our_changes.patch

diff --git a/python/ray/data/_internal/block_batching/pinned_staging.py b/python/ray/data/_internal/block_batching/pinned_staging.py
new file mode 100644
index 0000000000..8a3c285f02
--- /dev/null
+++ b/python/ray/data/_internal/block_batching/pinned_staging.py
@@ -0,0 +1,219 @@
+"""One-batch pinned-memory prefetch for synchronous map_batches GPU actors."""
+
+from concurrent.futures import Future, ThreadPoolExecutor
+from threading import RLock
+
+
+class PinnedPrefetch:
+    """Stage concrete NumPy batches on one producer thread, in FIFO order.
+
+    Only the consumer advances the upstream iterator. Each returned dictionary
+    owns fresh CUDA storage on actor-local cuda:0. There is at most one pending
+    batch in addition to the batch held by the consumer.
+
+    staging_collate_fn, if supplied, receives owned, writable NumPy arrays on
+    the producer thread. It must return a nonempty dict of numeric NumPy arrays
+    or dense CPU tensors. It must not capture the actor/model, launch CUDA work,
+    or call this iterator's methods.
+
+    Consume on one thread and use the returned tensors on that thread's
+    current CUDA stream. A UDF using additional streams must manage their
+    synchronization and record_stream calls itself. Call close() on early exit;
+    the map-task ExitStack and transform generator both do this.
+    """
+
+    def __init__(self, batches, staging_collate_fn=None):
+        if staging_collate_fn is not None and not callable(staging_collate_fn):
+            raise TypeError("staging_collate_fn must be callable or None")
+
+        # Importing this module does not import or initialize torch.
+        import torch
+
+        self._torch = torch
+        self._batches = iter(batches)
+        self._collate_fn = staging_collate_fn
+        self._device = torch.device("cuda", 0)
+        self._stream = torch.cuda.Stream(device=self._device)
+        self._pool = ThreadPoolExecutor(
+            max_workers=1, thread_name_prefix="ray-pinned-staging"
+        )
+        self._pending = None
+        self._started = False
+        self._exhausted = False
+        self._closed = False
+        # Serialize close against next and other close calls. The producer
+        # never acquires this lock; joining it while holding the lock is safe.
+        self._lock = RLock()
+
+    def __iter__(self):
+        return self
+
+    def _submit_next(self):
+        """Called only by the consumer, with the iterator lock held."""
+        if self._exhausted:
+            return None
+        try:
+            batch = next(self._batches)
+        except StopIteration:
+            self._exhausted = True
+            return None
+        except Exception as error:
+            self._exhausted = True
+            failed = Future()
+            failed.set_exception(error)
+            return failed
+
+        try:
+            # Never pass the iterator or next(upstream) to the executor.
+            return self._pool.submit(self._stage, batch)
+        except Exception as error:
+            # Even submission failure during lookahead must preserve batch N.
+            self._exhausted = True
+            failed = Future()
+            failed.set_exception(error)
+            return failed
+
+    @staticmethod
+    def _check_batch(batch):
+        if not isinstance(batch, dict) or not batch:
+            raise TypeError("Pinned staging requires a nonempty dict")
+        if not all(isinstance(key, str) for key in batch):
+            raise TypeError("Pinned staging requires string column names")
+
+    @staticmethod
+    def _copy_array(value):
+        import numpy as np
+
+        if not isinstance(value, np.ndarray) or value.dtype.kind not in "biufc":
+            raise TypeError("Pinned staging requires numeric or boolean arrays")
+        # An owned, writable, native-endian, C-contiguous copy also handles
+        # read-only object-store views and arrays with negative strides.
+        return np.array(
+            value, dtype=value.dtype.newbyteorder("="), order="C", copy=True
+        )
+
+    def _stage(self, batch):
+        import numpy as np
+
+        torch = self._torch
+        host, device = {}, {}
+        with torch.cuda.device(self._device), torch.no_grad():
+            try:
+                with torch.cuda.nvtx.range("ray::collate_pin"):
+                    self._check_batch(batch)
+                    if self._collate_fn is not None:
+                        batch = self._collate_fn(
+                            {
+                                key: self._copy_array(value)
+                                for key, value in batch.items()
+                            }
+                        )
+                        self._check_batch(batch)
+
+                    for key, value in batch.items():
+                        if isinstance(value, np.ndarray):
+                            cpu = torch.from_numpy(self._copy_array(value))
+                        elif (
+                            isinstance(value, torch.Tensor)
+                            and value.device.type == "cpu"
+                            and value.layout == torch.strided
