Messages in this thread | | | Subject | Re: [PATCH 5/5] dax: "Hotplug" persistent memory for use like normal RAM | From | Jane Chu <> | Date | Fri, 25 Jan 2019 15:30:10 -0800 |
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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
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