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

[Xℹ Ruoyao <xry111@xxxxxxxxxxxxxxxx>]

If the initial value of `num_entires` (calculated at line 1654) is not
an integral multiple of `AMDGPU_GPU_PAGES_IN_CPU_PAGE`, in line 1681 a
value greater than the initial value will be assigned to it.  That causes
`start > last + 1` after line 1708.  Then in the next iteration an
underflow happens at line 1654.  It causes message

    *ERROR* Couldn't update BO_VA (-12)

printed in kernel log, and GPU hanging.

Fortify the criteria of the loop to fix this issue.

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@xxxxxxxxxxxxxxxx>
Reported-by: Dan Horák <dan@xxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
---
 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
-- 
2.31.1