OK to mount multiple FS in one dir?

["Michael D. Crawford" <crawford@goingware.com>]

I was groping around my FAT/NTFS directories from Linux, mounting and 
unmounting them into
/mnt, and was suprised at some point that I got the message "/dev/sda5 
already mounted or
/mnt busy".  (I'm using a SCSI disk, use hda* for IDE).

Upon further examination, I found that I'd accidentally mounted 
/dev/sda1 (VFAT) on /mnt
while /dev/sda5 (NTFS) was still mounted there.  The NTFS files remained 
invisible until
I'd unmounted /dev/sda1 and then I could see them again.

This is with the 2.4.1 kernel on a Pentium III machine with an Adaptec 
29160 SCSI
controller.

I found I could mount three partitions on /mnt, and they'd all show up 
as mounted at
/mnt in the "mount" command, but if I unmounted one of them (only tried 
with the currently
visible one), then it appeared that there were no filesystems mounted 
there, but I could
continue umounting until the other two were gone.

I'm suprised this works.  Note that the kernel rejected an attempt to 
mount a filesystem
that was already mounted, but not to mount a filesystem at a point that 
was already in use.
It looks like there is a stack of mounts on the mount point.

Looking at Documentation/Changes, I see that I need util-linux 2.10o.  I 
had 2.10l.  But I
had the 2.10r util-linux sources on my machine and installed mount and 
umount from it, and
I find that it gets it right mostly when I mount and unmount multiple 
things, with the
exception that if /dev/sda5 was mounted before /dev/sda1, then if I give 
the command
"umount /dev/sda5", sda1 is the one that gets unmounted rather than 
sda5, so it takes the
most recently mounted filesystem rather than the one you specify.

Michael D. Crawford
GoingWare Inc. - Expert Software Development and Consulting
http://www.goingware.com/
crawford@goingware.com

   Tilting at Windmills for a Better Tomorrow.


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