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