+                            and not value.is_quantized
+                            and not value.is_nested
+                        ):
+                            # pin_memory() may alias an already-pinned tensor.
+                            # Start with fresh pageable storage even if the
+                            # collator keeps or reuses its pinned output.
+                            cpu = torch.empty(
+                                tuple(value.shape),
+                                dtype=value.dtype,
+                                device="cpu",
+                                pin_memory=False,
+                            )
+                            cpu.copy_(value.detach())
+                        else:
+                            raise TypeError(
+                                f"Column {key!r}: expected a numeric NumPy "
+                                "array or a dense, non-quantized CPU tensor"
+                            )
+                        host[key] = cpu.pin_memory()
+
+                with torch.cuda.stream(self._stream):
+                    with torch.cuda.nvtx.range("ray::H2D"):
+                        for key, pinned in host.items():
+                            device[key] = pinned.to(
+                                self._device, non_blocking=True
+                            )
+                        ready = torch.cuda.Event()
+                        ready.record(self._stream)
+
+                # The future completes only AFTER H2D completes. This wait is
+                # on the producer thread; compute for batch N can continue
+                # while the producer stages N+1. Keep all pinned sources alive.
+                ready.synchronize()
+                return device, ready
+            except BaseException as error:
+                # A later column's copy, event creation, or event recording
+                # can fail after earlier H2D work has already been issued.
+                # Drain before releasing any pinned source.
+                self._stream.synchronize()
+                if isinstance(error, StopIteration):
+                    raise RuntimeError(
+                        "staging_collate_fn raised StopIteration"
+                    ) from error
+                raise
+            finally:
+                host.clear()
+
+    def __next__(self):
+        with self._lock:
+            if self._closed:
+                raise StopIteration
+            try:
+                if not self._started:
+                    self._started = True
+                    self._pending = self._submit_next()
+                if self._pending is None:
+                    self.close()
+                    raise StopIteration
+
+                # result() waits for the producer's ready.synchronize().
+                device, ready = self._pending.result()
+                self._pending = None
+                stream = self._torch.cuda.current_stream(self._device)
+                stream.wait_event(ready)
+                for value in device.values():
+                    value.record_stream(stream)
+
+                # Start N+1 before handing N to the UDF. An upstream exception
+                # is stored in a Future and raised on the next consumption.
+                self._pending = self._submit_next()
+                return device
+            except BaseException:
+                self.close()
+                raise
+
+    def close(self):
+        """Join staging, drain issued H2D, and release unconsumed resources.
+
+        Idempotent, including calls through both finally and ExitStack. Does
+        not advance upstream or surface errors from unconsumed lookahead.
+        Returned CUDA tensors remain owned by the UDF and are never overwritten.
+        """
+        with self._lock:
+            if self._closed:
+                return
+            self._closed = True
+            try:
+                if self._pending is not None:
+                    self._pending.cancel()
+                # Running jobs cannot be cancelled. _stage drains both normal
+                # and partially-issued H2D before returning or raising, so
+                # joining the producer also drains all issued copies.
+                self._pool.shutdown(wait=True, cancel_futures=True)
+            finally:
+                self._pending = None
+                self._batches = iter(())
+                self._collate_fn = None
+                self._pool = None
+                self._stream = None
+                self._torch = None
diff --git a/python/ray/data/_internal/compute.py b/python/ray/data/_internal/compute.py
index d8eb354c0e..e120814bd4 100644
--- a/python/ray/data/_internal/compute.py
+++ b/python/ray/data/_internal/compute.py
@@ -116,6 +116,8 @@ class ActorPoolStrategy(ComputeStrategy):
         max_tasks_in_flight_per_actor: Optional[int] = None,
         max_concurrent_calls_per_actor: Optional[int] = None,
         enable_true_multi_threading: Optional[bool] = None,
+        pinned_staging: bool = False,
+        staging_collate_fn: Optional[Callable] = None,
     ):
         """Construct ActorPoolStrategy for a Dataset transform.
 
