linux-kernel - Re: [RFC Patch 0/2] Slimdump framework using CRASH_REASON

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Date:	Wed, 23 Nov 2011 23:03:18 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	"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,
	Valdis.Kletnieks@...edu, Nick Bowler <nbowler@...iptictech.com>
Subject: Re: [RFC Patch 0/2] Slimdump framework using CRASH_REASON - v2

On Mon, Nov 21, 2011 at 10:17:27AM -0500, Vivek Goyal wrote:
> On Mon, Nov 21, 2011 at 03:24:05PM +0530, K.Prasad wrote:
> > Hi All,
> > 	In furtherance of the previous discussion regarding 'slimdump'
> > (refer: http://article.gmane.org/gmane.linux.kernel/1204967), it was
> > decided that,
> > 
> > - An entry in VMCOREINFO elf-note be added to denote the cause of crash,
> >   instead of creating a new elf-note.
> > 
> > - Upstream tools such as 'makedumpfile' and 'crash' be modified to
> >   recognise this string and inform the user accordingly.
> > 
> > Accordingly, this new version of the patchset makes the following
> > changes 
> > 
> > Changelog - version 2
> > -----------------------
> > (First version posted here:
> > http://article.gmane.org/gmane.linux.kernel/1198435)
> > 
> > - Append VMCOREINFO elf-note with a new variable CRASH_REASON whose
> >   value will be populated using arch_add_crash_reason() function.
> > 
> > - Define arch_add_crash_reason() in the x86 MCE path to return "PANIC_MCE"
> >   in the panic path of MCE.
> > 
> > - 'makedumpfile' tool is taught to recognise PANIC_MCE string as one
> >   value of CRASH_REASON for which 'slimdump' must be captured.
> 
> So again, what is slimdump? I mean, what information is now being captured
> in the case of slimdump? Are you capturing atleast the kernel message
> buffers? I am assuming that any register info emitted on console will
> make into kernel buffers and that should be useful to figure out what
> MCE happened.
> 

The kernel message buffers can be obtained by using the --dump-dmesg
option of makedumpfile but again that's risky. We wouldn't know if it'll
cause access to the faulty memory (which is how the previous method of having
a new elf-notes in a pristine location is much safer).

The method in this patch is quite primitive in that informs the user
nothing more than a one-line cause of crash. One should take help from other
tools (such as service processor/firmware/ACPI logs, or previous corrected
error logs) to infer the location of bad memory. It would have been helpful
if we could provide a dump of related information such as contents of
"struct mce" (like in the very first iteration of the patch,
refer: http://article.gmane.org/gmane.linux.kernel/1146215), but that would
need some memory (possibly in the form of a new elf-note).

Thanks,
K.Prasad

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