Re: [patch 4/5] mm: add vm_insert_pfn helpler

[Thomas Hellström]

Nick Piggin wrote:

> On Wed, Oct 11, 2006 at 12:12:19PM +0200, Thomas Hellstrom wrote:
>  
>
> > Nick, I just realized: would it be possible to have a pgprot_t argument 
> > to this one, instead of it using vma->vm_pgprot?
> >
> > The motivation for this (DRM again) is that some architectures (powerpc) 
> > cannot map the AGP aperture through IO space, but needs to remap the 
> > page from memory with a nocache attribute set. Others need special 
> > pgprot settings for write-combined mappings.
> >
> > Now, there's a possibility to change vma->vm_pgprot during the first 
> > ->fault(), but again, we only have the mmap_sem in read mode.
> >    
>
> I don't see a problem with that. It would be nice if vm_pgprot could
> be kept in synch with the pte protections, but I guess a crazy
> driver should be allowed to do anything it wants ;)
>
>
>  
:).
Actually, the caching bits are sort of left out from the mm code anyway. 
For example, mprotect will reset them, which is sort of a security risc, 
since an unpriviliged user can call mprotect on uncached page mappings, 
causing inconsistent mappings and stability problems.

Thomas

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