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SubjectRe: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling
On Tue 03-12-19 11:48:53, Thomas Hellström (VMware) wrote:
> From: Thomas Hellstrom <thellstrom@vmware.com>
>
> 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@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: "Christian König" <christian.koenig@amd.com>
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
> Reviewed-by: Christian König <christian.koenig@amd.com>
> ---
> 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

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