Write from mmapped buffer to O_DIRECT output file

[Mikhail Karpenko]

Hello,

I have a devices which writes to a video buffer. This buffer is allocated
in system memory using CMA and I want to implement streaming write from
this buffer to a block device. My application opens video buffer with mmap
and I would like to use O_DIRECT write to avoid page cache related
overhead. Basically, the pseudo-code of the application looks like this:

    f_in = open("/dev/videobuf", O_RDONLY);
    f_mmap = mmap(0, BUFFER_SIZE, PROT_READ, MAP_SHARED, f_in, 0);
    f_out = open("/dev/sda", O_WRONLY | O_DIRECT);
    write(f_out, f_mmap, BLOCK_SIZE);

where BLOCK_SIZE is sector aligned value. f_out is opened without any
errors, but write results in EFAULT. I tried to track down this issue and
it turned out that mmap implementation in video buffer's driver uses
remap_pfn_range(), which sets VM_IO and VM_PFNMAP flags for VMA. The
O_DIRECT path in block device drivers checks these flags and returns
EFAULT. As far as I understand (from reading comments to VMA flags),
O_DIRECT writes need to pin the memory pages, but VMA flags indicate the
absence of "struct page" for underlying memory which causes an error. Am I
right here?
And the main question is how to correctly implement O_DIRECT write from
mmapped buffer? I have video buffer driver sources and can modify it
appropriately.

Mikhail
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