A write error on converting dax0.0 to kmem

A write error on converting dax0.0 to kmem

[Won-Kyo Choe]

Hi, Dave. I hope this message is sent appropriately in this time.

We've recently got a new machine which contains Optane DC memory and 
tried to use your patch set[1] in a recent kernel version(5.1.15). 
Unfortunately, we've failed on the last step[2] that describes 
converting device-dax driver as kmem. The main error is "echo: write error: No such device". 
We are certain that there must be a device and it should be recognized 
since we can see it in a path "/dev/dax0.0", however, somehow it keeps saying that error.

We've followed all your steps in the first patch[1] except a step about qemu configuration 
since we already have a persistent memory. We even checked that there is a region 
mapped with persistent memory from a command, `dmesg | grep e820` described in below.

BIOS-e820: [mem 0x0000000880000000-0x00000027ffffffff] persistent (type 7)

As the address is shown above, the thing is that in the qemu, the region is set as 
persistent (type 12) but in the native machine, it says persistent (type 7). 
We've still tried to find what type means and we simply guess that this is one 
of the reasons why we are not able to set the device as kmem.

We'd like to know why this error comes up and how can we handle this problem. 
We would really appreciate it if you are able to little bit help us.

Regards,

Won-Kyo

1. https://patchwork.kernel.org/cover/10829019/
2. https://patchwork.kernel.org/patch/10829041/

Re: A write error on converting dax0.0 to kmem

[Dan Williams]

[ this is on-topic for linux-nvdimm, but likely off-topic for
linux-mm. I leave it cc'd for now ]

On Thu, Jun 27, 2019 at 7:20 AM Won-Kyo Choe <heysid@ajou.ac.kr> wrote:
>
> Hi, Dave. I hope this message is sent appropriately in this time.
>
> We've recently got a new machine which contains Optane DC memory and
> tried to use your patch set[1] in a recent kernel version(5.1.15).
> Unfortunately, we've failed on the last step[2] that describes
> converting device-dax driver as kmem. The main error is "echo: write error: No such device".
> We are certain that there must be a device and it should be recognized
> since we can see it in a path "/dev/dax0.0", however, somehow it keeps saying that error.
>
> We've followed all your steps in the first patch[1] except a step about qemu configuration
> since we already have a persistent memory. We even checked that there is a region
> mapped with persistent memory from a command, `dmesg | grep e820` described in below.
>
> BIOS-e820: [mem 0x0000000880000000-0x00000027ffffffff] persistent (type 7)
>
> As the address is shown above, the thing is that in the qemu, the region is set as
> persistent (type 12) but in the native machine, it says persistent (type 7).
> We've still tried to find what type means and we simply guess that this is one
> of the reasons why we are not able to set the device as kmem.
>
> We'd like to know why this error comes up and how can we handle this problem.
> We would really appreciate it if you are able to little bit help us.

Before digging deeper let's first verify that you have switched
device-dax from using "class" devices to using "bus" devices. Yes,
that step is not included in the changelog instructions. Here is a man
page for a tool that can automate some of the steps for you:

http://pmem.io/ndctl/daxctl-migrate-device-model.html

You can validate that you're in "bus" mode by making sure a "dax0.0"
link appears under "/sys/bus/dax/devices".