Re: [PATCH v2 2/2] dax: add a sysfs knob to control memmap_on_memory behavior

[Verma, Vishal L]

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@xxxxxxxxxxxxxxx
> > +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.