> In message <20070109122644.GB1260@xxxxxxxxxxxxxxxxxxxxxxxx>, Jan Kara writes: > > > In message <20070108111852.ee156a90.akpm@xxxxxxxx>, Andrew Morton writes: > > > > On Sun, 7 Jan 2007 23:12:53 -0500 > > > > "Josef 'Jeff' Sipek" <jsipek@xxxxxxxxxxxxx> 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@xxxxxxx> SuSE CR Labs - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
