linux-kernel - Re: [PATCH 5/5] mce: recover from "action required" errors reported in data path in usermode

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Date:	Wed, 7 Sep 2011 15:25:00 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Chen Gong <gong.chen@...ux.intel.com>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Borislav Petkov <bp@...64.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Subject: Re: [PATCH 5/5] mce: recover from "action required" errors reported
 in data path in usermode

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...?

[..]

> > +	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.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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