runtime: use len(h.spans) to indicate mapped region

Currently we set the len and cap of h.spans to the full reserved
region of the address space and track the actual mapped region
separately in h.spans_mapped. Since we have both the len and cap at
our disposal, change things so len(h.spans) tracks how much of the
spans array is mapped and eliminate h.spans_mapped. This simplifies
mheap and means we'll get nice "index out of bounds" exceptions if we
do try to go off the end of the spans rather than a SIGSEGV.

Change-Id: I8ed9a1a9a844d90e9fd2e269add4704623dbdfe6
Reviewed-on: https://go-review.googlesource.com/30533
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mheap.go

@@ -54,8 +54,13 @@ type mheap struct {
 	// For free spans, only the lowest and highest pages map to the span itself.
 	// Internal pages map to an arbitrary span.
 	// For pages that have never been allocated, spans entries are nil.
-	spans        []*mspan
-	spans_mapped uintptr // bytes mapped starting at &spans[0]
+	//
+	// This is backed by a reserved region of the address space so
+	// it can grow without moving. The memory up to len(spans) is
+	// mapped. cap(spans) indicates the total reserved memory.
+	spans []*mspan
+
+	_ uint32 // align uint64 fields on 32-bit for atomics
 
 	// Proportional sweep
 	pagesInUse        uint64  // pages of spans in stats _MSpanInUse; R/W with mheap.lock
@@ -406,7 +411,7 @@ func (h *mheap) init(spansStart, spansBytes uintptr) {
 
 	sp := (*slice)(unsafe.Pointer(&h.spans))
 	sp.array = unsafe.Pointer(spansStart)
-	sp.len = int(spansBytes / sys.PtrSize)
+	sp.len = 0
 	sp.cap = int(spansBytes / sys.PtrSize)
 }
 
@@ -424,11 +429,13 @@ func (h *mheap) mapSpans(arena_used uintptr) {
 	n -= h.arena_start
 	n = n / _PageSize * sys.PtrSize
 	n = round(n, physPageSize)
-	if h.spans_mapped >= n {
+	need := n / unsafe.Sizeof(h.spans[0])
+	have := uintptr(len(h.spans))
+	if have >= need {
 		return
 	}
-	sysMap(add(unsafe.Pointer(&h.spans[0]), h.spans_mapped), n-h.spans_mapped, h.arena_reserved, &memstats.other_sys)
-	h.spans_mapped = n
+	h.spans = h.spans[:need]
+	sysMap(unsafe.Pointer(&h.spans[have]), (need-have)*unsafe.Sizeof(h.spans[0]), h.arena_reserved, &memstats.other_sys)
 }
 
 // Sweeps spans in list until reclaims at least npages into heap.
@@ -890,7 +897,7 @@ func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool, unusedsince i
 			h.spanalloc.free(unsafe.Pointer(t))
 		}
 	}
-	if (p+s.npages)*sys.PtrSize < h.spans_mapped {
+	if (p + s.npages) < uintptr(len(h.spans)) {
 		t := h.spans[p+s.npages]
 		if t != nil && t.state == _MSpanFree {
 			s.npages += t.npages