--- x/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ y/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -644,6 +644,8 @@ int drm_gem_shmem_mmap(struct drm_gem_sh
 		return ret;
 
 	vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
+	if ((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags))
+		return -EINVAL;
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	if (shmem->map_wc)
 		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);