[PATCH 5/7] drm/i915/gem/ttm: Respect the objection region in placement_from_obj

[Matthew Auld]

On Thu, 22 Jul 2021 at 10:49, Matthew Auld
<matthew.william.auld at gmail.com> wrote:
>
> On Wed, 21 Jul 2021 at 21:11, Jason Ekstrand <jason at jlekstrand.net> wrote:
> >
> > On Mon, Jul 19, 2021 at 8:35 AM Matthew Auld
> > <matthew.william.auld at gmail.com> wrote:
> > >
> > > On Fri, 16 Jul 2021 at 20:49, Jason Ekstrand <jason at jlekstrand.net> wrote:
> > > >
> > > > On Fri, Jul 16, 2021 at 1:45 PM Matthew Auld
> > > > <matthew.william.auld at gmail.com> wrote:
> > > > >
> > > > > On Fri, 16 Jul 2021 at 18:39, Jason Ekstrand <jason at jlekstrand.net> wrote:
> > > > > >
> > > > > > On Fri, Jul 16, 2021 at 11:00 AM Matthew Auld
> > > > > > <matthew.william.auld at gmail.com> wrote:
> > > > > > >
> > > > > > > On Fri, 16 Jul 2021 at 16:52, Matthew Auld
> > > > > > > <matthew.william.auld at gmail.com> wrote:
> > > > > > > >
> > > > > > > > On Fri, 16 Jul 2021 at 15:10, Jason Ekstrand <jason at jlekstrand.net> wrote:
> > > > > > > > >
> > > > > > > > > On Fri, Jul 16, 2021 at 8:54 AM Matthew Auld
> > > > > > > > > <matthew.william.auld at gmail.com> wrote:
> > > > > > > > > >
> > > > > > > > > > On Thu, 15 Jul 2021 at 23:39, Jason Ekstrand <jason at jlekstrand.net> wrote:
> > > > > > > > > > >
> > > > > > > > > > > Whenever we had a user object (n_placements > 0), we were ignoring
> > > > > > > > > > > obj->mm.region and always putting obj->placements[0] as the requested
> > > > > > > > > > > region.  For LMEM+SMEM objects, this was causing them to get shoved into
> > > > > > > > > > > LMEM on every i915_ttm_get_pages() even when SMEM was requested by, say,
> > > > > > > > > > > i915_gem_object_migrate().
> > > > > > > > > >
> > > > > > > > > > i915_ttm_migrate calls i915_ttm_place_from_region() directly with the
> > > > > > > > > > requested region, so there shouldn't be an issue with migration right?
> > > > > > > > > > Do you have some more details?
> > > > > > > > >
> > > > > > > > > With i915_ttm_migrate directly, no.  But, in the last patch in the
> > > > > > > > > series, we're trying to migrate LMEM+SMEM buffers into SMEM on
> > > > > > > > > attach() and pin it there.  This blows up in a very unexpected (IMO)
> > > > > > > > > way.  The flow goes something like this:
> > > > > > > > >
> > > > > > > > >  - Client attempts a dma-buf import from another device
> > > > > > > > >  - In attach() we call i915_gem_object_migrate() which calls
> > > > > > > > > i915_ttm_migrate() which migrates as requested.
> > > > > > > > >  - Once the migration is complete, we call i915_gem_object_pin_pages()
> > > > > > > > > which calls i915_ttm_get_pages() which depends on
> > > > > > > > > i915_ttm_placement_from_obj() and so migrates it right back to LMEM.
> > > > > > > >
> > > > > > > > The mm.pages must be NULL here, otherwise it would just increment the
> > > > > > > > pages_pin_count?
> > > > > >
> > > > > > Given that the test is using the ____four_underscores version, it
> > > > > > doesn't have that check.  However, this executes after we've done the
> > > > > > dma-buf import which pinned pages.  So we should definitely have
> > > > > > pages.
> > > > >
> > > > > We shouldn't call ____four_underscores() if we might already have
> > > > > pages though. Under non-TTM that would leak the pages, and in TTM we
> > > > > might hit the WARN_ON(mm->pages) in __i915_ttm_get_pages(), if for
> > > > > example nothing was moved. I take it we can't just call pin_pages()?
> > > > > Four scary underscores usually means "don't call this in normal code".
> > > >
> > > > I've switched the ____four_underscores call to a __two_underscores in
> > > > the selftests and it had no effect, good or bad.  But, still, probably
> > > > better to call that one.
> > > >
> > > > > >
> > > > > > > > >
> > > > > > > > > Maybe the problem here is actually that our TTM code isn't respecting
> > > > > > > > > obj->mm.pages_pin_count?
> > > > > > > >
> > > > > > > > I think if the resource is moved, we always nuke the mm.pages after
> > > > > > > > being notified of the move. Also TTM is also not allowed to move
> > > > > > > > pinned buffers.
> > > > > > > >
> > > > > > > > I guess if we are evicted/swapped, so assuming we are not holding the
> > > > > > > > object lock, and it's not pinned, the future call to get_pages() will
> > > > > > > > see mm.pages = NULL, even though the ttm_resource is still there, and
> > > > > > > > because we prioritise the placements[0], instead of mm.region we end
> > > > > > > > up moving it for no good reason. But in your case you are holding the
> > > > > > > > lock, or it's pinned? Also is this just with the selftest, or
> > > > > > > > something real?
> > > > > > >
> > > > > > > Or at least in the selftest I see ____i915_gem_object_get_pages()