@@ -137,6 +139,16 @@ class ActorPoolStrategy(ComputeStrategy):
                 than 1 UDF runs per actor. Otherwise, respects the `max_concurrent_calls_per_actor` argument.
                 By default, this flag is `None`, which gets translated to `False`.
                 For more details, see the `ActorPoolStrategy` class docstring.
+            pinned_staging: Experimental map_batches-only CUDA staging. Defaults
+                to False. Requires num_gpus=1, a positive integer batch_size,
+                batch_format="numpy", zero_copy_batch=False and a synchronous
+                UDF. Sets actor call concurrency to 1 and disables fusion.
+                The UDF receives a dict of CUDA tensors on local cuda:0 instead
+                of NumPy arrays. Prefetches one batch within each actor task.
+            staging_collate_fn: Optional deterministic CPU-only callable,
+                executed on the staging thread with an owned NumPy batch.
+                Return a nonempty dict of dense CPU tensors or numeric arrays.
+                Must not capture the actor/model or launch CUDA work.
         """
         if size is not None:
             if size < 1:
@@ -172,6 +184,19 @@ class ActorPoolStrategy(ComputeStrategy):
                 max_concurrent_calls_per_actor,
             )
 
+        if staging_collate_fn is not None and (
+            not pinned_staging or not callable(staging_collate_fn)
+        ):
+            raise ValueError("staging_collate_fn requires pinned_staging=True")
+        if pinned_staging:
+            if enable_true_multi_threading or max_concurrent_calls_per_actor not in (
+                None, 1
+            ):
+                raise ValueError("pinned_staging requires one synchronous actor call")
+            max_concurrent_calls_per_actor = 1
+        self.pinned_staging = pinned_staging
+        self.staging_collate_fn = staging_collate_fn
+
         self.min_size = min_size or 1
         self.max_size = max_size or float("inf")
 
@@ -213,6 +238,10 @@ class ActorPoolStrategy(ComputeStrategy):
             and self.max_size == other.max_size
             and self.initial_size == other.initial_size
             and self.enable_true_multi_threading == other.enable_true_multi_threading
+            and getattr(self, "pinned_staging", False)
+            == getattr(other, "pinned_staging", False)
+            and getattr(self, "staging_collate_fn", None)
+            == getattr(other, "staging_collate_fn", None)
             and self.max_tasks_in_flight_per_actor
             == other.max_tasks_in_flight_per_actor
             and self.max_concurrent_calls_per_actor
@@ -226,6 +255,7 @@ class ActorPoolStrategy(ComputeStrategy):
             f"initial_size={self.initial_size}, "
             f"max_tasks_in_flight_per_actor={self.max_tasks_in_flight_per_actor}, "
             f"max_concurrent_calls_per_actor={self.max_concurrent_calls_per_actor}, "
+            f"pinned_staging={getattr(self, 'pinned_staging', False)}, "
             f"num_workers={self.num_workers}, "
             f"enable_true_multi_threading={self.enable_true_multi_threading}, "
             f"ready_to_total_workers_ratio={self.ready_to_total_workers_ratio})"
diff --git a/python/ray/data/_internal/execution/interfaces/task_context.py b/python/ray/data/_internal/execution/interfaces/task_context.py
index 35ce6506ea..1701595883 100644
--- a/python/ray/data/_internal/execution/interfaces/task_context.py
+++ b/python/ray/data/_internal/execution/interfaces/task_context.py
@@ -47,6 +47,11 @@ class TaskContext:
     # Override of the target max-block-size for the task
     target_max_block_size_override: Optional[int] = None
 
+    # Worker-local cleanup, installed by _map_task (never sent from the driver).
+    _pinned_staging_cleanup: Optional[contextlib.ExitStack] = field(
+        default=None, init=False, repr=False, compare=False
+    )
+
     # Additional keyword arguments passed to the task.
     kwargs: Dict[str, Any] = field(default_factory=dict)
 
diff --git a/python/ray/data/_internal/execution/operators/map_operator.py b/python/ray/data/_internal/execution/operators/map_operator.py
index 818c62cf31..dcd43c335c 100644
--- a/python/ray/data/_internal/execution/operators/map_operator.py
+++ b/python/ray/data/_internal/execution/operators/map_operator.py
@@ -6,6 +6,7 @@ import logging
 import math
 import time
 from abc import ABC, abstractmethod
+from contextlib import ExitStack
 from dataclasses import replace
 from typing import (
     TYPE_CHECKING,
@@ -818,7 +819,12 @@ def _map_task(
 
     ctx.kwargs.update(kwargs)
 
-    with DataContext.current(data_context), TaskContext.current(ctx):
+    with (
+        DataContext.current(data_context),
+        TaskContext.current(ctx),
+        ExitStack() as staging_cleanup,
+    ):
+        ctx._pinned_staging_cleanup = staging_cleanup
         map_transformer.override_target_max_block_size(
             ctx.target_max_block_size_override
         )
@@ -843,6 +849,9 @@ def _map_task(
         udf_time_scope = UDFTimeScope()
 
