On Mon, Nov 30, 2009 at 03:46:29AM -0500, Erez Zadok wrote: > In message <1256152779-10054-21-git-send-email-vaurora@xxxxxxxxxx>, Valerie Aurora writes: > > From: Jan Blunck <jblunck@xxxxxxx> > > > > This patch adds the basic structures of VFS based union mounts. It is a new > > implementation based on some of my old ideas that influenced Bharata B Rao > > <bharata@xxxxxxxxxxxxxxxxxx> who came up with the proposal to let the > > union_mount struct only point to the next layer in the union stack. I rewrote > > nearly all of the central patches around lookup and the dcache interaction. > > > > Advantages of the new implementation: > > - the new union stack is no longer tied directly to one dentry > > - the union stack enables dentries to be part of more than one union > > (bind mounts) > > - it is unnecessary to traverse the union stack when de/referencing a dentry > > - caching of union stack information still driven by dentry cache > > > > XXX - is_unionized() is pretty heavy-weight for non-union file systems > > on a union mount-enabled kernel. May be simplified by assuming one or > > more of: > > > > - Two layers only > > - One-to-one association between layers (doesn't union submounts) > > - Writable layer mounted in only one place > > Yes, is_unionized() does appear to be heavy. Is it correct to assume that > every such dentry will have gotten looked up or traversed as part of a > union? If so, can we just set a flag in the dentry to mark it as > D_THIS_IS_PART_OF_A_UNION? Even if you could, what happens when a union r-w > layer is removed: could there be leftover dentries marked as part of a > union, which are no longer really part of it? dentries aren't themselves part of a unioned file system - they are shared among all the mounts of a superblock/device. A dentry is unioned or not only in the context of a particular mount. So we can't mark the dentry itself since it will be unioned in one mount (say, /mnt/union) and not in another (/mnt/ro). A dentry can be looked up in another mount of one of the components of the union before the union mount is created, so we can't mark a dentry at lookup anyway. The vfsmount for the top layer <dentry,mnt> pair is marked with MNT_UNION, but the vfsmount for the bottom layer is not. I think we could mark the lower vfsmount with a flag (probably not MNT_UNION but MNT_UNION_LOWER or something like that) and check for that. Al? Jan? Christoph? -VAL -- 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
