[PATCH v2] drm/ttm: Do not put non-struct page memory into PUD/PMDs

[Christian König]

Works for me.

Am 20.10.21 um 21:37 schrieb Jason Gunthorpe:
> On Wed, Oct 20, 2021 at 08:41:24AM +0200, Christian König wrote:
>
>>> I think the patch subject needs updating to reflect that we're disabling
>>> PUD/PMDs completely.
>>> With that fixed,
> Everyone is OK with this?
>
> drm/ttm: remove ttm_bo_vm_insert_huge()
>
> The huge page functionality in TTM does not work safely because PUD and
> PMD entries do not have a special bit.
>
> get_user_pages_fast() considers any page that passed pmd_huge() as
> usable:
>
> 	if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
> 		     pmd_devmap(pmd))) {
>
> And vmf_insert_pfn_pmd_prot() unconditionally sets
>
> 	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
>
> eg on x86 the page will be _PAGE_PRESENT | PAGE_PSE.
>
> As such gup_huge_pmd() will try to deref a struct page:
>
> 	head = try_grab_compound_head(pmd_page(orig), refs, flags);
>
> and thus crash.
>
> So, iomem cannot be installed using vmf_insert_pfn_pud/pmd_prot().
>
> Thomas further notices that the drivers are not expecting the struct page
> to be used by anything - in particular the refcount incr above will cause
> them to malfunction. This means even the struct page memory cannot be
> used.
>
> Therefore everything about this is not able to fully work correctly
> considering GUP_fast. Delete it entirely. It can return someday along with
> a proper PMD/PUD_SPECIAL bit in the page table itself to gate GUP_fast.
>
> Fixes: 314b6580adc5 ("drm/ttm, drm/vmwgfx: Support huge TTM pagefaults")
> Reviewed-by: Christian König <christian.koenig at amd.com>
> Reviewed-by: Thomas Hellström <thomas.helllstrom at linux.intel.com>
> Signed-off-by: Jason Gunthorpe <jgg at nvidia.com>