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.
