Subject: Re: multiple mounts on same directory
To: None <>
From: Bill Studenmund <>
List: tech-userlevel
Date: 07/18/2003 12:08:01
On Fri, 18 Jul 2003, Alan Barrett wrote:

> On Fri, 18 Jul 2003, Bill Studenmund wrote:
> > I think it's perfectly fine to just have the code in mount know that for
> > mfs mount (and mfs mounts _only_), f_mntfromname doesn't matter for
> > differences.
> >
> > The reason that "union" is special is that if you mount with the -b
> > option, then the f_mntfromname will get changed. As I recall, it'll become
> > "<below>:" in front of the name. So the matching code would also need to
> > synthesize a new union f_mntfromname. But once it's done that, it will
> > match perfectly well.
> The special cases that I know about so far are:
>     fstype       original name   f_mntfromname
>     ------       -------------   -------------
>     mfs          anything        mfs:123
>     kernfs       anything        kernfs
>     procfs       anything        procfs
>     union        /some/dir       <above>:/some/dir
>     union        /some/dir       <below>:/some/dir
> Is it OK to hard-code knowledge of these special cases in the duplicate
> detection part of "mount -a"?  The normal case would then be that
> f_mntfromname would have to be an exact match before a potential mount
> would be detected as a duplicate.

I'd say hard-coding is fine, since it's better than all the alternatives.
Only "mount -a" really needs to know about this, so I'd say keep the
changes close to it.

I'd say a switch (fstype) would be best, and have it: 1) note if the
fromname should be looked at (kern, proc, mfs would be no), and if the
from name needs to be different than what's in the fstab.

Take care,