diff --git a/README.md b/README.md
index 2dc8674c..09a0b003 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,7 @@ Usage: ./configure [OPTION]...
 
 # Usage:    
 ```
-ruri 3.2
+ruri 3.3
 
 Lightweight, User-friendly Linux-container Implementation
 
@@ -81,17 +81,19 @@ ARGS:
   -k, --keep [cap] ....................: Keep the specified cap
   -d, --drop [cap] ....................: Drop the specified cap
   -e, --env [env] [value] .............: Set environment variables to its value (**)
-  -m, --mount [dir/dev/img] [dir] .....: Mount dir/block-device/image to mountpoint
+  -m, --mount [dir/dev/img] [dir] .....: Mount dir/block-device/image to mountpoint (***)
   -M, --ro-mount [dir/dev/img] [dir] ..: Mount dir/block-device/image as read-only
   -S, --host-runtime ..................: Bind-mount /dev/, /sys/ and /proc/ from host
   -R, --read-only .....................: Mount / as read-only
-  -l, --limit [cpuset=cpu/memory=mem] .: Set cpuset/memory limit(***)
+  -l, --limit [cpuset=cpu/memory=mem] .: Set cpuset/memory limit(****)
   -w, --no-warnings ...................: Disable warnings
 
-(*)  :  `-a` option also need `-q` is set
-(**) : Will not work if [COMMAND [ARGS]...] is like `/bin/su -`
-(***): Each `-l` option can only set one of the cpuset/memory limits
-(***): for example: `ruri -l memory=1M -l cpuset=1 /test`
+Note:
+(*)   :  `-a` option also need `-q` is set
+(**)  : Will not work if [COMMAND [ARGS]...] is like `/bin/su -`
+(***) : You can use `-m [source] /` to mount a block device as root
+(****): Each `-l` option can only set one of the cpuset/memory limits
+        for example: `ruri -l memory=1M -l cpuset=1 /test`
 ```
 # Quick start(with rootfstool):
 ## Download and unpack a rootfs:
diff --git a/src/info.c b/src/info.c
index e4b06689..4a95e8d6 100644
--- a/src/info.c
+++ b/src/info.c
@@ -100,17 +100,19 @@ void show_helps(void)
 	cprintf("{base}  -k, --keep [cap] ....................: Keep the specified cap\n");
 	cprintf("{base}  -d, --drop [cap] ....................: Drop the specified cap\n");
 	cprintf("{base}  -e, --env [env] [value] .............: Set environment variables to its value (**)\n");
-	cprintf("{base}  -m, --mount [dir/dev/img] [dir] .....: Mount dir/block-device/image to mountpoint\n");
+	cprintf("{base}  -m, --mount [dir/dev/img] [dir] .....: Mount dir/block-device/image to mountpoint (***)\n");
 	cprintf("{base}  -M, --ro-mount [dir/dev/img] [dir] ..: Mount dir/block-device/image as read-only\n");
 	cprintf("{base}  -S, --host-runtime ..................: Bind-mount /dev/, /sys/ and /proc/ from host\n");
 	cprintf("{base}  -R, --read-only .....................: Mount / as read-only\n");
-	cprintf("{base}  -l, --limit [cpuset=cpu/memory=mem] .: Set cpuset/memory limit(***)\n");
+	cprintf("{base}  -l, --limit [cpuset=cpu/memory=mem] .: Set cpuset/memory limit(****)\n");
 	cprintf("{base}  -w, --no-warnings ...................: Disable warnings\n");
 	cprintf("\n");
-	cprintf("{base}(*)  :  `-a` option also need `-q` is set\n");
-	cprintf("{base}(**) : Will not work if [COMMAND [ARGS]...] is like `/bin/su -`\n");
-	cprintf("{base}(***): Each `-l` option can only set one of the cpuset/memory limits\n");
-	cprintf("{base}(***): for example: `ruri -l memory=1M -l cpuset=1 /test`\n");
+	cprintf("{base}Note:\n");
+	cprintf("{base}(*)   :  `-a` option also need `-q` is set\n");
+	cprintf("{base}(**)  : Will not work if [COMMAND [ARGS]...] is like `/bin/su -`\n");
+	cprintf("{base}(***) : You can use `-m [source] /` to mount a block device as root\n");
+	cprintf("{base}(****): Each `-l` option can only set one of the cpuset/memory limits\n");
+	cprintf("{base}        for example: `ruri -l memory=1M -l cpuset=1 /test`\n");
 	cprintf("{base}{clear}\n");
 }
 // For `ruri -H`.
diff --git a/src/mount.c b/src/mount.c
index ffd53116..8cdcacfb 100644
--- a/src/mount.c
+++ b/src/mount.c
@@ -155,6 +155,7 @@ static char *losetup(const char *img)
 // Mount dev/dir/img to target.
 int trymount(const char *source, const char *target, unsigned int mountflags)
 {
+	// umount target before mount(2), to avoid `device or resource busy`.
 	umount2(target, MNT_DETACH | MNT_FORCE);
 	int ret = 0;
 	// Check if mountpoint exists.