linux-kernel - Re: Oops with DCACHE_WORD

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Date:	Wed, 02 May 2012 23:38:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Nick Piggin <npiggin@...il.com>, Jana Saout <jana@...ut.de>,
	Joel Becker <jlbec@...lplan.org>, linux-kernel@...r.kernel.org
Subject: Re: Oops with DCACHE_WORD_ACCESS and ocfs2, autofs4

On 05/02/2012 10:57 PM, Linus Torvalds wrote:
> 
> There are multiple ways to fix this, including just marking that
> unaligned word access as being able to take an exception, but I had
> hoped to avoid having to do that. There are alternatives, like always
> padding allocations up by 7 bytes, but those are nasty too. So I'd
> like to understand what triggers this for Jana, it's possible we can
> just work around that particular issue.
> 

Can we do the trick of aligning the pointer and ignoring the start?
That would allow even architectures that don't have unaligned accesses
to work, too.

> Apr 30 14:02:46 web5 kernel: RIP: e030:[<ffffffff8113c29b>]
[<ffffffff8113c29b>] link_path_walk+0xab/0x890
> Apr 30 14:02:46 web5 kernel: RSP: e02b:ffff88001e7a3bc8  EFLAGS: 00010257
> Apr 30 14:02:46 web5 kernel: CS:  e033

These segment values look odd in the extreme...

> Apr 30 14:02:46 web5 kernel:  [<ffffffff810056c9>] ?
__raw_callee_save_xen_pmd_val+0x11/0x1e

... because he's running under Xen-PV.  So his memory map can be
arbitrarily screwed seven ways to Sunday.

	-hpa

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