Write from mmapped buffer to `O_DIRECT` output file - io

Question

I have a device 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, 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 and can modify it appropriately.
I found similar question, but these is no clear answer there.

Answer 1

remap_pfn_range will set your vma as special via pte_mkspecial and add VM_IO/VM_PFNMAP to vma, so you cannot pass the following checks when do Direct I/O.
You say your memory comes from CMA, that's good because cma memory already has struct page support, so you can just use vm_insert_pages as the following steps:
declare cma region from kernel argument or dts
get struct pages from cma:
dma_page = dma_alloc_contiguous(&pdev->dev, size, GFP_KERNEL);
if (!dma_page) {
pr_err("%s %d, dma_alloc_contiguous fail\n", __func__, __LINE__);
return -ENOMEM;
}
nr_pages = DIV_ROUND_UP(size, PAGE_SIZE);
pages = kvmalloc_array(nr_pages, sizeof(*pages), GFP_KERNEL);
for (i = 0; i < nr_pages; i++)
pages[i] = &dma_page[i];
insert pages to vma when mmap
int your_mmap(struct file *file, struct vm_area_struct *vma) {
int ret = 0;
unsigned long temp_nr_pages;
if (vma->vm_end - vma->vm_start > size)
return -EINVAL;
/* duplicitate nr_pages in that vm_insert_pages can change nr_pages */
temp_nr_pages = nr_pages;
ret = vm_insert_pages(vma, vma->vm_start, pages, &temp_nr_pages);
if (ret < 0)
pr_err("%s vm_insert_pages fail, error is %d\n", __func__, ret);
return ret;
}
export dma_alloc_contiguous(the only mm code change, but not so bad).
modified kernel/dma/contiguous.c
## -332,6 +332,7 ## struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp)
return cma_alloc_aligned(dma_contiguous_default_area, size, gfp); }
+EXPORT_SYMBOL(dma_alloc_contiguous);
/**
* dma_free_contiguous() - release allocated pages