Map multiple kernel buffer into contiguous userspace buffer? - linux

Question

I have allocated multiple kernel accessible buffers using dma_alloc_coherent, each 4MiB in size. The goal is to map these buffers into a contiguous userspace virtual memory. The issue is that remap_pfn_range doesn't seem to be working, as the userspace memory sometimes works and sometimes doesn't, or sometimes duplicates the page mappings of the buffers.
// in probe() function
dma_alloc_coherent(&pcie->dev, BUF_SIZE, &bus_addr0, GFP_KERNEL);
dma_alloc_coherent(&pcie->dev, BUF_SIZE, &bus_addr1, GFP_KERNEL);
// ...
// in mmap() function
vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
pfn = dma_to_phys(&pcie->dev, &bus_addr0) >> PAGE_SHIFT;
remap_pfn_range(pfn, vma->vm_start + 0, pfn, BUF_SIZE, vma->vm_page_prot);
pfn = dma_to_phys(&pcie->dev, &bus_addr1) >> PAGE_SHIFT;
remap_pfn_range(pfn, vma->vm_start + BUF_SIZE, pfn, BUF_SIZE, vma->vm_page_prot);
I'm not really sure of the best way to map multiple kernel buffers to contiguous userspace memory, but I have a feeling I am doing it wrong. Thanks in advance.

Answer 1

I have no idea why there isn't a better interface to map multiple buffers contiguously into user space. In theory you can use multiple calls to remap_pfn_range() but getting the correct pfn for memory allocated by dma_alloc_coherent() is essentially impossible on some platforms (e.g. ARM).
I have come up with a solution to this problem that might not be considered "good" but seems to work well enough in my usage on multiple platforms (x86_64, and various ARM). The solution is to temporarily modify the start and end addresses in the struct vm_area_struct while calling dma_mmap_coherent() multiple times, once for each buffer. As long as you reset the VMA start and end addresses to their original values, everything seems to work okay (see my prior disclaimer).
Here is an example:
static int mmap(struct file *file, struct vm_area_struct *vma)
{
. . .
int rc;
unsigned long vm_start_orig = vma->vm_start;
unsigned long vm_end_orig = vma->vm_end;
for (int idx = 0; idx < buffer_list_size; idx++) {
buffer_entry = &buffer_list[idx];
/* Temporarily modify VMA start and end addresses */
if (idx > 0) {
vma->vm_start = vma->vm_end;
}
vma->vm_end = vma->vm_start + buffer_entry->size;
rc = dma_mmap_coherent(dev, vma,
buffer_entry->virt_address,
buffer_entry->phys_addr,
buffer_entry->size);
if (rc != 0) {
pr_err("dma_mmap_coherent: %d (IDX = %d)\n", rc, idx);
return -EAGAIN;
}
}
/* Restore VMA addresses */
vma->vm_start = vm_start_orig;
vma->vm_end = vm_end_orig;
return rc;
}

Answer 2

Unfortunately, the only currently supported method for mmap()ing DMA coherent memory is the macro dma_mmap_coherent() or the function dma_mmap_attrs() (which is called by dma_mmap_coherent()). Unfortunately, that does not support splitting a single VMA across multiple, individually allocated blocks of DMA coherent memory.
(I wish there was a supported way to split the mmap()ing of a VMA across multiple allocations of DMA coherent memory because it affects the buffer allocation in a kernel subsystem that I help maintain. I had to change it to allocate the buffer as a single block of DMA coherent memory instead of many page-sized blocks.)