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

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Date:	Wed, 5 Oct 2011 14:53:50 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...en8.de>, 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 09:33:13AM +0200, Borislav Petkov wrote:
> 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?
>

It refers to uncorrected memory errors that are not consumed and the
corresponding 'struct page's are marked PG_hwpoison. Typically the SRAO
type errors that are handled in mm/memory-failure.c.

If MCE is enabled, during a kdump, we will deliberately trigger a read
operation over the poisoned memory and make the UCE fatal. It is not
clear what would happen if MCE is disabled in the above case.

Thanks,
K.Prasad

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