Re: Help trying to use /dev/pmem for dax debugging?

[Dave Jiang <dave.jiang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On 7/31/2018 12:36 PM, Ross Zwisler wrote:
> On Mon, Jul 30, 2018 at 07:53:12PM -0400, Theodore Y. Ts'o wrote:
>> In newer kernels, it looks like you can't use /dev/pmem0 for DAX
>> unless it's marked as being DAX capable.  This appears to require
>> CONFIG_NVDIMM_PFN.  But when I tried to build a kernel with that
>> configured, I get the following BUG:
>>
>> [    0.000000] Linux version 4.18.0-rc4-xfstests-00031-g7c2d77aa7d80 (tytso@cwcc) (gcc version 7.3.0 (Debian 7.3.0-27)) #460 SMP Mon Jul 30 19:38:44 EDT 2018
>> [    0.000000] Command line: systemd.show_status=auto systemd.log_level=crit root=/dev/vda console=ttyS0,115200 cmd=maint fstesttz=America/New_York fstesttyp=ext4 fstestapi=1.4 memmap=4G!9G memmap=9G!14G
> Hey Ted,
>
> You're using the memmap kernel command line parameter to reserve normal
> memory to be treated as normal memory, but you've also got kernel address
> randomization turned on in your kernel config:
>
> CONFIG_RANDOMIZE_BASE=y
> CONFIG_RANDOMIZE_MEMORY=y
>
> You need to turn these off for the memmap kernel command line parameter, else
> the memory we're using could overlap with addresses used for other things.

I believe this issue was fixed a while back. Although we probably can 
see if that is the issue or something else.


>
> Once that is off you probably want to double check that the addresses you're
> reserving are marked as 'usable' in the e820 table.  Gory details here, sorry
> for the huge link:
>
> https://nvdimm.wiki.kernel.org/how_to_choose_the_correct_memmap_kernel_parameter_for_pmem_on_your_system
>
> - Ross
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