From mboxrd@z Thu Jan 1 00:00:00 1970 From: Valerie Aurora Subject: Re: [PATCH 6/7 v3] overlay: hybrid overlay filesystem prototype Date: Mon, 27 Sep 2010 14:47:47 -0400 Message-ID: <20100927184747.GC8089@shell> References: <20100920180404.939991832@szeredi.hu> <20100920180447.854260354@szeredi.hu> <20100924175653.GB25129@shell> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: agruen@suse.de, linuxram@us.ibm.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, neilb@suse.de, viro@zeniv.linux.org.uk To: Miklos Szeredi Return-path: Received: from mx1.redhat.com ([209.132.183.28]:46773 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757124Ab0I0SsV (ORCPT ); Mon, 27 Sep 2010 14:48:21 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Mon, Sep 27, 2010 at 10:11:53AM +0200, Miklos Szeredi wrote: > On Fri, 24 Sep 2010, Valerie Aurora wrote: > > On Mon, Sep 20, 2010 at 08:04:10PM +0200, Miklos Szeredi wrote: > > > From: Miklos Szeredi > > > > > > This overlay filesystem is a hybrid of entirely filesystem based > > > (unionfs, aufs) and entierly VFS based (union mounts) solutions. > > > > [...] > > > > > +static int ovl_create_object(struct dentry *dentry, int mode, dev_t rdev, > > > + const char *link) > > > +{ > > > + int err; > > > + struct dentry *newdentry; > > > + struct dentry *upperdir; > > > + struct inode *inode; > > > + struct kstat stat = { > > > + .mode = mode, > > > + .rdev = rdev, > > > + }; > > > + > > > + err = -ENOMEM; > > > + inode = ovl_new_inode(dentry->d_sb, mode); > > > + if (!inode) > > > + goto out; > > > + > > > + err = ovl_copy_up(dentry->d_parent); > > > + if (err) > > > + goto out_iput; > > > + > > > + upperdir = ovl_dentry_upper(dentry->d_parent); > > > + mutex_lock_nested(&upperdir->d_inode->i_mutex, I_MUTEX_PARENT); > > > + > > > + newdentry = ovl_upper_create(upperdir, dentry, &stat, link); > > > + err = PTR_ERR(newdentry); > > > + if (IS_ERR(newdentry)) > > > + goto out_unlock; > > > + > > > + if (ovl_dentry_is_opaque(dentry) && S_ISDIR(mode)) { > > > + err = ovl_set_opaque(newdentry); > > > + if (err) > > > + goto out_dput; > > > + } > > > > Andreas Gruenbacher just convinced me that every single new directory > > created in the unioned file system should be marked opaque. "New" > > means either it replaces a whiteout or has no matching directory on > > the lower layer. The theory is that the topmost file system changes > > should take precedence and override any changes (off-line) in the > > lower file system. > > That's logical. However marking new directories opaque is only a half > solution. E.g. consider the case when we have /a/b/c/ on the lower fs > and /a/b/ on the upper, which is not opaque. Then /a/b/c/ is created > on the upper fs off-line. The union logic can't notice that "c" > should really be opaque and will merge with the contents of the lower > layer. Maybe I don't understand. It seems like directories created when the file system is *not* union mounted should definitely be merged with matching directories on the lower layer. Take the case of /etc/fstab. The first union mount never touches /etc and it doesn't exist on the topmost layer. Then we unmount the upper layer, mount it somewhere else as a plain mount, and create /etc/ and /etc/fstab. When we union mount it back over the lower layer again, we still want the lower layer /etc/ to be merged with the topmost /etc/, or else init.d will disappear. However, if while the file system is union mounted, /etc/ doesn't exist, and /etc/ is created, a later mount shouldn't merge a newly created /etc/ on the lower layer. > The real solution to this problem is to make opaque the default and > only mark *non* opaque directories. These are only created on copy-up > or by explicit admin action on the upper fs. Again, maybe I'm misunderstanding, but this doesn't make much sense to me. Say I create: /upper/a_dir/upper_file /lower/a_dir/lower_file Then when I union mount them, I want a_dir/ to be transparent automatically and show both upper_file and lower_file, without marking it manually. -VAL