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Date: Fri, 6 Jul 2012 07:50:19 -0400
From: Chris Mason <chris.mason@fusionio.com>
To: Jan Schmidt <list.btrfs@jan-o-sch.net>
CC: "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: btrfs GPF in read_extent_buffer() while scrubbing with kernel
 3.4.2
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On Fri, Jul 06, 2012 at 04:42:50AM -0600, Jan Schmidt wrote:
> ... down here in the stack. The warning is printed from two levels above,
> __readahead_hook.
> 
> Either I'm absolutely blind and there's code along the (rather short) road
> between those two that might do this I haven't seen. Or someone else messes
> with our extent buffers or the underlying pages. What really confuses me is
> that it happens so reproducibly.
> 
> I've no good idea at the moment how to go on. It might help to get a feeling if
> it's shifting around at least a little bit or really constant in the timing of
> occurrence. So can you please apply the next patch on top of the other two and
> give it some more failure tries? The "checksum mismatch [1234]" line will be of
> most interest. I'm also curious what the additional debug variables will say in
> the extended version of the very first printk. You can leave out the stack
> traces if you like, they won't matter much anyway.

I would suggest turning on slab debug and CONFIG_DEBUG_PAGEALLOC.
Something really strange is happening here.

-chris