Subject: Re: [concept & "good taste" review] persistent store
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Tony Luck <tony.luck@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        tglx@linutronix.de, mingo@elte.hu, greg@kroah.com,
        akpm@linux-foundation.org, ying.huang@intel.com,
        Borislav Petkov <bp@alien8.de>, David Miller <davem@davemloft.net>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Kyungmin Park <kmpark@infradead.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        "H. Peter Anvin" <hpa@zytor.com>
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Date: Fri, 17 Dec 2010 13:19:12 -0500
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On Fri, 2010-12-17 at 10:09 -0800, Tony Luck wrote:
> On Thu, Dec 16, 2010 at 10:28 PM, Tony Luck <tony.luck@gmail.com> wrote:
> >> The _only_ valid reason for persistent storage is for things like
> >> oopses that kill the machine.
> >
> > Maybe I misunderstood what "KMSG_DUMP_OOPS" meant ... it
> > looked to me like this code is used for non-fatal OOPsen - ones
> > that will be logged to /var/log/messages.
> 
> Thinking about this a bit more I see my experiments with
> this were hopelessly naive. There is no way to know at
> "oops" time whether the problem is going to turn out to
> be minor or fatal.  So the right thing to do here is assume
> the worst and squirrel the data away safely just in case
> death is imminent.

To be honest, this is what I'd recommend even if you could tell the
difference.  A lot of the oopses I see were triggered by something
non-fatal (usually a WARN_ON()) earlier in the sequence.  Without seeing
the preceding WARN_ON() data, the oops is usually terrifically hard to
diagnose (often just a NULL or junk pointer deref).

James


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