Add callback for the soft memory limit handling

tcmalloc/malloc_extension.cc

@@ -466,6 +466,20 @@ absl::Duration MallocExtension::GetSkipSubreleaseInterval() {
 #endif
 }
 
+static std::atomic<MallocExtension::SoftMemoryLimitCallback*> SoftMemoryLimitHandler_;
+
+void MallocExtension::SetSoftMemoryLimitHandler(SoftMemoryLimitCallback* handler) {
+#if ABSL_INTERNAL_HAVE_WEAK_MALLOCEXTENSION_STUBS
+  SoftMemoryLimitHandler_.store(handler);
+#endif
+}
+
+MallocExtension::SoftMemoryLimitCallback* MallocExtension::GetSoftMemoryLimitHandler() {
+#if ABSL_INTERNAL_HAVE_WEAK_MALLOCEXTENSION_STUBS
+  return SoftMemoryLimitHandler_.load();
+#endif
+}
+
 void MallocExtension::SetSkipSubreleaseInterval(absl::Duration value) {
 #if ABSL_INTERNAL_HAVE_WEAK_MALLOCEXTENSION_STUBS
   if (MallocExtension_Internal_SetSkipSubreleaseInterval == nullptr) {

tcmalloc/malloc_extension.h

@@ -646,6 +646,14 @@ class MallocExtension final {
   // Specifies the release rate from the page heap.  ProcessBackgroundActions
   // must be called for this to be operative.
   static void SetBackgroundReleaseRate(BytesPerSecond rate);
+
+  // Specifies the pointer to a callback function to be invoked when the soft
+  // memory limit is reached.
+  using SoftMemoryLimitCallback = void();
+  static void SetSoftMemoryLimitHandler(SoftMemoryLimitCallback* handler);
+
+  // Gets the current handler of the soft memory limit event.
+  static SoftMemoryLimitCallback* GetSoftMemoryLimitHandler();
 };
 
 }  // namespace tcmalloc

tcmalloc/page_allocator.cc

@@ -138,6 +138,10 @@ void PageAllocator::ShrinkToUsageLimit(Length n) {
   warned = true;
   TC_LOG("Couldn't respect usage limit of %v and OOM is likely to follow.",
          limits_[kSoft]);
+
+  if (auto* handler = MallocExtension::GetSoftMemoryLimitHandler()) {
+    (*handler)();
+  }
 }
 
 bool PageAllocator::ShrinkHardBy(Length pages, LimitKind limit_kind) {