Re: [REGRESSION][BISECTED] drm/i915: GPU HANGs on v6.12.20+

> GPU hangs if i915_gem_mmap_gtt_version() returns 5. If set to return 4(as it was
>  in v6.12.19) no HANG occur. This happens on v6.12.20+ kernels ONLY, v6.6.x, v6.
> 18.x, v7.1.3 and v6.12.19 are unaffected.
>
> The actual commit introducing this is bfef148f3680e6b9d28e7fca46d9520f80c5e50e,
> but it doesn't seem to affect any other kernel tree.

Thanks for the report and the bisection. This looks like a missing
prerequisite rather than a bad commit: upstream merged the remap_io_sg()
fix 2e0438f9c3d25e ("drm/i915: ensure segment offset never exceeds
allowed max", v6.14-rc1) before it started advertising GTT mmap version
5, but 6.12.y picked up only the version bump. That leaves userspace
doing multiple partial mmaps against the unfixed remap path, which
matches the wrong-PFN page fault and GPU hangs you're seeing. It also
explains your version matrix: 6.6.x never got the bump, and 6.18.x/7.1.x
have both the bump and the fix in their base.

I've queued 2e0438f9c3d25e for 6.12, which keeps 6.12.y matching
upstream instead of diverging via a revert.

-- 
Thanks,
Sasha