Re: [PATCH 5/5] dax: "Hotplug" persistent memory for use like normal RAM

[Jane Chu <jane.chu@oracle.com>]

On 1/25/2019 11:15 AM, Dan Williams wrote:
> On Fri, Jan 25, 2019 at 11:10 AM Jane Chu <jane.chu@oracle.com> wrote:
>>
>>
>> On 1/25/2019 10:20 AM, Verma, Vishal L wrote:
>>>
>>> On Fri, 2019-01-25 at 09:18 -0800, Dan Williams wrote:
>>>> On Fri, Jan 25, 2019 at 12:20 AM Du, Fan <fan.du@intel.com> wrote:
>>>>> Dan
>>>>>
>>>>> Thanks for the insights!
>>>>>
>>>>> Can I say, the UCE is delivered from h/w to OS in a single way in
>>>>> case of machine
>>>>> check, only PMEM/DAX stuff filter out UC address and managed in its
>>>>> own way by
>>>>> badblocks, if PMEM/DAX doesn't do so, then common RAS workflow will
>>>>> kick in,
>>>>> right?
>>>>
>>>> The common RAS workflow always kicks in, it's just the page state
>>>> presented by a DAX mapping needs distinct handling. Once it is
>>>> hot-plugged it no longer needs to be treated differently than "System
>>>> RAM".
>>>>
>>>>> And how about when ARS is involved but no machine check fired for
>>>>> the function
>>>>> of this patchset?
>>>>
>>>> The hotplug effectively disconnects this address range from the ARS
>>>> results. They will still be reported in the libnvdimm "region" level
>>>> badblocks instance, but there's no safe / coordinated way to go clear
>>>> those errors without additional kernel enabling. There is no "clear
>>>> error" semantic for "System RAM".
>>>>
>>> Perhaps as future enabling, the kernel can go perform "clear error" for
>>> offlined pages, and make them usable again. But I'm not sure how
>>> prepared mm is to re-accept pages previously offlined.
>>>
>>
>> Offlining a DRAM backed page due to an UC makes sense because
>>    a. the physical DRAM cell might still have an error
>>    b. power cycle, scrubing could potentially 'repair' the DRAM cell,
>> making the page usable again.
>>
>> But for a PMEM backed page, neither is true. If a poison bit is set in
>> a page, that indicates the underlying hardware has completed the repair
>> work, all that's left is for software to recover.  Secondly, because
>> poison is persistent, unless software explicitly clear the bit,
>> the page is permanently unusable.
> 
> Not permanently... system-owner always has the option to use the
> device-DAX and ARS mechanisms to clear errors at the next boot.
> There's just no kernel enabling to do that automatically as a part of
> this patch set.
> 
> However, we should consider this along with the userspace enabling to
> control which device-dax instances are set aside for hotplug. It would
> make sense to have a "clear errors before hotplug" configuration
> option.
> 

Agreed, it would be nice to clear error prior to the hotplug operation,
better if that can be handled by the kernel.

thanks,
-jane