From 9a67c570a9143d554cf19898259cdb21cac704d5 Mon Sep 17 00:00:00 2001
From: Xin LI <delphij@FreeBSD.org>
Date: Tue, 7 Jan 2020 04:33:14 +0000
Subject: [PATCH] fsck_msdosfs.8: document -M.

Reported by:	mckusick
Reviewed by:	mckusick, emaste, bcr
MFC after:	28 days
X-MFC-with:	r356313
Differential Revision:	https://reviews.freebsd.org/D23049
---
 sbin/fsck_msdosfs/fsck_msdosfs.8 | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/sbin/fsck_msdosfs/fsck_msdosfs.8 b/sbin/fsck_msdosfs/fsck_msdosfs.8
index f953dbdf2b8f..602451b61374 100644
--- a/sbin/fsck_msdosfs/fsck_msdosfs.8
+++ b/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 @@ which seeks to determine whether the file system needs to be cleaned
 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