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

[Jason Gunthorpe <jgg@nvidia.com>]

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@amd.com>
Reviewed-by: Thomas Hellström <thomas.helllstrom@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>