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

[Mike Fara <mjfara@gmail.com>]

To: mathieu.desnoyers@efficios.com
Cc: peterz@infradead.org, mingo@redhat.com, linux-kernel@vger.kernel.org, mjfara@gmail.com
Subject: Re: [BUG] sched_mm_cid_exit+0xe2: page fault on CID bitmap write with nopti on 6.19.0

Hi Mathieu,

Confirmed. Rebuilt 6.19.0 with commit 1e83ccd5921a cherry-picked,
CONFIG_SCHED_MM_CID=y, and nopti still on the cmdline. Clean boot,
no oopses.

Verified the fix is compiled in by disassembling sched_mm_cid_exit
from the running kernel. The inlined mm_drop_cid_on_cpu() now has
the cid_on_cpu() guard before the lock btr:

  mm_drop_cid_on_cpu (inlined at sched_mm_cid_exit+0xc4):

    mov    (%rcx),%eax           # Load pcp->cid
    test   $0x40000000,%eax      # Test ONCPU bit (bit 30)
    je     <skip>                # Not CPU-owned? Skip drop entirely
    and    $0xbfffffff,%eax      # Clear ONCPU: cpu_cid_to_cid()
    mov    %eax,(%rcx)           # Store back
    ...
    lock btr %rax,(%rcx)         # mm_drop_cid (bitmap clear)
  <skip>:
    ...                          # Continue safely

Without the fix, the code would fall through to lock btr with a
garbage bit number derived from the TRANSIT flag (bit 29), causing
the out-of-bounds write we reported.

System info:

  # uname -a
  6.19.0-gce #2 SMP PREEMPT_DYNAMIC Thu Feb 12 21:42:52 UTC 2026 x86_64

  # grep SCHED_MM_CID /boot/config-$(uname -r)
  CONFIG_SCHED_MM_CID=y

  # Boot cmdline includes: nopti mitigations=off

  # dmesg | grep -i 'BUG\|oops\|sched_mm\|page.fault'
  (clean - no errors)

Will continue soak testing and report back if anything surfaces.

Tested-by: Mike Fara <mjfara@gmail.com>

Thanks,
Mike Fara
mjfara@gmail.com