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

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Date:   Fri, 10 Aug 2018 09:18:11 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-nvdimm@...ts.01.org,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: Help trying to use /dev/pmem for dax debugging?



On 08/09/2018 07:53 PM, Theodore Y. Ts'o wrote:
> On Tue, Jul 31, 2018 at 01:27:15PM -0700, Dave Jiang wrote:
>>
>> 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:
>>>
>>> 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.
> 
> I turned off RANDOMIZE_BASE and RANDOMIZE_MEMORY, but that didn't fix
> my problem.
> 
> It turns out the problem was KASAN.  It looks like using memmap to
> create test /dev/pmemX devices is not compatible with CONFIG_KASAN
> being enabled.
> 
> So I have a workaround for now, but it seems this to be a bug in
> KASAN, or at least an unfortunate interaction between KASAN and
> NVDIMM_PFN.

Thanks Ted. I have updated the wiki Ross mentioned to reflect that.