Oilpan: Don't use guard pages in NaCl

Source/platform/heap/Heap.h

Index: Source/platform/heap/Heap.h
diff --git a/Source/platform/heap/Heap.h b/Source/platform/heap/Heap.h
index fd8da32387b87e85ca04f5ca93267a7104388095..3b6a3b026e4e37f91f676b56c18db22cd4ef40d3 100644
--- a/Source/platform/heap/Heap.h
+++ b/Source/platform/heap/Heap.h
@@ -57,6 +57,17 @@ const size_t blinkPageBaseMask = ~blinkPageOffsetMask;
 // away the page tables and lead to bad performance.
 const size_t blinkPagesPerRegion = 10;
 
+// TODO(nya): Replace this with something like #if ENABLE_NACL.
+#if 0

[sof, 2015/08/16 16:41:56]

will be adjusted/fixed before landing, presumably?

+// NaCl's system page size is 64 KB. This causes a problem in Oilpan's heap
+// layout because Oilpan allocates two guard pages for each blink page
+// (whose size is 128 KB). After a couple of discussions, we decided not

[sof, 2015/08/16 16:41:56]

Just saying that the address space expenditure would be too large, would do, I
think.

btw, guard page usage like this isn't entirely novel - has it been disabled for
other codebases that use guard pages & target NaCl also? 

+// to use guard pages in NaCl.
+const size_t blinkGuardPageSize = 0;
+#else
+const size_t blinkGuardPageSize = WTF::kSystemPageSize;
+#endif
+
 // Double precision floats are more efficient when 8 byte aligned, so we 8 byte
 // align all allocations even on 32 bit.
 const size_t allocationGranularity = 8;
@@ -298,7 +309,7 @@ private:
 // Blink heap pages are set up with a guard page before and after the payload.
 inline size_t blinkPagePayloadSize()
 {
-    return blinkPageSize - 2 * WTF::kSystemPageSize;
+    return blinkPageSize - 2 * blinkGuardPageSize;
 }
 
 // Blink heap pages are aligned to the Blink heap page size.
@@ -331,7 +342,7 @@ inline bool vTableInitialized(void* objectPointer)
 // aligned.
 inline bool isPageHeaderAddress(Address address)
 {
-    return !((reinterpret_cast<uintptr_t>(address) & blinkPageOffsetMask) - WTF::kSystemPageSize);
+    return !((reinterpret_cast<uintptr_t>(address) & blinkPageOffsetMask) - blinkGuardPageSize);
 }
 #endif
 
@@ -806,7 +817,7 @@ private:
 PLATFORM_EXPORT inline BasePage* pageFromObject(const void* object)
 {
     Address address = reinterpret_cast<Address>(const_cast<void*>(object));
-    BasePage* page = reinterpret_cast<BasePage*>(blinkPageAddress(address) + WTF::kSystemPageSize);
+    BasePage* page = reinterpret_cast<BasePage*>(blinkPageAddress(address) + blinkGuardPageSize);
     ASSERT(page->contains(address));
     return page;
 }