linux-kernel - Re: [PATCH v2 2/2] dax: add a sysfs knob to control memmap_on

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Message-ID: <ad95e550-cdb3-448f-beb1-fc4899053639@fujitsu.com>
Date:   Fri, 8 Dec 2023 09:20:31 +0000
From:   "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To:     "Verma, Vishal L" <vishal.l.verma@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>
CC:     "david@...hat.com" <david@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>
Subject: Re: [PATCH v2 2/2] dax: add a sysfs knob to control memmap_on_memory
 behavior



On 08/12/2023 03:25, Verma, Vishal L wrote:
> On Thu, 2023-12-07 at 08:29 +0000, Zhijian Li (Fujitsu) wrote:
>> Hi Vishal,
>>
>>
>> On 07/12/2023 12:36, Vishal Verma wrote:
>>> +
>>> +What:          /sys/bus/dax/devices/daxX.Y/memmap_on_memory
>>> +Date:          October, 2023
>>> +KernelVersion: v6.8
>>> +Contact:       nvdimm@...ts.linux.dev
>>> +Description:
>>> +               (RW) Control the memmap_on_memory setting if the dax device
>>> +               were to be hotplugged as system memory. This determines whether
>>> +               the 'altmap' for the hotplugged memory will be placed on the
>>> +               device being hotplugged (memmap_on+memory=1) or if it will be
>>
>> s/memmap_on+memory=1/memmap_on_memory=1
> 
> Thanks, will fix.
>>
>>
>> I have a question here
>>
>> What relationship about memmap_on_memory and 'ndctl-create-namespace
>> -M' option which
>> specifies where is the vmemmap backed memory.
>> I'm confused that memmap_on_memory=1 and '-M dev' are the same for
>> nvdimm devdax mode ?
>>
> The main difference is that memory that comes from non-nvdimm sources,
> such as hmem, or cxl, doesn't have a chance to set up the altmaps as
> pmem can with '-M dev'. For these, memmap_on_memory does this as part
> of the memory hotplug.

Thanks for your explanation.
(I wrongly thought nvdimm.kmem was also controlled by '-M dev' before)

feel free add:
Tested-by: Li Zhijian <lizhijian@...itsu.com>