[PATCH] drm/mm: revert "Break long searches in fragmented address spaces"

[Chris Wilson]

Quoting Daniel Vetter (2020-03-31 11:38:50)
> On Tue, Mar 31, 2020 at 11:20 AM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > Quoting Daniel Vetter (2020-03-31 10:16:18)
> > > On Tue, Mar 31, 2020 at 10:59:45AM +0200, Christian König wrote:
> > > > A not so gentle ping, since this pretty much broke all TTM based drivers.
> > > >
> > > > Could we revert this for now?
> > >
> > > Always ack for revert.
> > >
> > > Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> >
> > So you didn't check the earlier patch either?
> 
> I did, but wasn't super sold on the idea of more flags to smack an r-b
> onto it, so figured I'll throw the default ack-for-revert on this
> meanwhile.

We allow userspace to poison the drm_mm at roughly 8K intervals, a
search space of 35b with typically O(N^2) behaviour and each node
traversal (rb_next/rb_prev) will itself be costly. Even our simple tests
can generate a search of several minutes before our patience runs out.o
Any drm_mm that allows for userspace to control alignment can be
arbitrarily fragmented, hence a raised eyebrow that this search would be
allowed in atomic context.
-Chris