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

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Date:	Wed, 5 Oct 2011 13:11:16 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	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,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [Patch 1/4][kernel][slimdump] Add new elf-note of type
 NT_NOCOREDUMP to capture slimdump

On Tue, Oct 04, 2011 at 10:30:12AM -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).
> > 
> > This patch introduces a framework called 'slimdump' enabled through a new
> > elf-note NT_NOCOREDUMP. Any error whose cause cannot be attributed to a
> > software error and cannot be detected by analysing the kernel memory may
> > decide to add this elf-note to the vmcore and indicate the futility of
> > such an exercise. Tools such as 'kexec', 'makedumpfile' and 'crash' are
> > also modified in tandem to recognise this new elf-note and capture
> > 'slimdump'.
> > 
> > The physical address and size of the NT_NOCOREDUMP are made available to the
> > user-space through a "/sys/kernel/nt_nocoredump" sysfs file (just like other
> > kexec related files).
> 
> Even if kernel has to signal to user space the reason for crash, why not
> add this info to existing vmcoreinfo note. Something like another filed.
> PANIC_MCE=1.
> 
> Secondly, the note name NT_NOCOREDUMP itself sounds binding. Kernel can
> export the reason of panic and then it is up to user space what do they
> want to do with it.
> 

Like I mentioned here:
http://article.gmane.org/gmane.linux.kernel/1199466, we can bring in fine-grained
message headers or note-types based on other users of this framework.

> So to me,  
> 
> > 
> > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> > ---
> >  arch/x86/kernel/cpu/mcheck/mce.c |   28 ++++++++++++++++++++++++++++
> >  include/linux/elf.h              |   18 ++++++++++++++++++
> >  include/linux/kexec.h            |    1 +
> >  kernel/kexec.c                   |   11 +++++++++++
> >  kernel/ksysfs.c                  |   10 ++++++++++
> >  5 files changed, 68 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> > index 08363b0..483b2fc 100644
> > --- a/arch/x86/kernel/cpu/mcheck/mce.c
> > +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> > @@ -238,6 +238,34 @@ static atomic_t mce_paniced;
> >  static int fake_panic;
> >  static atomic_t mce_fake_paniced;
> >  
> > +void arch_add_nocoredump_note(u32 *buf)
> > +{
> > +	struct elf_note note;
> > +	const char note_name[] = "PANIC_MCE";
> > +	const char desc_msg[] = "Crash induced due to a fatal machine "
> > +				"check error";
> > +
> 
> Again, note_name and desc_msg seem to be only two exports. Frankly desc
> string seems pretty obivious and we should be able to ignore it. So just
> exporting PANIC_MCE=true or something like that in case of MCE. 
>

Yes, adding a new field to the VMCOREINFO note would have been much
simpler but there's a second part to the kdump + fatal MCE problem which
will need a new elf-note to solve.

On a system containing 'poisoned' pages (generated as a result of
detecting UC errors which haven't been 'consumed'), if a software bug results
in crashing the machine, the ensuing kdump operation will read from the
faulty memory location. This will trigger a new crash within the context
of the kexec'ed kernel and we want to avoid this.

The plan is to pass-down the list of poisoned memory pages to the second
kernel using an elf-note so that these pages are left untouched during
dump capture. I'm working on an implementation of the same and should
have patches soon.

Thanks,
K.Prasad

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