Re: [PATCH 5/5] mce: recover from "action required" errors reportedin data path in usermode

[Chen Gong]

ä 9/7/2011 9:25 PM, Borislav Petkov åé:
> On Wed, Sep 07, 2011 at 02:05:38AM -0400, Chen Gong wrote:
>
> [..]
>
> > > +	/* known AR MCACODs: */
> > > +	MCESEV(
> > > +		KEEP, "HT thread notices Action required: data load error",
> > > +		SER, MASK(MCI_STATUS_OVER|MCI_UC_SAR|MCI_ADDR|MCACOD, MCI_UC_SAR|MCI_ADDR|0x0134),
> > > +		MCGMASK(MCG_STATUS_EIPV, 0)
> > > +		),
> > > +	MCESEV(
> > > +		AR, "Action required: data load error",
> > > +		SER, MASK(MCI_STATUS_OVER|MCI_UC_SAR|MCI_ADDR|MCACOD, MCI_UC_SAR|MCI_ADDR|0x0134),
> > > +		USER
> > > +		),
> >
> > I don't think *AR* makes sense here because the following codes have a
> > assumption that it means *user space* condition. If so, in the future a
> > new *AR* severity for kernel usage is created, we can't distinguish
> > which one can call "memory_failure" as below. At least, it should have a
> > suffix such as AR_USER/AR_KERN:
> >
> > enum severity_level {
> >           MCE_NO_SEVERITY,
> >           MCE_KEEP_SEVERITY,
> >           MCE_SOME_SEVERITY,
> >           MCE_AO_SEVERITY,
> >           MCE_UC_SEVERITY,
> >           MCE_AR_USER_SEVERITY,
> > 	MCE_AR_KERN_SEVERITY,
> >           MCE_PANIC_SEVERITY,
> > };
>
> Are you saying you need action required handling for when the data load
> error happens in kernel space? If so, I don't see how you can replay the
> data load (assuming this is a data load from DRAM). In that case, we're
> fatal and need to panic. If it is a different type of data load coming
> from a lower cache level, then we could be able to recover...?
>
> [..]

Yep, what I talk is data load error in kenel space. In fact, I'm not 
sure what we can do except panic :-), IIRC, Tony ever said in some 
situations kernel can be recovered. If it is true, we must distinguish
these two different scenarios. In *user space* case, memory_failure can
be called, but on the contrary, it can't.

> > > +	if (worst == MCE_AR_SEVERITY) {
> > > +		unsigned long pfn = m.addr>>   PAGE_SHIFT;
> > > +
> > > +		pr_err("Uncorrected hardware memory error in user-access at %llx",
> > > +			m.addr);
> >
> > print in the MCE handler maybe makes a deadlock ? say, when other CPUs
> > are printing something, suddently they received MCE broadcast from
> > Monarch CPU, when Monarch CPU runs above codes, a deadlock happens ?
> > Please fix me if I miss something :-)
>
> sounds like it can happen if the other CPUs have grabbed some console
> semaphore/mutex (I don't know what exactly we're using there) and the
> monarch tries to grab it.
>
> > > +		if (__memory_failure(pfn, MCE_VECTOR, 0)<   0) {
> > > +			pr_err("Memory error not recovered");
> > > +			force_sig(SIGBUS, current);
> > > +		} else
> > > +			pr_err("Memory error recovered");
> > > +	}
> >
> > as you mentioned in the comment, the biggest concern is that when
> > __memory_failure runs too long, if another MCE happens at the same
> > time, (assuming this MCE is happened on its sibling CPU which has the
> > same banks), the 2nd MCE will crash the system. Why not delaying the
> > process in a safer context, such as using user_return_notifer ?
>
> The user_return_notifier won't work, as we concluded in the last
> discussion round: http://marc.info/?l=linux-kernel&m=130765542330349
>
> AFAIR, we want to have a realtime thread dealing with that recovery
> so that we exit #MC context as fast as possible. The code then should
> be able to deal with a follow-up #MC. Tony, whatever happened to that
> approach?
>
> Thanks.

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