Appears to be fixed with commit:
"drm/nv50-nvc0: make sure vma is definitely unmapped when destroying bo"
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
                vm->spg_shift = 12;
                vm->lpg_shift = 17;
                pgt_bits = 27;
-
-               /* Should be 4096 everywhere, this is a hack that's
-                * currently necessary to avoid an elusive bug that
-                * causes corruption when mixing small/large pages
-                */
-               if (length < (1ULL << 40))
-                       block = 4096;
-               else {
-                       block = (1 << pgt_bits);
-                       if (length < block)
-                               block = length;
-               }
+               block = 4096;
        } else {
                kfree(vm);
                return -ENOSYS;