memory: only resize dirty bitmap when memory size increases

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Orit Wasserman <owasserm@redhat.com>

exec.c

@@ -1211,6 +1211,9 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
                                    MemoryRegion *mr)
 {
     RAMBlock *block, *new_block;
+    ram_addr_t old_ram_size, new_ram_size;
+
+    old_ram_size = last_ram_offset() >> TARGET_PAGE_BITS;
 
     size = TARGET_PAGE_ALIGN(size);
     new_block = g_malloc0(sizeof(*new_block));
@@ -1271,10 +1274,13 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
     ram_list.version++;
     qemu_mutex_unlock_ramlist();
 
-    ram_list.phys_dirty = g_realloc(ram_list.phys_dirty,
-                                       last_ram_offset() >> TARGET_PAGE_BITS);
-    memset(ram_list.phys_dirty + (new_block->offset >> TARGET_PAGE_BITS),
+    new_ram_size = last_ram_offset() >> TARGET_PAGE_BITS;
+
+    if (new_ram_size > old_ram_size) {
+        ram_list.phys_dirty = g_realloc(ram_list.phys_dirty, new_ram_size);
+        memset(ram_list.phys_dirty + (new_block->offset >> TARGET_PAGE_BITS),
            0, size >> TARGET_PAGE_BITS);
+    }
     cpu_physical_memory_set_dirty_range(new_block->offset, size);
 
     qemu_ram_setup_dump(new_block->host, size);