Messages in this thread | | | Subject | Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page | From | Marek Szyprowski <> | Date | Fri, 14 Apr 2017 09:56:07 +0200 |
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Hi Shuah,
On 2017-04-11 00:50, Shuah Khan wrote: > On 04/06/2017 06:01 AM, Marek Szyprowski wrote: >> On 2017-04-05 18:02, 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. Please see www.spinics.net/lists/stable/msg164204.html >>> for more information on this problem. >>> >>> This solution allows coherent DMA memory without struct page to be >>> shared by providing a way for the exporter to tag the DMA buffer as >>> a special buffer without struct page association and passing the >>> information in sg_table to the importer. This information is used >>> in attach/map_sg to avoid cleaning D-cache and mapping. >>> >>> The details of the change are: >>> >>> Framework: >>> - Add a new dma_attrs field to struct scatterlist. >>> - Add a new DMA_ATTR_DEV_COHERENT_NOPAGE attribute to clearly identify >>> Coherent memory without struct page. >>> - Add a new dma_check_dev_coherent() interface to check if memory is >>> the device coherent area. There is no way to tell where the memory >>> returned by dma_alloc_attrs() came from. >>> >>> Exporter logic: >>> - Add logic to vb2_dc_alloc() to call dma_check_dev_coherent() and set >>> DMA_ATTR_DEV_COHERENT_NOPAGE based the results of the check. This is >>> done in the exporter context. >>> - Add logic to arm_dma_get_sgtable() to identify memory without struct >>> page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute. If this attr is >>> set, arm_dma_get_sgtable() will set page as the cpu_addr and update >>> dma_address and dma_attrs fields in struct scatterlist for this sgl. >>> This is done in exporter context when buffer is exported. With this >>> Note: This change is made on top of Russell King's patch that added >>> !pfn_valid(pfn) check to arm_dma_get_sgtable() to error out on invalid >>> pages. Coherent memory without struct page will trigger this error. >>> >>> Importer logic: >>> - Add logic to vb2_dc_dmabuf_ops_attach() to identify memory without >>> struct page using DMA_ATTR_DEV_COHERENT_NOPAGE attribute when it copies >>> the sg_table from the exporter. It will copy dma_attrs and dma_address >>> fields. With this logic, dmabuf_ops_attach will no longer trip on an >>> invalid page. >>> - Add logic to arm_dma_map_sg() to avoid mapping the page when sg_table >>> has DMA_ATTR_DEV_COHERENT_NOPAGE buffer. >>> - Add logic to arm_dma_unmap_sg() to do nothing for sg entries with >>> DMA_ATTR_DEV_COHERENT_NOPAGE attribute. >>> >>> Without this change the following use-case that runs into kernel >>> pagefault when importer tries to attach the exported buffer. >>> >>> With this change it works: (what a relief after watching pagefaults for >>> weeks!!) >>> >>> gst-launch-1.0 filesrc location=~/GH3_MOV_HD.mp4 ! qtdemux ! h264parse ! v4l2video4dec capture-io-mode=dmabuf ! v4l2video7convert output-io-mode=dmabuf-import ! kmssink force-modesetting=true >>> >>> I am sending RFC patch to get feedback on the approach and see if I missed >>> anything. >> Frankly, once You decided to hack around dma-buf and issues with coherent, >> carved out memory, it might be a bit better to find the ultimate solution >> instead of the another hack. Please note that it will still not allow to >> share a buffer allocated from carved-out memory and a device, which is >> behind IOMMU. > With your patch s5p-mfc patch series does address the problem for this > use-case for 4.12 onwards. However I am still concerned about prior > release and this pagefault is bad.
Right. It should simply fail with error code instead of pagefault.
> Invalid page test partially solves the problem. Would it helpful to > at least prevent the pagfault with a definitive test. Please see my > response to Russell. Let me know your thoughts on that. > >> I thought a bit about this and the current shape of dma-buf code. >> >> IMHO the proper way of solving all those issues would be to replace >> dma-buf internal representation of the memory from struct scatter_list >> to pfn array. This would really solve the problem of buffers which >> cannot be properly represented by scatter lists/struct pages and would >> even allow sharing buffers between all kinds of devices. Scatter-lists >> are also quite over-engineered structures to represent a single buffer >> (pfn array is a bit more compact representation). Also there is a lots >> of buggy code which use scatter-list in a bit creative way (like >> assuming that each page maps to a single scatter list entry for >> example). The only missing piece, required for such change would be >> extending DMA-mapping with dma_map_pfn() interface. > I agree with you on scatterlists being clumsy. Changing over to pfn array > could simplify things. I am exploring a slightly different option that > might not require too many changes. I will respond with concrete ideas > later on this week.
It looks that a similar issue is being worked on, see the following thread: https://lkml.org/lkml/2017/4/13/710
>> This would be however quite large task, especially taking into account >> all current users of DMA-buf framework... > Yeah it will be a large task.
Maybe once scatterlist are switched to pfns, changing dmabuf internal memory representation to pfn array might be much easier.
Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland
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