From: Neil Brown <neilb@suse.de>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
vaurora@redhat.com, viro@zeniv.linux.org.uk, jblunck@suse.de,
hch@infradead.org
Subject: Re: [PATCH 0/5] hybrid union filesystem prototype
Date: Mon, 30 Aug 2010 21:40:27 +1000 [thread overview]
Message-ID: <20100830214027.77e197f5@notabene> (raw)
In-Reply-To: <E1Oq1RL-0003zF-Tw@pomaz-ex.szeredi.hu>
On Mon, 30 Aug 2010 12:18:11 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:
> On Sun, 29 Aug 2010, Neil Brown wrote:
> > On Fri, 27 Aug 2010 18:53:45 +0200
> > Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > > On Fri, 27 Aug 2010, Neil Brown wrote:
> > > > I was wondering why you even bothered to fiddle st_ino. Given
> > > > that lots of other things come from one fs or the other, having
> > > > the merged directories appear to be from the upper filesystem
> > > > doesn't seem like a problem. I don't really care either way
> > > > though.
> > >
> > > "rm -rf" complains if st_ino of a directory changes spontaneously.
> >
> > I see...
> > Doesn't that mean that you must always present a merged-directory if the
> > lower directory exists.
>
> Directory opens are never "forwarded" to the lower filesystem like
> other opens. One reason is that lookups continuing from the file's
> path need to be on the union filesystem instead on one of the
> underlying filesystems.
You're right - I misread the code there.
> > > I think it's best to leave that stuff until someone actually cares.
> > > The "people might find it useful" argument is not strong enough to put
> > > nontrivial effort into thinking about all the weird corner cases.
> >
> > My thought on this is that in order to describe the behaviour of the
> > filesystem accurately you need to think about all the weird corner cases.
> >
> > Then it becomes a question of: is it easier to document the weird behaviour,
> > or change the code so the documentation will be easier to understand?
> > Some cases will go one way, some the other.
> >
> > But as you suggest this isn't urgent.
>
> You didn't mention one possibility: add limitations that prevent the
> weird corner cases arising. I believe this is the simplest option.
Val has been following that approach and asking if it is possible to make an
NFS filesystem really-truly read-only. i.e. no changes.
I don't believe it is.
But I won't pursue this further without patches.
> > My comment about set-theory unions being commutative set me thinking. I
> > really don't think "union" is the right name for this thing. There is
> > nothing about it which really fits that proper definition of a union.
> > whiteouts mean that even the list of names in a directory is not the union of
> > the lists of names in the upper and lower directories.
> > "overlay" is a much more accurate name. But union seems to be the name
> > that is most used. I wonder if it is too late to change that.
>
> We could call it overlayfs. People learn new names quickly :)
+1
>
> > Also, dnotify (and presumably inotifiy) doesn't work reliably in the current
> > implementation.
> > It works for opaque directories and those which don't have a namesake in the
> > lower filesystem, but for others it never notifies of changes to any files in
> > the directory.
> > This is because dnotify will set an fsnotifier on the merged-directory in the
> > union-fs, but the change happens to the file in the upper fs, so
> > fsnotify_parent will only call notifiers on the parent in the upper fs.
>
> I think *notify will work correctly, every modificaton will be
> notified on both the union fs and the upper fs. But I haven't tested
> this yet.
I tried. It doesn't.
To be precise: directory changes like file creation (even creation of a file
that already exists) get notified, but purely file-based event like modifying
the contents of the file don't generate events back to the overlayfs
directory.
Because an open (for write) of a file is passed through to the upper
filesystem, the notifications of modification through that open only go to the
upper filesystem.
Thanks,
NeilBrown
next prev parent reply other threads:[~2010-08-30 11:40 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-26 18:33 [PATCH 0/5] hybrid union filesystem prototype Miklos Szeredi
2010-08-26 18:33 ` [PATCH 1/5] vfs: implement open "forwarding" Miklos Szeredi
2010-08-26 18:33 ` [PATCH 2/5] vfs: make i_op->permission take a dentry instead of an inode Miklos Szeredi
2010-08-26 20:24 ` David P. Quigley
2010-08-27 4:11 ` Neil Brown
2010-08-27 18:13 ` David P. Quigley
2010-08-27 19:21 ` Valerie Aurora
2010-08-27 18:31 ` David P. Quigley
2010-08-26 18:33 ` [PATCH 3/5] vfs: add flag to allow rename to same inode Miklos Szeredi
2010-08-26 18:33 ` [PATCH 4/5] vfs: export do_splice_direct() to modules Miklos Szeredi
2010-08-26 18:33 ` [PATCH 5/5] union: hybrid union filesystem prototype Miklos Szeredi
2010-09-01 21:42 ` Valerie Aurora
2010-09-02 9:19 ` Miklos Szeredi
2010-09-02 21:33 ` Valerie Aurora
2010-09-03 5:10 ` Neil Brown
2010-09-03 9:16 ` Miklos Szeredi
2010-09-09 16:02 ` David P. Quigley
2010-09-03 8:52 ` Miklos Szeredi
2010-09-02 21:42 ` Valerie Aurora
2010-09-03 12:31 ` Miklos Szeredi
2010-08-27 7:05 ` [PATCH 0/5] " Neil Brown
2010-08-27 8:47 ` Miklos Szeredi
2010-08-27 11:35 ` Neil Brown
2010-08-27 16:53 ` Miklos Szeredi
2010-08-29 4:42 ` Neil Brown
2010-08-30 10:18 ` Miklos Szeredi
2010-08-30 11:40 ` Neil Brown [this message]
2010-08-30 12:20 ` Miklos Szeredi
2010-08-31 19:18 ` Valerie Aurora
2010-08-31 20:19 ` Trond Myklebust
2010-09-01 1:56 ` Valerie Aurora
2010-09-01 4:04 ` Trond Myklebust
2010-09-01 4:33 ` Neil Brown
2010-09-01 20:11 ` Miklos Szeredi
2010-08-31 19:29 ` Valerie Aurora
2010-09-02 13:15 ` Jan Engelhardt
2010-09-02 13:32 ` Neil Brown
2010-09-02 14:25 ` Jan Engelhardt
2010-09-02 14:28 ` Miklos Szeredi
2010-09-08 19:47 ` David P. Quigley
2010-09-23 13:18 ` Jan Engelhardt
2010-09-23 19:22 ` Valerie Aurora
2010-08-30 18:38 ` Valerie Aurora
2010-08-30 23:12 ` Neil Brown
2010-08-31 11:00 ` Miklos Szeredi
2010-08-31 11:24 ` Neil Brown
2010-08-31 15:05 ` Kyle Moffett
2010-08-31 20:36 ` Valerie Aurora
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