diff --git a/src/runtime/lfstack_64bit.go b/src/runtime/lfstack_64bit.go
index 5367f08c56..cf6c02895f 100644
--- a/src/runtime/lfstack_64bit.go
+++ b/src/runtime/lfstack_64bit.go
@@ -16,16 +16,25 @@ const (
 	//
 	// Architecture  Name              Maximum Value (exclusive)
 	// ---------------------------------------------------------------------
-	// arm64         TASK_SIZE_64      Depends on configuration.
-	// ppc64{,le}    TASK_SIZE_USER64  0x400000000000UL (46 bit addresses)
-	// mips64{,le}   TASK_SIZE64       0x010000000000UL (40 bit addresses)
-	// s390x         TASK_SIZE         0x020000000000UL (41 bit addresses)
+	// amd64         TASK_SIZE_MAX     0x007ffffffff000 (47 bit addresses)
+	// arm64         TASK_SIZE_64      0x01000000000000 (48 bit addresses)
+	// ppc64{,le}    TASK_SIZE_USER64  0x00400000000000 (46 bit addresses)
+	// mips64{,le}   TASK_SIZE64       0x00010000000000 (40 bit addresses)
+	// s390x         TASK_SIZE         1<<64 (64 bit addresses)
 	//
-	// These values may increase over time.
+	// These values may increase over time. In particular, ppc64
+	// and mips64 support arbitrary 64-bit addresses in hardware,
+	// but Linux imposes the above limits. amd64 has hardware
+	// support for 57 bit addresses as of 2017 (56 bits for user
+	// space), but Linux only uses addresses above 1<<47 for
+	// mappings that explicitly pass a high hint address.
 	//
-	// On AMD64, virtual addresses are 48-bit numbers sign extended to 64.
+	// On AMD64, virtual addresses are 48-bit (or 57-bit) numbers sign extended to 64.
 	// We shift the address left 16 to eliminate the sign extended part and make
 	// room in the bottom for the count.
+	//
+	// On s390x, there's not much we can do, so we just hope that
+	// the kernel doesn't get to really high addresses.
 	addrBits = 48
 
 	// In addition to the 16 bits taken from the top, we can take 3 from the