Re: [PATCH v2] futex: Fix regression with read only mappings

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2011-06-27 at 11:40 -0500, Shawn Bohrer wrote:
> > >     if (PageAnon(page_head)) {
> > 
> > This bit needs a comment too (unless I am the only one to whom this
> was
> > non-obvious), maybe:
> > 
> >               
> >               /*
> >                * A read-only anonymous page implies a COW on a
> >                * MAP_PRIVATE mapping. There is no sane use-case
> >                * for this scenario, return -EFAULT to userspace.
> >                */
> 
> Your comment is wrong.  Unfortunately the code is completly
> non-obvious to me as well, and I have no idea why it is there.  This
> little snippet came from Peter's suggested fix in:
> 
> https://lkml.org/lkml/2011/6/6/368
> 
> Sadly Peter's gone silent and I'm left wondering if he knew some
> corner case that should return -EFAULT with a RO anonymous page or if
> he _thought_ this was preventing RO MAP_PRIVATE mappings.  If it is
> the latter then this block can be removed because it does NOT do that.
> > > +           if (ro) {
> > > +                   err = -EFAULT;
> > > +                   goto out;
> > > +           }
> > > + 

Peter simply gets too much email.. anyway, the reason I put that there
is that a RO Anon page will never change and is thus a little pointless
to use for futex ops.