linux-kernel - Re: [Linux-nvdimm] [PATCH v2 10/20] pmem: use ida

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Date:	Wed, 29 Apr 2015 12:53:53 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Linux-nvdimm] [PATCH v2 10/20] pmem: use ida

On Wed, 2015-04-29 at 11:59 -0700, Dan Williams wrote:
> On Wed, Apr 29, 2015 at 11:25 AM, Toshi Kani <toshi.kani@...com> wrote:
> > Hi Dan,
> >
> > Thanks for the update.  This version of the patchset enumerates our NFIT
> > table properly. :-)
> >
> > On Tue, 2015-04-28 at 14:25 -0400, Dan Williams wrote:
> >> In preparation for the pmem driver attaching to pmem-namespaces emitted
> >> by libnd, convert it to use an ida instead of an always increasing
> >> atomic index.  This provides a bit of stability to pmem device names in
> >> the presence of driver re-bind events.
> >   :
> >> @@ -122,20 +123,26 @@ static struct pmem_device *pmem_alloc(struct device *dev, struct resource *res)
> >>  {
> >>       struct pmem_device *pmem;
> >>       struct gendisk *disk;
> >> -     int idx, err;
> >> +     int err;
> >>
> >>       err = -ENOMEM;
> >>       pmem = kzalloc(sizeof(*pmem), GFP_KERNEL);
> >>       if (!pmem)
> >>               goto out;
> >>
> >> +     pmem->id = ida_simple_get(&pmem_ida, 0, 0, GFP_KERNEL);
> >
> > nd_pmem_probe() is called asynchronously via async_schedule_domain
> > ().  We have seen a case that the region#->pmem# binding becomes
> > inconsistent across a reboot when there are 8 NVDIMM cards (reported by
> > Robert Elliott).  This leads user to access a wrong device.
> >
> > I think pmem id needs to be assigned before async_schedule_domain(), and
> > cascaded to nd_pmem_probe().
> >
> 
> I'll take a look at making this better, but it will never be
> bulletproof.  For the same reason that root=UUID=<uuid> is preferred
> over root=/dev/sda userspace should never rely on consistent pmem
> device names from boot to boot.

I agree that constant unique IDs, such as UUIDs, are necessary to
guarantee their consistent numbering regardless of configuration
changes.  For now, /dev/pmem%d should have consistent numbering while
NFIT table entries are consistent.

Thanks,
-Toshi

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