Linux: Which process is causing "device busy" when doing umount?

Look at the lsof command (list open files) -- it can tell you which processes are holding what open. Sometimes it's tricky but often something as simple as sudo lsof | grep (your device name here) could do it for you.

Just in case... sometimes happens that you are calling umount from the terminal, and your current directory belongs to the mounted filesystem.

You should use the fuser command.

Eg. fuser /dev/cdrom will return the pid(s) of the process using /dev/cdrom.

If you are trying to unmount, you can kill theses process using the -k switch (see man fuser).

lsof +f -- /mountpoint

(as lists the processes using files on the mount mounted at /mountpoint. Particularly useful for finding which process(es) are using a mounted USB stick or CD/DVD.

lsof and fuser are indeed two ways to find the process that keeps a certain file open. If you just want umount to succeed, you should investigate its -f and -l options.

That's exactly why the "fuser -m /mount/point" exists.

BTW, I don't think "fuser" or "lsof" will indicate when a resource is held by kernel module, although I don't usually have that issue..

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.