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

From: Dan Williams <dan.j.williams@intel.com>
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>
Subject: Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem) zone
Date: Mon, 7 May 2018 12:17:05 -0700	[thread overview]
Message-ID: <CAPcyv4hekYsXFy1PHg7zMyoWtj1pYVfnANfrhpk-+Hr_NBV=BQ@mail.gmail.com> (raw)
In-Reply-To: <x49a7tbi8r3.fsf@segfault.boston.devel.redhat.com>

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> 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.