runtime: de-duplicate span scavenging

Currently, span scavenging was done nearly identically in two different
locations. This change deduplicates that into one shared routine.

For #14045.

Change-Id: I15006b2c9af0e70b7a9eae9abb4168d3adca3860
Reviewed-on: https://go-review.googlesource.com/c/139297
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mheap.go

@@ -351,6 +351,34 @@ func (s *mspan) layout() (size, n, total uintptr) {
 	return
 }
 
+func (s *mspan) scavenge() uintptr {
+	start := s.base()
+	end := start + s.npages<<_PageShift
+	if physPageSize > _PageSize {
+		// We can only release pages in
+		// physPageSize blocks, so round start
+		// and end in. (Otherwise, madvise
+		// will round them *out* and release
+		// more memory than we want.)
+		start = (start + physPageSize - 1) &^ (physPageSize - 1)
+		end &^= physPageSize - 1
+		if end <= start {
+			// start and end don't span a
+			// whole physical page.
+			return 0
+		}
+	}
+	len := end - start
+	released := len - (s.npreleased << _PageShift)
+	if physPageSize > _PageSize && released == 0 {
+		return 0
+	}
+	memstats.heap_released += uint64(released)
+	s.npreleased = len >> _PageShift
+	sysUnused(unsafe.Pointer(start), len)
+	return released
+}
+
 // recordspan adds a newly allocated span to h.allspans.
 //
 // This only happens the first time a span is allocated from
@@ -1087,35 +1115,12 @@ func (h *mheap) busyList(npages uintptr) *mSpanList {
 
 func scavengeTreapNode(t *treapNode, now, limit uint64) uintptr {
 	s := t.spanKey
-	var sumreleased uintptr
 	if (now-uint64(s.unusedsince)) > limit && s.npreleased != s.npages {
-		start := s.base()
-		end := start + s.npages<<_PageShift
-		if physPageSize > _PageSize {
-			// We can only release pages in
-			// physPageSize blocks, so round start
-			// and end in. (Otherwise, madvise
-			// will round them *out* and release
-			// more memory than we want.)
-			start = (start + physPageSize - 1) &^ (physPageSize - 1)
-			end &^= physPageSize - 1
-			if end <= start {
-				// start and end don't span a
-				// whole physical page.
-				return sumreleased
-			}
+		if released := s.scavenge(); released != 0 {
+			return released
 		}
-		len := end - start
-		released := len - (s.npreleased << _PageShift)
-		if physPageSize > _PageSize && released == 0 {
-			return sumreleased
-		}
-		memstats.heap_released += uint64(released)
-		sumreleased += released
-		s.npreleased = len >> _PageShift
-		sysUnused(unsafe.Pointer(start), len)
 	}
-	return sumreleased
+	return 0
 }
 
 func scavengelist(list *mSpanList, now, limit uint64) uintptr {
@@ -1128,32 +1133,7 @@ func scavengelist(list *mSpanList, now, limit uint64) uintptr {
 		if (now-uint64(s.unusedsince)) <= limit || s.npreleased == s.npages {
 			continue
 		}
-		start := s.base()
-		end := start + s.npages<<_PageShift
-		if physPageSize > _PageSize {
-			// We can only release pages in
-			// physPageSize blocks, so round start
-			// and end in. (Otherwise, madvise
-			// will round them *out* and release
-			// more memory than we want.)
-			start = (start + physPageSize - 1) &^ (physPageSize - 1)
-			end &^= physPageSize - 1
-			if end <= start {
-				// start and end don't span a
-				// whole physical page.
-				continue
-			}
-		}
-		len := end - start
-
-		released := len - (s.npreleased << _PageShift)
-		if physPageSize > _PageSize && released == 0 {
-			continue
-		}
-		memstats.heap_released += uint64(released)
-		sumreleased += released
-		s.npreleased = len >> _PageShift
-		sysUnused(unsafe.Pointer(start), len)
+		sumreleased += s.scavenge()
 	}
 	return sumreleased
 }