PartitionAlloc: Make in-place realloc() less likely to happen

Source/wtf/PartitionAlloc.cpp

Index: Source/wtf/PartitionAlloc.cpp
diff --git a/Source/wtf/PartitionAlloc.cpp b/Source/wtf/PartitionAlloc.cpp
index dc284080b30c65526498434099140c38135d2e0d..54bae29d25886b498955be4490f859924b12330d 100644
--- a/Source/wtf/PartitionAlloc.cpp
+++ b/Source/wtf/PartitionAlloc.cpp
@@ -810,6 +810,13 @@ bool partitionReallocDirectMappedInPlace(PartitionRootGeneric* root, PartitionPa
     char* charPtr = static_cast<char*>(partitionPageToPointer(page));
 
     if (newSize < currentSize) {
+        size_t mapSize = partitionPageToDirectMapExtent(page)->mapSize;
+
+        // Don't reallocate in-place if new size is less than 80 % of the full
+        // map size, to avoid holding on to too much unused address space.
+        if ((newSize / kSystemPageSize) * 5 < (mapSize / kSystemPageSize) * 4)
+            return false;
+
         // Shrink by decommitting unneeded pages and making them inaccessible.
         size_t decommitSize = currentSize - newSize;
         decommitSystemPages(charPtr + newSize, decommitSize);