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

[Dan Williams]

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.