Messages in this thread | | | Date | 6 Jan 2005 20:38:27 +0100 | Date | Thu, 6 Jan 2005 20:38:26 +0100 | From | Andi Kleen <> | Subject | Re: chasing the four level page table |
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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
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