Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem) zone

[Jeff Moyer]

Dan Williams <dan.j.williams@xxxxxxxxx> writes:

> On Mon, May 7, 2018 at 12:08 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>> Dan Williams <dan.j.williams@xxxxxxxxx> writes:
>>
>>> On Mon, May 7, 2018 at 11:46 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>>>> On Mon, May 07, 2018 at 10:50:21PM +0800, Huaisheng Ye wrote:
>>>>> Traditionally, NVDIMMs are treated by mm(memory management) subsystem as
>>>>> DEVICE zone, which is a virtual zone and both its start and end of pfn
>>>>> are equal to 0, mm wouldnât manage NVDIMM directly as DRAM, kernel uses
>>>>> corresponding drivers, which locate at \drivers\nvdimm\ and
>>>>> \drivers\acpi\nfit and fs, to realize NVDIMM memory alloc and free with
>>>>> memory hot plug implementation.
>>>>
>>>> You probably want to let linux-nvdimm know about this patch set.
>>>> Adding to the cc.
>>>
>>> Yes, thanks for that!
>>>
>>>> Also, I only received patch 0 and 4.  What happened
>>>> to 1-3,5 and 6?
>>>>
>>>>> With current kernel, many mmâs classical features like the buddy
>>>>> system, swap mechanism and page cache couldnât be supported to NVDIMM.
>>>>> What we are doing is to expand kernel mmâs capacity to make it to handle
>>>>> NVDIMM like DRAM. Furthermore we make mm could treat DRAM and NVDIMM
>>>>> separately, that means mm can only put the critical pages to NVDIMM
>>
>> Please define "critical pages."
>>
>>>>> zone, here we created a new zone type as NVM zone. That is to say for
>>>>> traditional(or normal) pages which would be stored at DRAM scope like
>>>>> Normal, DMA32 and DMA zones. But for the critical pages, which we hope
>>>>> them could be recovered from power fail or system crash, we make them
>>>>> to be persistent by storing them to NVM zone.
>>
>> [...]
>>
>>> I think adding yet one more mm-zone is the wrong direction. Instead,
>>> what we have been considering is a mechanism to allow a device-dax
>>> instance to be given back to the kernel as a distinct numa node
>>> managed by the VM. It seems it times to dust off those patches.
>>
>> What's the use case?
>
> Use NVDIMMs as System-RAM given their potentially higher capacity than
> DDR. The expectation in that case is that data is forfeit (not
> persisted) after a crash. Any persistent use case would need to go
> through the pmem driver, filesystem-dax or device-dax.

OK, but that sounds different from what was being proposed, here.  I'll
quote from above:

>>>>> But for the critical pages, which we hope them could be recovered
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>>> from power fail or system crash, we make them to be persistent by
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>>> storing them to NVM zone.

Hence my confusion.

Cheers,
Jeff