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

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Message-ID: <CAPcyv4jr0U-a521+wAL7RDca_AoUrtvO6yeY6d0UQpY7d=CHuQ@mail.gmail.com>
Date:   Thu, 29 Jun 2017 13:42:19 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linda Knippers <linda.knippers@....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 Thu, Jun 29, 2017 at 12:20 PM, Linda Knippers <linda.knippers@....com> wrote:
> On 06/29/2017 01:54 PM, Dan Williams wrote:
>> Allow volatile nfit ranges to participate in all the same infrastructure
>> provided for persistent memory regions.
>
> This seems to be a bit more than "other rework".

It's part of the rationale for having a "write_cache" control
attribute. There's only so much I can squeeze into the subject line,
but it is mentioned in the cover letter.

>> A resulting resulting namespace
>> device will still be called "pmem", but the parent region type will be
>> "nd_volatile".
>
> What does this look like to a user or admin?  How does someone know that
> /dev/pmemX is persistent memory and /dev/pmemY isn't?  Someone shouldn't
> have to weed through /sys or ndctl some other interface to figure that out
> in the future if they don't have to do that today.  We have different
> names for BTT namespaces.  Is there a different name for volatile ranges?

No, the block device name is still /dev/pmem. It's already the case
that you need to check behind just the name of the device to figure
out if something is actually volatile or not (see memmap=ss!nn
configurations), so I would not be in favor of changing the device
name if we think the memory might not be persistent. Moreover, I think
it was a mistake that we change the device name for btt or not, and
I'm glad Matthew talked me out of making the same mistake with
memory-mode vs raw-mode pmem namespaces. So, the block device name
just reflects the driver of the block device, not the properties of
the device, just like all other block device instances.