# Copyright (C) 2005-2011 Junjiro R. Okajima # # This program is free software; you can redistribute it and/or modify # it under the terms of the GNU General Public License as published by # the Free Software Foundation; either version 2 of the License, or # (at your option) any later version. # # This program is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the # GNU General Public License for more details. # # You should have received a copy of the GNU General Public License # along with this program; if not, write to the Free Software # Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA Lookup in a Branch ---------------------------------------------------------------------- Since aufs has a character of sub-VFS (see Introduction), it operates lookup for branches as VFS does. It may be a heavy work. Generally speaking struct nameidata is a bigger structure and includes many information. But almost all lookup operation in aufs is the simplest case, ie. lookup only an entry directly connected to its parent. Digging down the directory hierarchy is unnecessary. VFS has a function lookup_one_len() for that use, but it is not usable for a branch filesystem which requires struct nameidata. So aufs implements a simple lookup wrapper function. When a branch filesystem allows NULL as nameidata, it calls lookup_one_len(). Otherwise it builds a simplest nameidata and calls lookup_hash(). Here aufs applies "a principle in NFSD", ie. if the filesystem supports NFS-export, then it has to support NULL as a nameidata parameter for ->create(), ->lookup() and ->d_revalidate(). So the lookup wrapper in aufs tests if ->s_export_op in the branch is NULL or not. When a branch is a remote filesystem, aufs basically trusts its ->d_revalidate(), also aufs forces the hardest revalidate tests for them. For d_revalidate, aufs implements three levels of revalidate tests. See "Revalidate Dentry and UDBA" in detail. Loopback Mount ---------------------------------------------------------------------- Basically aufs supports any type of filesystem and block device for a branch (actually there are some exceptions). But it is prohibited to add a loopback mounted one whose backend file exists in a filesystem which is already added to aufs. The reason is to protect aufs from a recursive lookup. If it was allowed, the aufs lookup operation might re-enter a lookup for the loopback mounted branch in the same context, and will cause a deadlock. Revalidate Dentry and UDBA (User's Direct Branch Access) ---------------------------------------------------------------------- Generally VFS helpers re-validate a dentry as a part of lookup. 0. digging down the directory hierarchy. 1. lock the parent dir by its i_mutex. 2. lookup the final (child) entry. 3. revalidate it. 4. call the actual operation (create, unlink, etc.) 5. unlock the parent dir If the filesystem implements its ->d_revalidate() (step 3), then it is called. Actually aufs implements it and checks the dentry on a branch is still valid. But it is not enough. Because aufs has to release the lock for the parent dir on a branch at the end of ->lookup() (step 2) and ->d_revalidate() (step 3) while the i_mutex of the aufs dir is still held by VFS. If the file on a branch is changed directly, eg. bypassing aufs, after aufs released the lock, then the subsequent operation may cause something unpleasant result. This situation is a result of VFS architecture, ->lookup() and ->d_revalidate() is separated. But I never say it is wrong. It is a good design from VFS's point of view. It is just not suitable for sub-VFS character in aufs. Aufs supports such case by three level of revalidation which is selectable by user. 1. Simple Revalidate Addition to the native flow in VFS's, confirm the child-parent relationship on the branch just after locking the parent dir on the branch in the "actual operation" (step 4). When this validation fails, aufs returns EBUSY. ->d_revalidate() (step 3) in aufs still checks the validation of the dentry on branches. 2. Monitor Changes Internally by Inotify/Fsnotify Addition to above, in the "actual operation" (step 4) aufs re-lookup the dentry on the branch, and returns EBUSY if it finds different dentry. Additionally, aufs sets the inotify/fsnotify watch for every dir on branches during it is in cache. When the event is notified, aufs registers a function to kernel 'events' thread by schedule_work(). And the function sets some special status to the cached aufs dentry and inode private data. If they are not cached, then aufs has nothing to do. When the same file is accessed through aufs (step 0-3) later, aufs will detect the status and refresh all necessary data. In this mode, aufs has to ignore the event which is fired by aufs itself. 3. No Extra Validation This is the simplest test and doesn't add any additional revalidation test, and skip therevalidatin in step 4. It is useful and improves aufs performance when system surely hide the aufs branches from user, by over-mounting something (or another method).
