Re: [PATCH 0/9] Allow persistent memory to be used like normal RAM

Dan Williams
2018-10-27T04:45:43Z

On Mon, Oct 22, 2018 at 6:11 PM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Mon, Oct 22, 2018 at 6:05 PM Dan Williams <dan.j.williams@intel.com> wrote:
> >
> > On Mon, Oct 22, 2018 at 1:18 PM Dave Hansen <dave.hansen@linux.intel.com> wrote:
> > >
> > > Persistent memory is cool. But, currently, you have to rewrite
> > > your applications to use it. Wouldn't it be cool if you could
> > > just have it show up in your system like normal RAM and get to
> > > it like a slow blob of memory? Well... have I got the patch
> > > series for you!
> > >
> > > This series adds a new "driver" to which pmem devices can be
> > > attached. Once attached, the memory "owned" by the device is
> > > hot-added to the kernel and managed like any other memory. On
> > > systems with an HMAT (a new ACPI table), each socket (roughly)
> > > will have a separate NUMA node for its persistent memory so
> > > this newly-added memory can be selected by its unique NUMA
> > > node.
> > >
> > > This is highly RFC, and I really want the feedback from the
> > > nvdimm/pmem folks about whether this is a viable long-term
> > > perversion of their code and device mode. It's insufficiently
> > > documented and probably not bisectable either.
> > >
> > > Todo:
> > > 1. The device re-binding hacks are ham-fisted at best. We
> > > need a better way of doing this, especially so the kmem
> > > driver does not get in the way of normal pmem devices.
> > > 2. When the device has no proper node, we default it to
> > > NUMA node 0. Is that OK?
> > > 3. We muck with the 'struct resource' code quite a bit. It
> > > definitely needs a once-over from folks more familiar
> > > with it than I.
> > > 4. Is there a better way to do this than starting with a
> > > copy of pmem.c?
> >
> > So I don't think we want to do patch 2, 3, or 5. Just jump to patch 7
> > and remove all the devm_memremap_pages() infrastructure and dax_region
> > infrastructure.
> >
> > The driver should be a dead simple turn around to call add_memory()
> > for the passed in range. The hard part is, as you say, arranging for
> > the kmem driver to not stand in the way of typical range / device
> > claims by the dax_pmem device.
> >
> > To me this looks like teaching the nvdimm-bus and this dax_kmem driver
> > to require explicit matching based on 'id'. The attachment scheme
> > would look like this:
> >
> > modprobe dax_kmem
> > echo dax0.0 > /sys/bus/nd/drivers/dax_kmem/new_id
> > echo dax0.0 > /sys/bus/nd/drivers/dax_pmem/unbind
> > echo dax0.0 > /sys/bus/nd/drivers/dax_kmem/bind
> >
> > At step1 the dax_kmem drivers will match no devices and stays out of
> > the way of dax_pmem. It learns about devices it cares about by being
> > explicitly told about them. Then unbind from the typical dax_pmem
> > driver and attach to dax_kmem to perform the one way hotplug.
> >
> > I expect udev can automate this by setting up a rule to watch for
> > device-dax instances by UUID and call a script to do the detach /
> > reattach dance.
>
> The next question is how to support this for ranges that don't
> originate from the pmem sub-system. I expect we want dax_kmem to
> register a generic platform device representing the range and have a
> generic platofrm driver that turns around and does the add_memory().

I forgot I have some old patches that do something along these lines
and make device-dax it's own bus. I'll dust those off so we can
discern what's left.