Re: extfs reliability

From: Ted Ts'o
Date: Thu Jul 29 2010 - 14:59:16 EST

On Thu, Jul 29, 2010 at 05:00:10PM +0400, Vladislav Bolkhovitin wrote:
> Christoph Hellwig, on 07/29/2010 12:31 PM wrote:
> > My reading of the ext3/jbd code we explicitly wait on I/O completion
> > of dependent writes, and only require those to actually be stable
> > by issueing a flush. If that wasn't the case the default ext3
> > barriers off behaviour would not only be dangerous on devices with
> > volatile write caches, but also on devices that do not have them,
> > which in addition to the reading of the code is not what we've seen
> > in actual power fail testing, where ext3 does well as long as there
> > is no volatile write cache.
> Basically, it is so, but, unfortunately, not absolutely. I've just tried 2 tests on ext4 with iSCSI:

Well, this thread was talking about something else (which is how
various file systems handle barriers), and not bugs about what happen
when a disk disappears from a system due to attachment failure --- but
that's fine, we can deal with that here.

> Segmentation fault

OK, I've looked at your kernel messages, and it looks like the problem
comes from this:

/* Debugging code just in case the in-memory inode orphan list
* isn't empty. The on-disk one can be non-empty if we've
* detected an error and taken the fs readonly, but the
* in-memory list had better be clean by this point. */
if (!list_empty(&sbi->s_orphan))
dump_orphan_list(sb, sbi);
J_ASSERT(list_empty(&sbi->s_orphan)); <====

This is a "should never happen situation", and we crash so we can
figure out how we got there. For production kernels, arguably it
would probably be better to print a message and a WARN_ON(1), and then
not force a crash from a BUG_ON (which is what J_ASSERT is defined to

Looking at your messages and the ext4_delete_inode() warning, I think
I know what caused it. Can you try this patch (attached below) and
see if it fixes things for you?

> I already reported such issues some time ago, but my reports were
> not too much welcomed, so I gave up. Anyway, anybody can easily do
> my tests at any time.

My apologies. I've gone through the linux-ext4 mailing list logs, and
I can't find any mention of this problem from any username
I'm not sure where you reported it, and I'm sorry we dropped your bug
report. All I can say is that we do the best that we can, and our
team is relatively small and short-handed.

- Ted