Fix issue with allocating huge objects in the memory manager

We shouldn't allocate objects that are larger than the size of
a standard memory segment through the chunk allocator, as this
can lead to problems when freeing the segment and then re-using
it again.

Instead allocate a private MemorySegment for these objects, and
free it when the object gets garbage collected.

Task-number: QTBUG-66732
Change-Id: Ic24ff65d204977f313ab0adaf7a8132883e525f0
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
(cherry picked from commit c99abf1851bbbfcec11eb77173df620746940ab0)
Reviewed-by: Erik Verbruggen <erik.verbruggen@qt.io>

src/qml/memory/qv4mm.cpp

@@ -633,8 +633,21 @@ void BlockAllocator::freeAll()
 
 
 HeapItem *HugeItemAllocator::allocate(size_t size) {
-    Chunk *c = chunkAllocator->allocate(size);
-    chunks.push_back(HugeChunk{c, size});
+    MemorySegment *m = nullptr;
+    Chunk *c = nullptr;
+    if (size >= MemorySegment::SegmentSize/2) {
+        // too large to handle through the ChunkAllocator, let's get our own memory segement
+        size_t segmentSize = size + Chunk::HeaderSize; // space required for the Chunk header
+        size_t pageSize = WTF::pageSize();
+        segmentSize = (segmentSize + pageSize - 1) & ~(pageSize - 1); // align to page sizes
+        m = new MemorySegment(segmentSize);
+        size = (size + pageSize - 1) & ~(pageSize - 1); // align to page sizes
+        c = m->allocate(size);
+    } else {
+        c = chunkAllocator->allocate(size);
+    }
+    Q_ASSERT(c);
+    chunks.push_back(HugeChunk{m, c, size});
     Chunk::setBit(c->objectBitmap, c->first() - c->realBase());
     Q_V4_PROFILE_ALLOC(engine, size, Profiling::LargeItem);
 #ifdef V4_USE_HEAPTRACK
@@ -651,7 +664,13 @@ static void freeHugeChunk(ChunkAllocator *chunkAllocator, const HugeItemAllocato
         b->vtable()->destroy(b);
         b->_checkIsDestroyed();
     }
-    chunkAllocator->free(c.chunk, c.size);
+    if (c.segment) {
+        // own memory segment
+        c.segment->free(c.chunk, c.size);
+        delete c.segment;
+    } else {
+        chunkAllocator->free(c.chunk, c.size);
+    }
 #ifdef V4_USE_HEAPTRACK
     heaptrack_report_free(c.chunk);
 #endif

src/qml/memory/qv4mm_p.h

@@ -69,6 +69,7 @@ QT_BEGIN_NAMESPACE
 namespace QV4 {
 
 struct ChunkAllocator;
+struct MemorySegment;
 
 template<typename T>
 struct StackAllocator {
@@ -176,6 +177,7 @@ struct HugeItemAllocator {
     ChunkAllocator *chunkAllocator;
     ExecutionEngine *engine;
     struct HugeChunk {
+        MemorySegment *segment;
         Chunk *chunk;
         size_t size;
     };