linux-kernel - Re: Oops with DCACHE_WORD

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Date:	Wed, 02 May 2012 23:40:04 -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 11:38 PM, H. Peter Anvin wrote:
> 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.
> 

This almost makes me want to suggest adding a taint flag for PV.

	-hpa


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