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

[Dan Williams]

On Mon, Oct 22, 2018 at 6:05 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Mon, Oct 22, 2018 at 1:18 PM Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> 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().