runtime: add page tracer

This change adds a new GODEBUG flag called pagetrace that writes a
low-overhead trace of how pages of memory are managed by the Go runtime.

The page tracer is kept behind a GOEXPERIMENT flag due to a potential
security risk for setuid binaries.

Change-Id: I6f4a2447d02693c25214400846a5d2832ad6e5c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/444157
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/mheap.go

@@ -1306,6 +1306,7 @@ HaveSpan:
 	// There are a few very limited cirumstances where we won't have a P here.
 	// It's OK to simply skip scavenging in these cases. Something else will notice
 	// and pick up the tab.
+	var now int64
 	if pp != nil && bytesToScavenge > 0 {
 		// Measure how long we spent scavenging and add that measurement to the assist
 		// time so we can track it for the GC CPU limiter.
@@ -1321,7 +1322,7 @@ HaveSpan:
 		})
 
 		// Finish up accounting.
-		now := nanotime()
+		now = nanotime()
 		if track {
 			pp.limiterEvent.stop(limiterEventScavengeAssist, now)
 		}
@@ -1360,6 +1361,7 @@ HaveSpan:
 	}
 	memstats.heapStats.release()
 
+	pageTraceAlloc(pp, now, base, npages)
 	return s
 }
 
@@ -1535,6 +1537,8 @@ func (h *mheap) grow(npage uintptr) (uintptr, bool) {
 // Free the span back into the heap.
 func (h *mheap) freeSpan(s *mspan) {
 	systemstack(func() {
+		pageTraceFree(getg().m.p.ptr(), 0, s.base(), s.npages)
+
 		lock(&h.lock)
 		if msanenabled {
 			// Tell msan that this entire span is no longer in use.
@@ -1565,6 +1569,8 @@ func (h *mheap) freeSpan(s *mspan) {
 //
 //go:systemstack
 func (h *mheap) freeManual(s *mspan, typ spanAllocType) {
+	pageTraceFree(getg().m.p.ptr(), 0, s.base(), s.npages)
+
 	s.needzero = 1
 	lock(&h.lock)
 	h.freeSpanLocked(s, typ)