linux-kernel - Re: [PATCH 4/4] dax: "Hotplug" persistent memory for use like normal RAM

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Date:   Thu, 17 Jan 2019 08:56:08 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Du, Fan" <fan.du@...el.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "dave@...1.net" <dave@...1.net>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "tiwai@...e.de" <tiwai@...e.de>,
        "zwisler@...nel.org" <zwisler@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Wu, Fengguang" <fengguang.wu@...el.com>,
        "baiyaowei@...s.chinamobile.com" <baiyaowei@...s.chinamobile.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "bp@...e.de" <bp@...e.de>
Subject: Re: [PATCH 4/4] dax: "Hotplug" persistent memory for use like normal RAM

On Wed, Jan 16, 2019 at 9:21 PM Du, Fan <fan.du@...el.com> wrote:
[..]
> >From: Dave Hansen <dave.hansen@...ux.intel.com>
> >
> >Currently, a persistent memory region is "owned" by a device driver,
> >either the "Direct DAX" or "Filesystem DAX" drivers.  These drivers
> >allow applications to explicitly use persistent memory, generally
> >by being modified to use special, new libraries.
> >
> >However, this limits persistent memory use to applications which
> >*have* been modified.  To make it more broadly usable, this driver
> >"hotplugs" memory into the kernel, to be managed ad used just like
> >normal RAM would be.
> >
> >To make this work, management software must remove the device from
> >being controlled by the "Device DAX" infrastructure:
> >
> >       echo -n dax0.0 > /sys/bus/dax/drivers/device_dax/remove_id
> >       echo -n dax0.0 > /sys/bus/dax/drivers/device_dax/unbind
> >
> >and then bind it to this new driver:
> >
> >       echo -n dax0.0 > /sys/bus/dax/drivers/kmem/new_id
> >       echo -n dax0.0 > /sys/bus/dax/drivers/kmem/bind
>
> Is there any plan to introduce additional mode, e.g. "kmem" in the userspace
> ndctl tool to do the configuration?
>

Yes, but not to ndctl. The daxctl tool will grow a helper for this.
The policy of what device-dax instances should be hotplugged at system
init will be managed by a persistent configuration file and udev
rules.