Re: MPSAFE VFS -- List of upcoming actions

On Fri, Jul 13, 2012 at 12:18 AM, Attilio Rao <attilio_at_freebsd.org> wrote:
> 2012/7/4 Attilio Rao <attilio_at_freebsd.org>:
>> 2012/6/29 Attilio Rao <attilio_at_freebsd.org>:
>>> As already published several times, according to the following plan:
>>> http://wiki.freebsd.org/NONMPSAFE_DEORBIT_VFS
>>>
>>
>> I still haven't heard from Vivien or Edward, anyway as NTFS is
>> basically only used RO these days (also the mount_ntfs code just
>> permits RO mounting) I stripped all the uncomplete/bogus write support
>> with the following patch:
>> http://www.freebsd.org/~attilio/ntfs_remove_write.patch
>>
>> This is an attempt to make the code smaller and possibly just focus on
>> the locking that really matter (as read-only filesystem).
>> On some points of the patch I'm a bit less sure as we could easily
>> take into account also write for things like vaccess() arguments, and
>> make easier to re-add correct write support at some point in the
>> future, but still force RO, even if the approach used in the patch is
>> more correct IMHO.
>> As an added bonus this patch cleans some dirty code in the mount
>> operation and fixes a bug as vfs_mountedfrom() is called before real
>> mounting is completed and can still fail.
>
> A quick update on this.
> It looks like NTFS won't be completed for this GSoC thus I seriously
> need to find an alternative to not loose the NTFS support entirely.
>
> I tried to look into the NTFS implementation right now and it is
> really a poor support. As Peter has also verified, it can deadlock in
> no-time, it compeltely violates VFS rules, etc. IMHO it deserves a
> complete rewrite if we would still support in-kernel NTFS. I also
> tried to look at the NetBSD implementation. Their code is someway
> similar to our, but they used very complicated (and very dirty) code
> to do the locking. Even if I don't know well enough NetBSD VFS, I have
> the impression not all the races are correctly handled. Definitively,
> not something I would like to port.
>
> Considering all that the only viable option would be meaning an
> userland filesystem implementation. My preferred choice would be to
> import PUFFS and librefuse on top of it but honestly it requires a lot
> of time to be completed, time which I don't currently have as in 2
> months Giant must be gone by the VFS.
>
> I then decided to switch to gnn's rewamp of FUSE patches. You can find
> his initial e-mail here:
> http://lists.freebsd.org/pipermail/freebsd-fs/2012-March/013876.html
>
> I've precisely got the second version of George's patch and created
> this dolphin branch:
> svn://svn.freebsd.org/base/projects/fuse
>
> I'm fixing low hanging fruit for the moment (see r238411 for example)
> and I still have to make a throughful review.
> However my idea is to commit the support once:
> - ntfs-3g is well stress-tested and proves to be bug-free
> - there is no major/big technical issue pending after the reviews

In the last weeks Peter, Florian, Gustau and I have been working in
stabilizing fuse support. In the specific, Peter has worked hard on
producing several utilities to nit stress-test fuse and in particular
ntfs, Florian has improved fuse related ports (as explained later) and
Gustau has done sparse testing. I feel moderately satisfied by the
level of stability of fuse now to propose to wider usage, in
particular given the huge amount of complaints I'm hearing around
about occasional fuse users.

The final target of the project is to completely import into base the
content of fusefs-kmod starting from earlier posted patches by George.
So far, we took care only of importing in the fuse branch the kernel
part, so that fusefs-kmod userland part is still needed to be
installed from ports, but I was studying the mount_fusefs licensing
before to process with the import for the userland bits of it.

The fixing has been happening here:
svn://svn.freebsd.org/base/projects/fuse/

which is essentially an HEAD branch + fuse kernel components. In order
to get fuse, please compile a kernel from this branch with FUSE option
or simply build and load fuse module.
Alternatively, a kernel patch that should work with HEAD_at_240684 is here:
http://www.freebsd.org/~attilio/fuse_import/fuse_240684.patch

I guess the patch can easilly apply to all FreeBSD branches, really,
but it is not tested to anything else different then -CURRENT.

As said you still need currently to build fusefs-kmod port. However
you need these further patches, to be put in the fusefs-kmod/files/
directory::
http://www.freebsd.org/~attilio/fuse_import/patch-Makefile
http://www.freebsd.org/~attilio/fuse_import/patch-mount_fusefs__mount_fusefs2.c

They both disable the old kernel building/linking and import new
functionality to let the new kernel support work well in presence of
many consumers.

In addition to fusefs-kmod, Bryan and Florian have also updated
fusefs-lib and fusefs-ntfs ports. For instance, please refer to this
e-mail:
http://lists.freebsd.org/pipermail/freebsd-ports/2012-August/077950.html

Even if this work is someway independent by the fusefs-kmod import, I
warmly suggest to all of you to use their patches (and this what we
have been testing so far too).

At this point what I'm looking for are reviews and further testing.
I would like to spend some words on what you should expect from this work:
*Fuse is far from being perfect*.
I cannot stress this enough. Peter stress-tests could break also Fuse
on Linux generally and by Fuse authors admissions the modules can
never guarantee to be completely starvation-free. However, they tend
to be designed in a way that sleeps can be at least interrupted
easily, making at least easy to recover from deadlocks. This is mostly
retained also in FreeBSD, for what I can tell. Also, sometimes fuse
seems to leave a small amount of hidden files, when it find references
on files it wants to delete. This happens also under Linux and it is
part of FUSE design, not much we can do.
However, if deadlocks can be someway tollerated, things you should
really pay attention are dumps of fuse modules (like ntfs-3g binary)
and kernel panics. They must not happen and if they do they need to be
fixed promptly.
However, the good new is that ntfs seems doing exceptionally good.
Florian could use ntfs as a backend for postgresql test. I think this
is by far a big improvement if compared to current in-kernel ntfs
which is completely torned.

So far we have almost entirely tested only ntfs-3g. I know Gustau also
used other modules like sshfs and George used GlusterFS with his older
patches, but I encourage you to test as many modules as you want, as
they may expose different bugs. Of course, I don't plan to spend much
more time on FUSE, but I can occasionally look at bugs as they fall in
the filesystems category and I'm always interested in keeping a good
open eye on such issues.

A few operational informations:
- In the next days I will import the userland bits of fusefs-kmod to
the fuse project branch making the port obsolete. When this happens I
will make this clear to the user of this thread.
- If no major bug is remained by the early October, I will commit this
to -CURRENT
- I expect Bryan and Florian to commit libfuse and ntfs updates soon.
They can do independently from the fusefs-kmod retiral, but I would
prefer their patches to go on first.
- After that I will handover fusefs maintainership to gnn as agreed in
precedence but I will be around helping with analysis and fixing,
depending on time availability

In the end I have really 2 minor questions:
- One is about importing the mount_fusefs userland bits. I don't think
we need a vendor import at all because they were developed by a
FreeBSD GSoC student and kept in his git repo (or someone else's).
Anyway, i'd just commit as new files once I do a good sweep. I hope
nobody objects to that.
- Another one is: fusefs-kmod right now is only amd64/i386 specific. I
have no idea why as it has not any MD specific code. However I'm sure
it has not been tested on other arches so far. Anyway I left it usable
by all the arches. I think this is the correct choice. If someone
objects with valid argument I can bring it back to be usable only on
i386 and amd64.

That's all, for any question please don't hesitate to contact me and
the other people involved in this work.

Thanks,
Attilio


-- 
Peace can only be achieved by understanding - A. Einstein