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

[Christian König]

Am 20.11.20 um 11:05 schrieb Daniel Vetter:
> On Fri, Nov 20, 2020 at 11:04 AM Christian König
> <christian.koenig@xxxxxxx> 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@xxxxxxxxxx>
> > > 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@xxxxxxxxxx>
> > >       Reviewed-by: Brian Paul <brianp@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > Cc: Brian Paul <brianp@xxxxxxxxxx>
> > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > > Cc: Christian Koenig <christian.koenig@xxxxxxx>
> > > Cc: Huang Rui <ray.huang@xxxxxxx>
> > 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);