mounting /usr read-only -- didn't this *used* to work?

[Jacques B.]

On 10/21/07, Robert P. J. Day <rpjday at crashcourse.ca> wrote:
>
>   once upon a time, after i set up my fedora system, i had a habit of
> re-mounting the entire /usr filesystem read-only so that, even as
> root, i couldn't do something indescribably stupid and destroy
> valuable files.  (theoretically, this remounting should be fine since,
> according to the FHS, the contents of /usr should be static and
> shareable.)
>
>   all i would do (and demo to students in class, as well, since they
> thought it was tres cool), was to use mount with the remount option:
>
>     # mount -oro,remount /usr
>
>   if i try that nowadays, though, i get:
>
>   # mount -oro,remount /usr
>   mount: /usr is busy
>
> i can certainly do the above with one of my currently unused
> partitions like, say, /opt, but i'm not sure why the /usr filesystem
> is considered "busy."
>
>   i'm unsure of the semantics of remounting a FS as read-only -- will
> it fail if some file is currently opened with write access?  i've
> used "fuser" to (apparently) verify that nothing like that seems to be
> happening.  thoughts?  does anyone else remember doing this on earlier
> fedora systems, and does it work on your latest version of fedora?
> thanks.
>
> rday
>
> p.s.  obviously, if you're going to modify /usr by, say, installing
> new packages, you'd temporarily remount /usr RW, do the install, then
> switch it back.
>
> p.p.s.  this is with filesystems all running under LVM2.
>
> --
> ========================================================================
> Robert P. J. Day

Something I must try. Never tried the remount option.  Did you try an
lsof to see if there are any open files from /usr for some reason as
well as check the output of your ps for same?  I'm guessing you most
likely did so I'm probably stating the obvious.  But just in case.  I
remember a few years back when I was introduced to Linux and we used
to get that busy error we'd check lsof and ps outputs for indicators
of what was keeping that resource busy.

Jacques B.