PartitionAlloc: support in-place resize of directly mapped allocation

Source/wtf/PartitionAllocTest.cpp

Index: Source/wtf/PartitionAllocTest.cpp
diff --git a/Source/wtf/PartitionAllocTest.cpp b/Source/wtf/PartitionAllocTest.cpp
index 531fcd4f70a4547e4d6ee1ace34097ab26b30527..68a83a852df29f6b00d536ddd2690cec4b62bf86 100644
--- a/Source/wtf/PartitionAllocTest.cpp
+++ b/Source/wtf/PartitionAllocTest.cpp
@@ -692,6 +692,23 @@ TEST(WTF_PartitionAlloc, Realloc)
 
     partitionFreeGeneric(genericAllocator.root(), ptr);
 
+    // Test that shrinking a direct mapped allocation happens in-place.
+    size = WTF::kGenericMaxBucketed + 16 * WTF::kSystemPageSize;
+    EXPECT_TRUE(WTF::partitionSizeIsDirectMapped(size));
+    ptr = partitionAllocGeneric(genericAllocator.root(), size);
+    size_t actualSize = partitionAllocGetSize(ptr);
+    ptr2 = partitionReallocGeneric(genericAllocator.root(), ptr, size - WTF::kSystemPageSize);
+    EXPECT_EQ(ptr, ptr2);
+    EXPECT_EQ(actualSize - WTF::kSystemPageSize, partitionAllocGetSize(ptr2));
+
+    // Test that a previously in-place shrunk direct mapped allocation can be
+    // expanded up to its original size again.
+    ptr = partitionReallocGeneric(genericAllocator.root(), ptr2, size);
+    EXPECT_EQ(ptr2, ptr);
+    EXPECT_EQ(actualSize, partitionAllocGetSize(ptr));
+
+    partitionFreeGeneric(genericAllocator.root(), ptr);
+
     TestShutdown();
 }