diff --git a/src/runtime/mcentral.go b/src/runtime/mcentral.go
index 1e1f6fd13d..a09801a549 100644
--- a/src/runtime/mcentral.go
+++ b/src/runtime/mcentral.go
@@ -101,6 +101,10 @@ havespan:
 	if n == 0 {
 		throw("empty span")
 	}
+	usedBytes := uintptr(s.ref) * s.elemsize
+	if usedBytes > 0 {
+		reimburseSweepCredit(usedBytes)
+	}
 	if s.freelist.ptr() == nil {
 		throw("freelist empty")
 	}
diff --git a/src/runtime/mgcsweep.go b/src/runtime/mgcsweep.go
index fc92301857..84b6181b6e 100644
--- a/src/runtime/mgcsweep.go
+++ b/src/runtime/mgcsweep.go
@@ -354,6 +354,11 @@ func mSpan_Sweep(s *mspan, preserve bool) bool {
 // also sweep pages (e.g., for a large allocation), it can pass a
 // non-zero callerSweepPages to leave that many pages unswept.
 //
+// deductSweepCredit makes a worst-case assumption that all spanBytes
+// bytes of the ultimately allocated span will be available for object
+// allocation. The caller should call reimburseSweepCredit if that
+// turns out not to be the case once the span is allocated.
+//
 // deductSweepCredit is the core of the "proportional sweep" system.
 // It uses statistics gathered by the garbage collector to perform
 // enough sweeping so that all pages are swept during the concurrent
@@ -379,6 +384,17 @@ func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
 	}
 }
 
+// reimburseSweepCredit records that unusableBytes bytes of a
+// just-allocated span are not available for object allocation. This
+// offsets the worst-case charge performed by deductSweepCredit.
+func reimburseSweepCredit(unusableBytes uintptr) {
+	if mheap_.sweepPagesPerByte == 0 {
+		// Nobody cares about the credit. Avoid the atomic.
+		return
+	}
+	xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))
+}
+
 func dumpFreeList(s *mspan) {
 	printlock()
 	print("runtime: free list of span ", s, ":\n")