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

["Thomas Hellström" <thomas@tungstengraphics.com>]

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