[PATCH] drm/amdgpu: fix an underflow on non-4KB-page systems

[Xi Ruoyao]

On 2021-03-29 20:10 +0200, Christian König wrote:
> You need to identify the root cause of this, most likely start or last 
> are not a multiple of AMDGPU_GPU_PAGES_IN_CPU_PAGE.

I printk'ed the value of start & last, they are all a multiple of 4
(AMDGPU_GPU_PAGES_IN_CPU_PAGE).

However... `num_entries = last - start + 1` so it became some irrational
thing...  Either this line is wrong, or someone called
amdgpu_vm_bo_update_mapping with [start, last) instead of [start, last], which
is unexpected.

> > > > BugLink: https://gitlab.freedesktop.org/drm/amd/-/issues/1549
> > > > Fixes: a39f2a8d7066 ("drm/amdgpu: nuke amdgpu_vm_bo_split_mapping v2")
> > > > Reported-by: Xi Ruoyao <xry111 at mengyan1223.wang>
> > > > Reported-by: Dan Horák <dan at danny.cz>
> > > > Cc: stable at vger.kernel.org
> > > > Signed-off-by: Xi Ruoyao <xry111 at mengyan1223.wang>
> > > > ---
> > > >    drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > index ad91c0c3c423..cee0cc9c8085 100644
> > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > @@ -1707,7 +1707,7 @@ static int amdgpu_vm_bo_update_mapping(struct
> > > > amdgpu_device *adev,
> > > >                  }
> > > >                  start = tmp;
> > > >    
> > > > -       } while (unlikely(start != last + 1));
> > > > +       } while (unlikely(start < last + 1));
> > > >    
> > > >          r = vm->update_funcs->commit(&params, fence);
> > > >    
> > > > 
> > > > base-commit: a5e13c6df0e41702d2b2c77c8ad41677ebb065b3
> 

-- 
Xi Ruoyao <xry111 at mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University