Index: head/sbin/fsck_msdosfs/fsck_msdosfs.8
===================================================================
--- head/sbin/fsck_msdosfs/fsck_msdosfs.8
+++ head/sbin/fsck_msdosfs/fsck_msdosfs.8
@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 3, 2016
+.Dd January 6, 2020
 .Dt FSCK_MSDOSFS 8
 .Os
 .Sh NAME
@@ -37,7 +37,7 @@
 .Op Fl Cf
 .Ar filesystem ...
 .Nm
-.Op Fl Cny
+.Op Fl CMny
 .Ar filesystem ...
 .Sh DESCRIPTION
 The
@@ -84,6 +84,21 @@
 immediately in foreground, or if its cleaning can be deferred to background.
 FAT (MS-DOS) file systems must always be cleaned in the foreground.
 A non-zero exit code is always returned for this option.
+.It Fl M
+Causes
+.Nm
+to not use
+.Xr mmap 2
+when checking a FAT32 file system.
+This option is mainly for debugging purposes and is not normally necessary.
+The
+.Nm
+utility will automatically fall back to use a simple LRU cache of 4 MiB
+when it failed to perform
+.Xr mmap 2 ,
+or when
+.Fl M
+is specified.
 .It Fl f
 Force
 .Nm