Fix managed memory misclassified as kDLCUDAHost in DLPack device mapping

cuda_core/cuda/core/_dlpack.pyx

@@ -88,20 +88,28 @@ cdef inline int setup_dl_tensor_layout(DLTensor* dl_tensor, object buf) except -
     return 0
 
 
+def classify_dl_device(buf) -> tuple[int, int]:
+    """Classify a buffer into a DLPack (device_type, device_id) pair.
+
+    ``buf`` must expose ``is_device_accessible``, ``is_host_accessible``,
+    ``is_managed``, and ``device_id`` attributes.
+    """
+    cdef bint d = buf.is_device_accessible
+    cdef bint h = buf.is_host_accessible
+    if d and not h:
+        return (_kDLCUDA, buf.device_id)
+    if d and h:
+        return (_kDLCUDAManaged if buf.is_managed else _kDLCUDAHost, 0)
+    if not d and h:
+        return (_kDLCPU, 0)
+    raise BufferError("buffer is neither device-accessible nor host-accessible")
+
+
 cdef inline int setup_dl_tensor_device(DLTensor* dl_tensor, object buf) except -1:
     cdef DLDevice* device = &dl_tensor.device
-    # buf should be a Buffer instance
-    if buf.is_device_accessible and not buf.is_host_accessible:
-        device.device_type = _kDLCUDA
-        device.device_id = buf.device_id
-    elif buf.is_device_accessible and buf.is_host_accessible:
-        device.device_type = _kDLCUDAHost
-        device.device_id = 0
-    elif not buf.is_device_accessible and buf.is_host_accessible:
-        device.device_type = _kDLCPU
-        device.device_id = 0
-    else:  # not buf.is_device_accessible and not buf.is_host_accessible
-        raise BufferError("invalid buffer")
+    dev_type, dev_id = classify_dl_device(buf)
+    device.device_type = <_DLDeviceType>dev_type
+    device.device_id = <int32_t>dev_id
     return 0
 
 

cuda_core/cuda/core/_memory/_buffer.pxd

@@ -12,6 +12,7 @@ cdef struct _MemAttrs:
     int device_id
     bint is_device_accessible
     bint is_host_accessible
+    bint is_managed
 
 
 cdef class Buffer:

cuda_core/cuda/core/_memory/_buffer.pyx

@@ -34,7 +34,7 @@ if sys.version_info >= (3, 12):
 else:
     BufferProtocol = object
 
-from cuda.core._dlpack import DLDeviceType, make_py_capsule
+from cuda.core._dlpack import classify_dl_device, make_py_capsule
 from cuda.core._utils.cuda_utils import driver
 from cuda.core._device import Device
 
@@ -323,16 +323,7 @@ cdef class Buffer:
         return capsule
 
     def __dlpack_device__(self) -> tuple[int, int]:
-        cdef bint d = self.is_device_accessible
-        cdef bint h = self.is_host_accessible
-        if d and (not h):
-            return (DLDeviceType.kDLCUDA, self.device_id)
-        if d and h:
-            # TODO: this can also be kDLCUDAManaged, we need more fine-grained checks
-            return (DLDeviceType.kDLCUDAHost, 0)
-        if (not d) and h:
-            return (DLDeviceType.kDLCPU, 0)
-        raise BufferError("buffer is neither device-accessible nor host-accessible")
+        return classify_dl_device(self)
 
     def __buffer__(self, flags: int, /) -> memoryview:
         # Support for Python-level buffer protocol as per PEP 688.
@@ -396,6 +387,12 @@ cdef class Buffer:
         _init_mem_attrs(self)
         return self._mem_attrs.is_host_accessible
 
+    @property
+    def is_managed(self) -> bool:
+        """Return True if this buffer is CUDA managed (unified) memory, otherwise False."""
+        _init_mem_attrs(self)
+        return self._mem_attrs.is_managed
+
     @property
     def is_mapped(self) -> bool:
         """Return True if this buffer is mapped into the process via IPC."""
@@ -459,6 +456,7 @@ cdef inline int _query_memory_attrs(
         out.is_host_accessible = True
         out.is_device_accessible = False
         out.device_id = -1
+        out.is_managed = False
     elif (
         is_managed
         or memory_type == cydriver.CUmemorytype.CU_MEMORYTYPE_HOST
@@ -467,10 +465,12 @@ cdef inline int _query_memory_attrs(
         out.is_host_accessible = True
         out.is_device_accessible = True
         out.device_id = device_id
+        out.is_managed = is_managed
     elif memory_type == cydriver.CUmemorytype.CU_MEMORYTYPE_DEVICE:
         out.is_host_accessible = False
         out.is_device_accessible = True
         out.device_id = device_id
+        out.is_managed = False
     else:
         with cython.gil:
             raise ValueError(f"Unsupported memory type: {memory_type}")

