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

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Message-ID: <m33al7ciwr.fsf@crossbow.pond.sub.org>
Date:	Thu, 14 Aug 2008 18:48:36 -0400
From:	Markus Armbruster <armbru@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Hugh Dickins <hugh@...itas.com>, Ian Campbell <ijc@...lion.org.uk>,
	linux-kernel@...r.kernel.org, Kel Modderman <kel@...ku42.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jaya Kumar <jayakumar.lkml@...il.com>
Subject: Re: kernel BUG at lib/radix-tree.c:473!

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> Hugh Dickins wrote:
>> As you can see, I'm still groping towards the right answer.
>> The driver probably needs to provide its own backing_dev_info
>> (or point to a suitable default), and its own address_space_ops,
>> and perhaps more (there should be examples elsewhere).  But whether
>> it is actually wrong, or whether I was wrong to mess it up, I've
>> not yet decided.
>>   
>
> My understanding is that the driver is doing something a bit clever:
> it uses the page dirty flags to determine which parts of the
> framebuffer have been written to, and uses that information to
> minimize the amount of stuff that needs to be copied out.  The writes

Yes.

> to the pages are not expected to generate actual page faults.
>
> But I haven't really looked at it closely, and I'm not at all familiar
> with the vm at this layer.  I'm not sure how it actually allocates the
> framebuffer memory for example (vmalloc?  incrementally on faults?).

vmalloc()

> I'm hoping Markus will leap in, since wrote this stuff.  Or, gasp,
> I'll read the code myself.

The actual cleverness is in fb_defio[*], which was written by Jaya
Kumar (cc'ed).  I merely ripped out the old, somewhat racy cleverness
I inherited from Anthony Liguori (which you can still admire in Xen's
2.6.18 kernel), and switched over to use fb_defio instead.  Because
one instance of clever code is enough.

My understanding of fb_defio's inner workings is rather limited I
fear.  I'm just using it.

Jaya, could you help?

[...]


[*] Documentation/fb/deferred_io.txt drivers/video/fb_defio.c
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