         def transform_iter_factory():
+            # Close the previous attempt before creating a fresh producer.
+            # The outer ExitStack also handles cancellation/GeneratorExit.
+            staging_cleanup.close()
             # Clear any per-task custom stats before each attempt (the reporter
             # is reused across retries of this task), so a prior attempt's stats
             # can't leak into this one. A producing transform repopulates it
diff --git a/python/ray/data/_internal/planner/plan_udf_map_op.py b/python/ray/data/_internal/planner/plan_udf_map_op.py
index 8d0af2b4d4..9d5e740a78 100644
--- a/python/ray/data/_internal/planner/plan_udf_map_op.py
+++ b/python/ray/data/_internal/planner/plan_udf_map_op.py
@@ -297,6 +297,32 @@ def plan_udf_map_op(
     )
 
     compute = get_compute(op.compute)
+    pinned_staging = getattr(compute, "pinned_staging", False)
+    if pinned_staging:
+        from ray.data._internal.utils.torch_inference import (
+            _BaseTorchInferenceUDFWrapper,
+        )
+
+        user_fn = op.fn.__call__ if isinstance(op.fn, CallableClass) else op.fn
+        if (
+            not isinstance(op, MapBatches)
+            or op.zero_copy_batch
+            or op.batch_format != "numpy"
+            or type(op.batch_size) is not int
+            or op.batch_size < 1
+            or op.ray_remote_args.get("num_gpus") != 1
+            or op.ray_remote_args_fn is not None
+            or _is_async_udf(user_fn)
+            or (
+                isinstance(op.fn, type)
+                and issubclass(op.fn, _BaseTorchInferenceUDFWrapper)
+            )
+        ):
+            raise ValueError(
+                "pinned_staging requires synchronous map_batches, "
+                "zero_copy_batch=False, batch_format='numpy', integer batch_size, "
+                "num_gpus=1, and no TorchInference wrapper or ray_remote_args_fn"
+            )
     udf_is_callable_class = isinstance(op.fn, CallableClass)
     fn, init_fn = _get_udf(
         op.fn,
@@ -309,10 +335,15 @@ def plan_udf_map_op(
 
     if isinstance(op, MapBatches):
         transform_fn = BatchMapTransformFn(
-            _generate_transform_fn_for_map_batches(fn),
+            _generate_transform_fn_for_map_batches(
+                fn, pinned_staging=pinned_staging,
+                staging_collate_fn=getattr(compute, "staging_collate_fn", None),
+            ),
             batch_size=op.batch_size,
             batch_format=op.batch_format,
-            zero_copy_batch=op.zero_copy_batch,
+            # Private CPU views are never given to the UDF. Staging makes
+            # owned pinned/device copies, and copies collator input if needed.
+            zero_copy_batch=True if pinned_staging else op.zero_copy_batch,
             is_udf=True,
             output_block_size_option=output_block_size_option,
         )
@@ -343,6 +374,7 @@ def plan_udf_map_op(
         ray_remote_args_fn=op.ray_remote_args_fn,
         ray_remote_args=op.ray_remote_args,
         per_block_limit=op.per_block_limit,
+        supports_fusion=not pinned_staging,
     )
 
 
@@ -376,6 +408,9 @@ def _get_udf(
             not is_async_udf
             and isinstance(compute, ActorPoolStrategy)
             and not compute.enable_true_multi_threading
+            # Staging fixes actor call concurrency at one; keep the UDF on
+            # the thread/stream where staged inputs are handed off.
+            and not getattr(compute, "pinned_staging", False)
         ):
             # NOTE: By default Actor-based UDFs are restricted to run within a
             # single-thread (when enable_true_multi_threading=False).
@@ -621,9 +656,25 @@ class _TransformingBatchIterator(Iterator[DataBatch]):
 
 def _generate_transform_fn_for_map_batches(
     fn: UserDefinedFunction,
+    *,
+    pinned_staging: bool = False,
+    staging_collate_fn: Optional[Callable] = None,
 ) -> MapTransformCallable[DataBatch, DataBatch]:
 
