Re: dax pmd fault handler never returns to userspace

[Toshi Kani <toshi.kani@hpe.com>]

On Wed, 2015-11-18 at 17:36 -0700, Ross Zwisler wrote:
> On Wed, Nov 18, 2015 at 03:04:41PM -0700, Toshi Kani wrote:
> > 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.
> 
> Here's a fix - please let me know if this seems incomplete or incorrect for
> some reason.

My test looks working now. :-)  I will do more testing and submit the gup patch 
as well.

Thanks,  
-Toshi