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    Subject[PATCH 4.14 011/190] sched/fair: Dont NUMA balance for kthreads
    Date
    From: Jens Axboe <axboe@kernel.dk>

    [ Upstream commit 18f855e574d9799a0e7489f8ae6fd8447d0dd74a ]

    Stefano reported a crash with using SQPOLL with io_uring:

    BUG: kernel NULL pointer dereference, address: 00000000000003b0
    CPU: 2 PID: 1307 Comm: io_uring-sq Not tainted 5.7.0-rc7 #11
    RIP: 0010:task_numa_work+0x4f/0x2c0
    Call Trace:
    task_work_run+0x68/0xa0
    io_sq_thread+0x252/0x3d0
    kthread+0xf9/0x130
    ret_from_fork+0x35/0x40

    which is task_numa_work() oopsing on current->mm being NULL.

    The task work is queued by task_tick_numa(), which checks if current->mm is
    NULL at the time of the call. But this state isn't necessarily persistent,
    if the kthread is using use_mm() to temporarily adopt the mm of a task.

    Change the task_tick_numa() check to exclude kernel threads in general,
    as it doesn't make sense to attempt ot balance for kthreads anyway.

    Reported-by: Stefano Garzarella <sgarzare@redhat.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Link: https://lore.kernel.org/r/865de121-8190-5d30-ece5-3b097dc74431@kernel.dk
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/fair.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 0b4e997fea1a..4d8add44fffb 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -2643,7 +2643,7 @@ void task_tick_numa(struct rq *rq, struct task_struct *curr)
    /*
    * We don't care about NUMA placement if we don't have memory.
    */
    - if (!curr->mm || (curr->flags & PF_EXITING) || work->next != work)
    + if ((curr->flags & (PF_EXITING | PF_KTHREAD)) || work->next != work)
    return;

    /*
    --
    2.25.1


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