-    if _is_async_udf(fn):
+    if pinned_staging:
+        def transform_fn(batches, ctx):
+            from ray.data._internal.block_batching.pinned_staging import PinnedPrefetch
+
+            if ctx._pinned_staging_cleanup is None:
+                raise RuntimeError("Pinned staging requires a map task cleanup scope")
+            staged = PinnedPrefetch(batches, staging_collate_fn)
+            ctx._pinned_staging_cleanup.callback(staged.close)
+            try:
+                yield from _TransformingBatchIterator(staged, fn)
+            finally:
+                staged.close()
+
+    elif _is_async_udf(fn):
         transform_fn = _generate_transform_fn_for_async_map(
             fn,
             _validate_batch_output,
diff --git a/python/ray/data/tests/block_batching/test_pinned_staging.py b/python/ray/data/tests/block_batching/test_pinned_staging.py
new file mode 100644
index 0000000000..4718d0ffb6
--- /dev/null
+++ b/python/ray/data/tests/block_batching/test_pinned_staging.py
@@ -0,0 +1,133 @@
+import sys
+import threading
+from concurrent.futures import ThreadPoolExecutor
+from contextlib import ExitStack
+from types import SimpleNamespace
+
+import pytest
+
+from ray.data._internal.block_batching.pinned_staging import PinnedPrefetch
+
+
+@pytest.fixture
+def fake_prefetch(monkeypatch):
+    stream = SimpleNamespace(wait_event=lambda event: None)
+    torch = SimpleNamespace(
+        device=lambda *args: "cuda:0",
+        cuda=SimpleNamespace(
+            Stream=lambda **kwargs: stream,
+            current_stream=lambda device: stream,
+        ),
+    )
+    monkeypatch.setitem(sys.modules, "torch", torch)
+
+    class Prefetch(PinnedPrefetch):
+        def _stage(self, batch):
+            return {
+                "x": SimpleNamespace(value=batch, record_stream=lambda stream: None)
+            }, None
+
+    return Prefetch
+
+
+def test_fifo_thread_boundary_and_lookahead(fake_prefetch):
+    owner = threading.get_ident()
+    produced = []
+    second_started = threading.Event()
+
+    class Prefetch(fake_prefetch):
+        def _stage(self, batch):
+            assert threading.get_ident() != owner
+            produced.append(batch)
+            if batch == 1:
+                second_started.set()
+            return super()._stage(batch)
+
+    def upstream():
+        for i in range(4):
+            assert threading.get_ident() == owner
+            yield i
+
+    p = Prefetch(upstream())
+    try:
+        assert next(p)["x"].value == 0
+        # N+1 staging is running while the consumer still owns N.
+        assert second_started.wait(3)
+        assert [b["x"].value for b in p] == [1, 2, 3]
+        assert produced == [0, 1, 2, 3]
+    finally:
+        p.close()
+
+
+def test_lookahead_error_preserves_good_batch(fake_prefetch):
+    def upstream():
+        yield 7
+        raise ValueError("upstream failed")
+
+    p = fake_prefetch(upstream())
+    assert next(p)["x"].value == 7
+    with pytest.raises(ValueError, match="upstream failed"):
+        next(p)
+    assert p._closed
+
+
+def test_cleanup_joins_running_producer_before_retry(fake_prefetch):
+    started, release, drained = (threading.Event() for _ in range(3))
+
+    class Prefetch(fake_prefetch):
+        def _stage(self, batch):
+            if batch == 1:
+                started.set()
+                assert release.wait(3)
+                drained.set()
+            return super()._stage(batch)
+
+    p = Prefetch(iter([0, 1, 2]))
+    cleanup = ExitStack()
+    cleanup.callback(p.close)
+    assert next(p)["x"].value == 0
+    assert started.wait(3)
+    with ThreadPoolExecutor(max_workers=1) as closer:
+        closing = closer.submit(cleanup.close)
+        try:
+            assert not drained.is_set()
+            assert not closing.done()
+        finally:
+            release.set()
+        closing.result(timeout=3)
+    assert drained.is_set()
+    assert p._closed and p._pending is None
+    retry = fake_prefetch(iter([0, 1, 2]))
+    assert [b["x"].value for b in retry] == [0, 1, 2]
+
+
+def test_cuda_owned_buffers_mutating_collator_and_tail():
+    import numpy as np
+
+    torch = pytest.importorskip("torch")
+    if not torch.cuda.is_available():
+        pytest.skip("CUDA required")
+
+    source = np.arange(8, dtype=np.float32).reshape(4, 2)
+    source.flags.writeable = False
+
+    def collate(batch):
+        batch["x"] += 1
+        return batch
+
+    compute = torch.cuda.Stream()
+    retained = []
+    with torch.cuda.stream(compute):
+        p = PinnedPrefetch(iter([{"x": source}, {"x": source[:1]}]), collate)
+        try:
+            for batch in p:
+                retained.append(batch["x"])
+                # Exercise mutation outside torch.inference_mode.
+                batch["x"].add_(2)
+        finally:
+            p.close()
+    compute.synchronize()
+    np.testing.assert_array_equal(source, np.arange(8).reshape(4, 2))
+    np.testing.assert_array_equal(retained[0].cpu().numpy(), source + 3)
+    np.testing.assert_array_equal(retained[1].cpu().numpy(), source[:1] + 3)
+    assert retained[0].data_ptr() != retained[1].data_ptr()