> > > > > > > which doesn't even consider the mm.pages AFAIK.
> > > > > >
> > > > > > The bogus migration is happening as part of the
> > > > > > __i915_gem_object_get_pages() (2 __underscores) call in
> > > > > > i915_gem_dmabuf_attach (see last patch).  That code is attempting to
> > > > > > migrate the BO to SMEM and then pin it there using the obvious calls
> > > > > > to do so.  However, in the pin_pages call, it gets implicitly migrated
> > > > > > back to LMEM thanks to i915_ttm_get_pages().  Why is _get_pages()
> > > > > > migrating things at all?
> > > > >
> > > > > Not sure yet, but __two_underscores() checks if
> > > > > i915_gem_object_has_pages() before actually calling into
> > > > > i915_ttm_get_pages(), so the mm.pages would have to be NULL here for
> > > > > some reason, so best guess is something to do with move_notify().
> > > >
> > > > Did a bit of experimenting along those lines and added the following
> > > > to the self-test BEFORE the export/import:
> > > >
> > > >     i915_gem_object_lock(obj, NULL);
> > > >     err = __i915_gem_object_get_pages(obj);
> > > >     __i915_gem_object_unpin_pages(obj);
> > > >     i915_gem_object_unlock(obj);
> > > >     if (err) {
> > > >         pr_err("__i915_gem_object_get_pages failed with err=%d\n", err);
> > > >         goto out_ret;
> > > >     }
> > > >
> > > > This seems to make the migration happen as expected without this
> > > > patch.  So it seems the problem only exists on buffers that haven't
> > > > gotten any backing storage yet (if I'm understanding get_pages
> > > > correctly).
> > > >
> > > > One potential work-around (not sure if this is a good idea or not!)
> > > > would be to do this inside dmabuf_attach().  Is this reliable?  Once
> > > > it has pages will it always have pages?  Or are there crazy races I
> > > > need to be worried about here?
> > >
> > > It turns out that the i915_ttm_adjust_gem_after_move() call in
> > > ttm_object_init will always update the mm.region to system memory(so
> > > that it matches the ttm resource), which seems reasonable given the
> > > default system placeholder thing, but does seem slightly iffy since we
> > > haven't actually moved/allocated anything.
> > >
> > > So effectively i915_ttm_migrate(SYSTEM) becomes a noop here since
> > > mm.region == mr. Which ofc means when we actually call get_pages() all
> > > that happens is that we allocate the pages in system memory(or without
> > > this patch placements[0]). Also with this patch lmem+smem, will always
> > > be placed in smem first, regardless of the placements ordering.
> > >
> > > For now we could maybe just split i915_ttm_adjust_gem_after_move() so
> > > we skip the part which updates the mm.region here in the init portion,
> > > since that should only happen when we try to place the object for
> > > real?
> >
> > Doesn't that mean we would end up with obj->mm.region and
> > obj->mm.res->mem_type are out-of-sync?  That seems bad.  I would think
> > we'd want the two in sync at all times.
>
> It likely doesn't matter since all roads lead to i915_ttm_get_pages()
> when we need to actually use the object?
>
> Also updating the mm.region in ttm_object_init() to reflect the dummy
> ttm resource seems a little scary, since any existing is_lmem() check
> now needs to happen after we place the object. Or at least the
> existing callers(for kernel internal objects) might not have expected
> that behaviour. Not sure if we checked all the callers.
>
> >
> > It seems like the fundamental problem here is that, when it's created,
> > the object isn't really in any memory region at all.  While I don't
> > think obj->mm.region == NULL is allowed or a good idea, it does seem
> > closer to the ground truth.
>
> Yeah, seems reasonable, especially for create_user where we don't know
> the placement until we actually call get_pages(). I think for internal
> users like with create_lmem() setting the mm.region early still makes
> some sense?
>
> >
> > Perhaps what we really want is for i915_gem_object_migrate to
> > get_pages before it does the migration to ensure that pages exist.
> > The only call to i915_gem_object_migrate in the code-base today is in
> > the display code and it's immediately followed by pin_pages().  For
> > that matter, maybe the call we actually want is
> > i915_object_migrate_and_pin that does the whole lot.
>
> I guess the only downside is that we might end up doing a real
> migration, with mempy or the blitter vs just changing the preferred
> placement for later? I think just go with whatever you feel is the
> simplest for now.

Another cheapo could be to drop the mr == mm.region noop, and just try
to place the object at mr anyway?

>
> >
> > Thoughts?
> >
> > --Jason
> >
> > P.S.  I'm going to go ahead and send another version with your other
> > comments addressed.  We can keep this discussion going here for now.