My brain hurts. (Messing with mount.)

[Rob Landley]

There is no mount spec.  Only endless questions.

It is possible to --bind mount a directory onto itself.  (Oddly, the system 
survived.  It seems to be a NOP, I could still list the contents of the 
directory, but it shows up in /proc/mounts.)

Judging by what the gnu one does for "mount -a /dev/hda" and "mount 
-a /dev/hda /thingy", the -a should be silently dropped when you supply 
blockdev and/or directory arguments.

When to take absolute paths of things and when to preserve relative paths is 
confusing.  (Yes, you can have relative paths such as "../woot" in fstab.)  
Some things (calling losetup ioctls, creating an entry in /etc/mtab) insist 
on an absolute path.  Other times the "blockdev" argument is a comment, or 
something else entirely (such as a network address, or an MTD device 
identifier) and taking the absolute path would screw things up.  For 
iterating through /etc/fstab, I need to check for matches against _both_,
because /etc/fstab doesn't just have /dev/hda1, it can also have something 
like:
  walrus.img    walrus   ext2 loop,ro

Which can be mounted via:
  mount walrus.img
But not via:
  mount ./walrus.img  # Still expects "walrus" in the current directory...

Oddly, after mounting that, I can umount ./walrus.img and it works.

When you supply two arguments, mount doesn't check fstab at all.  With the 
above fstab entry:
  mount walrus.img walrus

  The mount is rw, not ro, and I have to say -t ext2 -o loop with the gnu
  mount.

"mount -o remount,rw" does require at least one argument.

Using the above fstab entry, and doing this:
  mount walrus.img walrus
  mount -o remount,rw walrus
Complains that block device walrus.img is write protected, and it's mounting 
read only.  (Understandable, since it did a read-only association on the 
loopback device.)

Following up with
  mount -o remount,rw /dev/does-not-exist walrus
Complains that /dev/does-not-exist is mounted read only.  Amusing. :)

Ooh, if I add an entry:
  /path/to/walrus.img potato ext2 loop,rw 1 2

Then _that_ always resolves the paths, and can be mounted via a relative path.  
(In fact if _both_ are in there, that one wins even if it's after the first 
one.  Strange.)  Right, my brain hurts.

I consider "mount -t procfs /dev /proc" and then trying to umount /dev to be a 
"doctor, it hurts when I do this" case.

Apparently, when you mount a symlink on another symlink, the kernel 
dereferences both for you, which makes sense I suppose.  (Of course this is 
another case where /etc/mtab can get confused...)

Dinner time.

Rob
-- 
Never bet against the cheap plastic solution.