Re: [BUG] sched_mm_cid_exit+0xe2: page fault on CID bitmap write with nopti on 6.19.0

[Mathieu Desnoyers]

On 2026-02-12 16:12, root wrote:
> To: mathieu.desnoyers@xxxxxxxxxxxx
> Cc: peterz@xxxxxxxxxxxxx, mingo@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Subject: [BUG] sched_mm_cid_exit+0xe2: page fault on CID bitmap write with nopti on 6.19.0
>
> Hi Mathieu,
>
> I'm hitting a repeatable page fault in sched_mm_cid_exit() on 6.19.0
> when booting with nopti. The crash occurs during process exit
> (do_exit -> sched_mm_cid_exit) on an atomic bit-clear (lock btr) of
> the CID bitmap. The faulting address is within a 2MB huge page that
> returns a permissions violation on supervisor write access.
>
> The bug triggered 8 times over ~20 hours on a single boot, hitting
> multiple unrelated processes (git, gce_workload_ce). Eventually D-Bus
> died and systemd became non-functional, requiring a hard power-off.

Can you confirm whether the following fix in Linus' tree fixes your issue ?

commit 1e83ccd5921a ("sched/mmcid: Don't assume CID is CPU owned on mode switch")

I suspect that it will soon be cherry picked into stable for an eventual v6.19.1.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com