory. >> + */ >> + if (!pfn_valid(pfn) && dma_check_dev_coherent(dev, handle, cpu_addr)) >> + return -EINVAL; > > Right, so what this says is: > > if we do not haev a valid PFN > _and_ if the memory is from the coherent section > _then_ fail > > Why the extra check? Under what circunstances do we end up with memory > where the PFN is valid, but we do not have a valid struct page. It > seems to me that such a scenario is a bug in pfn_valid() and not > something that should be worked around like this. >
DMA_ATTR_NO_KERNEL_MAPPING case is the one I am concerned about. pfn_valid() would fail on this if I am understanding it correctly. A few drm drivers set this attr and use sg_table for passing buffers.
My reasoning behind adding this check is to not have this fail on NO_KERNEL_MAPPING cases. So I thought adding a restrictive check for just the per-device memory would help. However, I don't have a good understanding of the drm case, hence I could be trying to address a case that doesn't need to be addressed.
In any case, thanks for your patience and a quick reply.
-- Shuah
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