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

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Date:   Thu, 7 Dec 2023 19:25:38 +0000
From:   "Verma, Vishal L" <vishal.l.verma@...el.com>
To:     "lizhijian@...itsu.com" <lizhijian@...itsu.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 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.