[tip: sched/urgent] sched/fair: Don't NUMA balance for kthreads

[tip-bot2 for Jens Axboe]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     18f855e574d9799a0e7489f8ae6fd8447d0dd74a
Gitweb:        https://git.kernel.org/tip/18f855e574d9799a0e7489f8ae6fd8447d0dd74a
Author:        Jens Axboe <axboe@xxxxxxxxx>
AuthorDate:    Tue, 26 May 2020 09:38:31 -06:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 26 May 2020 18:34:58 +02:00

sched/fair: Don't NUMA balance for kthreads

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@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/865de121-8190-5d30-ece5-3b097dc74431@xxxxxxxxx
---
 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 538ba5d..da3e5b5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2908,7 +2908,7 @@ static 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;
 
 	/*