Make sure memory threshold callback always fires at least once

Summary:
If live==false the first time we notice memory exceeded the threshold,
we'll never trigger the callback.

Reviewed By: paulbiss

Differential Revision: D4824509

fbshipit-source-id: 795b905eda5d1ec8b13d78a549eb499bf3c415c2

hphp/runtime/base/memory-manager.cpp

@@ -410,9 +410,8 @@ void MemoryManager::refreshStatsImpl(MemoryUsageStats& stats) {
       refreshStatsHelperStop();
     }
 #endif
-    if (live &&
-        stats.usage() > m_memThresholdCallbackPeakUsage &&
-        stats.peakUsage <= m_memThresholdCallbackPeakUsage) {
+    if (live && stats.usage() > m_memThresholdCallbackPeakUsage) {
+      m_memThresholdCallbackPeakUsage = SIZE_MAX;
       setSurpriseFlag(MemThresholdFlag);
     }