unfriendly handling of system-ram mode devdaxes

[kilobyte]

Environment Information

• PMDK package version(s): any
• OS(es) version(s): Debian
• ndctl version(s): 67
• kernel version(s): 5.5-rc2

Please provide a reproduction of the bug:

daxctl reconfigure-device --mode=system-ram dax0.0 Have /dev/dax0.0 in your test config, run a test that uses it.

Actual behavior:

Errors that happen are misleading to the user: error: cannot remove file '/dev/dax0.0' or pmemdetect -z successfully returns 18446744073709551615

Expected behavior:

For non-tests, a better error message would be wanted.

For tests, it's debatable whether we should skip that devdax (like we do when some other parameter of that device, such as size or alignment is no good), or return a clear error.

Details

system-ram mode is a new feature of {nd,dax}ctl. It's especially often found on machines of people roped into messing with HMEM stuff.