[PATCH v7 8/9] powerpc/mce: Add sysctl control for recovery action on MCE.

[Aneesh Kumar K.V]

On 08/08/2018 08:26 PM, Michael Ellerman wrote:
> Mahesh J Salgaonkar <mahesh at linux.vnet.ibm.com> writes:
>> From: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
>>
>> Introduce recovery action for recovered memory errors (MCEs). There are
>> soft memory errors like SLB Multihit, which can be a result of a bad
>> hardware OR software BUG. Kernel can easily recover from these soft errors
>> by flushing SLB contents. After the recovery kernel can still continue to
>> function without any issue. But in some scenario's we may keep getting
>> these soft errors until the root cause is fixed. To be able to analyze and
>> find the root cause, best way is to gather enough data and system state at
>> the time of MCE. Hence this patch introduces a sysctl knob where user can
>> decide either to continue after recovery or panic the kernel to capture the
>> dump.
> 
> I'm not convinced we want this.
> 
> As we've discovered it's often not possible to reconstruct what happened
> based on a dump anyway.
> 
> The key thing you need is the content of the SLB and that's not included
> in a dump.
> 
> So I think we should dump the SLB content when we get the MCE (which
> this series does) and any other useful info, and then if we can recover
> we should.
> 

The reasoning there is what if we got multi-hit due to some corruption 
in slb_cache_ptr. ie. some part of kernel is wrongly updating the paca 
data structure due to wrong pointer. Now that is far fetched, but then 
possible right?. Hence the idea that, if we don't have much insight into 
why a slb multi-hit occur from the dmesg which include slb content, 
slb_cache contents etc, there should be an easy way to force a dump that 
might assist in further debug.

-aneesh