linux-ext4 - Re: [PATCH 1/2] ext4: call WARN

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Message-ID: <alpine.LFD.2.00.1301280810210.23664@dhcp-1-104.brq.redhat.com>
Date:	Mon, 28 Jan 2013 08:10:40 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Dave Chinner <david@...morbit.com>
cc:	"Theodore Ts'o" <tytso@....edu>,
	Lukáš Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/2] ext4: call WARN_ON after the debug message

On Sat, 26 Jan 2013, Dave Chinner wrote:

> Date: Sat, 26 Jan 2013 12:00:55 +1100
> From: Dave Chinner <david@...morbit.com>
> To: Theodore Ts'o <tytso@....edu>
> Cc: Lukáš Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org
> Subject: Re: [PATCH 1/2] ext4: call WARN_ON after the debug message
> 
> On Fri, Jan 25, 2013 at 10:09:48AM -0500, Theodore Ts'o wrote:
> > On Fri, Jan 25, 2013 at 10:22:44AM +0100, Lukáš Czerner wrote:
> > > 
> > > we can get the EIO error from ext4_map_blocks not only in the case
> > > of hardware error. The extent tree might not be in consistent state,
> > > or we could even ask for blocks outside the file system itself (I
> > > believe I've seen this before) and I think that in those cases it
> > > might be worth to all WARN_ON.
> > 
> > Sure, but in those cases, the file system is corrupt, and we should
> > have thrown an ext4_error() in ext4_map_blocks().  The point is that a
> > WARN_ON is only useful if there is a potential programming bug.  If we
> > know for sure that it's caused by a file system corruption, then we
> > don't want to throw a WARN_ON.
> 
> FWIW, XFS handles this problem with an error level sysctl. For
> situations like this, the default level doesn't throw stack traces
> (just the error message), but there are situations where diagnosis
> requires emitting the stack trace.
> 
> Hence if it is necessary for bug triage, users can then turn the
> sysctl up to 11 and the stack trace will be emitted after the
> warning message.  CONFIG_XFS_DEBUG sets the default log level to 11,
> because developers always want to know where an error came from ;)
> 
> Cheers,
> 
> Dave.
> 

That might actually be very useful. Thanks Dave!

-Lukas