Re: [RFC] union-mount stuff

From: Alexander Viro (viro@math.psu.edu)
Date: Wed Jun 07 2000 - 05:01:52 EST


On Wed, 7 Jun 2000, James Sutherland wrote:

> > A) suppose we have a bunch of filesystems union-mounted on
> > /foo/bar. We do chdir("/foo/bar"), what should become busy? Variants:
> > mountpoint, first element, last element, all of them.
>
> The key question, presumably, is which FSs can we unmount from the union
> without /foo/bar breaking? If all the FSs can be unmounted safely, none
> should be busy.

There is only one tricky situation: when we umount everything but the last
element (and thus union-mount degenerates into the plain mount) && there
is something standing on the mountpoint && aside of that mounted fs is not
busy. Then current code will refuse to umount the thing, but the new one
(for consistency) probably ought to (see the (E)). Yes, we can umount
anything without breakage.

> > Doing replace (i.e. if everything can be umounted - do it and mount
> > the new fs in place of the union) is attractive - we probably might
> > treat the normal mount same way, which kills the "I've clicked in my
> > point'n'drool krapplication ten times and it mounted CD ten times,
> > waaaaaah" bug reports.
>
> Making the point'n'drool krapplication fail the second time it tries to
> mount anything makes more sense, IMO - that way, however much the user
> points and/or drools, they only have one thing mounted. (Ideally, the

Erm... In the case of "replacement" semantics it would go

<first point&drool> mounted /dev/cdrom on /cdrom
<next point&drool> woops, already mounted something? umount and put /dev/cdrom
 on that (/cdrom) place. We are back to the same situation.
ad infinitum...

[sweet memories of printer asking to insert paper, seventy identical
~30-page turds produced by TeX in the queue, very unhappy little Sun
holding said queue and very annoyed luser sitting in 3 feet from printer
and keeping fornic^Wclick on "print"...]

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