Bug#615998: linux-image-2.6.32-5-xen-amd64: Repeatable "kernel BUG at fs/jbd2/commit.c:534" from Postfix on ext4

[Lukas Czerner <lczerner@redhat.com>]

On Mon, 27 Jun 2011, Jan Kara wrote:

> On Mon 27-06-11 13:16:50, Lukas Czerner wrote:
> > On Fri, 24 Jun 2011, Jan Kara wrote:
> > 
> > > On Fri 24-06-11 11:03:52, Moffett, Kyle D wrote:
> > > > On Jun 24, 2011, at 09:46, Jan Kara wrote:
> > > > > On Thu 23-06-11 16:19:08, Moffett, Kyle D wrote:
> > > > >> Besides which, line 534 in the Debian 2.6.32 kernel I am using is this
> > > > >> one:
> > > > >> 
> > > > >>  J_ASSERT(commit_transaction->t_nr_buffers <=
> > > > >>           commit_transaction->t_outstanding_credits);
> > > > > 
> > > > >  Hmm, OK, so we've used more metadata buffers than we told JBD2 to
> > > > > reserve. I suppose you are not using data=journal mode and the filesystem
> > > > > was created as ext4 (i.e. not converted from ext3), right? Are you using
> > > > > quotas?
> > > > 
> > > > The filesystem *is* using data=journal mode.  If I switch to data=ordered
> > > > or data=writeback, the problem goes away.
> > >   Ah, OK. Then bug https://bugzilla.kernel.org/show_bug.cgi?id=34642 is
> > > probably ext3 incarnation of the same problem and it seems it's still
> > > present even in the current kernel - that ext3 assertion triggered even
> > > with 2.6.39 kernel. Frankly data=journal mode is far less tested than the
> > > other two modes especially with ext4, so I'm not sure how good idea is to
> > > use it in production.
>  
>   Hi Lukas,
> 
> > if it is so (and it probably is, since I am not testing this mode as
> > well:), it would be interesting to find out whether there are many users
> > of this and if there are not, which is probably the case, deprecate it now,
> > so we can remove it later. If we are openly suggesting not to use this,
> > then there is probably no point in having this option in the first
> > place.
>   For about one year I'm trying to look for people using data=journal and
> I've found some. So although data=journal users are minority, there are
> some. That being said I agree with you we should do something about it
> - either state that we want to fully support data=journal - and then we
> should really do better with testing it or deprecate it and remove it
> (which would save us some complications in the code).
> 
> I would be slightly in favor of removing it (code simplicity, less options
> to configure for admin, less options to test for us, some users I've come
> across actually were not quite sure why they are using it - they just
> thought it looks safer).
> 
> 								Honza
> 

I completely agree with you. Also I find it really dangerous that the
option which looks much safer is in fact less safe, because just a
minority of people (including developers) are testing it.

Thanks!
-Lukas