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.
> 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
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