linux-kernel - [PATCH 5.6 017/161] sched/fair: Dont NUMA balance for kthreads

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Message-Id: <20200616153107.230078982@linuxfoundation.org>
Date:   Tue, 16 Jun 2020 17:33:27 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Stefano Garzarella <sgarzare@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.6 017/161] sched/fair: Dont NUMA balance for kthreads

From: Jens Axboe <axboe@...nel.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@...hat.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Peter Zijlstra <peterz@...radead.org>
Link: https://lore.kernel.org/r/865de121-8190-5d30-ece5-3b097dc74431@kernel.dk
Signed-off-by: Sasha Levin <sashal@...nel.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 603d3d3cbf77..efb15f0f464b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2682,7 +2682,7 @@ 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;
 
 	/*
-- 
2.25.1