linux-kernel - Re: [PATCH v5 2/2] dax/kmem: allow kmem to add memory with memmap_on

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Message-ID: <874jip94k5.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Tue, 17 Oct 2023 13:54:50 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     "Verma, Vishal L" <vishal.l.verma@...el.com>
Cc:     "david@...hat.com" <david@...hat.com>,
        "Hocko, Michal" <mhocko@...e.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "aneesh.kumar@...ux.ibm.com" <aneesh.kumar@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "jmoyer@...hat.com" <jmoyer@...hat.com>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH v5 2/2] dax/kmem: allow kmem to add memory with
 memmap_on_memory

"Verma, Vishal L" <vishal.l.verma@...el.com> writes:

> On Tue, 2023-10-17 at 13:18 +0800, Huang, Ying wrote:
>> "Verma, Vishal L" <vishal.l.verma@...el.com> writes:
>>
>> > On Thu, 2023-10-05 at 14:16 -0700, Dan Williams wrote:
>> > > Vishal Verma wrote:
>> > > >
>> > <..>
>> >
>> > > > +
>> > > > +       rc = kstrtobool(buf, &val);
>> > > > +       if (rc)
>> > > > +               return rc;
>> > >
>> > > Perhaps:
>> > >
>> > > if (dev_dax->memmap_on_memory == val)
>> > >         return len;
>> > >
>> > > ...and skip the check below when it is going to be a nop
>> > >
>> > > > +
>> > > > +       device_lock(dax_region->dev);
>> > > > +       if (!dax_region->dev->driver) {
>> > >
>> > > Is the polarity backwards here? I.e. if the device is already
>> > > attached to
>> > > the kmem driver it is too late to modify memmap_on_memory policy.
>> >
>> > Hm this sounded logical until I tried it. After a reconfigure-
>> > device to
>> > devdax (i.e. detach kmem), I get the -EBUSY if I invert this check.
>>
>> Can you try to unbind the device via sysfs by hand and retry?
>>
> I think what is happening maybe is while kmem gets detached, the device
> goes back to another dax driver (hmem in my tests). So either way, the
> check for if (driver) or if (!driver) won't distinguish between kmem
> vs. something else.
>
> Maybe we just remove this check? Or add an explicit kmem check somehow?

I think it's good to check kmem explicitly here.

--
Best Regards,
Huang, Ying