[RFC] drm/vgem: Don't use get_page in fault handler.

[Chris Wilson]

Quoting Christian König (2020-07-07 11:58:26)
> Am 07.07.20 um 10:59 schrieb Daniel Vetter:
> > On Tue, Jul 7, 2020 at 9:27 AM Lepton Wu <ytht.net at gmail.com> wrote:
> >> For pages which are allocated in ttm with transparent huge pages,
> >> tail pages have zero as reference count. The current vgem code use
> >> get_page on them and it will trigger BUG when release_pages get called
> >> on those pages later.
> >>
> >> Here I attach the minimal code to trigger this bug on a qemu VM which
> >> enables virtio gpu (card1) and vgem (card0). BTW, since the upstream
> >> virtio gpu has changed to use drm gem and moved away from ttm. So we
> >> have to use an old kernel like 5.4 to reproduce it. But I guess
> >> same thing could happen for a real GPU if the real GPU use similar code
> >> path to allocate pages from ttm. I am not sure about two things: first, do we
> >> need to fix this? will a real GPU hit this code path? Second, suppose this
> >> need to be fixed, should this be fixed in ttm side or vgem side?  If we remove
> >> "huge_flags &= ~__GFP_COMP" from ttm_get_pages, then get_page in vgem works
> >> fine. But it's there for fix another bug:
> >> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.freedesktop.org%2Fshow_bug.cgi%3Fid%3D103138&data=02%7C01%7Cchristian.koenig%40amd.com%7Cfc0831be8fd848abfd8908d82254266d%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637297092113216357&sdata=S%2BtLJyB8mqyyE%2BSIjbfHM6FGFuFjrfI50tahpaoJ3wU%3D&reserved=0
> >> It could also be "fixed" with this patch. But I am really not sure if this is
> >> the way to go.
> > For imported dma-buf objects, vgem should not handle this itself, but
> > forward to the exporter through the dma_buf_mmap stuff. We have
> > helpers for this all now, probably just not wired up correctly. Trying
> > to ensure that all combinations of mmap code across all drivers work
> > the same doesn't work.
> 
> Yes, Daniel is right what vgem does here is fundamentally broken in many 
> ways.
> 
> First of all nobody should ever call get_page() on the pages TTM has 
> allocated. Those pages come out of a block box and should not be touched 
> by anybody.

It doesn't. It's only used on the pages vgem allocates (from shmemfs). So
I'm really confused as to how ttm gets involved here.
-Chris