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Subject: Re: dax pmd fault handler never returns to userspace
From: Toshi Kani <toshi.kani@hpe.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>,
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	Ross Zwisler <ross.zwisler@intel.com>,
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Date: Wed, 18 Nov 2015 15:04:41 -0700
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On Wed, 2015-11-18 at 13:57 -0800, Dan Williams wrote:
> On Wed, Nov 18, 2015 at 1:33 PM, Toshi Kani <toshi.kani@hpe.com> wrote:
> > I am seeing a similar/same problem in my test.  I think the problem is that
> > in
> > case of a WP fault, wp_huge_pmd() -> __dax_pmd_fault() ->
> > vmf_insert_pfn_pmd(),
> > which is a no-op since the PMD is mapped already.  We need WP handling for
> > this
> > PMD map.
> > 
> > If it helps, I have attached change for follow_trans_huge_pmd().  I have not
> > tested much, though.
> 
> Interesting, I didn't get this far because my tests were crashing the
> kernel.  I'll add this case the pmd fault test in ndctl.

I hit this one with mmap(MAP_POPULATE).  With this change, I then hit the WP
fault loop when writing to the range.

Thanks,
-Toshi