diff --git a/src/runtime/lfstack.go b/src/runtime/lfstack.go
index 4a20fff9d80..8a36a67b357 100644
--- a/src/runtime/lfstack.go
+++ b/src/runtime/lfstack.go
@@ -12,6 +12,10 @@ import "unsafe"
 func lfstackpush(head *uint64, node *lfnode) {
 	node.pushcnt++
 	new := lfstackPack(node, node.pushcnt)
+	if node1, _ := lfstackUnpack(new); node1 != node {
+		println("runtime: lfstackpush invalid packing: node=", node, " cnt=", hex(node.pushcnt), " packed=", hex(new), " -> node=", node1, "\n")
+		gothrow("lfstackpush")
+	}
 	for {
 		old := atomicload64(head)
 		node.next, _ = lfstackUnpack(old)
diff --git a/src/runtime/lfstack_linux_power64x.go b/src/runtime/lfstack_linux_power64x.go
index 7a122bf92c1..89e389fc726 100644
--- a/src/runtime/lfstack_linux_power64x.go
+++ b/src/runtime/lfstack_linux_power64x.go
@@ -9,18 +9,24 @@ package runtime
 
 import "unsafe"
 
-// On Power64, Linux limits the user address space to 43 bits.
-// (https://www.kernel.org/doc/ols/2001/ppc64.pdf)
-// In addition to the 21 bits taken from the top, we can take 3 from the
-// bottom, because node must be pointer-aligned, giving a total of 24 bits
+// On Power64, Linux limits the user address space to 46 bits (see
+// TASK_SIZE_USER64 in the Linux kernel).  This has grown over time,
+// so here we allow 48 bit addresses.
+//
+// In addition to the 16 bits taken from the top, we can take 3 from the
+// bottom, because node must be pointer-aligned, giving a total of 19 bits
 // of count.
+const (
+	addrBits = 48
+	cntBits  = 64 - addrBits + 3
+)
 
 func lfstackPack(node *lfnode, cnt uintptr) uint64 {
-	return uint64(uintptr(unsafe.Pointer(node)))<<21 | uint64(cnt&(1<<24-1))
+	return uint64(uintptr(unsafe.Pointer(node)))<<(64-addrBits) | uint64(cnt&(1<<cntBits-1))
 }
 
 func lfstackUnpack(val uint64) (node *lfnode, cnt uintptr) {
-	node = (*lfnode)(unsafe.Pointer(uintptr(val >> 24 << 3)))
-	cnt = uintptr(val & (1<<24 - 1))
+	node = (*lfnode)(unsafe.Pointer(uintptr(val >> cntBits << 3)))
+	cnt = uintptr(val & (1<<cntBits - 1))
 	return
 }