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

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Date:   Thu, 24 Jan 2019 22:27:58 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Michal Hocko <mhocko@...e.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Takashi Iwai <tiwai@...e.de>,
        Ross Zwisler <zwisler@...nel.org>,
        Linux MM <linux-mm@...ck.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 5/5] dax: "Hotplug" persistent memory for use like normal RAM

On Thu, Jan 24, 2019 at 10:13 PM Jane Chu <jane.chu@...cle.com> wrote:
>
> Hi, Dave,
>
> While chatting with my colleague Erwin about the patchset, it occurred
> that we're not clear about the error handling part. Specifically,
>
> 1. If an uncorrectable error is detected during a 'load' in the hot
> plugged pmem region, how will the error be handled?  will it be
> handled like PMEM or DRAM?

DRAM.

> 2. If a poison is set, and is persistent, which entity should clear
> the poison, and badblock(if applicable)? If it's user's responsibility,
> does ndctl support the clearing in this mode?

With persistent memory advertised via a static logical-to-physical
storage/dax device mapping, once an error develops it destroys a
physical *and* logical part of a device address space. That loss of
logical address space makes error clearing a necessity. However, with
the DRAM / "System RAM" error handling model, the OS can just offline
the page and map a different one to repair the logical address space.
So, no, ndctl will not have explicit enabling to clear volatile
errors, the OS will just dynamically offline problematic pages.