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

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Date:   Wed, 5 Jul 2017 23:46:00 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "Knippers, Linda" <linda.knippers@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hch@....de" <hch@....de>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "x86@...nel.org" <x86@...nel.org>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "jack@...e.cz" <jack@...e.cz>
Subject: Re: [PATCH v4 12/16] libnvdimm, nfit: enable support for volatile
 ranges

On Thu, 2017-06-29 at 18:28 -0700, Dan Williams wrote:
> On Thu, Jun 29, 2017 at 4:14 PM, Linda Knippers <linda.knippers@....c
> om> wrote:
> > On 06/29/2017 06:58 PM, Dan Williams wrote:
> > > On Thu, Jun 29, 2017 at 3:49 PM, Linda Knippers <linda.knippers@h
> > > pe.com> wrote:
> > > > > 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?
> > > > 
> > > 
> > >     cat $(readlink -f /sys/block/pmem0/device)/../devtype
> > > 
> > > ...this is where 'ndctl list' will get the information.
> > > 
> > 
> > Thanks.
> > 
> > I think we need a section 4 pmem manpage like exists for
> > mem, sd, fd, md, etc., where we can put stuff like this, as well
> > as providing some overview information that will point people to
> > other resources.  I'll give that some thought unless there is one
> > already that I'm not finding.
> > 
> 
> A "pmem" man page sounds like a great idea, I wasn't aware we even
> had an sd man page.

Sorry for being late to respond, but I agree with Linda that this 
naming policy is likely to confuse users.  I also care less about the
current users who use memmap option.  This case is pmem-emulation and
they know what they are doing.

Assuming block device interface is needed (in addition to device-dax)
for volatile range for use-cases like swap device, I wonder if user can
actually specify a right pmem device for swap from OS-install GUI when
both volatile and persistent block devices are listed as /dev/pmemN. 
Sometimes we are restricted with GUI menu.  Some users use GUI all the
time like Windows as well.

Can we differentiate the naming by adding 'v' like 'pmemNv' (if you
can't go with 'vmemN')?  I don't think having 's' for BTT was that bad.
 It's been helpful to tell users that these pmem devices are not byte-
addressable.  I also think that BTT for volatile range makes no sense
(unless emulated as persistent memory by memmap option).

Thanks,
-Toshi