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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Mon, 7 May 2018 12:17:05 -0700
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Subject: Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem) zone
To:     Jeff Moyer <jmoyer@redhat.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Huaisheng Ye <yehs1@lenovo.com>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        chengnt@lenovo.com, Dave Hansen <dave.hansen@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        pasha.tatashin@oracle.com, Linux MM <linux-mm@kvack.org>,
        colyli@suse.de, Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sasha Levin <alexander.levin@verizon.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>
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On Mon, May 7, 2018 at 12:08 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
> Dan Williams <dan.j.williams@intel.com> writes:
>
>> On Mon, May 7, 2018 at 11:46 AM, Matthew Wilcox <willy@infradead.org> wr=
ote:
>>> 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 wouldn=E2=80=99t manage NVDIMM directly as DRAM, ke=
rnel uses
>>>> corresponding drivers, which locate at \drivers\nvdimm\ and
>>>> \drivers\acpi\nfit and fs, to realize NVDIMM memory alloc and free wit=
h
>>>> 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 mm=E2=80=99s classical features like the bud=
dy
>>>> system, swap mechanism and page cache couldn=E2=80=99t be supported to=
 NVDIMM.
>>>> What we are doing is to expand kernel mm=E2=80=99s 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.