diff --git a/sbin/fsck_ffs/fsck.h b/sbin/fsck_ffs/fsck.h
index 55a57bc6959f..32f9cfaa7eae 100644
--- a/sbin/fsck_ffs/fsck.h
+++ b/sbin/fsck_ffs/fsck.h
@@ -272,6 +272,7 @@ int	bflag;			/* location of alternate super block */
 int	debug;			/* output debugging info */
 int	cvtlevel;		/* convert to newer file system format */
 int	bkgrdcheck;		/* determine if background check is possible */
+int	bkgrdsumadj;		/* whether the kernel have ability to adjust superblock summary */
 char	usedsoftdep;		/* just fix soft dependency inconsistencies */
 char	preen;			/* just fix normal inconsistencies */
 char	rerun;			/* rerun fsck. Only used in non-preen mode */
diff --git a/sbin/fsck_ffs/pass5.c b/sbin/fsck_ffs/pass5.c
index 796b546b9516..693831fe1578 100644
--- a/sbin/fsck_ffs/pass5.c
+++ b/sbin/fsck_ffs/pass5.c
@@ -358,7 +358,7 @@ pass5(void)
 		if (cmd.value != 0) {
 			if (debug)
 				printf("adjndir by %+" PRIi64 "\n", cmd.value);
-			if (sysctl(adjndir, MIBSIZE, 0, 0,
+			if (bkgrdsumadj == 0 || sysctl(adjndir, MIBSIZE, 0, 0,
 			    &cmd, sizeof cmd) == -1)
 				rwerror("ADJUST NUMBER OF DIRECTORIES", cmd.value);
 		}
@@ -367,7 +367,7 @@ pass5(void)
 		if (cmd.value != 0) {
 			if (debug)
 				printf("adjnbfree by %+" PRIi64 "\n", cmd.value);
-			if (sysctl(adjnbfree, MIBSIZE, 0, 0,
+			if (bkgrdsumadj == 0 || sysctl(adjnbfree, MIBSIZE, 0, 0,
 			    &cmd, sizeof cmd) == -1)
 				rwerror("ADJUST NUMBER OF FREE BLOCKS", cmd.value);
 		}
@@ -376,7 +376,7 @@ pass5(void)
 		if (cmd.value != 0) {
 			if (debug)
 				printf("adjnifree by %+" PRIi64 "\n", cmd.value);
-			if (sysctl(adjnifree, MIBSIZE, 0, 0,
+			if (bkgrdsumadj == 0 || sysctl(adjnifree, MIBSIZE, 0, 0,
 			    &cmd, sizeof cmd) == -1)
 				rwerror("ADJUST NUMBER OF FREE INODES", cmd.value);
 		}
@@ -385,7 +385,7 @@ pass5(void)
 		if (cmd.value != 0) {
 			if (debug)
 				printf("adjnffree by %+" PRIi64 "\n", cmd.value);
-			if (sysctl(adjnffree, MIBSIZE, 0, 0,
+			if (bkgrdsumadj == 0 || sysctl(adjnffree, MIBSIZE, 0, 0,
 			    &cmd, sizeof cmd) == -1)
 				rwerror("ADJUST NUMBER OF FREE FRAGS", cmd.value);
 		}
@@ -394,7 +394,7 @@ pass5(void)
 		if (cmd.value != 0) {
 			if (debug)
 				printf("adjnumclusters by %+" PRIi64 "\n", cmd.value);
-			if (sysctl(adjnumclusters, MIBSIZE, 0, 0,
+			if (bkgrdsumadj == 0 || sysctl(adjnumclusters, MIBSIZE, 0, 0,
 			    &cmd, sizeof cmd) == -1)
 				rwerror("ADJUST NUMBER OF FREE CLUSTERS", cmd.value);
 		}
diff --git a/sbin/fsck_ffs/setup.c b/sbin/fsck_ffs/setup.c
index e95a6adbb4ad..f6b736d2ec79 100644
--- a/sbin/fsck_ffs/setup.c
+++ b/sbin/fsck_ffs/setup.c
@@ -140,6 +140,24 @@ setup(char *dev)
 			pfatal("kernel lacks background fsck support\n");
 			exit(EEXIT);
 		}
+		/*
+		 * When kernel is lack of runtime bgfsck superblock summary
+		 * adjustment functionality, it does not mean we can not
+		 * continue, as old kernels will recompute the summary at
+		 * mount time.  However, it will be an unexpected softupdates
+		 * inconsistency if it turns out that the summary is still
+		 * incorrect.  Set a flag so subsequent operation can know
+		 * this.
+		 */
+		bkgrdsumadj = 1;
+		if (sysctlnametomib("vfs.ffs.adjndir", adjndir, &size) < 0 ||
+		    sysctlnametomib("vfs.ffs.adjnbfree", adjnbfree, &size) < 0 ||
+		    sysctlnametomib("vfs.ffs.adjnifree", adjnifree, &size) < 0 ||
+		    sysctlnametomib("vfs.ffs.adjnffree", adjnffree, &size) < 0 ||
+		    sysctlnametomib("vfs.ffs.adjnumclusters", adjnumclusters, &size) < 0) {
+			bkgrdsumadj = 0;
+			pwarn("kernel lacks runtime superblock summary adjustment support");
+		}
 		cmd.version = FFS_CMD_VERSION;
 		cmd.handle = fsreadfd;
 		fswritefd = -1;