[sandbox] Schedule GC when EPT utilization reaches certain thresholds

During ExternalPointerTable::Grow, if we cross one of a handful of
predefined utilization thresholds, we now request a (major) GC to free
up entries that are no longer used in the table.

Bug: v8:10391
Change-Id: Id2d262f0f1d4dc37aec1e4978a8be2d223fb2b2b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3890971
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83155}

src/sandbox/external-pointer-table.cc

@@ -309,6 +309,18 @@ uint32_t ExternalPointerTable::Grow(Isolate* isolate) {
 
   set_capacity(new_capacity);
 
+  // Schedule GC when the table's utilization crosses one of these thresholds.
+  constexpr double kGCThresholds[] = {0.5, 0.75, 0.9, 0.95, 0.99};
+  constexpr double kMaxCapacity = static_cast<double>(kMaxExternalPointers);
+  double old_utilization = static_cast<double>(old_capacity) / kMaxCapacity;
+  double new_utilization = static_cast<double>(new_capacity) / kMaxCapacity;
+  for (double threshold : kGCThresholds) {
+    if (old_utilization < threshold && new_utilization >= threshold) {
+      isolate->heap()->ReportExternalMemoryPressure();
+      break;
+    }
+  }
+
   // Build freelist bottom to top, which might be more cache friendly.
   uint32_t start = std::max<uint32_t>(old_capacity, 1);  // Skip entry zero
   uint32_t last = new_capacity - 1;