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

[Xishi Qiu]

Hi Dan,

How about let the BIOS report a new type for kmem in e820 table?
e.g.
#define E820_PMEM	7
#define E820_KMEM	8

Then pmem and kmem are separately, and we can easily hotadd kmem
to the memory subsystem, no disturb the existing code (e.g. pmem,
nvdimm, dax...).

I don't know whether Intel will change some hardware features for
pmem which used like a volatility memory in the future. Perhaps
faster than pmem, cheaper, but volatility, and no need to care
about atomicity, consistency, L2/L3 cache...

Another question, why call it kmem? what does the "k" mean?

Thanks,
Xishi Qiu
On 2018/10/23 09:11, Dan Williams wrote:
> 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().
>