On Thu, Nov 5, 2020 at 3:31 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Thu, Nov 5, 2020 at 2:22 PM Christian König <christian.koenig@xxxxxxx> wrote:
> >
> > Am 05.11.20 um 14:20 schrieb Daniel Vetter:
> > > On Thu, Nov 05, 2020 at 01:56:22PM +0100, Christian König wrote:
> > >> Am 05.11.20 um 13:50 schrieb Daniel Vetter:
> > >>> 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@xxxxxxx> 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@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.
> > >>>>>> 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?
> > >> When IOMMU is disabled or uses an 1 to 1 mapping we would otherwise give the
> > >> same access as /dev/mem to system memory and that is forbidden. But as I
> > >> noted this is just for the debugfs file.
> > > Ah, there's a config option for that. Plus it's debugfs, anything goes in
> > > debugfs, but if you're worried about that hole we should just disable the
> > > entire debugfs file for CONFIG_STRICT_DEVMEM. I can perhaps throw that on
> > > top, that follow_pfn patch series I'm baking is all about this kind of
> > > fun.
> >
> > And exactly that would get a NAK from us.
> >
> > We have specially created that debugfs file as an alternative when
> > CONFIG_STRICT_DEVMEM is set.
>
> Uh that doesn't work if you work around core restrictions with your
> own debugfs paths. Maybe you can do fun like this in your dkms, but
> not in upstream. Like if this was specifically created to work around
> CONFIG_STRICT_DEVMEM (and it sounds like that) then I think this
> should never have landed in upstream to begin with.
I'm also kinda confused that there's distros with CONFIG_STRICT_DEVMEM
which allow debugfs. debugfs is a pretty bad root hole all around, or
at least that's been the assumption all the time.
-Daniel
> > >> When I tried a few years ago to not set the page->mapping I immediately ran
> > >> into issues with our eviction test. So I think that this is used elsewhere
> > >> as well.
> > > That's the kind of interaction I'm worried about here tbh. If this does
> > > some kind of shrinking of some sorts, I think a real shrinker should take
> > > over.
> > >
> > > An improved grep shows nothing else, so the only the above is the only
> > > thing I can think of. What kind of eviction test goes boom if you clear
> > > ->mapping here? I'd be happy to type up the clever trick for the debugfs
> > > files.
> > > -Daniel
> > >
> > >> Regards,
> > >> Christian.
> > >>
> > >>> 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@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>
> > >>>>>>> ---
> > >>>>>>> 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
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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