memory: Fix qemu crash on starting dirty log twice with stopped VM

QEMU can now easily crash with two continuous migration carried out:

(qemu) migrate -d exec:cat>out
(qemu) migrate_cancel
(qemu) migrate -d exec:cat>out
[crash] ../softmmu/memory.c:2782: memory_global_dirty_log_start: Assertion
`!(global_dirty_tracking & flags)' failed.

It's because memory API provides a way to postpone dirty log stop if the VM is
stopped, and that'll be re-done until the next VM start.  It was added in 2017
with commit 1931076077 ("migration: optimize the downtime", 2017-08-01).

However the recent work on allowing dirty tracking to be bitmask broke it,
which is commit 63b41db4bc ("memory: make global_dirty_tracking a bitmask",
2021-11-01).

The fix proposed in this patch contains two things:

  (1) Instead of passing over the flags to postpone stop dirty track, we add a
      global variable (along with current vmstate_change variable) to record
      what flags to stop dirty tracking.

  (2) When start dirty tracking, instead if remove the vmstate hook directly,
      we also execute the postponed stop process so that we make sure all the
      starts and stops will be paired.

This procedure is overlooked in the bitmask-ify work in 2021.

Cc: Hyman Huang <huangy81@chinatelecom.cn>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2044818
Fixes: 63b41db4bc ("memory: make global_dirty_tracking a bitmask")
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220207123019.27223-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 45 insertions, 16 deletions

diff --git a/softmmu/memory.c b/softmmu/memory.c
index 678dc62f06..8060c6de78 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -2790,19 +2790,32 @@ void memory_global_after_dirty_log_sync(void)
     MEMORY_LISTENER_CALL_GLOBAL(log_global_after_sync, Forward);
 }
 
+/*
+ * Dirty track stop flags that are postponed due to VM being stopped.  Should
+ * only be used within vmstate_change hook.
+ */
+static unsigned int postponed_stop_flags;
 static VMChangeStateEntry *vmstate_change;
+static void memory_global_dirty_log_stop_postponed_run(void);
 
 void memory_global_dirty_log_start(unsigned int flags)
 {
-    unsigned int old_flags = global_dirty_tracking;
+    unsigned int old_flags;
+
+    assert(flags && !(flags & (~GLOBAL_DIRTY_MASK)));
 
     if (vmstate_change) {
-        qemu_del_vm_change_state_handler(vmstate_change);
-        vmstate_change = NULL;
+        /* If there is postponed stop(), operate on it first */
+        postponed_stop_flags &= ~flags;
+        memory_global_dirty_log_stop_postponed_run();
     }
 
-    assert(flags && !(flags & (~GLOBAL_DIRTY_MASK)));
-    assert(!(global_dirty_tracking & flags));
+    flags &= ~global_dirty_tracking;
+    if (!flags) {
+        return;
+    }
+
+    old_flags = global_dirty_tracking;
     global_dirty_tracking |= flags;
     trace_global_dirty_changed(global_dirty_tracking);
 
@@ -2830,29 +2843,45 @@ static void memory_global_dirty_log_do_stop(unsigned int flags)
     }
 }
 
+/*
+ * Execute the postponed dirty log stop operations if there is, then reset
+ * everything (including the flags and the vmstate change hook).
+ */
+static void memory_global_dirty_log_stop_postponed_run(void)
+{
+    /* This must be called with the vmstate handler registered */
+    assert(vmstate_change);
+
+    /* Note: postponed_stop_flags can be cleared in log start routine */
+    if (postponed_stop_flags) {
+        memory_global_dirty_log_do_stop(postponed_stop_flags);
+        postponed_stop_flags = 0;
+    }
+
+    qemu_del_vm_change_state_handler(vmstate_change);
+    vmstate_change = NULL;
+}
+
 static void memory_vm_change_state_handler(void *opaque, bool running,
                                            RunState state)
 {
-    unsigned int flags = (unsigned int)(uintptr_t)opaque;
     if (running) {
-        memory_global_dirty_log_do_stop(flags);
-
-        if (vmstate_change) {
-            qemu_del_vm_change_state_handler(vmstate_change);
-            vmstate_change = NULL;
-        }
+        memory_global_dirty_log_stop_postponed_run();
     }
 }
 
 void memory_global_dirty_log_stop(unsigned int flags)
 {
     if (!runstate_is_running()) {
+        /* Postpone the dirty log stop, e.g., to when VM starts again */
         if (vmstate_change) {
-            return;
+            /* Batch with previous postponed flags */
+            postponed_stop_flags |= flags;
+        } else {
+            postponed_stop_flags = flags;
+            vmstate_change = qemu_add_vm_change_state_handler(
+                memory_vm_change_state_handler, NULL);
         }
-        vmstate_change = qemu_add_vm_change_state_handler(
-                                memory_vm_change_state_handler,
-                                (void *)(uintptr_t)flags);
         return;
     }