linux-ext4 - Re: dax pmd fault handler never returns to userspace

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Date:	Wed, 18 Nov 2015 15:04:41 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-nvdimm <linux-nvdimm@...1.01.org>,
	Ross Zwisler <ross.zwisler@...el.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: dax pmd fault handler never returns to userspace

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@....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 

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