linux-kernel - Re: Warning and BUG with btrfs and corrupted image

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Date:	Thu, 5 Feb 2009 19:59:16 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Pavel Machek <pavel@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Eric Sesterhenn <snakebyte@....de>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: Warning and BUG with btrfs and corrupted image

On Wed, Feb 04, 2009 at 07:29:51PM +0100, Pavel Machek wrote:
> On Sun 2009-02-01 12:40:50, Dave Chinner wrote:
> > On Mon, Jan 26, 2009 at 05:27:11PM +0100, Pavel Machek wrote:
> > > On Wed 2009-01-21 15:00:42, Dave Chinner wrote:
> > > +	  Turning this option on will result in kernel panicking any time
> > > +	  it detects on-disk corruption.
> > 
> > Thin end of a wedge. There's a couple of thousand conditions that
> > CONFIG_XFS_DEBUG introduces kernel panics on:
> > 
> > $ grep -r ASSERT fs/xfs |wc -l
> > 2095
> > 
> > 
> > CONFIG_*_DEBUG means include *debug* code there to help developers,
> > including adding additional failure tests into the kernel. Besides,
> > which bit of "don't turn it on unless you are an XFS developer"
> > don't you understand?
> 
> Yes, but DEBUG code is normally to help debugging, not to crash
> kernels.

Crashing the kernel at exactly the point a problem is detected
is often the simplest way of debugging the problem.

e.g. CONFIG_VM_DEBUG=y turns on VM_BUG_ON() which crashes the kernel
whenever it detects something wrong. Do I turn it on? Yes. Do i
complain about it when I hit a VM_BUG_ON()? No, I report the
bug and move on. If you turn on a DEBUG option, then you are
asking the system to behave in a way useful to a developer,
not an end user. That includes panicing when something wrong
is detected.

> IMO xfs should use errors=panic mount option as ext3 does,
> but...

We already have an equivalent:

/proc/sys/fs/xfs/panic_mask

The mask is empty on production kernels and can be selectively
turned on (depending on what error type you want to panic on).
CONFIG_XFS_DEBUG turns them all on by default so we can, weļl, panic
the system and debug any problem that occurs....

Cheers,

Dave,
-- 
Dave Chinner
david@...morbit.com
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