Re: More info for DSM w/r/t sunffb on 2.6.15-rc6

[Hugh Dickins <hugh@veritas.com>]

On Sat, 24 Dec 2005, Linus Torvalds wrote:
> On Fri, 23 Dec 2005, David S. Miller wrote:
> >                                          
> > +	if (!(vma->vm_flags & VM_SHARED))
> > +		return -EINVAL;
> > +
> 
> Side note - as I explained to Nick the other week, VM_SHARED really means 
> "shared _writable_" mapping, so you're now disallowing a shared read-only 
> open too.
> 
> Which may be fine, of course. Especially if sbusfb always ends up giving a 
> writable pfn-mapping. But I wanted to check that that was what you meant 
> to do.
> 
> To test for MAP_SHARED, either do the is_cow_mapping() thing, or check 
> the VM_MAYSHARE bit.

I wondered over that too.  Concluded that at the moment it's right for
the test to be on VM_SHARED, because it's secretly giving away shared
write permission.  But in future, once that's been corrected, such checks
would do better to check for VM_MAYSHARE (i.e. reject all MAP_PRIVATE).

Hugh