Re: [Intel-gfx] [PATCH v3 06/12] drm/ttm: add TTM_PAGE_FLAG_EXTERNAL_MAPPABLE

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On 9/16/21 11:58 AM, Matthew Auld wrote:
> On 16/09/2021 10:03, Thomas Hellström wrote:
>> On Thu, 2021-09-16 at 08:55 +0200, Christian König wrote:
>>>
>>>
>>> Am 15.09.21 um 20:59 schrieb Matthew Auld:
>>>> In commit:
>>>>
>>>> commit 667a50db0477d47fdff01c666f5ee1ce26b5264c
>>>> Author: Thomas Hellstrom <thellstrom@vmware.com>
>>>> Date:   Fri Jan 3 11:17:18 2014 +0100
>>>>
>>>>       drm/ttm: Refuse to fault (prime-) imported pages
>>>>
>>>> we introduced the restriction that imported pages should not be
>>>> directly
>>>> mappable through TTM(this also extends to userptr). In the next
>>>> patch we
>>>> want to introduce a shmem_tt backend, which should follow all the
>>>> existing rules with TTM_PAGE_FLAG_EXTERNAL, since it will need to
>>>> handle
>>>> swapping itself, but with the above mapping restriction lifted.
>>>>
>>>> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
>>>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>>>> Cc: Christian König <christian.koenig@amd.com>
>>>> ---
>>>>    drivers/gpu/drm/ttm/ttm_bo_vm.c | 6 ++++--
>>>>    include/drm/ttm/ttm_tt.h        | 7 +++++++
>>>>    2 files changed, 11 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> index 708390588c7c..fd6e18f12f50 100644
>>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>> @@ -163,8 +163,10 @@ vm_fault_t ttm_bo_vm_reserve(struct
>>>> ttm_buffer_object *bo,
>>>>           * (if at all) by redirecting mmap to the exporter.
>>>>           */
>>>>          if (bo->ttm && (bo->ttm->page_flags &
>>>> TTM_PAGE_FLAG_EXTERNAL)) {
>>>> -               dma_resv_unlock(bo->base.resv);
>>>> -               return VM_FAULT_SIGBUS;
>>>> +               if (!(bo->ttm->page_flags &
>>>> TTM_PAGE_FLAG_EXTERNAL_MAPPABLE)) {
>>
>> I saw there was previously a thought about testing for ttm_bo_type_sg
>> here. Was that a dead end?
>
> I guess the issue was userptr, which is also EXTERNAL and needs to be 
> rejected here, but underneath it's not type_sg.


OK, I thought userptr was using type_sg. I wonder what it's for if 
that's not the case...


>
> Just having an explicit MAPPABLE flag or similar seemed the more 
> flexible approach? Happy to try out other ideas.
>
I'm fine with a flag as well.

/Thomas