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

[Christian König]

Am 31.03.20 um 15:19 schrieb 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.

Wow, that is indeed quite a lot.

What is the criteria use for ordering the tree? Just the size or is that 
size+alignment?

Never looked into this, but maybe we have a low hanging fruit for an 
improvement here?

I'm not 100% sure, but moving away from atomic context wouldn't be that 
easy.

Christian.

> -Chris