Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling

[Michal Hocko]

On Tue 03-12-19 11:48:53, Thomas Hellström (VMware) wrote:
> From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> 
> TTM graphics buffer objects may, transparently to user-space,  move
> between IO and system memory. When that happens, all PTEs pointing to the
> old location are zapped before the move and then faulted in again if
> needed. When that happens, the page protection caching mode- and
> encryption bits may change and be different from those of
> struct vm_area_struct::vm_page_prot.
> 
> We were using an ugly hack to set the page protection correctly.
> Fix that and instead use vmf_insert_mixed_prot() and / or
> vmf_insert_pfn_prot().
> Also get the default page protection from
> struct vm_area_struct::vm_page_prot rather than using vm_get_page_prot().
> This way we catch modifications done by the vm system for drivers that
> want write-notification.

So essentially this should have any new side effect on functionality it
is just making a hacky/ugly code less so? In other words what are the
consequences of having page protection inconsistent from vma's?

> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> Cc: "Christian König" <christian.koenig@xxxxxxx>
> Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> Reviewed-by: Christian König <christian.koenig@xxxxxxx>
> ---
>  drivers/gpu/drm/ttm/ttm_bo_vm.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index e6495ca2630b..2098f8d4dfc5 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -173,7 +173,6 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>  				    pgoff_t num_prefault)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
> -	struct vm_area_struct cvma = *vma;
>  	struct ttm_buffer_object *bo = vma->vm_private_data;
>  	struct ttm_bo_device *bdev = bo->bdev;
>  	unsigned long page_offset;
> @@ -244,7 +243,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>  		goto out_io_unlock;
>  	}
>  
> -	cvma.vm_page_prot = ttm_io_prot(bo->mem.placement, prot);
> +	prot = ttm_io_prot(bo->mem.placement, prot);
>  	if (!bo->mem.bus.is_iomem) {
>  		struct ttm_operation_ctx ctx = {
>  			.interruptible = false,
> @@ -260,7 +259,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>  		}
>  	} else {
>  		/* Iomem should not be marked encrypted */
> -		cvma.vm_page_prot = pgprot_decrypted(cvma.vm_page_prot);
> +		prot = pgprot_decrypted(prot);
>  	}
>  
>  	/*
> @@ -284,10 +283,11 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>  		}
>  
>  		if (vma->vm_flags & VM_MIXEDMAP)
> -			ret = vmf_insert_mixed(&cvma, address,
> -					__pfn_to_pfn_t(pfn, PFN_DEV));
> +			ret = vmf_insert_mixed_prot(vma, address,
> +						    __pfn_to_pfn_t(pfn, PFN_DEV),
> +						    prot);
>  		else
> -			ret = vmf_insert_pfn(&cvma, address, pfn);
> +			ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
>  
>  		/* Never error on prefaulted PTEs */
>  		if (unlikely((ret & VM_FAULT_ERROR))) {
> @@ -319,7 +319,7 @@ vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
>  	if (ret)
>  		return ret;
>  
> -	prot = vm_get_page_prot(vma->vm_flags);
> +	prot = vma->vm_page_prot;
>  	ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT);
>  	if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
>  		return ret;
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs