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SubjectRe: [PATCH 01/24] Unionfs: Documentation
> In message <20070109122644.GB1260@atrey.karlin.mff.cuni.cz>, Jan Kara writes:
> > > In message <20070108111852.ee156a90.akpm@osdl.org>, Andrew Morton writes:
> > > > On Sun, 7 Jan 2007 23:12:53 -0500
> > > > "Josef 'Jeff' Sipek" <jsipek@cs.sunysb.edu> wrote:
> > > >
> > > > > +Modifying a Unionfs branch directly, while the union is mounted, is
> > > > > +currently unsupported.
> > > >
> > > > Does this mean that if I have /a/b/ and /c/d/ unionised under /mnt/union, I
> > > > am not allowed to alter anything under /a/b/ and /c/d/? That I may only
> > > > alter stuff under /mnt/union?
> > > >
> > > > If so, that sounds like a significant limitation.
> > <snip>
> > > Now, we've discussed a number of possible solutions. Thanks to suggestions
> > > we got at OLS, we discussed a way to hide the lower namespace, or make it
> > > readonly, using existing kernel facilities. But my understanding is that
> > > even it'd work, it'd only address new processes: if an existing process has
> > > an open fd in a lower branch before we "lock up" the lower branch's name
> > > space, that process may still be able to make lower-level changes.
> > > Detecting such processes may not be easy. What to do with them, once
> > > detected, is also unclear. We welcome suggestions.
> > Yes, making fs readonly at VFS level would not work for already opened
> > files. But you if you create new union, you could lock down the
> > filesystems you are unioning (via s_umount semaphore), go through lists
> > of all open fd's on those filesystems and check whether they are open
> > for write or not. If some fd is open for writing, you simply fail to
> > create the union (and it's upto user to solve the problem). Otherwise
> > you mark filesystems as RO and safely proceed with creating the union.
> > I guess you must have come up with this solution. So what is the problem
> > with it?
>
> Jan, all of it is duable: we can downgrade the f/s to readonly, grab various
> locks, search through various lists looking for open fd's and such, then
> decide if to allow the mount or not. And hopefully all of that can be done
> in a non-racy manner. But it feels just rather hacky and ugly to me. If
> this community will endorse such a solution, we'll be happy to develop it.
> But right now my impression is that if we posted such patches today, some
> people will have to wipe the vomit off of their monitors... :-)
I see :). To me it just sounds as if you want to do remount-read-only
for source filesystems, which is operation we support perfectly fine,
and after that create union mount. But I agree you cannot do quite that
since you need to have write access later from your union mount. So
maybe it's not so easy as I thought.
On the other hand, there was some effort to support read-only bind-mounts of
read-write filesystems (there were even some patches floating around but
I don't think they got merged) and that should be even closer to what
you'd need...

Honza
--
Jan Kara <jack@suse.cz>
SuSE CR Labs
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