linux-kernel - Re: [BUG] kernel BUG at arch/i386/mm/highmem.c:15! on 2.6.23-rc8/rc9

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Date:	Thu, 4 Oct 2007 09:38:28 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hugh@...itas.com>
cc:	gurudas pai <gurudas.pai@...cle.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] kernel BUG at arch/i386/mm/highmem.c:15!  on
 2.6.23-rc8/rc9



On Thu, 4 Oct 2007, Hugh Dickins wrote:
> 
> Very helpful, thanks.  Guru, please try the appended patch, I think
> you'll find it fixes it for you (it did for me, once I'd puzzled out
> why I was failing to reproduce the problem - tests on ext3 don't work).
> Thank you so much for reporting this just in time!

Ouch.

The patch looks obviously correct, to the point that I don't understand 
how this bug happened in the first place.  It seems to have been 
introduced by Nick in d0217ac04ca6591841e5665f518e38064f4e65bd ("mm: fault 
feedback #1") if I read it right.

So "do_nonlinear_fault()" would effectively end up unmapping the PTE 
twice, right? And if I understand the problem right, this wasn't noticed 
immediately, because it probably only matters on:

 - CONFIG_HIGHPTE
*and*
 - a filesystem that allows VM_NONLINEAR, which became very rare with 
   dirty accounting.

Correct?

			Linus
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