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

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Message-ID: <1447895109.21443.169.camel@hpe.com>
Date:	Wed, 18 Nov 2015 18:05:09 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	Dan Williams <dan.j.williams@...el.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 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@....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
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