linux-kernel - Re: kernel BUG at lib/radix-tree.c:473!

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Message-ID: <Pine.LNX.4.64.0808150049110.17216@blonde.site>
Date:	Fri, 15 Aug 2008 01:00:54 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Johannes Weiner <hannes@...urebad.de>
cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ian Campbell <ijc@...lion.org.uk>,
	linux-kernel@...r.kernel.org, Kel Modderman <kel@...ku42.de>,
	Markus Armbruster <armbru@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: kernel BUG at lib/radix-tree.c:473!

On Fri, 15 Aug 2008, Johannes Weiner wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
> > Hugh Dickins wrote:
> >
> >> An additional useful input would be: what happens if you replace
> >> that /dev/fb0 by a symlink /dev/fb0 pointing to an fb0 device node in
> >> one of your disk filesystems?  I rather expect that to cause the same
> >> trouble, which would argue that the driver is wrong and shmem right.
> >
> > I don't follow. Do you mean make /dev/fb0 a plain file on a
> > filesystem?  Or make it a disk device node?  Something else?
> 
> Creating a device node on a different filesystem to see if the driver
> only worked with the safe shmem set_page_dirty and now breaks due to
> exposure to the generic version.  Or if the driver works with the
> generic version through other mappings and the shmem code screws it up
> somewhere else.

Yes, that's it.  I think it was ext2 I referred to, when I worried
about this when making the change to tmpfs; and my reading of it
was that ext2 left a device node's a_ops unset, as I was changing
tmpfs to do.  (Looking at it again, ext2 doesn't even specify its
.set_page_dirty, so even if it had assigned an a_ops, it wouldn't
have avoided the default behaviour.)  But I'd like to hear what
actually happens in practice, rather than relying on my reading.

Hugh
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