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

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Date:   Fri, 20 Nov 2020 11:08:31 +0100
From:   Christian König <christian.koenig@....com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Linux MM <linux-mm@...ck.org>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Brian Paul <brianp@...are.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Huang Rui <ray.huang@....com>
Subject: Re: [PATCH] drm/ttm: don't set page->mapping

Am 20.11.20 um 11:05 schrieb Daniel Vetter:
> On Fri, Nov 20, 2020 at 11:04 AM Christian König
> <christian.koenig@....com> wrote:
>> Am 20.11.20 um 10:54 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@...are.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@...are.com>
>>>       Reviewed-by: Brian Paul <brianp@...are.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.
>>>
>>> Cc: Thomas Hellstrom <thellstrom@...are.com>
>>> Cc: Brian Paul <brianp@...are.com>
>>> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
>>> Cc: Christian Koenig <christian.koenig@....com>
>>> Cc: Huang Rui <ray.huang@....com>
>> This is still a NAK as long as we can't come up with a better way to
>> track TTMs page allocations.
>>
>> Additional to that page_mapping() is used quite extensively in the mm
>> code and I'm not sure if that isn't needed for other stuff as well.
> Apologies, I'm honestly not quite sure how this lone patch here ended
> up in this submission. I didn't want to send it out.

No problem.

But looking a bit deeper into the mm code that other drm drivers don't 
set this correctly and still use unmap_mapping_range() sounds like quite 
a bug to me.

Going to track down what exactly that is used for.

Christian.

> -Daniel
>
>> Regards,
>> Christian.
>>
>>> ---
>>>    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 da9eeffe0c6d..5b2eb6d58bb7 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_tt.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
>>> @@ -284,17 +284,6 @@ 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)
>>>    {
>>> @@ -313,7 +302,6 @@ 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);
>