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

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Oct 21, 2021 at 01:41:39PM +0200, Daniel Vetter wrote:
> On Wed, Oct 20, 2021 at 04:37:02PM -0300, Jason Gunthorpe wrote:
> > 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>
> 
> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> I think we also want cc: stable here.

Ok
 
> Do you plan to land this through some dedicated pull for -rc? I think that
> makes sense to highlight it, but I can also smash this into some
> drm-fixes.

I was hoping you'd take it? Do want a v3?

Thanksm
Jason