cuda_core/cuda/core/_memoryview.pyx

@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 from ._dlpack cimport *
+from ._dlpack import classify_dl_device
 from libc.stdint cimport intptr_t
 from cuda.core._layout cimport _StridedLayout, get_strides_ptr
 from cuda.core._stream import Stream
@@ -590,8 +591,6 @@ cdef inline int _smv_get_dl_device(
     cdef _DLDeviceType device_type
     cdef int32_t device_id
     cdef object buf
-    cdef bint d
-    cdef bint h
     if view.dl_tensor != NULL:
         device_type = view.dl_tensor.device.device_type
         if device_type == _kDLCUDA:
@@ -601,20 +600,9 @@ cdef inline int _smv_get_dl_device(
             device_id = 0
     elif view.is_device_accessible:
         buf = view.get_buffer()
-        d = buf.is_device_accessible
-        h = buf.is_host_accessible
-        if d and (not h):
-            device_type = _kDLCUDA
-            device_id = buf.device_id
-        elif d and h:
-            # We do not currently differentiate pinned vs managed here.
-            device_type = _kDLCUDAHost
-            device_id = 0
-        elif (not d) and h:
-            device_type = _kDLCPU
-            device_id = 0
-        else:
-            raise BufferError("buffer is neither device-accessible nor host-accessible")
+        dev_type, dev_id = classify_dl_device(buf)
+        device_type = <_DLDeviceType>dev_type
+        device_id = <int32_t>dev_id
     else:
         device_type = _kDLCPU
         device_id = 0

cuda_core/tests/test_memory.py

@@ -556,7 +556,7 @@ def test_buffer_dunder_dlpack():
     [
         (DummyDeviceMemoryResource, (DLDeviceType.kDLCUDA, 0)),
         (DummyHostMemoryResource, (DLDeviceType.kDLCPU, 0)),
-        (DummyUnifiedMemoryResource, (DLDeviceType.kDLCUDAHost, 0)),
+        (DummyUnifiedMemoryResource, (DLDeviceType.kDLCUDAManaged, 0)),
         (DummyPinnedMemoryResource, (DLDeviceType.kDLCUDAHost, 0)),
     ],
 )
@@ -579,7 +579,7 @@ def test_buffer_dlpack_failure_clean_up():
     dummy_mr = NullMemoryResource()
     buffer = dummy_mr.allocate(size=1024)
     before = sys.getrefcount(buffer)
-    with pytest.raises(BufferError, match="invalid buffer"):
+    with pytest.raises(BufferError, match="buffer is neither device-accessible nor host-accessible"):
         buffer.__dlpack__()
     after = sys.getrefcount(buffer)
     # we use the buffer refcount as sentinel for proper clean-up here,
@@ -588,6 +588,23 @@ def test_buffer_dlpack_failure_clean_up():
     assert after == before
 
 
+def test_managed_buffer_dlpack_roundtrip_device_type():
+    """Verify that a managed Buffer round-trips through DLPack with kDLCUDAManaged."""
+    device = Device()
+    device.set_current()
+    skip_if_managed_memory_unsupported(device)
+    mr = DummyUnifiedMemoryResource(device)
+    buf = mr.allocate(size=1024)
+
+    # Buffer-level classification should report managed.
+    assert buf.__dlpack_device__() == (DLDeviceType.kDLCUDAManaged, 0)
+
+    # The end-to-end path: Buffer -> DLPack capsule -> StridedMemoryView
+    # must preserve kDLCUDAManaged rather than downgrading to kDLCUDAHost.
+    view = StridedMemoryView.from_any_interface(buf, stream_ptr=-1)
+    assert view.__dlpack_device__() == (int(DLDeviceType.kDLCUDAManaged), 0)
+
+
 @pytest.mark.parametrize("use_device_object", [True, False])
 def test_device_memory_resource_initialization(use_device_object):
     """Test that DeviceMemoryResource can be initialized successfully.