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

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Message-ID: <20180810025339.GB21087@thunk.org>
Date:   Thu, 9 Aug 2018 22:53:40 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Dave Jiang <dave.jiang@...el.com>
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 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.

						- Ted