[SNIP]

But Jason pointed me to the right piece of code. See this comment in in mmap_region():
		/* ->mmap() can change vma->vm_file, but must guarantee that
		 * vma_link() below can deny write-access if VM_DENYWRITE is set
		 * and map writably if VM_SHARED is set. This usually means the
		 * new file must not have been exposed to user-space, yet.
		 */
		vma->vm_file = get_file(file);
		error = call_mmap(file, vma);
So changing vma->vm_file is allowed at least under certain circumstances.

Only the "file must not have been exposed to user-space, yet" part still needs double checking. Currently working on that.

Ok, I think we can guarantee for all DMA-bufs what is required here.

While searching the code I've found that at least vgem_prime_mmap() and i915_gem_dmabuf_mmap() are doing the same thing of modifying vma->vm_file.

So I'm leaning towards that this works as expected and we should just document this properly.

Daniel and Jason what do you think?

Christian.