Messages in this thread | | | Date | Thu, 6 Jan 2005 13:36:46 -0500 | From | Jon Smirl <> | Subject | Re: chasing the four level page table |
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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.
On Thu, 06 Jan 2005 19:12:15 +0100, Andi Kleen <ak@muc.de> wrote: > Jon Smirl <jonsmirl@gmail.com> writes: > > > The DRM driver contains this routine: > > > > drivers/char/drm/drm_memory.h > > > > static inline unsigned long > > drm_follow_page (void *vaddr) > > { > > pgd_t *pgd = pgd_offset_k((unsigned long) vaddr); > > pud_t *pud = pud_offset(pgd, (unsigned long) vaddr); > > pmd_t *pmd = pmd_offset(pud, (unsigned long) vaddr); > > pte_t *ptep = pte_offset_kernel(pmd, (unsigned long) vaddr); > > return pte_pfn(*ptep) << PAGE_SHIFT; > > } > > > > No other driver needs to chase the page table like this so there is > > probably some other way to achieve this. Can someone who knows more > > about the VM system tell me if there is a way to eliminate this code? > > 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. > > The function is also not quite correct, it should already least take > the page_table_lock (depending on where you call it from) and check > p*_none() on each level. > > -Andi >
-- Jon Smirl jonsmirl@gmail.com - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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