Re: [Crash-utility] [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(a)linux.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