memory: Change memory_region_set_ram_discard_manager() to return the result

Modify memory_region_set_ram_discard_manager() to return -EBUSY if a
RamDiscardManager is already set in the MemoryRegion. The caller must
handle this failure, such as having virtio-mem undo its actions and fail
the realize() process. Opportunistically move the call earlier to avoid
complex error handling.

This change is beneficial when introducing a new RamDiscardManager
instance besides virtio-mem. After
ram_block_coordinated_discard_require(true) unlocks all
RamDiscardManager instances, only one instance is allowed to be set for
one MemoryRegion at present.

Suggested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Tested-by: Alexey Kardashevskiy <aik@amd.com>
Reviewed-by: Alexey Kardashevskiy <aik@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Link: https://lore.kernel.org/r/20250612082747.51539-3-chenyi.qiang@intel.com
Signed-off-by: Peter Xu <peterx@redhat.com>

hw/virtio/virtio-mem.c

@@ -1047,6 +1047,17 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
         return;
     }
 
+    /*
+     * Set ourselves as RamDiscardManager before the plug handler maps the
+     * memory region and exposes it via an address space.
+     */
+    if (memory_region_set_ram_discard_manager(&vmem->memdev->mr,
+                                              RAM_DISCARD_MANAGER(vmem))) {
+        error_setg(errp, "Failed to set RamDiscardManager");
+        ram_block_coordinated_discard_require(false);
+        return;
+    }
+
     /*
      * We don't know at this point whether shared RAM is migrated using
      * QEMU or migrated using the file content. "x-ignore-shared" will be
@@ -1061,6 +1072,7 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
         ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
         if (ret) {
             error_setg_errno(errp, -ret, "Unexpected error discarding RAM");
+            memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
             ram_block_coordinated_discard_require(false);
             return;
         }
@@ -1122,13 +1134,6 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
     vmem->system_reset = VIRTIO_MEM_SYSTEM_RESET(obj);
     vmem->system_reset->vmem = vmem;
     qemu_register_resettable(obj);
-
-    /*
-     * Set ourselves as RamDiscardManager before the plug handler maps the
-     * memory region and exposes it via an address space.
-     */
-    memory_region_set_ram_discard_manager(&vmem->memdev->mr,
-                                          RAM_DISCARD_MANAGER(vmem));
 }
 
 static void virtio_mem_device_unrealize(DeviceState *dev)
@@ -1136,12 +1141,6 @@ static void virtio_mem_device_unrealize(DeviceState *dev)
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
     VirtIOMEM *vmem = VIRTIO_MEM(dev);
 
-    /*
-     * The unplug handler unmapped the memory region, it cannot be
-     * found via an address space anymore. Unset ourselves.
-     */
-    memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
-
     qemu_unregister_resettable(OBJECT(vmem->system_reset));
     object_unref(OBJECT(vmem->system_reset));
 
@@ -1154,6 +1153,11 @@ static void virtio_mem_device_unrealize(DeviceState *dev)
     virtio_del_queue(vdev, 0);
     virtio_cleanup(vdev);
     g_free(vmem->bitmap);
+    /*
+     * The unplug handler unmapped the memory region, it cannot be
+     * found via an address space anymore. Unset ourselves.
+     */
+    memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
     ram_block_coordinated_discard_require(false);
 }
 

include/system/memory.h

@@ -2499,13 +2499,13 @@ static inline bool memory_region_has_ram_discard_manager(MemoryRegion *mr)
  *
  * This function must not be called for a mapped #MemoryRegion, a #MemoryRegion
  * that does not cover RAM, or a #MemoryRegion that already has a
- * #RamDiscardManager assigned.
+ * #RamDiscardManager assigned. Return 0 if the rdm is set successfully.
  *
  * @mr: the #MemoryRegion
  * @rdm: #RamDiscardManager to set
  */
-void memory_region_set_ram_discard_manager(MemoryRegion *mr,
-                                           RamDiscardManager *rdm);
+int memory_region_set_ram_discard_manager(MemoryRegion *mr,
+                                          RamDiscardManager *rdm);
 
 /**
  * memory_region_find: translate an address/size relative to a

system/memory.c

@@ -2106,12 +2106,16 @@ RamDiscardManager *memory_region_get_ram_discard_manager(MemoryRegion *mr)
     return mr->rdm;
 }
 
-void memory_region_set_ram_discard_manager(MemoryRegion *mr,
-                                           RamDiscardManager *rdm)
+int memory_region_set_ram_discard_manager(MemoryRegion *mr,
+                                          RamDiscardManager *rdm)
 {
     g_assert(memory_region_is_ram(mr));
-    g_assert(!rdm || !mr->rdm);
+    if (mr->rdm && rdm) {
+        return -EBUSY;
+    }
+
     mr->rdm = rdm;
+    return 0;
 }
 
 uint64_t ram_discard_manager_get_min_granularity(const RamDiscardManager *rdm,