diff --git a/libgo/runtime/malloc.goc b/libgo/runtime/malloc.goc
index 8ccaa6b..e5ab1e5 100644
--- a/libgo/runtime/malloc.goc
+++ b/libgo/runtime/malloc.goc
@@ -339,12 +339,14 @@ runtime_mallocinit(void)
 	extern byte _end[];
 	byte *want;
 	uintptr limit;
+	bool is_reserved;
 
 	runtime_sizeof_C_MStats = sizeof(MStats);
 
 	p = nil;
 	arena_size = 0;
 	bitmap_size = 0;
+	is_reserved = false;
 	
 	// for 64-bit build
 	USED(p);
@@ -389,7 +391,8 @@ runtime_mallocinit(void)
 		// If this fails we fall back to the 32 bit memory mechanism
 		arena_size = MaxMem;
 		bitmap_size = arena_size / (sizeof(void*)*8/4);
-		p = runtime_SysReserve((void*)(0x00c0ULL<<32), bitmap_size + arena_size);
+		p = runtime_SysReserve((void*)(0x00c0ULL<<32), bitmap_size + arena_size, false);
+		is_reserved = false;
 	}
 	if (p == nil) {
 		// On a 32-bit machine, we can't typically get away
@@ -428,7 +431,8 @@ runtime_mallocinit(void)
 		want = (byte*)(((uintptr)_end + (1<<18) + (1<<20) - 1)&~((1<<20)-1));
 		if(0xffffffff - (uintptr)want <= bitmap_size + arena_size)
 		  want = 0;
-		p = runtime_SysReserve(want, bitmap_size + arena_size);
+		p = runtime_SysReserve(want, bitmap_size + arena_size, true);
+		is_reserved = true;
 		if(p == nil)
 			runtime_throw("runtime: cannot reserve arena virtual address space");
 		if((uintptr)p & (((uintptr)1<<PageShift)-1))
@@ -441,6 +445,7 @@ runtime_mallocinit(void)
 	runtime_mheap->arena_start = p + bitmap_size;
 	runtime_mheap->arena_used = runtime_mheap->arena_start;
 	runtime_mheap->arena_end = runtime_mheap->arena_start + arena_size;
+	runtime_mheap->is_reserved = is_reserved;
 
 	// Initialize the rest of the allocator.	
 	runtime_MHeap_Init(runtime_mheap, runtime_SysAlloc);
@@ -462,12 +467,15 @@ runtime_MHeap_SysAlloc(MHeap *h, uintptr n)
 		byte *new_end;
 		uintptr needed;
 
+                if (!h->is_reserved) {
+			runtime_throw("runtime: cannot allocate memory");
+		}
 		needed = (uintptr)h->arena_used + n - (uintptr)h->arena_end;
 		// Round wanted arena size to a multiple of 256MB.
 		needed = (needed + (256<<20) - 1) & ~((256<<20)-1);
 		new_end = h->arena_end + needed;
 		if(new_end <= h->arena_start + MaxArena32) {
-			p = runtime_SysReserve(h->arena_end, new_end - h->arena_end);
+			p = runtime_SysReserve(h->arena_end, new_end - h->arena_end, true);
 			if(p == h->arena_end)
 				h->arena_end = new_end;
 		}
@@ -475,7 +483,7 @@ runtime_MHeap_SysAlloc(MHeap *h, uintptr n)
 	if(n <= (uintptr)(h->arena_end - h->arena_used)) {
 		// Keep taking from our reservation.
 		p = h->arena_used;
-		runtime_SysMap(p, n);
+		runtime_SysMap(p, n, h->is_reserved);
 		h->arena_used += n;
 		runtime_MHeap_MapBits(h);
 		if(raceenabled)
diff --git a/libgo/runtime/malloc.h b/libgo/runtime/malloc.h
index ebea34e..da6845c 100644
--- a/libgo/runtime/malloc.h
+++ b/libgo/runtime/malloc.h
@@ -177,8 +177,8 @@ struct MLink
 void*	runtime_SysAlloc(uintptr nbytes);
 void	runtime_SysFree(void *v, uintptr nbytes);
 void	runtime_SysUnused(void *v, uintptr nbytes);
-void	runtime_SysMap(void *v, uintptr nbytes);
-void*	runtime_SysReserve(void *v, uintptr nbytes);
+void	runtime_SysMap(void *v, uintptr nbytes, bool do_reserve);
+void*	runtime_SysReserve(void *v, uintptr nbytes, bool is_reserved);
 
 // FixAlloc is a simple free-list allocator for fixed size objects.
 // Malloc uses a FixAlloc wrapped around SysAlloc to manages its
@@ -434,6 +434,7 @@ struct MHeap
 
 	FixAlloc spanalloc;	// allocator for Span*
 	FixAlloc cachealloc;	// allocator for MCache*
+	bool is_reserved;       // is the all the address space for this heap actually reserved by us?
 };
 extern MHeap *runtime_mheap;
 
diff --git a/libgo/runtime/mem.c b/libgo/runtime/mem.c
index 8481e95..b45534d 100644
--- a/libgo/runtime/mem.c
+++ b/libgo/runtime/mem.c
@@ -110,7 +110,7 @@ runtime_SysFree(void *v, uintptr n)
 }
 
 void*
-runtime_SysReserve(void *v, uintptr n)
+runtime_SysReserve(void *v, uintptr n, bool do_reserve)
 {
 	int fd = -1;
 	void *p;
@@ -130,7 +130,7 @@ runtime_SysReserve(void *v, uintptr n)
 	// much address space.  Instead, assume that the reservation is okay
 	// if we can reserve at least 64K and check the assumption in SysMap.
 	// Only user-mode Linux (UML) rejects these requests.
-	if(sizeof(void*) == 8 && (uintptr)v >= 0xffffffffU) {
+	if(!do_reserve) {
 		p = mmap_fixed(v, 64<<10, PROT_NONE, MAP_ANON|MAP_PRIVATE, fd, 0);
 		if (p != v)
 			return nil;
@@ -149,7 +149,7 @@ runtime_SysReserve(void *v, uintptr n)
 }
 
 void
-runtime_SysMap(void *v, uintptr n)
+runtime_SysMap(void *v, uintptr n, bool is_reserved)
 {
 	void *p;
 	int fd = -1;
@@ -168,7 +168,7 @@ runtime_SysMap(void *v, uintptr n)
 #endif
 
 	// On 64-bit, we don't actually have v reserved, so tread carefully.
-	if(sizeof(void*) == 8 && (uintptr)v >= 0xffffffffU) {
+	if(!is_reserved) {
 		p = mmap_fixed(v, n, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, fd, 0);
 		if(p == MAP_FAILED && errno == ENOMEM)
 			runtime_throw("runtime: out of memory");
diff --git a/libgo/runtime/mgc0.c b/libgo/runtime/mgc0.c
index c3b3211..bd2157c 100644
--- a/libgo/runtime/mgc0.c
+++ b/libgo/runtime/mgc0.c
@@ -2483,6 +2483,6 @@ runtime_MHeap_MapBits(MHeap *h)
 	page_size = getpagesize();
 	n = (n+page_size-1) & ~(page_size-1);
 
-	runtime_SysMap(h->arena_start - n, n - h->bitmap_mapped);
+	runtime_SysMap(h->arena_start - n, n - h->bitmap_mapped, h->is_reserved);
 	h->bitmap_mapped = n;
 }