Re: [PATCH v2 3/3] DAX: mmap write readonly file

Ross Zwisler
2017-04-11T22:45:44Z

On Tue, Apr 11, 2017 at 12:52:35PM -0600, Ross Zwisler wrote:
> On Tue, Apr 11, 2017 at 09:56:14AM -0400, Jeff Moyer wrote:
> > Eryu Guan <eguan@redhat.com> writes:
> >
> > > On Mon, Apr 10, 2017 at 02:05:53PM +0800, Xiong Zhou wrote:
> > >> Regression case that one can write to read-only
> > >> file in a DAX mountpoint.
> > >>
> > >> Signed-off-by: Xiong Zhou <xzhou@redhat.com>
> > >> ---
> > >>
> > >> v2:
> > >> compile test programme manually in this test because default
> > >> cc option -O2 prevents this issue reproduction;
> > >
> > > Hmm, this looks.. ugly to me :) Better to find out the exact reason that
> > > prevents the bug from reproducing and update the c program accordingly.
> >
> > The compiler probably optimizes this bit out:
> >
> > + /* fault in the page */
> > + foo = *buf;
>
> Yep, verified this with objdump. You can prevent the compiler from
> optimizing out this bit by making 'foo' volatile. Patch at the end of this
> mail, and I also verified that this works with objdump.
>
> In my setup at least this test passes both with v4.10.0 (which does not
> contain the fix we are testing for, and should fail) and with v4.10.3 (which
> does contain the kernel fix, and should pass).
>
> So, I think the test still needs a little love. :)

Umm...never mind, my testing was broken. I was accidentally testing with the
upstream generic/422. :( Will retest tomorrow.