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

On Mon, Oct 22, 2018 at 6:05 PM Dan Williams <dan.j.williams(a)intel.com&gt; wrote: > > On Mon, Oct 22, 2018 at 1:18 PM Dave Hansen <dave.hansen(a)linux.intel.com&gt; 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().