Re: [PATCH] drm/xe: Fix unnecessary host-side population of ttm_tt on non-TT resources

[Dave Airlie]

On Wed, 2 Sept 2026 at 18:08, Matthew Brost <matthew.brost@xxxxxxxxx> wrote:
>
> On Wed, Aug 26, 2026 at 07:01:21PM -0700, Matthew Brost wrote:
>
> Dave ping. Question below.
>
> > On Thu, Aug 20, 2026 at 10:34:49PM -0400, Nathan Bourgeois wrote:
> > > > Shouldn't we just be calling xe_bo_validate() here instead of
> > > > ttm_tt_populate? (With the correct xe_validation_guard() wrapping).
> > >
> > > > Yes, this might be a better solution, making ttm_bo_setup_export()
> > > > completely unnecessary.
> > >
> > > If ttm_bo_setup_export() is unnecessary, I'm happy to change the patch
> > > or make a new patch. I will attempt to implement and test this locally.
> > >
> > > > This part looks good as different patch from what I'm assuming will be a
> > > > TTM fix.
> > >
> > > Regarding this, what do you recommend I do, assuming the patch
> > > remains local to drm/xe? I'm still learning the ropes of contributing.
> > >
> >
> >
> > For Xe I believe Thomas and I aligned a xe_bo_validate with a correct
> > xe_validation_guard is the Xe preferred solution in the existing
> > design... But a question to Dave below before I commit to anything.
> >
> > > Nathan
> > >
> > > On Thu, Aug 20, 2026 at 9:08 PM Dave Airlie <airlied@xxxxxxxxx> wrote:
> > > >
> > > > > Yes, this might be a better solution, making ttm_bo_setup_export()
> > > > > completely unnecessary.
> > > > >
> > > > > It's also a bit odd that, in flows where we don't have backing storage
> > > > > on export, we populate with pages and charge the system memory cgroup,
> > > > > only to move the data to VRAM when the import attach is triggered,
> > > > > resulting in a copy and a change in cgroup charging.
> > > > >
> > > > > I guess the question is why was ttm_bo_setup_export() introduced over
> > > > > just a validation at export?
> > > > >
> > > >
> > > > I'd like to think I had an answer for that, but I don't. Likely
> > > > because I wasn't thinking about VRAM charging at all, and just
> > > > worrying about making sure we had populated some pages for system
> > > > memory ones, so the other side couldn't DoS us.
> > > >
> >
> > Dave:
> >
> > We don't charge any cgroups yet, right? This would only come into play
> > once a version of [1] merges, correct?
> >
> > What would prevent the pages populated for a TTM BO from being
> > immediately reclaimed and discarded? I'm fairly certain Xe's shrinker
> > could do exactly that, since we don't pin those pages. This seems to
> > imply that we'd need to store the cgroup associated with the TTM BO at
> > creation time and charge allocations to that cgroup, regardless of which
> > task ultimately triggers the page allocation.

This was actually to fix a non-cgroup bug with a possible priority
inversion problems.

i.e. a client could allocate a BO export it to a compositor, and then
the compositor would populate it for the first time and get ENOMEM.

This was to avoid that case by making sure a client had tried to
allocate all the pages for the BO before exporting it, so it would get
the failure at that time.

I don't believe xe should just be reclaiming and discarding these
pages without swapping them to shmem first?

Validating is probably fine as well.

Dave.