From 698ba577565a1484600cf605097904c9c974d49e Mon Sep 17 00:00:00 2001
From: Matthew Dillon <dillon@apollo.backplane.com>
Date: Sun, 27 May 2018 08:45:28 -0700
Subject: [PATCH] kernel - Fix rare pmap_scan_callback() panic

* Fix a panic where pmap_scan_callback() can sometimes call
  pv_put() with a NULL pd_pv.
---
 sys/platform/pc64/x86_64/pmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/sys/platform/pc64/x86_64/pmap.c b/sys/platform/pc64/x86_64/pmap.c
index 3d4f621d9d..02b58dc4f9 100644
--- a/sys/platform/pc64/x86_64/pmap.c
+++ b/sys/platform/pc64/x86_64/pmap.c
@@ -4668,8 +4668,10 @@ kernel_skip:
 			pte_pv = pv_get_try(pmap, pmap_pte_pindex(sva),
 					    &pte_placemark, &error);
 			if (error) {
-				pv_put(pd_pv);		/* lock order */
-				pd_pv = NULL;
+				if (pd_pv) {
+					pv_put(pd_pv);	/* lock order */
+					pd_pv = NULL;
+				}
 				if (pt_pv) {
 					pv_put(pt_pv);	/* lock order */
 					pt_pv = NULL;
-- 
2.41.0