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

[Thomas Hellström]

Hi,

On 10/19/21 20:21, Jason Gunthorpe wrote:
> 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.
>
> Prevent this by never using IO memory with vmf_insert_pfn_pud/pmd_prot().

Actually I think if fast gup will break even page backed memory since 
the backing drivers don't assume anybody else takes a refcount / 
pincount. Normal pages have PTE_SPECIAL and VM_PFNMAP to block that.

(Side note I was recommended to introduce a PTE_HUGESPECIAL bit for this 
and basically had  a patch ready but got scared off when trying to 
handle 64-bit PTE atomic updates on x86-32)

It might be that we (Intel) try to resurrect this code using 
PTE_HUGESPECIAL in the near future for i915, but until that, I think 
it's the safest option to disable it completely.

Thanks,

Thomas

>
> Fixes: 314b6580adc5 ("drm/ttm, drm/vmwgfx: Support huge TTM pagefaults")
> Signed-off-by: Jason Gunthorpe <jgg at nvidia.com>
> ---
>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
>
> AFAICT only the vmwgfx driver makes use of this, and I can't tell which path
> it is taking.
>
> This function will also try to install a PUD - does vmwgfx have a case where
> it has obtained a 1GB high order page - or is that dead code now?
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index f56be5bc0861ec..91d02c345fbba8 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -203,10 +203,13 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
>   	if (page_offset + fault_page_size > bo->resource->num_pages)
>   		goto out_fallback;
>   
> +	/*
> +	 * vmf_insert_pfn_pud/pmd_prot() can only be called with struct page
> +	 * backed memory
> +	 */
>   	if (bo->resource->bus.is_iomem)
> -		pfn = ttm_bo_io_mem_pfn(bo, page_offset);
> -	else
> -		pfn = page_to_pfn(ttm->pages[page_offset]);
> +		goto out_fallback;
> +	pfn = page_to_pfn(ttm->pages[page_offset]);
>   
>   	/* pfn must be fault_page_size aligned. */
>   	if ((pfn & (fault_page_size - 1)) != 0)
>
> base-commit: 519d81956ee277b4419c723adfb154603c2565ba