[PATCH] drm/ttm: use compound pages for THP

[David Stevens]

On Wed, Jan 13, 2021 at 5:58 PM Christian König
<christian.koenig at amd.com> wrote:
>
> Am 13.01.21 um 09:47 schrieb David Stevens:
> > Compound page metadata is necessary for page reference counting to work
> > properly on pages besides the head page. Without it, put_page
> > corresponding to the last outstanding get_page call on a tail page will
> > end up freeing that page, even if the bo still references the page.
>
> NAK, I should add a comment since I can't count any more how many times
> people came up with this.
>
> Calling put_page() on a TTM allocated page is strictly illegal in the
> first place since they are not reference counted.
>
> Please don't tell me somebody once more tried to mmap() pages from a
> DMA-buf created sg_table into a process address space?

I ran into this on a system using the currently in development
virtio-gpu blob resources [1]. The implementation passes dma buffers
allocated by the host gpu to KVM_SET_USER_MEMORY_REGION, so the guest
can directly access the buffers without the need for an intermediate
copy.

[1] https://patchwork.kernel.org/project/dri-devel/cover/20200814024000.2485-1-gurchetansingh@chromium.org/

> Regards,
> Christian.
>
> >
> > Signed-off-by: David Stevens <stevensd at chromium.org>
> > ---
> >   drivers/gpu/drm/ttm/ttm_pool.c | 1 -
> >   1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
> > index 7b2f60616750..09239b93dc2c 100644
> > --- a/drivers/gpu/drm/ttm/ttm_pool.c
> > +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> > @@ -83,7 +83,6 @@ static struct page *ttm_pool_alloc_page(struct ttm_pool *pool, gfp_t gfp_flags,
> >               gfp_flags |= GFP_TRANSHUGE_LIGHT | __GFP_NORETRY |
> >                       __GFP_KSWAPD_RECLAIM;
> >               gfp_flags &= ~__GFP_MOVABLE;
> > -             gfp_flags &= ~__GFP_COMP;
> >       }
> >
> >       if (!pool->use_dma_alloc) {
>