Revert "UBUNTU: SAUCE: Import aufs driver"

[mirror_ubuntu-zesty-kernel.git] / Documentation / filesystems / aufs / design / 03atomic_open.txt

diff --git a/Documentation/filesystems/aufs/design/03atomic_open.txt b/Documentation/filesystems/aufs/design/03atomic_open.txt
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-
-# Copyright (C) 2015-2016 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, see <http://www.gnu.org/licenses/>.
-
-Support for a branch who has its ->atomic_open()
-----------------------------------------------------------------------
-The filesystems who implement its ->atomic_open() are not majority. For
-example NFSv4 does, and aufs should call NFSv4 ->atomic_open,
-particularly for open(O_CREAT|O_EXCL, 0400) case. Other than
-->atomic_open(), NFSv4 returns an error for this open(2). While I am not
-sure whether all filesystems who have ->atomic_open() behave like this,
-but NFSv4 surely returns the error.
-
-In order to support ->atomic_open() for aufs, there are a few
-approaches.
-
-A. Introduce aufs_atomic_open()
-   - calls one of VFS:do_last(), lookup_open() or atomic_open() for
-     branch fs.
-B. Introduce aufs_atomic_open() calling create, open and chmod. this is
-   an aufs user Pip Cet's approach
-   - calls aufs_create(), VFS finish_open() and notify_change().
-   - pass fake-mode to finish_open(), and then correct the mode by
-     notify_change().
-C. Extend aufs_open() to call branch fs's ->atomic_open()
-   - no aufs_atomic_open().
-   - aufs_lookup() registers the TID to an aufs internal object.
-   - aufs_create() does nothing when the matching TID is registered, but
-     registers the mode.
-   - aufs_open() calls branch fs's ->atomic_open() when the matching
-     TID is registered.
-D. Extend aufs_open() to re-try branch fs's ->open() with superuser's
-   credential
-   - no aufs_atomic_open().
-   - aufs_create() registers the TID to an internal object. this info
-     represents "this process created this file just now."
-   - when aufs gets EACCES from branch fs's ->open(), then confirm the
-     registered TID and re-try open() with superuser's credential.
-
-Pros and cons for each approach.
-
-A.
-   - straightforward but highly depends upon VFS internal.
-   - the atomic behavaiour is kept.
-   - some of parameters such as nameidata are hard to reproduce for
-     branch fs.
-   - large overhead.
-B.
-   - easy to implement.
-   - the atomic behavaiour is lost.
-C.
-   - the atomic behavaiour is kept.
-   - dirty and tricky.
-   - VFS checks whether the file is created correctly after calling
-     ->create(), which means this approach doesn't work.
-D.
-   - easy to implement.
-   - the atomic behavaiour is lost.
-   - to open a file with superuser's credential and give it to a user
-     process is a bad idea, since the file object keeps the credential
-     in it. It may affect LSM or something. This approach doesn't work
-     either.
-
-The approach A is ideal, but it hard to implement. So here is a
-variation of A, which is to be implemented.
-
-A-1. Introduce aufs_atomic_open()
-     - calls branch fs ->atomic_open() if exists. otherwise calls
-       vfs_create() and finish_open().
-     - the demerit is that the several checks after branch fs
-       ->atomic_open() are lost. in the ordinary case, the checks are
-       done by VFS:do_last(), lookup_open() and atomic_open(). some can
-       be implemented in aufs, but not all I am afraid.