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

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Date:   Fri, 25 Jan 2019 18:20:56 +0000
From:   "Verma, Vishal L" <vishal.l.verma@...el.com>
To:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "Du, Fan" <fan.du@...el.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...e.de" <bp@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "tiwai@...e.de" <tiwai@...e.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        "zwisler@...nel.org" <zwisler@...nel.org>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "baiyaowei@...s.chinamobile.com" <baiyaowei@...s.chinamobile.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "Wu, Fengguang" <fengguang.wu@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "jane.chu@...cle.com" <jane.chu@...cle.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
Subject: Re: [PATCH 5/5] dax: "Hotplug" persistent memory for use like
 normal RAM


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@...el.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.