runtime: drop to 32 bit malloc if 64 bit will not work

On 64 bit UML it is not possible to reserve memory at 0xF8<<32.
Detect when linux cannot use these high virtual memory addresses
and drop back to the 32 bit memory allocator.

R=rsc, cw
CC=golang-dev
https://golang.org/cl/5634050

src/pkg/runtime/malloc.goc

@@ -289,12 +289,13 @@ runtime·mallocinit(void)
 		// Actually we reserve 17 GB (because the bitmap ends up being 1 GB)
 		// but it hardly matters: fc is not valid UTF-8 either, and we have to
 		// allocate 15 GB before we get that far.
+		//
+		// If this fails we fall back to the 32 bit memory mechanism
 		arena_size = 16LL<<30;
 		bitmap_size = arena_size / (sizeof(void*)*8/4);
 		p = runtime·SysReserve((void*)(0x00f8ULL<<32), bitmap_size + arena_size);
-		if(p == nil)
-			runtime·throw("runtime: cannot reserve arena virtual address space");
-	} else {
+	}
+	if (p == nil) {
 		// On a 32-bit machine, we can't typically get away
 		// with a giant virtual address space reservation.
 		// Instead we map the memory information bitmap
@@ -359,8 +360,8 @@ runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
 		return p;
 	}
 	
-	// On 64-bit, our reservation is all we have.
-	if(sizeof(void*) == 8)
+	// If using 64-bit, our reservation is all we have.
+	if(sizeof(void*) == 8 && (uintptr)h->bitmap >= 0xffffffffU)
 		return nil;
 
 	// On 32-bit, once the reservation is gone we can

src/pkg/runtime/mem_linux.c

@@ -73,9 +73,18 @@ runtime·SysReserve(void *v, uintptr n)
 
 	// On 64-bit, people with ulimit -v set complain if we reserve too
 	// much address space.  Instead, assume that the reservation is okay
-	// and check the assumption in SysMap.
-	if(sizeof(void*) == 8)
+	// 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) {
+		p = runtime·mmap(v, 64<<10, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
+		if (p != v) {
+			return nil;
+		}
+		runtime·munmap(p, 64<<10);
+		
+		
 		return v;
+	}
 	
 	p = runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
 	if((uintptr)p < 4096 || -(uintptr)p < 4096) {
@@ -92,7 +101,7 @@ runtime·SysMap(void *v, uintptr n)
 	mstats.sys += n;
 
 	// On 64-bit, we don't actually have v reserved, so tread carefully.
-	if(sizeof(void*) == 8) {
+	if(sizeof(void*) == 8 && (uintptr)v >= 0xffffffffU) {
 		p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, -1, 0);
 		if(p != v && addrspace_free(v, n)) {
 			// On some systems, mmap ignores v without