Quoting Christian König (2020-04-01 08:29:34) > 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxx> > >>> 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? The tree is just size. Alignment is a little used parameter, but there's a requirement for userspace to be able to control it -- although it is strictly the older interface, it is still open to abuse. Converting the tree to [size, ffs(addr)] would help for many, but on top of that we have zones in the drm_mm, so search-in-range can be abused on top of search-for-alignment. > Never looked into this, but maybe we have a low hanging fruit for an > improvement here? A bit -- alignment is so rarely used in practice, optimising it was not a concern, just someone else has now noticed the potential for abuse. They ran a test, get bored and complained that it didn't respond to ^C for a long period of time and from that derive a proof-of-concept test to show how it can be used by one client to upset another :| > I'm not 100% sure, but moving away from atomic context wouldn't be that > easy. Fair enough. I would not worry unless the layout is controllable by the user -- but we probably want some other means of bounding the search. -Chris _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
