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

[Daniel Vetter <daniel@ffwll.ch>]

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?

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);
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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