--- a/sys/dev/drm/amd/amdgpu/amdgpu_mn.c
+++ b/sys/dev/drm/amd/amdgpu/amdgpu_mn.c
@@ -83,6 +83,7 @@
 	/* objects protected by lock */
 	struct rw_semaphore	lock;
 	struct rb_root_cached	objects;
 	struct lock		read_lock;
 	atomic_t		recursion;
+	struct kref		kref;
 };
@@ -394,8 +395,10 @@
 	hash_for_each_possible(adev->mn_hash, amn, node, key)
 		if (AMDGPU_MN_KEY(amn->mm, amn->type) == key)
+			kref_get(&amn->kref);
 			goto release_locks;
 
 	amn = kzalloc(sizeof(*amn), GFP_KERNEL);
@@ -410,6 +413,7 @@
 	amn->mn.ops = &amdgpu_mn_ops[type];
 	amn->objects = RB_ROOT_CACHED;
 	mutex_init(&amn->read_lock);
 	atomic_set(&amn->recursion, 0);
+	kref_init(&amn->kref);
 
 	r = __mmu_notifier_register(&amn->mn, mm);
--- a/sys/dev/drm/amd/amdgpu/amdgpu_mn.h
+++ b/sys/dev/drm/amd/amdgpu/amdgpu_mn.h
@@ -44,6 +44,12 @@
 #else
+/*
+ * WARNING: DragonFly has no MMU-notifier implementation behind
+ * <linux/mmu_notifier.h>; amdgpu_mn.c is deliberately NOT in the module
+ * build.  These stubs keep userptr BOs uncreatable (-ENODEV) and the CS
+ * path lock-free on a NULL mn.  Do not define CONFIG_MMU_NOTIFIER until
+ * sys/vm grows real invalidate_range_start/end + release hooks that the
+ * shim's registration API actually wires into.
+ */
 static inline void amdgpu_mn_lock(struct amdgpu_mn *mn) {}
 static inline void amdgpu_mn_unlock(struct amdgpu_mn *mn) {}