linux-kernel - Re: [Patch 1/4][kernel][slimdump] Add new elf-note of type NT

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Date:	Wed, 5 Oct 2011 09:33:13 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org,
	crash-utility@...hat.com, kexec@...ts.infradead.org,
	Andi Kleen <andi@...stfloor.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, anderson@...hat.com,
	tachibana@....nes.nec.co.jp, oomichi@....nes.nec.co.jp
Subject: Re: [Patch 1/4][kernel][slimdump] Add new elf-note of type
 NT_NOCOREDUMP to capture slimdump

On Wed, Oct 05, 2011 at 12:48:44PM +0530, K.Prasad wrote:
> On Tue, Oct 04, 2011 at 10:04:37AM -0400, Vivek Goyal wrote:
> > On Mon, Oct 03, 2011 at 01:02:03PM +0530, K.Prasad wrote:
> > > There are certain types of crashes induced by faulty hardware in which
> > > capturing crashing kernel's memory (through kdump) makes no sense (or sometimes
> > > dangerous).
> > > 
> > > A case in point, is unrecoverable memory errors (resulting in fatal machine
> > > check exceptions) in which reading from the faulty memory location from the
> > > kexec'ed kernel will cause double fault and system reset (leaving no
> > > information for the user).
> > 
> > Prasad,
> > 
> > I am just trying to remember what was wrong with Andi's approach of
> > disable MCE while copying the dump?
> >
> 
> Hi Vivek,
> 	The behaviour upon a read operation on an UC memory location is
> undefined and so we want to avoid it (previously discussed here:
> http://article.gmane.org/gmane.linux.kernel/1146799). When we disable
> MCE and copy the dump, we will invariably read the faulty memory
> location.

Right, from the message above:

"- To disable MCE exceptions as done by the patches cited above. However
  the result of a read operation on corrupted memory is unknown and the
  system behaviour is undefined. We're unsure if this is a safe thing to
  do."

Can you elaborate more on that? Are we talking poisoned memory here or
undetected and uncorrectable memory errors?

Thanks.

-- 
Regards/Gruss,
    Boris.
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