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Due to an sd card corruption I decided to mount the root fs as read only. Therefore, I added the 'ro' argument in /etc/fstab 

/dev/mmcblk0p1  /boot           vfat    defaults,ro          0       2
/dev/mmcblk0p2  /               ext4    defaults,ro,noatime  0       1

After the device is booted, the filesystem is mounted read-only - so that's fine. However, sometimes I need write access to the file system. So, I remount the root fs with

sudo mount / -o remount,rw

as read/write. However, if I want to remount the filesystem with

sudo mount / -o remount,ro

I get the following error:

mount: / is busy

If I run:

fuser -v ./

I get the following output: 

USER PID ACCESS COMMAND
pi   2421 ..c.. sftp-server
pi   2477 ..c.. bash

Does anybody know what's causing the problem? Why can the filesystem be mounted as read only at startup, but not if I remount it manually?

RPiAwesomeness
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user9760
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6 Answers6

4

You can emulate a magic sysrq keypress by executing

# echo "u" > /proc/sysrq-trigger

This causes the kernel to low-level try to remount all filesystems read only, see here.

dpq
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2

This is most likely because something (by the looks of that command it is the BASH shell and SFTP-Server) is/are accessing the root filesystem. During boot nothing is accessing anything in / yet, so it can be set to read-only.

However, once boot-up is complete, some software may want a lock on the filesystem or some files are being changed within. Obviously, as all the files that Raspbian uses - software, personal files, logs, etc. - are within the root filesystem. Thus you get the mount: / is busy error.

If you were able to kill both BASH and SFTP-server and then run the command, I suspect that it would work fine. However, because BASH is the way you are entering the commands, and BASH is stored on the / partition, that can't happen.

RPiAwesomeness
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2

If you still can not unmount or remount your device after stopping all services and processes with open files, then there may be a swap file or swap partition keeping your device busy. This will not show up with fuser or lsof. Turn off swapping with:

sudo swapoff -a

You could check beforehand and show a summary of any swap partitions or swap files with:

swapon -s

or:

cat /proc/swaps

As an alternative to using the command sudo swapoff -a, you might also be able to disable the swap by stopping a service or systemd unit. For example:

sudo systemctl stop dphys-swapfile

or:

sudo systemctl stop var-swap.swap

In my case, turning off swap was necessary, in addition to stopping any services and processes with files open for writing, so that I could remount my root partition as read only in order to run fsck on my root partition without rebooting. This was necessary on a Raspberry Pi running Raspbian Jessie.

Simon Gould
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On many SD cards, there is a switch prohibiting write access. If it is off, switch it the other way to disable write protection.

Image Courtesy of jackenhack.com

If the switch is not to blame, I would advise following the advice here for rescuing.

Qu0rk
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1

I was banging the same wall for a long time, until I found this: https://wiki.debian.org/ReadonlyRoot#Find_processes_blocking_the_remount_readonly

When lsof and fuser do not show any files opened for writing on / (which seems your case), there could still be processes that have opened files that were deleted since, and they are blocking the remount.

The following command shows these proceses:

lsof +L1; lsof|sed -n '/SYSV/d; /DEL\|(path /p;' |grep -Ev '/(dev|home|tmp|var)'

After restarting deamons shown in the list and killing the rest, mount -o remount,ro / went fine.

Osman-pasha
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0

you probably have some app writing to the filesystem... i suspect the sftp-server is transferring something.

So until the file is not write closed or killed, it's probably locking up the remount as ro

higuita
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