Quoting Lionel Landwerlin (2018-05-11 16:43:02) > On 11/05/18 15:18, Chris Wilson wrote: > > Quoting Lionel Landwerlin (2018-05-11 15:14:13) > >> My understanding of the virtual memory addressing from the GPU is limited... > >> But how can the GPU poke at the kernel's allocated data? > >> I thought we mapped into the GPU's address space only what is allocated > >> through gem. > > Correct. The HW should only be accessing the pages through the GTT and > > the GTT should only contain known pages (or a pointer to the scratch > > page). There is maybe a hole where we are freeing the memory before > > the HW has finished using it (still writing through stale TLB and > > whatnot even though the system has reallocated the pages), but other > > than that quite, quite scary. Hence this awooga. > > -Chris > > > > I managed to reproduce a kasan backtrace on the same test. > So it's not just the CI machine. > > But I can't even startup a gdm on that machine with drm-tip. So maybe > there is some much more broken... Don't leave us in suspense... > i915/perf unpins the object correctly before freeing (at which point it > could be reused). Sure, but does perf know that the OA unit has stopped writing at that point... That's not so clear (from my pov). > Should we ensure i915_vma_destroy() i915/perf maybe? > > It almost seems like this is an issue that could arise in other part of > the driver too. The problem of the HW continuing to access the pages after unbinding is inherent to the system (and what actually happens if we change PTE in flight is usually undefined), hence the great care we go to track HW activity and try not to release pages while it is still using them. -Chris
