From c68fa39ced8b4b44bc453d6e45bb7d714201309f Mon Sep 17 00:00:00 2001
From: youkaichao <youkaichao@gmail.com>
Date: Mon, 27 Apr 2026 09:37:22 +0800
Subject: [PATCH] Auto-disable expandable_segments around cumem memory pool
 (#40812)

Signed-off-by: youkaichao <youkaichao@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
(cherry picked from commit 2ce95a761b9acd925d1bc69cfd1d4fc13de9e2b7)
---
 vllm/device_allocator/cumem.py | 72 +++++++++++++++++++---------------
 1 file changed, 40 insertions(+), 32 deletions(-)

diff --git a/vllm/device_allocator/cumem.py b/vllm/device_allocator/cumem.py
index 554a34b6a68..b2202b7d08d 100644
--- a/vllm/device_allocator/cumem.py
+++ b/vllm/device_allocator/cumem.py
@@ -128,13 +128,6 @@ class CuMemAllocator:
         return CuMemAllocator.instance
 
     def __init__(self):
-        conf = os.environ.get("PYTORCH_CUDA_ALLOC_CONF", "")
-        assert "expandable_segments:True" not in conf, (
-            "Expandable segments are not compatible with memory pool. "
-            "Please track https://github.com/pytorch/pytorch/issues/147851 "
-            "for the latest updates."
-        )
-
         self.pointer_to_data: dict[int, AllocationData] = {}
         self.current_tag: str = CuMemAllocator.default_tag
         self.allocator_and_pools: dict[str, Any] = {}
@@ -264,34 +257,49 @@ class CuMemAllocator:
 
         assert isinstance(tag, str)
 
+        # Expandable segments are incompatible with the memory pool used for
+        # sleep mode (see https://github.com/pytorch/pytorch/issues/147851).
+        # If the user has enabled expandable segments via
+        # PYTORCH_CUDA_ALLOC_CONF, temporarily disable them for the duration
+        # of the memory pool context and restore on exit.
+        conf = os.environ.get("PYTORCH_CUDA_ALLOC_CONF", "")
+        expandable_was_enabled = "expandable_segments:True" in conf
+        if expandable_was_enabled:
+            torch.cuda.memory._set_allocator_settings("expandable_segments:False")
+
         old_tag = self.current_tag
         self.current_tag = tag
-        with use_memory_pool_with_allocator(
-            self.python_malloc_callback, self.python_free_callback
-        ) as data:
-            # start to hit another PyTorch bug in PyTorch 2.6,
-            # possibly because of gc-related issue w.r.t. the allocator and
-            # the memory pool.
-            # to avoid the issue, we keep a reference of the data.
-            # see https://github.com/pytorch/pytorch/issues/146431 .
-            self.allocator_and_pools[tag] = data
-            yield
-            # PyTorch's bug, calling torch.cuda.empty_cache() will error
-            # when using pluggable allocator, see
-            # https://github.com/pytorch/pytorch/issues/145168 .
-            # if we have some memory allocated and then freed,
-            # the memory will not be released, e.g. in online quantization,
-            # where the model is created in higher precision, and then
-            # quantized in lower precision.
-            # Find all unused allocations and manually release them.
-            # TODO: we should expose `empty_cache` method in the memory pool.
-            # TODO: ask for help from PyTorch team to expose this method.
-            allocations = data[0].snapshot()
-            for allocation in allocations:
-                if allocation["allocated_size"] == 0:
-                    handle = self._python_free_callback(allocation["address"])
-                    unmap_and_release(handle)
+        try:
+            with use_memory_pool_with_allocator(
+                self.python_malloc_callback, self.python_free_callback
+            ) as data:
+                # start to hit another PyTorch bug in PyTorch 2.6,
+                # possibly because of gc-related issue w.r.t. the allocator
+                # and the memory pool.
+                # to avoid the issue, we keep a reference of the data.
+                # see https://github.com/pytorch/pytorch/issues/146431 .
+                self.allocator_and_pools[tag] = data
+                yield
+                # PyTorch's bug, calling torch.cuda.empty_cache() will error
+                # when using pluggable allocator, see
+                # https://github.com/pytorch/pytorch/issues/145168 .
+                # if we have some memory allocated and then freed,
+                # the memory will not be released, e.g. in online
+                # quantization, where the model is created in higher
+                # precision, and then quantized in lower precision.
+                # Find all unused allocations and manually release them.
+                # TODO: we should expose `empty_cache` method in the memory
+                # pool.
+                # TODO: ask for help from PyTorch team to expose this method.
+                allocations = data[0].snapshot()
+                for allocation in allocations:
+                    if allocation["allocated_size"] == 0:
+                        handle = self._python_free_callback(allocation["address"])
+                        unmap_and_release(handle)
+        finally:
             self.current_tag = old_tag
+            if expandable_was_enabled:
+                torch.cuda.memory._set_allocator_settings("expandable_segments:True")
 
     def get_current_usage(self) -> int:
         """