[PATCH 2/2] drm/ttm: Fix COW check

[Daniel Vetter]

On Fri, Jul 9, 2021 at 8:38 AM Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
>
>
>
> Am 08.07.21 um 21:36 schrieb Alex Deucher:
> > From: Felix Kuehling <Felix.Kuehling at amd.com>
> >
> > KFD Thunk maps invisible VRAM BOs with PROT_NONE, MAP_PRIVATE.
> > is_cow_mapping returns true for these mappings. Add a check for
> > vm_flags & VM_WRITE to avoid mmap failures on private read-only or
> > PROT_NONE mappings.
>
> I'm pretty sure that this is not working as expected.
>
> >
> > Fixes: f91142c62161 ("drm/ttm: nuke VM_MIXEDMAP on BO mappings v3")
> > Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>
> > Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
> > ---
> > drivers/gpu/drm/ttm/ttm_bo_vm.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > index f56be5bc0861..a75e90c7d4aa 100644
> > --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > @@ -552,7 +552,7 @@ static const struct vm_operations_struct ttm_bo_vm_ops = {
> >   int ttm_bo_mmap_obj(struct vm_area_struct *vma, struct ttm_buffer_object *bo)
> >   {
> >       /* Enforce no COW since would have really strange behavior with it. */
> > -     if (is_cow_mapping(vma->vm_flags))
> > +     if (is_cow_mapping(vma->vm_flags) && (vma->vm_flags & VM_WRITE))
>
> is_cow_mapping() already checks for VM_MAYWRITE, so this here shouldn't
> be necessary.

MAYWRITE != WRITE

But then you need to make sure you do catch mprotect() calls to catch
the cow, and I'm not sure that's even possible. Otherwise it'll go
boom again on the page refcount.
-Daniel

>
> Christian.
>
> >               return -EINVAL;
> >
> >       ttm_bo_get(bo);
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch