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

[Jane Chu]

On 1/25/2019 11:15 AM, Dan Williams wrote:
> On Fri, Jan 25, 2019 at 11:10 AM Jane Chu <jane.chu@xxxxxxxxxx> 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@xxxxxxxxx> 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