I have a problem with a mountpoint being too busy to unmount. Normally, lsof or fuser would tell me what process is using it, but this is a bind mount, so lsof and fuser show what is using both the original mountpoint, and the bind mountpoint.


Mount the filesystems:

$ sudo mkdir /mnt/mount /mnt/bind
$ sudo mount /dev/sdb1 /mnt/mount
$ sudo mount -o bind /mnt/mount /mnt/bind

Start a process that keeps /mnt/mount too busy to unmount

$ sudo dd if=/dev/zero of=/mnt/mount/testfile bs=1 oflag=dsync

Check what fuser says

$ sudo fuser -m /mnt/mount
/mnt/mount:           4022
$ sudo fuser -m /mnt/bind
/mnt/bind:            4022

Check what lsof says

$ sudo lsof +D /mnt/mount
dd      4022 root    1w   REG   8,17    28545   12 /mnt/mount/testfile

$ sudo lsof +D /mnt/bind
dd      4022 root    1w   REG   8,17    40682   12 /mnt/bind/testfile

Try to unmount both

$ sudo umount -v *
umount: /mnt/bind (/mnt/mount) unmounted
umount: /mnt/mount: target is busy

If you follow the same steps, but tell dd to write the file to /mnt/bind/testfile instead, unmounting /mnt/mount will be successful, but unmounting /mnt/bind will fail.

Clearly it makes a difference which mountpoint the process (dd in this case) is using, but lsof and fuser do not differentiate. Is there something that does?

  • 1
    Did you try lsof -pn | grep /mnt/bind and lsof -pn | grep /mnt/mount ? – wurtel Dec 1 '14 at 11:11

Thanks to wurtel, this became apparent:

While both fuser and lsof as used in my question both show the same process using both mountpoints, after either of those commands tell you the PID, running:

lsof -p $PID

does reveal exactly which mountpoint is being used. A bit of grepping and you're set.

  • @mikeserv: Maybe I'm using it wrong, but -M does not appear to help. Do you have an example of how to use it to tell if a process is writing to /mnt/mount or /mnt/bind? – Tal Dec 5 '14 at 8:07
  • thats cause its the wrong one - got it mixed up with -m. -M only works if the named file arg is a mountpoint but behaves identically otherwise - only listing process using that specific arg. -m, on the other hand, lists all process using any file on a mounted filesystem. With -v it prints to stderr a ps like list of things they might be doing with the files - m specifies a mount point. So try fuser -v /mnt/point and fuser -vm /mnt/point and hopefully youll see what i mean. – mikeserv Dec 5 '14 at 8:20
  • But you know what - i dont think that does what youre asking anyway. lsof definitely does, of course. w/ fuser maybe like fuser -vMm $(findmnt -UnRloTARGET /mnt/point) - but there findmnt does moat of the work anyway. – mikeserv Dec 5 '14 at 8:54
  • Neither fuser -v, fuser -vm, or fuser -vMm $(findmnt -UnRloTARGET /mnt/point) appear to be able to tell the difference between dd writing to /mnt/mount and dd writing to /mnt/bind. So far, the only way that works seems to be lsof -p – Tal Dec 9 '14 at 2:01
  • yeah - typically lsof will have the leg-up there - it's a more complicated tool. I'm sure what the difference is between your mnts bind and mnt - you haven't said - but I 'm not surprised if lsof provides more information. – mikeserv Dec 9 '14 at 2:10

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