Re: [PATCH 1/6] mm: tracking shared dirty pages

[Hugh Dickins <hugh@veritas.com>]

On Thu, 22 Jun 2006, Peter Zijlstra wrote:
> On Wed, 2006-06-21 at 22:56 -0700, Andrew Morton wrote:
> 
> > > +		vma->vm_page_prot =
> > > +			__pgprot(pte_val
> > > +				(pte_wrprotect
> > > +				 (__pte(pgprot_val(vma->vm_page_prot)))));
> > > +
> > 
> > Is there really no simpler way?
>.... 
> Awell, thoughts, comments?

Just note vma->vm_page_prot before calling the ->mmap and check after.
If the driver has messed with it at all, it's not a mapping we want to
apply dirty capping to anyway.  If it's unchanged, and the other tests
pass, go ahead and reset readonly vm_page_prot via protection_map[].

This is of course a hack, as is the pgprotting code from drm.
The correct solution would be to set the proper backing_dev_info
in a number of drivers - we were too lazy when setting the default
in the first place; but even if we caught all the intree drivers,
we'd miss out-of-tree ones and suffer unnecessary pain from them.
So for now a hack is necessary, and I haven't thought of a better.

Other comments to follow...

Hugh

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