[PATCH] drm/ttm: nuke VM_MIXEDMAP on BO mappings v2

[Thomas Hellström (Intel)]

On 6/7/21 3:58 PM, Christian König wrote:
> We discussed if that is really the right approach for quite a while now, but
> digging deeper into a bug report on arm turned out that this is actually
> horrible broken right now.
>
> The reason for this is that vmf_insert_mixed_prot() always tries to grab
> a reference to the underlaying page on architectures without
> ARCH_HAS_PTE_SPECIAL and as far as I can see also enabled GUP.
>
> So nuke using VM_MIXEDMAP here and use VM_PFNMAP instead.
>
> Also make sure to reject mappings without VM_SHARED.
>
> v2: reject COW mappings, merge function with only caller
>
> Signed-off-by: Christian König <christian.koenig at amd.com>
> Bugs: https://gitlab.freedesktop.org/drm/amd/-/issues/1606#note_936174
> ---
>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 44 +++++++++++----------------------
>   1 file changed, 14 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index 61828488ae2b..c9edb75626d9 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -359,12 +359,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>   		 * at arbitrary times while the data is mmap'ed.
>   		 * See vmf_insert_mixed_prot() for a discussion.
>   		 */
> -		if (vma->vm_flags & VM_MIXEDMAP)
> -			ret = vmf_insert_mixed_prot(vma, address,
> -						    __pfn_to_pfn_t(pfn, PFN_DEV),
> -						    prot);
> -		else
> -			ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
> +		ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>   
>   		/* Never error on prefaulted PTEs */
>   		if (unlikely((ret & VM_FAULT_ERROR))) {
> @@ -411,15 +406,9 @@ vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot)
>   	pfn = page_to_pfn(page);
>   
>   	/* Prefault the entire VMA range right away to avoid further faults */
> -	for (address = vma->vm_start; address < vma->vm_end; address += PAGE_SIZE) {
> -
> -		if (vma->vm_flags & VM_MIXEDMAP)
> -			ret = vmf_insert_mixed_prot(vma, address,
> -						    __pfn_to_pfn_t(pfn, PFN_DEV),
> -						    prot);
> -		else
> -			ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
> -	}
> +	for (address = vma->vm_start; address < vma->vm_end;
> +	     address += PAGE_SIZE)
> +		ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>   
>   	return ret;
>   }
> @@ -560,8 +549,16 @@ static const struct vm_operations_struct ttm_bo_vm_ops = {
>   	.access = ttm_bo_vm_access,
>   };
>   
> -static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_struct *vma)
> +int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
>   {
> +	/* Enforce VM_SHARED here since without it we would have really strange
> +	 * behavior on COW.
> +	 */

Nit: Perhaps "Enforce no COW.." since mappings are allowed with 
VM_SHARED iff VM_MAYWRITE is not set. Also style consistency with 
comments: First /* followed by line-break or are you adapting the above 
style for ttm?

With that fixed,

Reviewed-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>


> +	if (is_cow_mapping(vma->vm_flags))
> +		return -EINVAL;
> +
> +	ttm_bo_get(bo);
> +
>   	/*
>   	 * Drivers may want to override the vm_ops field. Otherwise we
>   	 * use TTM's default callbacks.
> @@ -576,21 +573,8 @@ static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_s
>   
>   	vma->vm_private_data = bo;
>   
> -	/*
> -	 * We'd like to use VM_PFNMAP on shared mappings, where
> -	 * (vma->vm_flags & VM_SHARED) != 0, for performance reasons,
> -	 * but for some reason VM_PFNMAP + x86 PAT + write-combine is very
> -	 * bad for performance. Until that has been sorted out, use
> -	 * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
> -	 */
> -	vma->vm_flags |= VM_MIXEDMAP;
> +	vma->vm_flags |= VM_PFNMAP;
>   	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
> -}
> -
> -int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
> -{
> -	ttm_bo_get(bo);
> -	ttm_bo_mmap_vma_setup(bo, vma);
>   	return 0;
>   }
>   EXPORT_SYMBOL(ttm_bo_mmap_obj);