linux-kernel - Re: [v1 2/2] device-dax: "Hotremove" persistent memory that is used like normal RAM

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Date:   Sat, 20 Apr 2019 13:01:51 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     James Morris <jmorris@...ei.org>, Sasha Levin <sashal@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Keith Busch <keith.busch@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ross Zwisler <zwisler@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Takashi Iwai <tiwai@...e.de>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [v1 2/2] device-dax: "Hotremove" persistent memory that is used
 like normal RAM

> > Thank you for looking at this.  Are you saying, that if drv.remove()
> > returns a failure it is simply ignored, and unbind proceeds?
>
> Yeah, that's the problem. I've looked at making unbind able to fail,
> but that can lead to general bad behavior in device-drivers. I.e. why
> spend time unwinding allocated resources when the driver can simply
> fail unbind? About the best a driver can do is make unbind wait on
> some event, but any return results in device-unbind.

Hm, just tested, and it is indeed so.

I see the following options:

1. Move hot remove code to some other interface, that can fail. Not
sure what that would be, but outside of unbind/remove_id. Any
suggestion?
2. Option two is don't attept to offline memory in unbind. Do
hot-remove memory in unbind if every section is already offlined.
Basically, do a walk through memblocks, and if every section is
offlined, also do the cleanup.

Pasha