Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page

[Shuah Khan]

On 04/06/2017 06:01 AM, Marek Szyprowski wrote:
> Hi Shuah,
> 
> 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.

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.

> 
> 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.

thanks,
-- Shuah

> 
>> Signed-off-by: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
>> ---
>>   arch/arm/mm/dma-mapping.c                      | 34 ++++++++++++++++++++++----
>>   drivers/base/dma-coherent.c                    | 25 +++++++++++++++++++
>>   drivers/media/v4l2-core/videobuf2-dma-contig.c |  6 +++++
>>   include/linux/dma-mapping.h                    |  8 ++++++
>>   include/linux/scatterlist.h                    |  1 +
>>   5 files changed, 69 insertions(+), 5 deletions(-)
>> [...]
> 
> Best regards