runtime: allocate page table lazily

This removes the 256MB memory allocation at startup,
which conflicts with ulimit.
Also will allow to eliminate an unnecessary memory dereference in GC,
because the page table is usually mapped at known address.
Update #5049.
Update #5236.

R=golang-dev, khr, r, khr, rsc
CC=golang-dev
https://golang.org/cl/9791044

src/pkg/runtime/malloc.goc

@@ -323,7 +323,7 @@ void
 runtime·mallocinit(void)
 {
 	byte *p;
-	uintptr arena_size, bitmap_size;
+	uintptr arena_size, bitmap_size, spans_size;
 	extern byte end[];
 	byte *want;
 	uintptr limit;
@@ -331,11 +331,13 @@ runtime·mallocinit(void)
 	p = nil;
 	arena_size = 0;
 	bitmap_size = 0;
-	
+	spans_size = 0;
+
 	// for 64-bit build
 	USED(p);
 	USED(arena_size);
 	USED(bitmap_size);
+	USED(spans_size);
 
 	if((runtime·mheap = runtime·SysAlloc(sizeof(*runtime·mheap))) == nil)
 		runtime·throw("runtime: cannot allocate heap metadata");
@@ -375,7 +377,8 @@ 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);
+		spans_size = arena_size / PageSize * sizeof(runtime·mheap->map[0]);
+		p = runtime·SysReserve((void*)(0x00c0ULL<<32), bitmap_size + spans_size + arena_size);
 	}
 	if (p == nil) {
 		// On a 32-bit machine, we can't typically get away
@@ -397,11 +400,13 @@ runtime·mallocinit(void)
 		// of address space, which is probably too much in a 32-bit world.
 		bitmap_size = MaxArena32 / (sizeof(void*)*8/4);
 		arena_size = 512<<20;
-		if(limit > 0 && arena_size+bitmap_size > limit) {
+		spans_size = MaxArena32 / PageSize * sizeof(runtime·mheap->map[0]);
+		if(limit > 0 && arena_size+bitmap_size+spans_size > limit) {
 			bitmap_size = (limit / 9) & ~((1<<PageShift) - 1);
 			arena_size = bitmap_size * 8;
+			spans_size = arena_size / PageSize * sizeof(runtime·mheap->map[0]);
 		}
-		
+
 		// SysReserve treats the address we ask for, end, as a hint,
 		// not as an absolute requirement.  If we ask for the end
 		// of the data segment but the operating system requires
@@ -412,17 +417,19 @@ runtime·mallocinit(void)
 		// away from the running binary image and then round up
 		// to a MB boundary.
 		want = (byte*)(((uintptr)end + (1<<18) + (1<<20) - 1)&~((1<<20)-1));
-		p = runtime·SysReserve(want, bitmap_size + arena_size);
+		p = runtime·SysReserve(want, bitmap_size + spans_size + arena_size);
 		if(p == nil)
 			runtime·throw("runtime: cannot reserve arena virtual address space");
 		if((uintptr)p & (((uintptr)1<<PageShift)-1))
-			runtime·printf("runtime: SysReserve returned unaligned address %p; asked for %p", p, bitmap_size+arena_size);
+			runtime·printf("runtime: SysReserve returned unaligned address %p; asked for %p", p,
+				bitmap_size+spans_size+arena_size);
 	}
 	if((uintptr)p & (((uintptr)1<<PageShift)-1))
 		runtime·throw("runtime: SysReserve returned unaligned address");
 
-	runtime·mheap->bitmap = p;
-	runtime·mheap->arena_start = p + bitmap_size;
+	runtime·mheap->map = (MSpan**)p;
+	runtime·mheap->bitmap = p + spans_size;
+	runtime·mheap->arena_start = p + spans_size + bitmap_size;
 	runtime·mheap->arena_used = runtime·mheap->arena_start;
 	runtime·mheap->arena_end = runtime·mheap->arena_start + arena_size;
 
@@ -461,6 +468,7 @@ runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
 		runtime·SysMap(p, n);
 		h->arena_used += n;
 		runtime·MHeap_MapBits(h);
+		runtime·MHeap_MapSpans(h);
 		if(raceenabled)
 			runtime·racemapshadow(p, n);
 		return p;
@@ -489,6 +497,7 @@ runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
 		if(h->arena_used > h->arena_end)
 			h->arena_end = h->arena_used;
 		runtime·MHeap_MapBits(h);
+		runtime·MHeap_MapSpans(h);
 		if(raceenabled)
 			runtime·racemapshadow(p, n);
 	}

src/pkg/runtime/malloc.h

@@ -411,7 +411,8 @@ struct MHeap
 	uint32	nspancap;
 
 	// span lookup
-	MSpan *map[1<<MHeapMap_Bits];
+	MSpan**	map;
+	uintptr	spans_mapped;
 
 	// range of addresses we might see in the heap
 	byte *bitmap;
@@ -442,6 +443,7 @@ MSpan*	runtime·MHeap_LookupMaybe(MHeap *h, void *v);
 void	runtime·MGetSizeClassInfo(int32 sizeclass, uintptr *size, int32 *npages, int32 *nobj);
 void*	runtime·MHeap_SysAlloc(MHeap *h, uintptr n);
 void	runtime·MHeap_MapBits(MHeap *h);
+void	runtime·MHeap_MapSpans(MHeap *h);
 void	runtime·MHeap_Scavenger(void);
 
 void*	runtime·mallocgc(uintptr size, uint32 flag, int32 dogc, int32 zeroed);

src/pkg/runtime/mgc0.c

@@ -2403,7 +2403,7 @@ runtime·MHeap_MapBits(MHeap *h)
 	uintptr n;
 
 	n = (h->arena_used - h->arena_start) / wordsPerBitmapWord;
-	n = (n+bitmapChunk-1) & ~(bitmapChunk-1);
+	n = ROUND(n, bitmapChunk);
 	if(h->bitmap_mapped >= n)
 		return;
 

src/pkg/runtime/mheap.c

@@ -65,6 +65,24 @@ runtime·MHeap_Init(MHeap *h, void *(*alloc)(uintptr))
 		runtime·MCentral_Init(&h->central[i], i);
 }
 
+void
+runtime·MHeap_MapSpans(MHeap *h)
+{
+	uintptr n;
+
+	// Map spans array, PageSize at a time.
+	n = (uintptr)h->arena_used;
+	if(sizeof(void*) == 8)
+		n -= (uintptr)h->arena_start;
+	// Coalescing code reads spans past the end of mapped arena, thus +1.
+	n = (n / PageSize + 1) * sizeof(h->map[0]);
+	n = ROUND(n, PageSize);
+	if(h->spans_mapped >= n)
+		return;
+	runtime·SysMap((byte*)h->map + h->spans_mapped, n - h->spans_mapped);
+	h->spans_mapped = n;
+}
+
 // Allocate a new span of npage pages from the heap
 // and record its size class in the HeapMap and HeapMapCache.
 MSpan*