Re: chasing the four level page table

[Andi Kleen <ak@muc.de>]

On Thu, Jan 06, 2005 at 01:36:46PM -0500, Jon Smirl wrote:
> On Thu, 06 Jan 2005 19:12:15 +0100, Andi Kleen <ak@muc.de> wrote:
> > Yes, you should use get_user_pages() instead if you access real memory.
> > If you try to find hardware mappings using that there is no ready
> > function for you right now, although I guess it could be added.
> 
> drm_follow_page is used like this:
> 
> offset = drm_follow_page(pt) | ((unsigned long) pt & ~PAGE_MASK);
> map = drm_lookup_map(offset, size, dev);
> if (map && map->type == _DRM_AGP) {
> 	vunmap(pt);
> 	return;
> }
> 
> I think pt is a user space address. In DRM AGP memory is mapped into
> kernel and user space so the user space address is being converted
> into a kernel space one. The kernel space one is used to verify that
> the address is a valid mapping to AGP space, if so the page is
> unmapped.  I didn't write this code so I'm not 100% sure of what is
> going on.

You can't use get_user_pages in this case because the AGP aperture
can be above mem_map.  If none of the callers take page_table_lock
already you would need to add that too. I guess from the context the lock
is not taken, but better double check.

Perhaps we should add a get_user_phys() or somesuch for this.

-Andi