linux-ext4 - Re: Help trying to use /dev/pmem for dax debugging?

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Date:   Tue, 31 Jul 2018 13:36:42 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-nvdimm@...ts.01.org, linux-ext4@...r.kernel.org
Subject: Re: Help trying to use /dev/pmem for dax debugging?

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@...c) (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.

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