Re: [PATCH] drm/ttm: don't set page->mapping

["Christian König" <christian.koenig@amd.com>]

Am 05.11.20 um 13:50 schrieb Daniel Vetter:
> On Thu, Nov 05, 2020 at 01:29:50PM +0100, Christian König wrote:
>> Am 05.11.20 um 10:11 schrieb Daniel Vetter:
>>> On Thu, Nov 5, 2020 at 9:00 AM Christian König <christian.koenig@amd.com> wrote:
>>>> Am 04.11.20 um 17:50 schrieb Daniel Vetter:
>>>>> Random observation while trying to review Christian's patch series to
>>>>> stop looking at struct page for dma-buf imports.
>>>>>
>>>>> This was originally added in
>>>>>
>>>>> commit 58aa6622d32af7d2c08d45085f44c54554a16ed7
>>>>> Author: Thomas Hellstrom <thellstrom@vmware.com>
>>>>> Date:   Fri Jan 3 11:47:23 2014 +0100
>>>>>
>>>>>        drm/ttm: Correctly set page mapping and -index members
>>>>>
>>>>>        Needed for some vm operations; most notably unmap_mapping_range() with
>>>>>        even_cows = 0.
>>>>>
>>>>>        Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
>>>>>        Reviewed-by: Brian Paul <brianp@vmware.com>
>>>>>
>>>>> but we do not have a single caller of unmap_mapping_range with
>>>>> even_cows == 0. And all the gem drivers don't do this, so another
>>>>> small thing we could standardize between drm and ttm drivers.
>>>>>
>>>>> Plus I don't really see a need for unamp_mapping_range where we don't
>>>>> want to indiscriminately shoot down all ptes.
>>>> NAK, we use this to determine if a pages belongs to the driver or not in
>>>> amdgpu for example.
>>>>
>>>> Mostly used for debugging, but I would really like to keep that.
>>> Can you pls point me at that code? A quick grep hasn't really found much at all.
>> See amdgpu_iomem_read() for an example:
> Why do you reject this?

When IOMMU is disabled or uses an 1 to 1 mapping we would otherwise give 
the same access as /dev/mem to system memory and that is forbidden. But 
as I noted this is just for the debugfs file.

When I tried a few years ago to not set the page->mapping I immediately 
ran into issues with our eviction test. So I think that this is used 
elsewhere as well.

Regards,
Christian.

>
> If this is to avoid issues with userptr, then I think there's a simple
> trick:
> - grab page reference
> - recheck that the iova still points at the same address
> - do read/write, safe in the knowledge that this page cannot be reused for
>    anything else
> - drop page reference
>
> Of course this can still race against iova updates, but that seems to be a
> fundamental part of your debug interface here.
>
> Or am I missing something?
>
> Just pondering this more since setting the page->mapping pointer for just
> this seems somewhat wild abuse of ->mapping semantics :-)
> -Daniel
>
>>>                  if (p->mapping != adev->mman.bdev.dev_mapping)
>>>                          return -EPERM;
>> Christian.
>>
>>> -Daniel
>>>
>>>> Christian.
>>>>
>>>>> Cc: Thomas Hellstrom <thellstrom@vmware.com>
>>>>> Cc: Brian Paul <brianp@vmware.com>
>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>>>>> Cc: Christian Koenig <christian.koenig@amd.com>
>>>>> Cc: Huang Rui <ray.huang@amd.com>
>>>>> ---
>>>>>     drivers/gpu/drm/ttm/ttm_tt.c | 12 ------------
>>>>>     1 file changed, 12 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
>>>>> index 8861a74ac335..438ea43fd8c1 100644
>>>>> --- a/drivers/gpu/drm/ttm/ttm_tt.c
>>>>> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
>>>>> @@ -291,17 +291,6 @@ int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm)
>>>>>         return ret;
>>>>>     }
>>>>>
>>>>> -static void ttm_tt_add_mapping(struct ttm_bo_device *bdev, struct ttm_tt *ttm)
>>>>> -{
>>>>> -     pgoff_t i;
>>>>> -
>>>>> -     if (ttm->page_flags & TTM_PAGE_FLAG_SG)
>>>>> -             return;
>>>>> -
>>>>> -     for (i = 0; i < ttm->num_pages; ++i)
>>>>> -             ttm->pages[i]->mapping = bdev->dev_mapping;
>>>>> -}
>>>>> -
>>>>>     int ttm_tt_populate(struct ttm_bo_device *bdev,
>>>>>                     struct ttm_tt *ttm, struct ttm_operation_ctx *ctx)
>>>>>     {
>>>>> @@ -320,7 +309,6 @@ int ttm_tt_populate(struct ttm_bo_device *bdev,
>>>>>         if (ret)
>>>>>                 return ret;
>>>>>
>>>>> -     ttm_tt_add_mapping(bdev, ttm);
>>>>>         ttm->page_flags |= TTM_PAGE_FLAG_PRIV_POPULATED;
>>>>>         if (unlikely(ttm->page_flags & TTM_PAGE_FLAG_SWAPPED)) {
>>>>>                 ret = ttm_tt_swapin(ttm);

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