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

[Marek Szyprowski <m.szyprowski@samsung.com>]

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