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

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Date:   Mon, 07 May 2018 15:28:29 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Huaisheng Ye <yehs1@...ovo.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        chengnt@...ovo.com, Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        pasha.tatashin@...cle.com, Linux MM <linux-mm@...ck.org>,
        colyli@...e.de, Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem) zone

Dan Williams <dan.j.williams@...el.com> writes:

> On Mon, May 7, 2018 at 12:08 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> Dan Williams <dan.j.williams@...el.com> writes:
>>
>>> On Mon, May 7, 2018 at 11:46 AM, Matthew Wilcox <willy@...radead.org> 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 wouldn’t 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 mm’s classical features like the buddy
>>>>> system, swap mechanism and page cache couldn’t be supported to NVDIMM.
>>>>> What we are doing is to expand kernel mm’s 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