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Date: Wed, 26 Feb 2003 13:32:46 +0000 (GMT)
From: Hugh Dickins <hugh@veritas.com>
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To: Jonah Sherman <jsherman@stuy.edu>
cc: Andrew Morton <akpm@digeo.com>, <linux-kernel@vger.kernel.org>
Subject: Re: [OOPS] 2.5.63 - NULL pointer dereference in loop device
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On Tue, 25 Feb 2003, Jonah Sherman wrote:
> On Tue, Feb 25, 2003 at 09:15:56PM +0000, Hugh Dickins wrote:
> > If you "losetup /dev/loop0 /dev/hdN", then it's LO_FLAGS_BH_REMAP
> > and doesn't even call bio_copy: it doesn't copy bio or buffers or
> 
> It appears this way if you just look at none_status, but you didn't look
> at loop_init_xfer().  Notice that it doesn't call xfer->init unless
> type != 0, so that flag is infact never set.

Hah!  Indeed, thank you for setting me straight.

> > Can you shed more light on how to reproduce this?
> 
> The block dev it is being used on must be larger than your RAM.  I don't
> have any swap on this machine, so I don't know if it must be bigger than
> that too.  Maybe disabling swap before testing this oops will make it
> work?

Yes, I'd tried block dev larger than RAM, with and without swap:
no luck.  Perhaps my disks somehow keep up with the stuff coming
down from /dev/zero but yours don't?  (No disgrace!)  But never mind,
Andrew has a surer way to hit it.

> In any case, the patch sent by Andrew Morton fixed this bug.

Andrew's patch for this is a thing of great beauty, a shining beacon
to inspire all whose eyes are graced by it.  But I'd still have liked
to understand why it's needed in your case not mine: it's a bit
embarrassing for me to claim I have patches for various loopmem
hangs, yet be unable to reproduce this.

Hugh

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