On Wed, 3 May 2000, Jeremy Fitzhardinge wrote:
> I know hpa has been thinking about how to stack dentries. How does this
> compare?
Orthogonal. IIRC, hpa wanted them as a way to do loopbacks. Well, as soon
as tree scanning in autofs4 switches to new linkage/goes away[1] we are
getting much cheaper way to do loopbacks without mucking with dentries.
So the only stacking of any kind is in the mounpoint and you hardly can
get out without that... There may be other applications of dentry
stacking, but that's completely different story - these things are
independent and stacking would be a serious overhead for autofs* needs.
[1] I would really prefer the latter, but if it will be hard to do fast -
fine, it will be switch to new linkage; I have that code.
> BTW, what happens if you umount a filesystem which has these scattered about
> its namespace? Do they get cleaned up as part of the umount (appropriate
> callback, etc), or do you need to clear them out before the umount? I prefer
> the former.
Hrrrmmm... Probably the former, but I can argue it both ways ;-)
> Also, what happens if you attach one to a non-directory? Could you use it to
> put arbiary "special files" into the namespace without having to do anything
> special? It would make thinks like Pavel's podfuk more useful without having
> to do horrible namespace hacks as he does now.
Ummm... I'm not sure that I like the idea. Reason: I'm very suspicious of
the situations when file turns into directory and back. I never seen it
done right and in all cases when it had been done it was full of nasty
special cases, kludges, etc. Mostly on the userland side of things, BTW.
If you can do it in clean way and nothing will break I'll be only glad
about that. Mechanism itself doesn't care for the type of that stuff, so I
have no objections on that side. Just a nasty gut feeling...
> Also, when one is inserted between two real filesystems, it still needs to be
> able to mediate namespace lookups. Autofs may need this to block access to a
> filesystem while the daemon is umounting it.
It depends. How much are you going to do with the filesystem before
umount(8)?
> > These objects are not filesystems - they rather look like a traps
> > set in the unified tree. Notice that they do not waste anon device like
> > "one node autofs" would do.
>
> That's not a huge issue, since you run out pretty quickly with NFS's
> consumption.
<shrug> two times slower. At least something...
