linux-kernel - Re: [PATCH v4 12/16] libnvdimm, nfit: enable support for volatile ranges

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Date:   Thu, 29 Jun 2017 18:49:52 -0400
From:   Linda Knippers <linda.knippers@....com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 12/16] libnvdimm, nfit: enable support for volatile
 ranges



On 6/29/2017 6:43 PM, Dan Williams wrote:
> On Thu, Jun 29, 2017 at 3:35 PM, Linda Knippers <linda.knippers@....com> wrote:
>> On 06/29/2017 06:28 PM, Dan Williams wrote:
>>> On Thu, Jun 29, 2017 at 3:12 PM, Linda Knippers <linda.knippers@....com> wrote:
>>> [..]
>>>>> The /dev/pmem
>>>>> device name just tells you that your block device is hosted by a
>>>>> driver that knows how to handle persistent memory constraints, but any
>>>>> other details about the nature of the address range need to come from
>>>>> other sources of information, and potentially information sources that
>>>>> the kernel does not know about.
>>>>
>>>>
>>>> I'm asking about the other source of information in this specific case
>>>> where we're exposing pmem devices that will never ever be persistent.
>>>> Before we add these devices, I think we should be able to tell the user
>>>> how they can know the properties of the underlying device.
>>>
>>> The only way I can think to indicate this is with a platform + device
>>> whitelist in a tool like ndctl. Where the tool says "yes, these
>>> xyz-vendor DIMMs on this abc-vendor platform with this 123-version
>>> BIOS" is a known good persistent configuration.
>>
>> Doesn't the kernel know that something will never ever be persistent
>> because the NFIT type says NFIT_SPA_VDISK, NFIT_SPA_VCD, or NFIT_SPA_VOLATILE?
>> That's the case I'm asking about here.   In this patch, you're adding support
>> for creating /dev/pmem devices for those address ranges.  My question is
>> how the admin/user knows that those devices will never ever be persistent.
>
> The parent region of the namespace will have a 'volatile' type:
>
> # cat /sys/bus/nd/devices/region0/devtype
> nd_volatile

If all I know is the /dev/pmem device name, how do I find that?

-- ljk
>
>> I don't think we need ndctl to know which vendors' hardware/firmware
>> actually works as advertised.
>
> Certification stickers is what I was thinking, but I was missing your
> direction question.
>