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

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Message-ID: <CAPcyv4jrxMNEEeUZZG3=CdkYSyX7OtJLv_ZQ1gbML7bscePiQA@mail.gmail.com>
Date:   Sat, 20 Apr 2019 09:36:42 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Pavel Tatashin <pasha.tatashin@...een.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

On Sat, Apr 20, 2019 at 9:30 AM Pavel Tatashin
<pasha.tatashin@...een.com> wrote:
>
> > > +
> > > +       /* Walk and offline every singe memory_block of the dax region. */
> > > +       lock_device_hotplug();
> > > +       rc = walk_memory_range(start_pfn, end_pfn, dev, offline_memblock_cb);
> > > +       unlock_device_hotplug();
> > > +       if (rc)
> > > +               return rc;
> >
> > This potential early return is the reason why memory hotremove is not
> > reliable vs the driver-core. If this walk fails to offline the memory
> > it will still be online, but the driver-core has no consideration for
> > device-unbind failing. The ubind will proceed while the memory stays
> > pinned.
>
> Hi Dan,
>
> 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.