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

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Message-ID: <20201120150148.GJ401619@phenom.ffwll.local>
Date:   Fri, 20 Nov 2020 16:01:48 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König <christian.koenig@....com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        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

On Fri, Nov 20, 2020 at 11:08:31AM +0100, Christian König wrote:
> 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.

Pagecache shootdown. unmap_mapping_range only shoots down from the virtual
side. Since that's all we care about, we don't need to set up the
address_space in the page.
-Daniel

> 
> 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);
> > 
> 

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