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

[Zhijian Li (Fujitsu)]

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


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 ?

ndctl-create-namespace
...
        -M, --map=
            A pmem namespace in "fsdax" or "devdax" mode requires allocation of per-page metadata. The allocation
            can be drawn from either:

            •   "mem": typical system memory

            •   "dev": persistent memory reserved from the namespace :: Given relative capacities of "Persistent
                Memory" to "System RAM" the allocation defaults to reserving space out of the namespace directly
                ("--map=dev"). The overhead is 64-bytes per 4K (16GB per 1TB) on x86.


Thanks
Zhijian

> +		placed on regular memory (memmap_on_memory=0). This attribute
> +		must be set before the device is handed over to the 'kmem'